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-rw-r--r--tvix/eval/src/builtins/hash.rs29
-rw-r--r--tvix/eval/src/builtins/impure.rs110
-rw-r--r--tvix/eval/src/builtins/mod.rs1728
-rw-r--r--tvix/eval/src/builtins/to_xml.rs154
-rw-r--r--tvix/eval/src/builtins/versions.rs163
-rw-r--r--tvix/eval/src/chunk.rs273
-rw-r--r--tvix/eval/src/compiler.rs267
-rw-r--r--tvix/eval/src/compiler/bindings.rs826
-rw-r--r--tvix/eval/src/compiler/import.rs120
-rw-r--r--tvix/eval/src/compiler/mod.rs1684
-rw-r--r--tvix/eval/src/compiler/optimiser.rs125
-rw-r--r--tvix/eval/src/compiler/scope.rs378
-rw-r--r--tvix/eval/src/errors.rs1096
-rw-r--r--tvix/eval/src/eval.rs20
-rw-r--r--tvix/eval/src/io.rs164
-rw-r--r--tvix/eval/src/lib.rs394
-rw-r--r--tvix/eval/src/main.rs54
-rw-r--r--tvix/eval/src/nix_search_path.rs256
-rw-r--r--tvix/eval/src/observer.rs318
-rw-r--r--tvix/eval/src/opcode.rs270
-rw-r--r--tvix/eval/src/pretty_ast.rs468
-rw-r--r--tvix/eval/src/properties.rs164
-rw-r--r--tvix/eval/src/source.rs65
-rw-r--r--tvix/eval/src/spans.rs109
-rw-r--r--tvix/eval/src/systems.rs351
-rw-r--r--tvix/eval/src/test_utils.rs8
-rw-r--r--tvix/eval/src/tests/mod.rs203
-rw-r--r--tvix/eval/src/tests/nix_tests/README.md8
-rw-r--r--tvix/eval/src/tests/nix_tests/binary-databin0 -> 1024 bytes
-rw-r--r--tvix/eval/src/tests/nix_tests/data1
-rw-r--r--tvix/eval/src/tests/nix_tests/dir1/a.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/dir2/a.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/dir2/b.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/dir3/a.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/dir3/b.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/dir3/c.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/dir4/a.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/dir4/c.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-abort.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-assert.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-bad-antiquote-1.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-bad-antiquote-3.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-blackhole.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-deepseq.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-foldlStrict-strict-op-application.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-hashfile-missing.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-missing-arg.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-path-slash.nix6
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-remove.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-seq.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-substring.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-fail-to-path.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-any-all.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-any-all.nix11
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-arithmetic.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-arithmetic.nix59
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrnames.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrnames.nix11
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrs.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrs.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrs2.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrs2.nix10
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrs3.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrs3.nix22
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrs4.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrs4.nix7
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrs5.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-attrs5.nix21
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-1.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-1.nix2
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-2.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-2.nix2
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-builtins-add.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-builtins-add.nix8
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-builtins.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-builtins.nix12
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-callable-attrs.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-callable-attrs.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-catattrs.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-catattrs.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-closure.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-closure.exp.xml343
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-closure.nix13
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-comments.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-comments.nix59
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-concat.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-concat.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-concatmap.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-concatmap.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-concatstringssep.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-concatstringssep.nix8
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-deepseq.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-deepseq.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-delayed-with-inherit.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-delayed-with-inherit.nix24
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-bare.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-bare.nix17
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.nix17
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-elem.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-elem.nix6
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-empty-args.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-empty-args.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-eq.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-eq.nix3
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-filter.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-filter.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-flatten.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-flatten.nix8
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-float.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-float.nix6
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-elements.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-elements.nix9
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.nix6
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict.nix3
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-fromTOML.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-fromTOML.nix208
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-fromjson-escapes.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-fromjson-escapes.nix3
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-fromjson.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-fromjson.nix35
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-functionargs.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-functionargs.exp.xml15
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-functionargs.nix80
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-getenv.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-getenv.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-groupBy.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-groupBy.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-hash.exp0
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-hashfile.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-hashfile.nix4
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-hashstring.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-hashstring.nix4
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-if.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-if.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-ind-string.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-ind-string.nix128
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-intersectAttrs.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-intersectAttrs.nix50
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-let.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-let.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-list.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-list.nix7
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-listtoattrs.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-listtoattrs.nix11
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-logic.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-logic.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-map.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-map.nix3
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-mapattrs.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-mapattrs.nix3
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-nested-with.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-nested-with.nix3
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-new-let.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-new-let.nix14
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-null-dynamic-attrs.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-null-dynamic-attrs.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-partition.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-partition.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-path.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-pathexists.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-pathexists.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-patterns.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-patterns.nix16
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-readfile.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-readfile.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-redefine-builtin.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-redefine-builtin.nix3
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-regex-match.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-regex-match.nix29
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-regex-split.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-regex-split.nix48
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-regression-20220122.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-regression-20220122.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-regression-20220125.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-regression-20220125.nix2
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-remove.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-remove.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-1.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-1.nix6
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-2.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-2.nix6
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-3.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-3.nix6
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-4.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-4.nix10
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-6.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-6.nix7
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-7.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-scope-7.nix6
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-seq.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-seq.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-sort.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-sort.nix20
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-splitversion.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-splitversion.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-string.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-string.nix12
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-strings-as-attrs-names.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-strings-as-attrs-names.nix20
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-substring.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-substring.nix21
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-tail-call-1.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-tail-call-1.nix3
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-tojson.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-tojson.nix13
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-toxml.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-toxml.nix3
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-toxml2.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-toxml2.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-tryeval.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-tryeval.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-types.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-types.nix37
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-versions.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-versions.nix43
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-with.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-with.nix19
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-xml.exp.xml52
-rw-r--r--tvix/eval/src/tests/nix_tests/eval-okay-xml.nix21
-rw-r--r--tvix/eval/src/tests/nix_tests/imported.nix3
-rw-r--r--tvix/eval/src/tests/nix_tests/imported2.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/lib.nix61
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-bad-antiquote-2.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-fromTOML-timestamps.nix130
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-nonexist-path.nix4
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-scope-5.nix10
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-undeclared-arg.nix1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.nix4
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.flags1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.nix15
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.nix41
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context.nix6
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-curpos.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-curpos.nix5
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-delayed-with.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-delayed-with.nix29
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-eq-derivations.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-eq-derivations.nix10
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-floor-ceil.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-floor-ceil.nix9
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.exp1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.flags1
-rw-r--r--tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.nix130
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-rw-r--r--tvix/eval/src/vm/macros.rs93
-rw-r--r--tvix/eval/src/vm/mod.rs1368
-rw-r--r--tvix/eval/src/warnings.rs152
897 files changed, 20800 insertions, 834 deletions
diff --git a/tvix/eval/src/builtins/hash.rs b/tvix/eval/src/builtins/hash.rs
new file mode 100644
index 0000000000..d0145f1e7d
--- /dev/null
+++ b/tvix/eval/src/builtins/hash.rs
@@ -0,0 +1,29 @@
+use bstr::ByteSlice;
+use data_encoding::HEXLOWER;
+use md5::Md5;
+use sha1::Sha1;
+use sha2::{digest::Output, Digest, Sha256, Sha512};
+
+use crate::ErrorKind;
+
+/// Reads through all data from the passed reader, and returns the resulting [Digest].
+/// The exact hash function used is left generic over all [Digest].
+fn hash<D: Digest + std::io::Write>(mut r: impl std::io::Read) -> Result<Output<D>, ErrorKind> {
+    let mut hasher = D::new();
+    std::io::copy(&mut r, &mut hasher)?;
+    Ok(hasher.finalize())
+}
+
+/// For a given algo "string" and reader for data, calculate the digest
+/// and return it as a hexlower encoded [String].
+pub fn hash_nix_string(algo: impl AsRef<[u8]>, s: impl std::io::Read) -> Result<String, ErrorKind> {
+    match algo.as_ref() {
+        b"md5" => Ok(HEXLOWER.encode(hash::<Md5>(s)?.as_bstr())),
+        b"sha1" => Ok(HEXLOWER.encode(hash::<Sha1>(s)?.as_bstr())),
+        b"sha256" => Ok(HEXLOWER.encode(hash::<Sha256>(s)?.as_bstr())),
+        b"sha512" => Ok(HEXLOWER.encode(hash::<Sha512>(s)?.as_bstr())),
+        _ => Err(ErrorKind::UnknownHashType(
+            algo.as_ref().as_bstr().to_string(),
+        )),
+    }
+}
diff --git a/tvix/eval/src/builtins/impure.rs b/tvix/eval/src/builtins/impure.rs
new file mode 100644
index 0000000000..c82b910f5f
--- /dev/null
+++ b/tvix/eval/src/builtins/impure.rs
@@ -0,0 +1,110 @@
+use builtin_macros::builtins;
+use genawaiter::rc::Gen;
+
+use std::{
+    env,
+    time::{SystemTime, UNIX_EPOCH},
+};
+
+use crate::{
+    self as tvix_eval,
+    errors::ErrorKind,
+    io::FileType,
+    value::NixAttrs,
+    vm::generators::{self, GenCo},
+    NixString, Value,
+};
+
+#[builtins]
+mod impure_builtins {
+    use std::ffi::OsStr;
+    use std::os::unix::ffi::OsStrExt;
+
+    use super::*;
+    use crate::builtins::{coerce_value_to_path, hash::hash_nix_string};
+
+    #[builtin("getEnv")]
+    async fn builtin_get_env(co: GenCo, var: Value) -> Result<Value, ErrorKind> {
+        Ok(env::var(OsStr::from_bytes(&var.to_str()?))
+            .unwrap_or_else(|_| "".into())
+            .into())
+    }
+
+    #[builtin("hashFile")]
+    async fn builtin_hash_file(co: GenCo, algo: Value, path: Value) -> Result<Value, ErrorKind> {
+        let path = match coerce_value_to_path(&co, path).await? {
+            Err(cek) => return Ok(Value::from(cek)),
+            Ok(p) => p,
+        };
+        let r = generators::request_open_file(&co, path).await;
+        hash_nix_string(algo.to_str()?, r).map(Value::from)
+    }
+
+    #[builtin("pathExists")]
+    async fn builtin_path_exists(co: GenCo, path: Value) -> Result<Value, ErrorKind> {
+        match coerce_value_to_path(&co, path).await? {
+            Err(cek) => Ok(Value::from(cek)),
+            Ok(path) => Ok(generators::request_path_exists(&co, path).await),
+        }
+    }
+
+    #[builtin("readDir")]
+    async fn builtin_read_dir(co: GenCo, path: Value) -> Result<Value, ErrorKind> {
+        match coerce_value_to_path(&co, path).await? {
+            Err(cek) => Ok(Value::from(cek)),
+            Ok(path) => {
+                let dir = generators::request_read_dir(&co, path).await;
+                let res = dir.into_iter().map(|(name, ftype)| {
+                    (
+                        // TODO: propagate Vec<u8> or bytes::Bytes into NixString.
+                        NixString::from(
+                            String::from_utf8(name.to_vec()).expect("parsing file name as string"),
+                        ),
+                        Value::from(match ftype {
+                            FileType::Directory => "directory",
+                            FileType::Regular => "regular",
+                            FileType::Symlink => "symlink",
+                            FileType::Unknown => "unknown",
+                        }),
+                    )
+                });
+
+                Ok(Value::attrs(NixAttrs::from_iter(res)))
+            }
+        }
+    }
+
+    #[builtin("readFile")]
+    async fn builtin_read_file(co: GenCo, path: Value) -> Result<Value, ErrorKind> {
+        match coerce_value_to_path(&co, path).await? {
+            Err(cek) => Ok(Value::from(cek)),
+            Ok(path) => {
+                let mut buf = Vec::new();
+                generators::request_open_file(&co, path)
+                    .await
+                    .read_to_end(&mut buf)?;
+                Ok(Value::from(buf))
+            }
+        }
+    }
+}
+
+/// Return all impure builtins, that is all builtins which may perform I/O
+/// outside of the VM and so cannot be used in all contexts (e.g. WASM).
+pub fn impure_builtins() -> Vec<(&'static str, Value)> {
+    let mut result = impure_builtins::builtins();
+
+    // currentTime pins the time at which evaluation was started
+    {
+        let seconds = match SystemTime::now().duration_since(UNIX_EPOCH) {
+            Ok(dur) => dur.as_secs() as i64,
+
+            // This case is hit if the system time is *before* epoch.
+            Err(err) => -(err.duration().as_secs() as i64),
+        };
+
+        result.push(("currentTime", Value::Integer(seconds)));
+    }
+
+    result
+}
diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
new file mode 100644
index 0000000000..04a0b3dd33
--- /dev/null
+++ b/tvix/eval/src/builtins/mod.rs
@@ -0,0 +1,1728 @@
+//! This module implements the builtins exposed in the Nix language.
+//!
+//! See //tvix/eval/docs/builtins.md for a some context on the
+//! available builtins in Nix.
+
+use bstr::{ByteSlice, ByteVec};
+use builtin_macros::builtins;
+use genawaiter::rc::Gen;
+use imbl::OrdMap;
+use regex::Regex;
+use std::cmp::{self, Ordering};
+use std::collections::VecDeque;
+use std::collections::{BTreeMap, HashSet};
+use std::path::PathBuf;
+
+use crate::arithmetic_op;
+use crate::value::PointerEquality;
+use crate::vm::generators::{self, GenCo};
+use crate::warnings::WarningKind;
+use crate::{
+    self as tvix_eval,
+    builtins::hash::hash_nix_string,
+    errors::{CatchableErrorKind, ErrorKind},
+    value::{CoercionKind, NixAttrs, NixList, NixString, Thunk, Value},
+};
+
+use self::versions::{VersionPart, VersionPartsIter};
+
+mod hash;
+mod to_xml;
+mod versions;
+
+#[cfg(test)]
+pub use to_xml::value_to_xml;
+
+#[cfg(feature = "impure")]
+mod impure;
+
+#[cfg(feature = "impure")]
+pub use impure::impure_builtins;
+
+// we set TVIX_CURRENT_SYSTEM in build.rs
+pub const CURRENT_PLATFORM: &str = env!("TVIX_CURRENT_SYSTEM");
+
+/// Coerce a Nix Value to a plain path, e.g. in order to access the
+/// file it points to via either `builtins.toPath` or an impure
+/// builtin. This coercion can _never_ be performed in a Nix program
+/// without using builtins (i.e. the trick `path: /. + path` to
+/// convert from a string to a path wouldn't hit this code).
+///
+/// This operation doesn't import a Nix path value into the store.
+pub async fn coerce_value_to_path(
+    co: &GenCo,
+    v: Value,
+) -> Result<Result<PathBuf, CatchableErrorKind>, ErrorKind> {
+    let value = generators::request_force(co, v).await;
+    if let Value::Path(p) = value {
+        return Ok(Ok(*p));
+    }
+
+    match generators::request_string_coerce(
+        co,
+        value,
+        CoercionKind {
+            strong: false,
+            import_paths: false,
+        },
+    )
+    .await
+    {
+        Ok(vs) => {
+            let path = vs.to_path()?.to_owned();
+            if path.is_absolute() {
+                Ok(Ok(path))
+            } else {
+                Err(ErrorKind::NotAnAbsolutePath(path))
+            }
+        }
+        Err(cek) => Ok(Err(cek)),
+    }
+}
+
+#[builtins]
+mod pure_builtins {
+    use std::ffi::OsString;
+
+    use bstr::{BString, ByteSlice, B};
+    use imbl::Vector;
+    use itertools::Itertools;
+    use os_str_bytes::OsStringBytes;
+
+    use crate::{value::PointerEquality, AddContext, NixContext, NixContextElement};
+
+    use super::*;
+
+    macro_rules! try_value {
+        ($value:expr) => {{
+            let val = $value;
+            if val.is_catchable() {
+                return Ok(val);
+            }
+            val
+        }};
+    }
+
+    #[builtin("abort")]
+    async fn builtin_abort(co: GenCo, message: Value) -> Result<Value, ErrorKind> {
+        // TODO(sterni): coerces to string
+        // Although `abort` does not make use of any context,
+        // we must still accept contextful strings as parameters.
+        // If `to_str` was used, this would err out with an unexpected type error.
+        // Therefore, we explicitly accept contextful strings and ignore their contexts.
+        Err(ErrorKind::Abort(message.to_contextful_str()?.to_string()))
+    }
+
+    #[builtin("add")]
+    async fn builtin_add(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        arithmetic_op!(&x, &y, +)
+    }
+
+    #[builtin("all")]
+    async fn builtin_all(co: GenCo, pred: Value, list: Value) -> Result<Value, ErrorKind> {
+        for value in list.to_list()?.into_iter() {
+            let pred_result = generators::request_call_with(&co, pred.clone(), [value]).await;
+            let pred_result = try_value!(generators::request_force(&co, pred_result).await);
+
+            if !pred_result.as_bool()? {
+                return Ok(Value::Bool(false));
+            }
+        }
+
+        Ok(Value::Bool(true))
+    }
+
+    #[builtin("any")]
+    async fn builtin_any(co: GenCo, pred: Value, list: Value) -> Result<Value, ErrorKind> {
+        for value in list.to_list()?.into_iter() {
+            let pred_result = generators::request_call_with(&co, pred.clone(), [value]).await;
+            let pred_result = try_value!(generators::request_force(&co, pred_result).await);
+
+            if pred_result.as_bool()? {
+                return Ok(Value::Bool(true));
+            }
+        }
+
+        Ok(Value::Bool(false))
+    }
+
+    #[builtin("attrNames")]
+    async fn builtin_attr_names(co: GenCo, set: Value) -> Result<Value, ErrorKind> {
+        let xs = set.to_attrs()?;
+        let mut output = Vec::with_capacity(xs.len());
+
+        for (key, _val) in xs.iter() {
+            output.push(Value::from(key.clone()));
+        }
+
+        Ok(Value::List(NixList::construct(output.len(), output)))
+    }
+
+    #[builtin("attrValues")]
+    async fn builtin_attr_values(co: GenCo, set: Value) -> Result<Value, ErrorKind> {
+        let xs = set.to_attrs()?;
+        let mut output = Vec::with_capacity(xs.len());
+
+        for (_key, val) in xs.iter() {
+            output.push(val.clone());
+        }
+
+        Ok(Value::List(NixList::construct(output.len(), output)))
+    }
+
+    #[builtin("baseNameOf")]
+    async fn builtin_base_name_of(co: GenCo, s: Value) -> Result<Value, ErrorKind> {
+        let span = generators::request_span(&co).await;
+        let s = s
+            .coerce_to_string(
+                co,
+                CoercionKind {
+                    strong: false,
+                    import_paths: false,
+                },
+                span,
+            )
+            .await?
+            .to_contextful_str()?;
+
+        let mut bs = (**s).to_owned();
+        if let Some(last_slash) = bs.rfind_char('/') {
+            bs = bs[(last_slash + 1)..].into();
+        }
+        Ok(NixString::new_inherit_context_from(&s, bs).into())
+    }
+
+    #[builtin("bitAnd")]
+    async fn builtin_bit_and(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Integer(x.as_int()? & y.as_int()?))
+    }
+
+    #[builtin("bitOr")]
+    async fn builtin_bit_or(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Integer(x.as_int()? | y.as_int()?))
+    }
+
+    #[builtin("bitXor")]
+    async fn builtin_bit_xor(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Integer(x.as_int()? ^ y.as_int()?))
+    }
+
+    #[builtin("catAttrs")]
+    async fn builtin_cat_attrs(co: GenCo, key: Value, list: Value) -> Result<Value, ErrorKind> {
+        let key = key.to_str()?;
+        let list = list.to_list()?;
+        let mut output = vec![];
+
+        for item in list.into_iter() {
+            let set = generators::request_force(&co, item).await.to_attrs()?;
+
+            if let Some(value) = set.select(&key) {
+                output.push(value.clone());
+            }
+        }
+
+        Ok(Value::List(NixList::construct(output.len(), output)))
+    }
+
+    #[builtin("ceil")]
+    async fn builtin_ceil(co: GenCo, double: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Integer(double.as_float()?.ceil() as i64))
+    }
+
+    #[builtin("compareVersions")]
+    async fn builtin_compare_versions(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        let s1 = x.to_str()?;
+        let s1 = VersionPartsIter::new_for_cmp((&s1).into());
+        let s2 = y.to_str()?;
+        let s2 = VersionPartsIter::new_for_cmp((&s2).into());
+
+        match s1.cmp(s2) {
+            std::cmp::Ordering::Less => Ok(Value::Integer(-1)),
+            std::cmp::Ordering::Equal => Ok(Value::Integer(0)),
+            std::cmp::Ordering::Greater => Ok(Value::Integer(1)),
+        }
+    }
+
+    #[builtin("concatLists")]
+    async fn builtin_concat_lists(co: GenCo, lists: Value) -> Result<Value, ErrorKind> {
+        let mut out = imbl::Vector::new();
+
+        for value in lists.to_list()? {
+            let list = try_value!(generators::request_force(&co, value).await).to_list()?;
+            out.extend(list.into_iter());
+        }
+
+        Ok(Value::List(out.into()))
+    }
+
+    #[builtin("concatMap")]
+    async fn builtin_concat_map(co: GenCo, f: Value, list: Value) -> Result<Value, ErrorKind> {
+        let list = list.to_list()?;
+        let mut res = imbl::Vector::new();
+        for val in list {
+            let out = generators::request_call_with(&co, f.clone(), [val]).await;
+            let out = try_value!(generators::request_force(&co, out).await);
+            res.extend(out.to_list()?);
+        }
+        Ok(Value::List(res.into()))
+    }
+
+    #[builtin("concatStringsSep")]
+    async fn builtin_concat_strings_sep(
+        co: GenCo,
+        separator: Value,
+        list: Value,
+    ) -> Result<Value, ErrorKind> {
+        let mut separator = separator.to_contextful_str()?;
+        let mut context = NixContext::new();
+        if let Some(sep_context) = separator.context_mut() {
+            context = context.join(sep_context);
+        }
+        let list = list.to_list()?;
+        let mut res = BString::default();
+        for (i, val) in list.into_iter().enumerate() {
+            if i != 0 {
+                res.push_str(&separator);
+            }
+            match generators::request_string_coerce(
+                &co,
+                val,
+                CoercionKind {
+                    strong: false,
+                    import_paths: true,
+                },
+            )
+            .await
+            {
+                Ok(mut s) => {
+                    res.push_str(&s);
+                    if let Some(ref mut other_context) = s.context_mut() {
+                        // It is safe to consume the other context here
+                        // because the `list` and `separator` are originally
+                        // moved, here.
+                        // We are not going to use them again
+                        // because the result here is a string.
+                        context = context.join(other_context);
+                    }
+                }
+                Err(c) => return Ok(Value::Catchable(Box::new(c))),
+            }
+        }
+        // FIXME: pass immediately the string res.
+        Ok(NixString::new_context_from(context, res).into())
+    }
+
+    #[builtin("deepSeq")]
+    async fn builtin_deep_seq(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        generators::request_deep_force(&co, x).await;
+        Ok(y)
+    }
+
+    #[builtin("div")]
+    async fn builtin_div(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        arithmetic_op!(&x, &y, /)
+    }
+
+    #[builtin("dirOf")]
+    async fn builtin_dir_of(co: GenCo, s: Value) -> Result<Value, ErrorKind> {
+        let is_path = s.is_path();
+        let span = generators::request_span(&co).await;
+        let str = s
+            .coerce_to_string(
+                co,
+                CoercionKind {
+                    strong: false,
+                    import_paths: false,
+                },
+                span,
+            )
+            .await?
+            .to_contextful_str()?;
+        let result = str
+            .rfind_char('/')
+            .map(|last_slash| {
+                let x = &str[..last_slash];
+                if x.is_empty() {
+                    B("/")
+                } else {
+                    x
+                }
+            })
+            .unwrap_or(b".");
+        if is_path {
+            Ok(Value::Path(Box::new(PathBuf::from(
+                OsString::assert_from_raw_vec(result.to_owned()),
+            ))))
+        } else {
+            Ok(Value::from(NixString::new_inherit_context_from(
+                &str, result,
+            )))
+        }
+    }
+
+    #[builtin("elem")]
+    async fn builtin_elem(co: GenCo, x: Value, xs: Value) -> Result<Value, ErrorKind> {
+        for val in xs.to_list()? {
+            match generators::check_equality(&co, x.clone(), val, PointerEquality::AllowAll).await?
+            {
+                Ok(true) => return Ok(true.into()),
+                Ok(false) => continue,
+                Err(cek) => return Ok(Value::from(cek)),
+            }
+        }
+        Ok(false.into())
+    }
+
+    #[builtin("elemAt")]
+    async fn builtin_elem_at(co: GenCo, xs: Value, i: Value) -> Result<Value, ErrorKind> {
+        let xs = xs.to_list()?;
+        let i = i.as_int()?;
+        if i < 0 {
+            Err(ErrorKind::IndexOutOfBounds { index: i })
+        } else {
+            match xs.get(i as usize) {
+                Some(x) => Ok(x.clone()),
+                None => Err(ErrorKind::IndexOutOfBounds { index: i }),
+            }
+        }
+    }
+
+    #[builtin("filter")]
+    async fn builtin_filter(co: GenCo, pred: Value, list: Value) -> Result<Value, ErrorKind> {
+        let list: NixList = list.to_list()?;
+        let mut out = imbl::Vector::new();
+
+        for value in list {
+            let result = generators::request_call_with(&co, pred.clone(), [value.clone()]).await;
+            let verdict = try_value!(generators::request_force(&co, result).await);
+            if verdict.as_bool()? {
+                out.push_back(value);
+            }
+        }
+
+        Ok(Value::List(out.into()))
+    }
+
+    #[builtin("floor")]
+    async fn builtin_floor(co: GenCo, double: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Integer(double.as_float()?.floor() as i64))
+    }
+
+    #[builtin("foldl'")]
+    async fn builtin_foldl(
+        co: GenCo,
+        op: Value,
+        #[lazy] nul: Value,
+        list: Value,
+    ) -> Result<Value, ErrorKind> {
+        let mut nul = nul;
+        let list = list.to_list()?;
+        for val in list {
+            // Every call of `op` is forced immediately, but `nul` is not, see
+            // https://github.com/NixOS/nix/blob/940e9eb8/src/libexpr/primops.cc#L3069-L3070C36
+            // and our tests for foldl'.
+            nul = generators::request_call_with(&co, op.clone(), [nul, val]).await;
+            nul = generators::request_force(&co, nul).await;
+            if let c @ Value::Catchable(_) = nul {
+                return Ok(c);
+            }
+        }
+
+        Ok(nul)
+    }
+
+    #[builtin("functionArgs")]
+    async fn builtin_function_args(co: GenCo, f: Value) -> Result<Value, ErrorKind> {
+        let lambda = &f.as_closure()?.lambda();
+        let formals = if let Some(formals) = &lambda.formals {
+            formals
+        } else {
+            return Ok(Value::attrs(NixAttrs::empty()));
+        };
+        Ok(Value::attrs(NixAttrs::from_iter(
+            formals.arguments.iter().map(|(k, v)| (k.clone(), (*v))),
+        )))
+    }
+
+    #[builtin("fromJSON")]
+    async fn builtin_from_json(co: GenCo, json: Value) -> Result<Value, ErrorKind> {
+        let json_str = json.to_str()?;
+
+        serde_json::from_slice(&json_str).map_err(|err| err.into())
+    }
+
+    #[builtin("toJSON")]
+    async fn builtin_to_json(co: GenCo, val: Value) -> Result<Value, ErrorKind> {
+        match val.into_contextful_json(&co).await? {
+            Err(cek) => Ok(Value::from(cek)),
+            Ok((json_value, ctx)) => {
+                let json_str = serde_json::to_string(&json_value)?;
+                Ok(NixString::new_context_from(ctx, json_str).into())
+            }
+        }
+    }
+
+    #[builtin("fromTOML")]
+    async fn builtin_from_toml(co: GenCo, toml: Value) -> Result<Value, ErrorKind> {
+        let toml_str = toml.to_str()?;
+
+        toml::from_str(toml_str.to_str()?).map_err(|err| err.into())
+    }
+
+    #[builtin("filterSource")]
+    #[allow(non_snake_case)]
+    async fn builtin_filterSource(_co: GenCo, #[lazy] _e: Value) -> Result<Value, ErrorKind> {
+        // TODO: implement for nixpkgs compatibility
+        Ok(Value::from(CatchableErrorKind::UnimplementedFeature(
+            "filterSource".into(),
+        )))
+    }
+
+    #[builtin("genericClosure")]
+    async fn builtin_generic_closure(co: GenCo, input: Value) -> Result<Value, ErrorKind> {
+        let attrs = input.to_attrs()?;
+
+        // The work set is maintained as a VecDeque because new items
+        // are popped from the front.
+        let mut work_set: VecDeque<Value> =
+            generators::request_force(&co, attrs.select_required("startSet")?.clone())
+                .await
+                .to_list()?
+                .into_iter()
+                .collect();
+
+        let operator = attrs.select_required("operator")?;
+
+        let mut res = imbl::Vector::new();
+        let mut done_keys: Vec<Value> = vec![];
+
+        while let Some(val) = work_set.pop_front() {
+            let val = generators::request_force(&co, val).await;
+            let attrs = val.to_attrs()?;
+            let key = attrs.select_required("key")?;
+
+            let value_missing = bgc_insert_key(&co, key.clone(), &mut done_keys).await?;
+
+            if let Err(cek) = value_missing {
+                return Ok(Value::Catchable(Box::new(cek)));
+            }
+
+            if let Ok(false) = value_missing {
+                continue;
+            }
+
+            res.push_back(val.clone());
+
+            let op_result = generators::request_force(
+                &co,
+                generators::request_call_with(&co, operator.clone(), [val]).await,
+            )
+            .await;
+
+            work_set.extend(op_result.to_list()?.into_iter());
+        }
+
+        Ok(Value::List(NixList::from(res)))
+    }
+
+    #[builtin("genList")]
+    async fn builtin_gen_list(
+        co: GenCo,
+        // Nix 2.3 doesn't propagate failures here
+        #[lazy] generator: Value,
+        length: Value,
+    ) -> Result<Value, ErrorKind> {
+        let mut out = imbl::Vector::<Value>::new();
+        let len = length.as_int()?;
+        // the best span we can getโ€ฆ
+        let span = generators::request_span(&co).await;
+
+        for i in 0..len {
+            let val = Value::Thunk(Thunk::new_suspended_call(
+                generator.clone(),
+                i.into(),
+                span.clone(),
+            ));
+            out.push_back(val);
+        }
+
+        Ok(Value::List(out.into()))
+    }
+
+    #[builtin("getAttr")]
+    async fn builtin_get_attr(co: GenCo, key: Value, set: Value) -> Result<Value, ErrorKind> {
+        let k = key.to_str()?;
+        let xs = set.to_attrs()?;
+
+        match xs.select(&k) {
+            Some(x) => Ok(x.clone()),
+            None => Err(ErrorKind::AttributeNotFound {
+                name: k.to_string(),
+            }),
+        }
+    }
+
+    #[builtin("groupBy")]
+    async fn builtin_group_by(co: GenCo, f: Value, list: Value) -> Result<Value, ErrorKind> {
+        let mut res: BTreeMap<NixString, imbl::Vector<Value>> = BTreeMap::new();
+        for val in list.to_list()? {
+            let key = try_value!(
+                generators::request_force(
+                    &co,
+                    generators::request_call_with(&co, f.clone(), [val.clone()]).await,
+                )
+                .await
+            )
+            .to_str()?;
+
+            res.entry(key).or_default().push_back(val);
+        }
+        Ok(Value::attrs(NixAttrs::from_iter(
+            res.into_iter()
+                .map(|(k, v)| (k, Value::List(NixList::from(v)))),
+        )))
+    }
+
+    #[builtin("hasAttr")]
+    async fn builtin_has_attr(co: GenCo, key: Value, set: Value) -> Result<Value, ErrorKind> {
+        let k = key.to_str()?;
+        let xs = set.to_attrs()?;
+
+        Ok(Value::Bool(xs.contains(&k)))
+    }
+
+    #[builtin("hasContext")]
+    #[allow(non_snake_case)]
+    async fn builtin_hasContext(co: GenCo, e: Value) -> Result<Value, ErrorKind> {
+        if e.is_catchable() {
+            return Ok(e);
+        }
+
+        let v = e.to_contextful_str()?;
+        Ok(Value::Bool(v.has_context()))
+    }
+
+    #[builtin("getContext")]
+    #[allow(non_snake_case)]
+    async fn builtin_getContext(co: GenCo, e: Value) -> Result<Value, ErrorKind> {
+        if e.is_catchable() {
+            return Ok(e);
+        }
+
+        // also forces the value
+        let span = generators::request_span(&co).await;
+        let v = e
+            .coerce_to_string(
+                co,
+                CoercionKind {
+                    strong: true,
+                    import_paths: true,
+                },
+                span,
+            )
+            .await?;
+        let s = v.to_contextful_str()?;
+
+        let groups = s
+            .iter_context()
+            .flat_map(|context| context.iter())
+            // Do not think `group_by` works here.
+            // `group_by` works on consecutive elements of the iterator.
+            // Due to how `HashSet` works (ordering is not guaranteed),
+            // this can become a source of non-determinism if you `group_by` naively.
+            // I know I did.
+            .into_grouping_map_by(|ctx_element| match ctx_element {
+                NixContextElement::Plain(spath) => spath,
+                NixContextElement::Single { derivation, .. } => derivation,
+                NixContextElement::Derivation(drv_path) => drv_path,
+            })
+            .collect::<Vec<_>>();
+
+        let elements = groups
+            .into_iter()
+            .map(|(key, group)| {
+                let mut outputs: Vector<NixString> = Vector::new();
+                let mut is_path = false;
+                let mut all_outputs = false;
+
+                for ctx_element in group {
+                    match ctx_element {
+                        NixContextElement::Plain(spath) => {
+                            debug_assert!(spath == key, "Unexpected group containing mixed keys, expected: {:?}, encountered {:?}", key, spath);
+                            is_path = true;
+                        }
+
+                        NixContextElement::Single { name, derivation } => {
+                            debug_assert!(derivation == key, "Unexpected group containing mixed keys, expected: {:?}, encountered {:?}", key, derivation);
+                            outputs.push_back(name.clone().into());
+                        }
+
+                        NixContextElement::Derivation(drv_path) => {
+                            debug_assert!(drv_path == key, "Unexpected group containing mixed keys, expected: {:?}, encountered {:?}", key, drv_path);
+                            all_outputs = true;
+                        }
+                    }
+                }
+
+                // FIXME(raitobezarius): is there a better way to construct an attribute set
+                // conditionally?
+                let mut vec_attrs: Vec<(&str, Value)> = Vec::new();
+
+                if is_path {
+                    vec_attrs.push(("path", true.into()));
+                }
+
+                if all_outputs {
+                    vec_attrs.push(("allOutputs", true.into()));
+                }
+
+                if !outputs.is_empty() {
+                    outputs.sort();
+                    vec_attrs.push(("outputs", Value::List(outputs
+                                .into_iter()
+                                .map(|s| s.into())
+                                .collect::<Vector<Value>>()
+                                .into()
+                    )));
+                }
+
+                (key.clone(), Value::attrs(NixAttrs::from_iter(vec_attrs.into_iter())))
+            });
+
+        Ok(Value::attrs(NixAttrs::from_iter(elements)))
+    }
+
+    #[builtin("appendContext")]
+    #[allow(non_snake_case)]
+    async fn builtin_appendContext(
+        co: GenCo,
+        origin: Value,
+        added_context: Value,
+    ) -> Result<Value, ErrorKind> {
+        // `appendContext` is a "grow" context function.
+        // It cannot remove a context element, neither replace a piece of its contents.
+        //
+        // Growing context is always a safe operation, there's no loss of dependency tracking
+        // information.
+        //
+        // This is why this operation is not prefixed by `unsafe` and is deemed *safe*.
+        // Nonetheless, it is possible to craft nonsensical context elements referring
+        // to inexistent derivations, output paths or output names.
+        //
+        // In Nix, those nonsensical context elements are partially mitigated by checking
+        // that various parameters are indeed syntatically valid store paths in the context, i.e.
+        // starting with the same prefix as `builtins.storeDir`, or ending with `.drv`.
+        // In addition, if writing to the store is possible (evaluator not in read-only mode), Nix
+        // will realize some paths and ensures they are present in the store.
+        //
+        // In this implementation, we do none of that, no syntax checks, no realization.
+        // The next `TODO` are the checks that Nix implements.
+        let mut ctx_elements: HashSet<NixContextElement> = HashSet::new();
+        let span = generators::request_span(&co).await;
+        let origin = origin
+            .coerce_to_string(
+                co,
+                CoercionKind {
+                    strong: true,
+                    import_paths: true,
+                },
+                span,
+            )
+            .await?;
+        let mut origin = origin.to_contextful_str()?;
+
+        let added_context = added_context.to_attrs()?;
+        for (context_key, context_element) in added_context.into_iter() {
+            // Invariant checks:
+            // - TODO: context_key must be a syntactically valid store path.
+            // - Perform a deep force `context_element`.
+            let context_element = context_element.to_attrs()?;
+            if let Some(path) = context_element.select("path") {
+                if path.as_bool()? {
+                    ctx_elements.insert(NixContextElement::Plain(context_key.to_string()));
+                }
+            }
+            if let Some(all_outputs) = context_element.select("allOutputs") {
+                if all_outputs.as_bool()? {
+                    // TODO: check if `context_key` is a derivation path.
+                    // This may require realization.
+                    ctx_elements.insert(NixContextElement::Derivation(context_key.to_string()));
+                }
+            }
+            if let Some(some_outputs) = context_element.select("outputs") {
+                let some_outputs = some_outputs.to_list()?;
+                // TODO: check if `context_key` is a derivation path.
+                // This may require realization.
+                for output in some_outputs.into_iter() {
+                    let output = output.to_str()?;
+                    ctx_elements.insert(NixContextElement::Single {
+                        derivation: context_key.to_string(),
+                        name: output.to_string(),
+                    });
+                }
+            }
+        }
+
+        if let Some(origin_ctx) = origin.context_mut() {
+            // FUTUREWORK(performance): avoid this clone
+            // and extend in-place.
+            *origin_ctx = origin_ctx.clone().join(&mut ctx_elements.into());
+        }
+
+        Ok(origin.into())
+    }
+
+    #[builtin("hashString")]
+    async fn builtin_hash_string(co: GenCo, algo: Value, s: Value) -> Result<Value, ErrorKind> {
+        hash_nix_string(algo.to_str()?, std::io::Cursor::new(s.to_str()?)).map(Value::from)
+    }
+
+    #[builtin("head")]
+    async fn builtin_head(co: GenCo, list: Value) -> Result<Value, ErrorKind> {
+        if list.is_catchable() {
+            return Ok(list);
+        }
+
+        match list.to_list()?.get(0) {
+            Some(x) => Ok(x.clone()),
+            None => Err(ErrorKind::IndexOutOfBounds { index: 0 }),
+        }
+    }
+
+    #[builtin("intersectAttrs")]
+    async fn builtin_intersect_attrs(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        if x.is_catchable() {
+            return Ok(x);
+        }
+        if y.is_catchable() {
+            return Ok(y);
+        }
+        let left_set = x.to_attrs()?;
+        if left_set.is_empty() {
+            return Ok(Value::attrs(NixAttrs::empty()));
+        }
+        let mut left_keys = left_set.keys();
+
+        let right_set = y.to_attrs()?;
+        if right_set.is_empty() {
+            return Ok(Value::attrs(NixAttrs::empty()));
+        }
+        let mut right_keys = right_set.keys();
+
+        let mut out: OrdMap<NixString, Value> = OrdMap::new();
+
+        // Both iterators have at least one entry
+        let mut left = left_keys.next().unwrap();
+        let mut right = right_keys.next().unwrap();
+
+        // Calculate the intersection of the attribute sets by simultaneously
+        // advancing two key iterators, and inserting into the result set from
+        // the right side when the keys match. Iteration over Nix attribute sets
+        // is in sorted lexicographical order, so we can advance either iterator
+        // until it "catches up" with its counterpart.
+        //
+        // Only when keys match are the key and value clones actually allocated.
+        //
+        // We opted for this implementation over simpler ones because of the
+        // heavy use of this function in nixpkgs.
+        loop {
+            if left == right {
+                // We know that the key exists in the set, and can
+                // skip the check instructions.
+                unsafe {
+                    out.insert(
+                        right.clone(),
+                        right_set.select(right).unwrap_unchecked().clone(),
+                    );
+                }
+
+                left = match left_keys.next() {
+                    Some(x) => x,
+                    None => break,
+                };
+
+                right = match right_keys.next() {
+                    Some(x) => x,
+                    None => break,
+                };
+
+                continue;
+            }
+
+            if left < right {
+                left = match left_keys.next() {
+                    Some(x) => x,
+                    None => break,
+                };
+                continue;
+            }
+
+            if right < left {
+                right = match right_keys.next() {
+                    Some(x) => x,
+                    None => break,
+                };
+                continue;
+            }
+        }
+
+        Ok(Value::attrs(out.into()))
+    }
+
+    #[builtin("isAttrs")]
+    async fn builtin_is_attrs(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        // TODO(edef): make this beautiful
+        if value.is_catchable() {
+            return Ok(value);
+        }
+
+        Ok(Value::Bool(matches!(value, Value::Attrs(_))))
+    }
+
+    #[builtin("isBool")]
+    async fn builtin_is_bool(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        if value.is_catchable() {
+            return Ok(value);
+        }
+
+        Ok(Value::Bool(matches!(value, Value::Bool(_))))
+    }
+
+    #[builtin("isFloat")]
+    async fn builtin_is_float(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        if value.is_catchable() {
+            return Ok(value);
+        }
+
+        Ok(Value::Bool(matches!(value, Value::Float(_))))
+    }
+
+    #[builtin("isFunction")]
+    async fn builtin_is_function(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        if value.is_catchable() {
+            return Ok(value);
+        }
+
+        Ok(Value::Bool(matches!(
+            value,
+            Value::Closure(_) | Value::Builtin(_)
+        )))
+    }
+
+    #[builtin("isInt")]
+    async fn builtin_is_int(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        if value.is_catchable() {
+            return Ok(value);
+        }
+
+        Ok(Value::Bool(matches!(value, Value::Integer(_))))
+    }
+
+    #[builtin("isList")]
+    async fn builtin_is_list(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        if value.is_catchable() {
+            return Ok(value);
+        }
+
+        Ok(Value::Bool(matches!(value, Value::List(_))))
+    }
+
+    #[builtin("isNull")]
+    async fn builtin_is_null(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        if value.is_catchable() {
+            return Ok(value);
+        }
+
+        Ok(Value::Bool(matches!(value, Value::Null)))
+    }
+
+    #[builtin("isPath")]
+    async fn builtin_is_path(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        if value.is_catchable() {
+            return Ok(value);
+        }
+
+        Ok(Value::Bool(matches!(value, Value::Path(_))))
+    }
+
+    #[builtin("isString")]
+    async fn builtin_is_string(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        if value.is_catchable() {
+            return Ok(value);
+        }
+
+        Ok(Value::Bool(matches!(value, Value::String(_))))
+    }
+
+    #[builtin("length")]
+    async fn builtin_length(co: GenCo, list: Value) -> Result<Value, ErrorKind> {
+        if list.is_catchable() {
+            return Ok(list);
+        }
+        Ok(Value::Integer(list.to_list()?.len() as i64))
+    }
+
+    #[builtin("lessThan")]
+    async fn builtin_less_than(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        let span = generators::request_span(&co).await;
+        match x.nix_cmp_ordering(y, co, span).await? {
+            Err(cek) => Ok(Value::from(cek)),
+            Ok(Ordering::Less) => Ok(Value::Bool(true)),
+            Ok(_) => Ok(Value::Bool(false)),
+        }
+    }
+
+    #[builtin("listToAttrs")]
+    async fn builtin_list_to_attrs(co: GenCo, list: Value) -> Result<Value, ErrorKind> {
+        let list = list.to_list()?;
+        let mut map = BTreeMap::new();
+        for val in list {
+            let attrs = try_value!(generators::request_force(&co, val).await).to_attrs()?;
+            let name = try_value!(
+                generators::request_force(&co, attrs.select_required("name")?.clone()).await
+            )
+            .to_str()?;
+            let value = attrs.select_required("value")?.clone();
+            // Map entries earlier in the list take precedence over entries later in the list
+            map.entry(name).or_insert(value);
+        }
+        Ok(Value::attrs(NixAttrs::from_iter(map.into_iter())))
+    }
+
+    #[builtin("map")]
+    async fn builtin_map(co: GenCo, #[lazy] f: Value, list: Value) -> Result<Value, ErrorKind> {
+        let mut out = imbl::Vector::<Value>::new();
+
+        // the best span we can getโ€ฆ
+        let span = generators::request_span(&co).await;
+
+        for val in list.to_list()? {
+            let result = Value::Thunk(Thunk::new_suspended_call(f.clone(), val, span.clone()));
+            out.push_back(result)
+        }
+
+        Ok(Value::List(out.into()))
+    }
+
+    #[builtin("mapAttrs")]
+    async fn builtin_map_attrs(
+        co: GenCo,
+        #[lazy] f: Value,
+        attrs: Value,
+    ) -> Result<Value, ErrorKind> {
+        let attrs = attrs.to_attrs()?;
+        let mut out = imbl::OrdMap::new();
+
+        // the best span we can getโ€ฆ
+        let span = generators::request_span(&co).await;
+
+        for (key, value) in attrs.into_iter() {
+            let result = Value::Thunk(Thunk::new_suspended_call(
+                f.clone(),
+                key.clone().into(),
+                span.clone(),
+            ));
+            let result = Value::Thunk(Thunk::new_suspended_call(result, value, span.clone()));
+
+            out.insert(key, result);
+        }
+
+        Ok(Value::attrs(out.into()))
+    }
+
+    #[builtin("match")]
+    async fn builtin_match(co: GenCo, regex: Value, str: Value) -> Result<Value, ErrorKind> {
+        let s = str;
+        if s.is_catchable() {
+            return Ok(s);
+        }
+        let s = s.to_contextful_str()?;
+        let re = regex;
+        if re.is_catchable() {
+            return Ok(re);
+        }
+        let re = re.to_str()?;
+        let re: Regex = Regex::new(&format!("^{}$", re.to_str()?)).unwrap();
+        match re.captures(s.to_str()?) {
+            Some(caps) => Ok(Value::List(
+                caps.iter()
+                    .skip(1)
+                    .map(|grp| {
+                        // Surprisingly, Nix does not propagate
+                        // the original context here.
+                        // Though, it accepts contextful strings as an argument.
+                        // An example of such behaviors in nixpkgs
+                        // can be observed in make-initrd.nix when it comes
+                        // to compressors which are matched over their full command
+                        // and then a compressor name will be extracted from that.
+                        grp.map(|g| Value::from(g.as_str())).unwrap_or(Value::Null)
+                    })
+                    .collect::<imbl::Vector<Value>>()
+                    .into(),
+            )),
+            None => Ok(Value::Null),
+        }
+    }
+
+    #[builtin("mul")]
+    async fn builtin_mul(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        arithmetic_op!(&x, &y, *)
+    }
+
+    #[builtin("parseDrvName")]
+    async fn builtin_parse_drv_name(co: GenCo, s: Value) -> Result<Value, ErrorKind> {
+        if s.is_catchable() {
+            return Ok(s);
+        }
+
+        // This replicates cppnix's (mis?)handling of codepoints
+        // above U+007f following 0x2d ('-')
+        let s = s.to_str()?;
+        let slice: &[u8] = s.as_ref();
+        let (name, dash_and_version) = slice.split_at(
+            slice
+                .windows(2)
+                .enumerate()
+                .find_map(|x| match x {
+                    (idx, [b'-', c1]) if !c1.is_ascii_alphabetic() => Some(idx),
+                    _ => None,
+                })
+                .unwrap_or(slice.len()),
+        );
+        let version = dash_and_version
+            .split_first()
+            .map(|x| core::str::from_utf8(x.1))
+            .unwrap_or(Ok(""))?;
+        Ok(Value::attrs(NixAttrs::from_iter(
+            [("name", core::str::from_utf8(name)?), ("version", version)].into_iter(),
+        )))
+    }
+
+    #[builtin("partition")]
+    async fn builtin_partition(co: GenCo, pred: Value, list: Value) -> Result<Value, ErrorKind> {
+        let mut right: imbl::Vector<Value> = Default::default();
+        let mut wrong: imbl::Vector<Value> = Default::default();
+
+        let list: NixList = list.to_list()?;
+        for elem in list {
+            let result = generators::request_call_with(&co, pred.clone(), [elem.clone()]).await;
+
+            if try_value!(generators::request_force(&co, result).await).as_bool()? {
+                right.push_back(elem);
+            } else {
+                wrong.push_back(elem);
+            };
+        }
+
+        let res = [
+            ("right", Value::List(NixList::from(right))),
+            ("wrong", Value::List(NixList::from(wrong))),
+        ];
+
+        Ok(Value::attrs(NixAttrs::from_iter(res.into_iter())))
+    }
+
+    #[builtin("removeAttrs")]
+    async fn builtin_remove_attrs(
+        co: GenCo,
+        attrs: Value,
+        keys: Value,
+    ) -> Result<Value, ErrorKind> {
+        let attrs = attrs.to_attrs()?;
+        let keys = keys
+            .to_list()?
+            .into_iter()
+            .map(|v| v.to_str())
+            .collect::<Result<HashSet<_>, _>>()?;
+        let res = attrs.iter().filter_map(|(k, v)| {
+            if !keys.contains(k) {
+                Some((k.clone(), v.clone()))
+            } else {
+                None
+            }
+        });
+        Ok(Value::attrs(NixAttrs::from_iter(res)))
+    }
+
+    #[builtin("replaceStrings")]
+    async fn builtin_replace_strings(
+        co: GenCo,
+        from: Value,
+        to: Value,
+        s: Value,
+    ) -> Result<Value, ErrorKind> {
+        let from = from.to_list()?;
+        for val in &from {
+            try_value!(generators::request_force(&co, val.clone()).await);
+        }
+
+        let to = to.to_list()?;
+        for val in &to {
+            try_value!(generators::request_force(&co, val.clone()).await);
+        }
+
+        let mut string = s.to_contextful_str()?;
+
+        let mut res = BString::default();
+
+        let mut i: usize = 0;
+        let mut empty_string_replace = false;
+        let mut context = NixContext::new();
+
+        if let Some(string_context) = string.context_mut() {
+            context = context.join(string_context);
+        }
+
+        // This can't be implemented using Rust's string.replace() as
+        // well as a map because we need to handle errors with results
+        // as well as "reset" the iterator to zero for the replacement
+        // everytime there's a successful match.
+        // Also, Rust's string.replace allocates a new string
+        // on every call which is not preferable.
+        'outer: while i < string.len() {
+            // Try a match in all the from strings
+            for elem in std::iter::zip(from.iter(), to.iter()) {
+                let from = elem.0.to_contextful_str()?;
+                let mut to = elem.1.to_contextful_str()?;
+
+                if i + from.len() > string.len() {
+                    continue;
+                }
+
+                // We already applied a from->to with an empty from
+                // transformation.
+                // Let's skip it so that we don't loop infinitely
+                if empty_string_replace && from.is_empty() {
+                    continue;
+                }
+
+                // if we match the `from` string, let's replace
+                if string[i..i + from.len()] == *from {
+                    res.push_str(&to);
+                    i += from.len();
+                    if let Some(to_ctx) = to.context_mut() {
+                        context = context.join(to_ctx);
+                    }
+
+                    // remember if we applied the empty from->to
+                    empty_string_replace = from.is_empty();
+
+                    continue 'outer;
+                }
+            }
+
+            // If we don't match any `from`, we simply add a character
+            res.push_str(&string[i..i + 1]);
+            i += 1;
+
+            // Since we didn't apply anything transformation,
+            // we reset the empty string replacement
+            empty_string_replace = false;
+        }
+
+        // Special case when the string is empty or at the string's end
+        // and one of the from is also empty
+        for elem in std::iter::zip(from.iter(), to.iter()) {
+            let from = elem.0.to_contextful_str()?;
+            // We mutate `to` by consuming its context
+            // if we perform a successful replacement.
+            // Therefore, it's fine if `to` was mutate and we reuse it here.
+            // We don't need to merge again the context, it's already in the right state.
+            let mut to = elem.1.to_contextful_str()?;
+
+            if from.is_empty() {
+                res.push_str(&to);
+                if let Some(to_ctx) = to.context_mut() {
+                    context = context.join(to_ctx);
+                }
+                break;
+            }
+        }
+
+        Ok(Value::from(NixString::new_context_from(context, res)))
+    }
+
+    #[builtin("seq")]
+    async fn builtin_seq(co: GenCo, _x: Value, y: Value) -> Result<Value, ErrorKind> {
+        // The builtin calling infra has already forced both args for us, so
+        // we just return the second and ignore the first
+        Ok(y)
+    }
+
+    #[builtin("split")]
+    async fn builtin_split(co: GenCo, regex: Value, str: Value) -> Result<Value, ErrorKind> {
+        if str.is_catchable() {
+            return Ok(str);
+        }
+
+        if regex.is_catchable() {
+            return Ok(regex);
+        }
+
+        let s = str.to_contextful_str()?;
+        let text = s.to_str()?;
+        let re = regex.to_str()?;
+        let re = Regex::new(re.to_str()?).unwrap();
+        let mut capture_locations = re.capture_locations();
+        let num_captures = capture_locations.len();
+        let mut ret = imbl::Vector::new();
+        let mut pos = 0;
+
+        while let Some(thematch) = re.captures_read_at(&mut capture_locations, text, pos) {
+            // push the unmatched characters preceding the match
+            ret.push_back(Value::from(NixString::new_inherit_context_from(
+                &s,
+                &text[pos..thematch.start()],
+            )));
+
+            // Push a list with one element for each capture
+            // group in the regex, containing the characters
+            // matched by that capture group, or null if no match.
+            // We skip capture 0; it represents the whole match.
+            let v: imbl::Vector<Value> = (1..num_captures)
+                .map(|i| capture_locations.get(i))
+                .map(|o| {
+                    o.map(|(start, end)| {
+                        // Here, a surprising thing happens: we silently discard the original
+                        // context. This is as intended, Nix does the same.
+                        Value::from(&text[start..end])
+                    })
+                    .unwrap_or(Value::Null)
+                })
+                .collect();
+            ret.push_back(Value::List(NixList::from(v)));
+            pos = thematch.end();
+        }
+
+        // push the unmatched characters following the last match
+        // Here, a surprising thing happens: we silently discard the original
+        // context. This is as intended, Nix does the same.
+        ret.push_back(Value::from(&text[pos..]));
+
+        Ok(Value::List(NixList::from(ret)))
+    }
+
+    #[builtin("sort")]
+    async fn builtin_sort(co: GenCo, comparator: Value, list: Value) -> Result<Value, ErrorKind> {
+        let list = list.to_list()?;
+        let mut len = list.len();
+        let mut data = list.into_inner();
+
+        // Asynchronous sorting algorithm in which the comparator can make use of
+        // VM requests (required as `builtins.sort` uses comparators written in
+        // Nix).
+        //
+        // This is a simple, optimised bubble sort implementation. The choice of
+        // algorithm is constrained by the comparator in Nix not being able to
+        // yield equality, and us being unable to use the standard library
+        // implementation of sorting (which is a lot longer, but a lot more
+        // efficient) here.
+        // TODO(amjoseph): Investigate potential impl in Nix code, or Tvix bytecode.
+        loop {
+            let mut new_len = 0;
+            for i in 1..len {
+                if try_value!(
+                    generators::request_force(
+                        &co,
+                        generators::request_call_with(
+                            &co,
+                            comparator.clone(),
+                            [data[i].clone(), data[i - 1].clone()],
+                        )
+                        .await,
+                    )
+                    .await
+                )
+                .as_bool()
+                .context("evaluating comparator in `builtins.sort`")?
+                {
+                    data.swap(i, i - 1);
+                    new_len = i;
+                }
+            }
+
+            if new_len == 0 {
+                break;
+            }
+
+            len = new_len;
+        }
+
+        Ok(Value::List(data.into()))
+    }
+
+    #[builtin("splitVersion")]
+    async fn builtin_split_version(co: GenCo, s: Value) -> Result<Value, ErrorKind> {
+        if s.is_catchable() {
+            return Ok(s);
+        }
+        let s = s.to_str()?;
+        let s = VersionPartsIter::new((&s).into());
+
+        let parts = s
+            .map(|s| {
+                Value::from(match s {
+                    VersionPart::Number(n) => n,
+                    VersionPart::Word(w) => w,
+                })
+            })
+            .collect::<Vec<Value>>();
+        Ok(Value::List(NixList::construct(parts.len(), parts)))
+    }
+
+    #[builtin("stringLength")]
+    async fn builtin_string_length(co: GenCo, #[lazy] s: Value) -> Result<Value, ErrorKind> {
+        // also forces the value
+        let span = generators::request_span(&co).await;
+        let s = s
+            .coerce_to_string(
+                co,
+                CoercionKind {
+                    strong: false,
+                    import_paths: true,
+                },
+                span,
+            )
+            .await?;
+
+        if s.is_catchable() {
+            return Ok(s);
+        }
+
+        Ok(Value::Integer(s.to_contextful_str()?.len() as i64))
+    }
+
+    #[builtin("sub")]
+    async fn builtin_sub(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        arithmetic_op!(&x, &y, -)
+    }
+
+    #[builtin("substring")]
+    async fn builtin_substring(
+        co: GenCo,
+        start: Value,
+        len: Value,
+        s: Value,
+    ) -> Result<Value, ErrorKind> {
+        let beg = start.as_int()?;
+        let len = len.as_int()?;
+        let span = generators::request_span(&co).await;
+        let x = s
+            .coerce_to_string(
+                co,
+                CoercionKind {
+                    strong: false,
+                    import_paths: true,
+                },
+                span,
+            )
+            .await?;
+        if x.is_catchable() {
+            return Ok(x);
+        }
+        let x = x.to_contextful_str()?;
+
+        if beg < 0 {
+            return Err(ErrorKind::IndexOutOfBounds { index: beg });
+        }
+        let beg = beg as usize;
+
+        // Nix doesn't assert that the length argument is
+        // non-negative when the starting index is GTE the
+        // string's length.
+        if beg >= x.len() {
+            return Ok(Value::from(NixString::new_inherit_context_from(
+                &x,
+                BString::default(),
+            )));
+        }
+
+        let end = if len < 0 {
+            x.len()
+        } else {
+            cmp::min(beg + (len as usize), x.len())
+        };
+
+        Ok(Value::from(NixString::new_inherit_context_from(
+            &x,
+            &x[beg..end],
+        )))
+    }
+
+    #[builtin("tail")]
+    async fn builtin_tail(co: GenCo, list: Value) -> Result<Value, ErrorKind> {
+        if list.is_catchable() {
+            return Ok(list);
+        }
+
+        let xs = list.to_list()?;
+
+        if xs.is_empty() {
+            Err(ErrorKind::TailEmptyList)
+        } else {
+            let output = xs.into_iter().skip(1).collect::<Vec<_>>();
+            Ok(Value::List(NixList::construct(output.len(), output)))
+        }
+    }
+
+    #[builtin("throw")]
+    async fn builtin_throw(co: GenCo, message: Value) -> Result<Value, ErrorKind> {
+        // If it's already some error, let's propagate it immediately.
+        if message.is_catchable() {
+            return Ok(message);
+        }
+        // TODO(sterni): coerces to string
+        // We do not care about the context here explicitly.
+        Ok(Value::from(CatchableErrorKind::Throw(
+            message.to_contextful_str()?.to_string().into(),
+        )))
+    }
+
+    #[builtin("toString")]
+    async fn builtin_to_string(co: GenCo, #[lazy] x: Value) -> Result<Value, ErrorKind> {
+        // TODO(edef): please fix me w.r.t. to catchability.
+        // coerce_to_string forces for us
+        // FIXME: should `coerce_to_string` preserve context?
+        // it does for now.
+        let span = generators::request_span(&co).await;
+        x.coerce_to_string(
+            co,
+            CoercionKind {
+                strong: true,
+                import_paths: false,
+            },
+            span,
+        )
+        .await
+    }
+
+    #[builtin("toXML")]
+    async fn builtin_to_xml(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        let value = generators::request_deep_force(&co, value).await;
+        if value.is_catchable() {
+            return Ok(value);
+        }
+
+        let mut buf: Vec<u8> = vec![];
+        to_xml::value_to_xml(&mut buf, &value)?;
+        Ok(String::from_utf8(buf)?.into())
+    }
+
+    #[builtin("placeholder")]
+    async fn builtin_placeholder(co: GenCo, #[lazy] _x: Value) -> Result<Value, ErrorKind> {
+        generators::emit_warning_kind(&co, WarningKind::NotImplemented("builtins.placeholder"))
+            .await;
+        Ok("<builtins.placeholder-is-not-implemented-in-tvix-yet>".into())
+    }
+
+    #[builtin("trace")]
+    async fn builtin_trace(co: GenCo, message: Value, value: Value) -> Result<Value, ErrorKind> {
+        // TODO(grfn): `trace` should be pluggable and capturable, probably via a method on
+        // the VM
+        eprintln!("trace: {} :: {}", message, message.type_of());
+        Ok(value)
+    }
+
+    #[builtin("toPath")]
+    async fn builtin_to_path(co: GenCo, s: Value) -> Result<Value, ErrorKind> {
+        if s.is_catchable() {
+            return Ok(s);
+        }
+
+        match coerce_value_to_path(&co, s).await? {
+            Err(cek) => Ok(Value::from(cek)),
+            Ok(path) => {
+                let path: Value = crate::value::canon_path(path).into();
+                let span = generators::request_span(&co).await;
+                Ok(path
+                    .coerce_to_string(
+                        co,
+                        CoercionKind {
+                            strong: false,
+                            import_paths: false,
+                        },
+                        span,
+                    )
+                    .await?)
+            }
+        }
+    }
+
+    #[builtin("tryEval")]
+    async fn builtin_try_eval(co: GenCo, #[lazy] e: Value) -> Result<Value, ErrorKind> {
+        let res = match generators::request_try_force(&co, e).await {
+            Value::Catchable(_) => [("value", false.into()), ("success", false.into())],
+            value => [("value", value), ("success", true.into())],
+        };
+
+        Ok(Value::attrs(NixAttrs::from_iter(res.into_iter())))
+    }
+
+    #[builtin("typeOf")]
+    async fn builtin_type_of(co: GenCo, x: Value) -> Result<Value, ErrorKind> {
+        if x.is_catchable() {
+            return Ok(x);
+        }
+
+        Ok(Value::from(x.type_of()))
+    }
+}
+
+/// Internal helper function for genericClosure, determining whether a
+/// value has been seen before.
+async fn bgc_insert_key(
+    co: &GenCo,
+    key: Value,
+    done: &mut Vec<Value>,
+) -> Result<Result<bool, CatchableErrorKind>, ErrorKind> {
+    for existing in done.iter() {
+        match generators::check_equality(
+            co,
+            existing.clone(),
+            key.clone(),
+            // TODO(tazjin): not actually sure which semantics apply here
+            PointerEquality::ForbidAll,
+        )
+        .await?
+        {
+            Ok(true) => return Ok(Ok(false)),
+            Ok(false) => (),
+            Err(cek) => return Ok(Err(cek)),
+        }
+    }
+
+    done.push(key);
+    Ok(Ok(true))
+}
+
+/// The set of standard pure builtins in Nix, mostly concerned with
+/// data structure manipulation (string, attrs, list, etc. functions).
+pub fn pure_builtins() -> Vec<(&'static str, Value)> {
+    let mut result = pure_builtins::builtins();
+
+    // Pure-value builtins
+    result.push(("nixVersion", Value::from("2.3-compat-tvix-0.1")));
+    result.push(("langVersion", Value::Integer(6)));
+    result.push(("null", Value::Null));
+    result.push(("true", Value::Bool(true)));
+    result.push(("false", Value::Bool(false)));
+
+    result.push((
+        "currentSystem",
+        crate::systems::llvm_triple_to_nix_double(CURRENT_PLATFORM).into(),
+    ));
+
+    // TODO: implement for nixpkgs compatibility
+    result.push((
+        "__curPos",
+        Value::from(CatchableErrorKind::UnimplementedFeature("__curPos".into())),
+    ));
+
+    result
+}
+
+#[builtins]
+mod placeholder_builtins {
+    use super::*;
+
+    #[builtin("unsafeDiscardStringContext")]
+    async fn builtin_unsafe_discard_string_context(
+        co: GenCo,
+        s: Value,
+    ) -> Result<Value, ErrorKind> {
+        let span = generators::request_span(&co).await;
+        let mut v = s
+            .coerce_to_string(
+                co,
+                // It's weak because
+                // lists, integers, floats and null are not
+                // accepted as parameters.
+                CoercionKind {
+                    strong: false,
+                    import_paths: true,
+                },
+                span,
+            )
+            .await?
+            .to_contextful_str()?;
+        v.clear_context();
+        Ok(Value::from(v))
+    }
+
+    #[builtin("unsafeDiscardOutputDependency")]
+    async fn builtin_unsafe_discard_output_dependency(
+        co: GenCo,
+        s: Value,
+    ) -> Result<Value, ErrorKind> {
+        let span = generators::request_span(&co).await;
+        let mut v = s
+            .coerce_to_string(
+                co,
+                // It's weak because
+                // lists, integers, floats and null are not
+                // accepted as parameters.
+                CoercionKind {
+                    strong: false,
+                    import_paths: true,
+                },
+                span,
+            )
+            .await?
+            .to_contextful_str()?;
+
+        // If there's any context, we will swap any ... by a path one.
+        if let Some(ctx) = v.context_mut() {
+            let new_context: tvix_eval::NixContext = ctx
+                .iter()
+                .map(|elem| match elem {
+                    // FUTUREWORK(performance): ideally, we should either:
+                    // (a) do interior mutation of the existing context.
+                    // (b) let the structural sharing make those clones cheap.
+                    crate::NixContextElement::Derivation(drv_path) => {
+                        crate::NixContextElement::Plain(drv_path.to_string())
+                    }
+                    elem => elem.clone(),
+                })
+                .collect::<HashSet<_>>()
+                .into();
+
+            *ctx = new_context;
+        }
+
+        Ok(Value::from(v))
+    }
+
+    #[builtin("addErrorContext")]
+    async fn builtin_add_error_context(
+        co: GenCo,
+        #[lazy] _context: Value,
+        #[lazy] val: Value,
+    ) -> Result<Value, ErrorKind> {
+        generators::emit_warning_kind(&co, WarningKind::NotImplemented("builtins.addErrorContext"))
+            .await;
+        Ok(val)
+    }
+
+    #[builtin("unsafeGetAttrPos")]
+    async fn builtin_unsafe_get_attr_pos(
+        co: GenCo,
+        _name: Value,
+        _attrset: Value,
+    ) -> Result<Value, ErrorKind> {
+        generators::emit_warning_kind(
+            &co,
+            WarningKind::NotImplemented("builtins.unsafeGetAttrsPos"),
+        )
+        .await;
+        let res = [
+            ("line", 42.into()),
+            ("column", 42.into()),
+            ("file", Value::String("/deep/thought".into())),
+        ];
+        Ok(Value::attrs(NixAttrs::from_iter(res.into_iter())))
+    }
+}
+
+pub fn placeholders() -> Vec<(&'static str, Value)> {
+    placeholder_builtins::builtins()
+}
diff --git a/tvix/eval/src/builtins/to_xml.rs b/tvix/eval/src/builtins/to_xml.rs
new file mode 100644
index 0000000000..bb12cebfc9
--- /dev/null
+++ b/tvix/eval/src/builtins/to_xml.rs
@@ -0,0 +1,154 @@
+//! This module implements `builtins.toXML`, which is a serialisation
+//! of value information as well as internal tvix state that several
+//! things in nixpkgs rely on.
+
+use bstr::ByteSlice;
+use std::{io::Write, rc::Rc};
+use xml::writer::events::XmlEvent;
+use xml::writer::EmitterConfig;
+use xml::writer::EventWriter;
+
+use crate::{ErrorKind, Value};
+
+/// Recursively serialise a value to XML. The value *must* have been
+/// deep-forced before being passed to this function.
+pub fn value_to_xml<W: Write>(mut writer: W, value: &Value) -> Result<(), ErrorKind> {
+    let config = EmitterConfig {
+        perform_indent: true,
+        pad_self_closing: true,
+
+        // Nix uses single-quotes *only* in the document declaration,
+        // so we need to write it manually.
+        write_document_declaration: false,
+        ..Default::default()
+    };
+
+    // Write a literal document declaration, using C++-Nix-style
+    // single quotes.
+    writeln!(writer, "<?xml version='1.0' encoding='utf-8'?>")?;
+
+    let mut writer = EventWriter::new_with_config(writer, config);
+
+    writer.write(XmlEvent::start_element("expr"))?;
+    value_variant_to_xml(&mut writer, value)?;
+    writer.write(XmlEvent::end_element())?;
+
+    // Unwrap the writer to add the final newline that C++ Nix adds.
+    writeln!(writer.into_inner())?;
+
+    Ok(())
+}
+
+fn write_typed_value<W: Write, V: ToString>(
+    w: &mut EventWriter<W>,
+    name: &str,
+    value: V,
+) -> Result<(), ErrorKind> {
+    w.write(XmlEvent::start_element(name).attr("value", &value.to_string()))?;
+    w.write(XmlEvent::end_element())?;
+    Ok(())
+}
+
+fn value_variant_to_xml<W: Write>(w: &mut EventWriter<W>, value: &Value) -> Result<(), ErrorKind> {
+    match value {
+        Value::Thunk(t) => return value_variant_to_xml(w, &t.value()),
+
+        Value::Null => {
+            w.write(XmlEvent::start_element("null"))?;
+            w.write(XmlEvent::end_element())
+        }
+
+        Value::Bool(b) => return write_typed_value(w, "bool", b),
+        Value::Integer(i) => return write_typed_value(w, "int", i),
+        Value::Float(f) => return write_typed_value(w, "float", f),
+        Value::String(s) => return write_typed_value(w, "string", s.to_str()?),
+        Value::Path(p) => return write_typed_value(w, "path", p.to_string_lossy()),
+
+        Value::List(list) => {
+            w.write(XmlEvent::start_element("list"))?;
+
+            for elem in list.into_iter() {
+                value_variant_to_xml(w, elem)?;
+            }
+
+            w.write(XmlEvent::end_element())
+        }
+
+        Value::Attrs(attrs) => {
+            w.write(XmlEvent::start_element("attrs"))?;
+
+            for elem in attrs.iter() {
+                w.write(XmlEvent::start_element("attr").attr("name", &elem.0.to_str_lossy()))?;
+                value_variant_to_xml(w, elem.1)?;
+                w.write(XmlEvent::end_element())?;
+            }
+
+            w.write(XmlEvent::end_element())
+        }
+
+        Value::Closure(c) => {
+            w.write(XmlEvent::start_element("function"))?;
+
+            match &c.lambda.formals {
+                Some(formals) => {
+                    let mut attrspat = XmlEvent::start_element("attrspat");
+                    if formals.ellipsis {
+                        attrspat = attrspat.attr("ellipsis", "1");
+                    }
+                    if let Some(ref name) = &formals.name {
+                        attrspat = attrspat.attr("name", name.as_str());
+                    }
+
+                    w.write(attrspat)?;
+
+                    for arg in formals.arguments.iter() {
+                        w.write(
+                            XmlEvent::start_element("attr").attr("name", &arg.0.to_str_lossy()),
+                        )?;
+                        w.write(XmlEvent::end_element())?;
+                    }
+
+                    w.write(XmlEvent::end_element())?;
+                }
+                None => {
+                    // TODO(tazjin): tvix does not currently persist function
+                    // argument names anywhere (whereas we do for formals, as
+                    // that is required for other runtime behaviour). Because of
+                    // this the implementation here is fake, always returning
+                    // the same argument name.
+                    //
+                    // If we don't want to persist the data, we can re-parse the
+                    // AST from the spans of the lambda's bytecode and figure it
+                    // out that way, but it needs some investigating.
+                    w.write(XmlEvent::start_element("varpat").attr("name", /* fake: */ "x"))?;
+                    w.write(XmlEvent::end_element())?;
+                }
+            }
+
+            w.write(XmlEvent::end_element())
+        }
+
+        Value::Builtin(_) => {
+            w.write(XmlEvent::start_element("unevaluated"))?;
+            w.write(XmlEvent::end_element())
+        }
+
+        Value::AttrNotFound
+        | Value::Blueprint(_)
+        | Value::DeferredUpvalue(_)
+        | Value::UnresolvedPath(_)
+        | Value::Json(..)
+        | Value::FinaliseRequest(_) => {
+            return Err(ErrorKind::TvixBug {
+                msg: "internal value variant encountered in builtins.toXML",
+                metadata: Some(Rc::new(value.clone())),
+            })
+        }
+
+        Value::Catchable(_) => {
+            panic!("tvix bug: value_to_xml() called on a value which had not been deep-forced")
+        }
+    }?;
+
+    Ok(())
+}
diff --git a/tvix/eval/src/builtins/versions.rs b/tvix/eval/src/builtins/versions.rs
new file mode 100644
index 0000000000..6de5121424
--- /dev/null
+++ b/tvix/eval/src/builtins/versions.rs
@@ -0,0 +1,163 @@
+use std::cmp::Ordering;
+use std::iter::{once, Chain, Once};
+use std::ops::RangeInclusive;
+
+use bstr::{BStr, ByteSlice, B};
+
+/// Version strings can be broken up into Parts.
+/// One Part represents either a string of digits or characters.
+/// '.' and '_' represent deviders between parts and are not included in any part.
+#[derive(PartialEq, Eq, Clone, Debug)]
+pub enum VersionPart<'a> {
+    Word(&'a BStr),
+    Number(&'a BStr),
+}
+
+impl PartialOrd for VersionPart<'_> {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for VersionPart<'_> {
+    fn cmp(&self, other: &Self) -> Ordering {
+        match (self, other) {
+            (VersionPart::Number(s1), VersionPart::Number(s2)) => {
+                // Note: C++ Nix uses `int`, but probably doesn't make a difference
+                // We trust that the splitting was done correctly and parsing will work
+                let n1: u64 = s1.to_str_lossy().parse().unwrap();
+                let n2: u64 = s2.to_str_lossy().parse().unwrap();
+                n1.cmp(&n2)
+            }
+
+            // `pre` looses unless the other part is also a `pre`
+            (VersionPart::Word(x), VersionPart::Word(y)) if *x == B("pre") && *y == B("pre") => {
+                Ordering::Equal
+            }
+            (VersionPart::Word(x), _) if *x == B("pre") => Ordering::Less,
+            (_, VersionPart::Word(y)) if *y == B("pre") => Ordering::Greater,
+
+            // Number wins against Word
+            (VersionPart::Number(_), VersionPart::Word(_)) => Ordering::Greater,
+            (VersionPart::Word(_), VersionPart::Number(_)) => Ordering::Less,
+
+            (VersionPart::Word(w1), VersionPart::Word(w2)) => w1.cmp(w2),
+        }
+    }
+}
+
+/// Type used to hold information about a VersionPart during creation
+enum InternalPart {
+    Number { range: RangeInclusive<usize> },
+    Word { range: RangeInclusive<usize> },
+    Break,
+}
+
+/// An iterator which yields the parts of a version string.
+///
+/// This can then be directly used to compare two versions
+pub struct VersionPartsIter<'a> {
+    cached_part: InternalPart,
+    iter: bstr::CharIndices<'a>,
+    version: &'a BStr,
+}
+
+impl<'a> VersionPartsIter<'a> {
+    pub fn new(version: &'a BStr) -> Self {
+        Self {
+            cached_part: InternalPart::Break,
+            iter: version.char_indices(),
+            version,
+        }
+    }
+
+    /// Create an iterator that yields all version parts followed by an additional
+    /// `VersionPart::Word("")` part (i.e. you can think of this as
+    /// `builtins.splitVersion version ++ [ "" ]`). This is necessary, because
+    /// Nix's `compareVersions` is not entirely lexicographical: If we have two
+    /// equal versions, but one is longer, the longer one is only considered
+    /// greater if the first additional part of the longer version is not `pre`,
+    /// e.g. `2.3 > 2.3pre`. It is otherwise lexicographical, so peculiar behavior
+    /// like `2.3 < 2.3.0pre` ensues. Luckily for us, this means that we can
+    /// lexicographically compare two version strings, _if_ we append an extra
+    /// component to both versions.
+    pub fn new_for_cmp(version: &'a BStr) -> Chain<Self, Once<VersionPart>> {
+        Self::new(version).chain(once(VersionPart::Word("".into())))
+    }
+}
+
+impl<'a> Iterator for VersionPartsIter<'a> {
+    type Item = VersionPart<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let char = self.iter.next();
+
+        if char.is_none() {
+            let cached_part = std::mem::replace(&mut self.cached_part, InternalPart::Break);
+            match cached_part {
+                InternalPart::Break => return None,
+                InternalPart::Number { range } => {
+                    return Some(VersionPart::Number(&self.version[range]))
+                }
+                InternalPart::Word { range } => {
+                    return Some(VersionPart::Word(&self.version[range]))
+                }
+            }
+        }
+
+        let (start, end, char) = char.unwrap();
+        match char {
+            // Divider encountered
+            '.' | '-' => {
+                let cached_part = std::mem::replace(&mut self.cached_part, InternalPart::Break);
+                match cached_part {
+                    InternalPart::Number { range } => {
+                        Some(VersionPart::Number(&self.version[range]))
+                    }
+                    InternalPart::Word { range } => Some(VersionPart::Word(&self.version[range])),
+                    InternalPart::Break => self.next(),
+                }
+            }
+
+            // digit encountered
+            _ if char.is_ascii_digit() => {
+                let cached_part = std::mem::replace(
+                    &mut self.cached_part,
+                    InternalPart::Number {
+                        range: start..=(end - 1),
+                    },
+                );
+                match cached_part {
+                    InternalPart::Number { range } => {
+                        self.cached_part = InternalPart::Number {
+                            range: *range.start()..=*range.end() + 1,
+                        };
+                        self.next()
+                    }
+                    InternalPart::Word { range } => Some(VersionPart::Word(&self.version[range])),
+                    InternalPart::Break => self.next(),
+                }
+            }
+
+            // char encountered
+            _ => {
+                let mut cached_part = InternalPart::Word {
+                    range: start..=(end - 1),
+                };
+                std::mem::swap(&mut cached_part, &mut self.cached_part);
+                match cached_part {
+                    InternalPart::Word { range } => {
+                        self.cached_part = InternalPart::Word {
+                            range: *range.start()..=*range.end() + char.len_utf8(),
+                        };
+                        self.next()
+                    }
+                    InternalPart::Number { range } => {
+                        Some(VersionPart::Number(&self.version[range]))
+                    }
+                    InternalPart::Break => self.next(),
+                }
+            }
+        }
+    }
+}
diff --git a/tvix/eval/src/chunk.rs b/tvix/eval/src/chunk.rs
index 3475e58f18..f1a35a6ce1 100644
--- a/tvix/eval/src/chunk.rs
+++ b/tvix/eval/src/chunk.rs
@@ -1,26 +1,289 @@
+use std::io::Write;
+use std::ops::{Index, IndexMut};
+
 use crate::opcode::{CodeIdx, ConstantIdx, OpCode};
 use crate::value::Value;
+use crate::SourceCode;
+
+/// Represents a source location from which one or more operations
+/// were compiled.
+///
+/// The span itself is an index into a [codemap::CodeMap], and the
+/// structure tracks the number of operations that were yielded from
+/// the same span.
+///
+/// At error reporting time, it becomes possible to either just fetch
+/// the textual representation of that span from the codemap, or to
+/// even re-parse the AST using rnix to create more semantically
+/// interesting errors.
+#[derive(Clone, Debug, PartialEq)]
+struct SourceSpan {
+    /// Span into the [codemap::CodeMap].
+    span: codemap::Span,
+
+    /// Index of the first operation covered by this span.
+    start: usize,
+}
 
+/// A chunk is a representation of a sequence of bytecode
+/// instructions, associated constants and additional metadata as
+/// emitted by the compiler.
 #[derive(Debug, Default)]
 pub struct Chunk {
     pub code: Vec<OpCode>,
-    constants: Vec<Value>,
+    pub constants: Vec<Value>,
+    spans: Vec<SourceSpan>,
+}
+
+impl Index<ConstantIdx> for Chunk {
+    type Output = Value;
+
+    fn index(&self, index: ConstantIdx) -> &Self::Output {
+        &self.constants[index.0]
+    }
+}
+
+impl Index<CodeIdx> for Chunk {
+    type Output = OpCode;
+
+    fn index(&self, index: CodeIdx) -> &Self::Output {
+        &self.code[index.0]
+    }
+}
+
+impl IndexMut<CodeIdx> for Chunk {
+    fn index_mut(&mut self, index: CodeIdx) -> &mut Self::Output {
+        &mut self.code[index.0]
+    }
 }
 
 impl Chunk {
-    pub fn add_op(&mut self, data: OpCode) -> CodeIdx {
+    pub fn push_op(&mut self, data: OpCode, span: codemap::Span) -> CodeIdx {
         let idx = self.code.len();
         self.code.push(data);
+        self.push_span(span, idx);
         CodeIdx(idx)
     }
 
-    pub fn add_constant(&mut self, data: Value) -> ConstantIdx {
+    /// Get the first span of a chunk, no questions asked.
+    pub fn first_span(&self) -> codemap::Span {
+        self.spans[0].span
+    }
+
+    /// Return a reference to the last op in the chunk, if any
+    pub fn last_op(&self) -> Option<&OpCode> {
+        self.code.last()
+    }
+
+    /// Pop the last operation from the chunk and clean up its tracked
+    /// span. Used when the compiler backtracks.
+    pub fn pop_op(&mut self) {
+        // Simply drop the last op.
+        self.code.pop();
+
+        if let Some(span) = self.spans.last() {
+            // If the last span started at this op, drop it.
+            if span.start == self.code.len() {
+                self.spans.pop();
+            }
+        }
+    }
+
+    pub fn push_constant(&mut self, data: Value) -> ConstantIdx {
         let idx = self.constants.len();
         self.constants.push(data);
         ConstantIdx(idx)
     }
 
-    pub fn constant(&self, idx: ConstantIdx) -> &Value {
-        &self.constants[idx.0]
+    /// Return a reference to the constant at the given [`ConstantIdx`]
+    pub fn get_constant(&self, constant: ConstantIdx) -> Option<&Value> {
+        self.constants.get(constant.0)
+    }
+
+    // Span tracking implementation
+
+    fn push_span(&mut self, span: codemap::Span, start: usize) {
+        match self.spans.last_mut() {
+            // We do not need to insert the same span again, as this
+            // instruction was compiled from the same span as the last
+            // one.
+            Some(last) if last.span == span => {}
+
+            // In all other cases, this is a new source span.
+            _ => self.spans.push(SourceSpan { span, start }),
+        }
+    }
+
+    /// Retrieve the [codemap::Span] from which the instruction at
+    /// `offset` was compiled.
+    pub fn get_span(&self, offset: CodeIdx) -> codemap::Span {
+        let position = self
+            .spans
+            .binary_search_by(|span| span.start.cmp(&offset.0));
+
+        let span = match position {
+            Ok(index) => &self.spans[index],
+            Err(index) => {
+                if index == 0 {
+                    &self.spans[0]
+                } else {
+                    &self.spans[index - 1]
+                }
+            }
+        };
+
+        span.span
+    }
+
+    /// Write the disassembler representation of the operation at
+    /// `idx` to the specified writer.
+    pub fn disassemble_op<W: Write>(
+        &self,
+        writer: &mut W,
+        source: &SourceCode,
+        width: usize,
+        idx: CodeIdx,
+    ) -> Result<(), std::io::Error> {
+        write!(writer, "{:#width$x}\t ", idx.0, width = width)?;
+
+        // Print continuation character if the previous operation was at
+        // the same line, otherwise print the line.
+        let line = source.get_line(self.get_span(idx));
+        if idx.0 > 0 && source.get_line(self.get_span(CodeIdx(idx.0 - 1))) == line {
+            write!(writer, "   |\t")?;
+        } else {
+            write!(writer, "{:4}\t", line)?;
+        }
+
+        match self[idx] {
+            OpCode::OpConstant(idx) => {
+                let val_str = match &self[idx] {
+                    Value::Thunk(t) => t.debug_repr(),
+                    Value::Closure(c) => format!("closure({:p})", c.lambda),
+                    val => format!("{}", val),
+                };
+
+                writeln!(writer, "OpConstant({}@{})", val_str, idx.0)
+            }
+            op => writeln!(writer, "{:?}", op),
+        }?;
+
+        Ok(())
+    }
+
+    /// Extend this chunk with the content of another, moving out of the other
+    /// in the process.
+    ///
+    /// This is used by the compiler when it detects that it unnecessarily
+    /// thunked a nested expression.
+    pub fn extend(&mut self, other: Self) {
+        // Some operations need to be modified in certain ways before being
+        // valid as part of the new chunk.
+        let const_count = self.constants.len();
+        for (idx, op) in other.code.iter().enumerate() {
+            let span = other.get_span(CodeIdx(idx));
+            match op {
+                // As the constants shift, the index needs to be moved relatively.
+                OpCode::OpConstant(ConstantIdx(idx)) => {
+                    self.push_op(OpCode::OpConstant(ConstantIdx(idx + const_count)), span)
+                }
+
+                // Other operations either operate on relative offsets, or no
+                // offsets, and are safe to keep as-is.
+                _ => self.push_op(*op, span),
+            };
+        }
+
+        self.constants.extend(other.constants);
+        self.spans.extend(other.spans);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::test_utils::dummy_span;
+
+    // Note: These tests are about the functionality of the `Chunk` type, the
+    // opcodes used below do *not* represent valid, executable Tvix code (and
+    // don't need to).
+
+    #[test]
+    fn push_op() {
+        let mut chunk = Chunk::default();
+        chunk.push_op(OpCode::OpAdd, dummy_span());
+        assert_eq!(chunk.code.last().unwrap(), &OpCode::OpAdd);
+    }
+
+    #[test]
+    fn extend_empty() {
+        let mut chunk = Chunk::default();
+        chunk.push_op(OpCode::OpAdd, dummy_span());
+
+        let other = Chunk::default();
+        chunk.extend(other);
+
+        assert_eq!(
+            chunk.code,
+            vec![OpCode::OpAdd],
+            "code should not have changed"
+        );
+    }
+
+    #[test]
+    fn extend_simple() {
+        let span = dummy_span();
+        let mut chunk = Chunk::default();
+        chunk.push_op(OpCode::OpAdd, span);
+
+        let mut other = Chunk::default();
+        other.push_op(OpCode::OpSub, span);
+        other.push_op(OpCode::OpMul, span);
+
+        let expected_code = vec![OpCode::OpAdd, OpCode::OpSub, OpCode::OpMul];
+
+        chunk.extend(other);
+
+        assert_eq!(chunk.code, expected_code, "code should have been extended");
+    }
+
+    #[test]
+    fn extend_with_constant() {
+        let span = dummy_span();
+        let mut chunk = Chunk::default();
+        chunk.push_op(OpCode::OpAdd, span);
+        let cidx = chunk.push_constant(Value::Integer(0));
+        assert_eq!(
+            cidx.0, 0,
+            "first constant in main chunk should have index 0"
+        );
+        chunk.push_op(OpCode::OpConstant(cidx), span);
+
+        let mut other = Chunk::default();
+        other.push_op(OpCode::OpSub, span);
+        let other_cidx = other.push_constant(Value::Integer(1));
+        assert_eq!(
+            other_cidx.0, 0,
+            "first constant in other chunk should have index 0"
+        );
+        other.push_op(OpCode::OpConstant(other_cidx), span);
+
+        chunk.extend(other);
+
+        let expected_code = vec![
+            OpCode::OpAdd,
+            OpCode::OpConstant(ConstantIdx(0)),
+            OpCode::OpSub,
+            OpCode::OpConstant(ConstantIdx(1)), // <- note: this was rewritten
+        ];
+
+        assert_eq!(
+            chunk.code, expected_code,
+            "code should have been extended and rewritten"
+        );
+
+        assert_eq!(chunk.constants.len(), 2);
+        assert!(matches!(chunk.constants[0], Value::Integer(0)));
+        assert!(matches!(chunk.constants[1], Value::Integer(1)));
     }
 }
diff --git a/tvix/eval/src/compiler.rs b/tvix/eval/src/compiler.rs
deleted file mode 100644
index 5b6f748dc7..0000000000
--- a/tvix/eval/src/compiler.rs
+++ /dev/null
@@ -1,267 +0,0 @@
-//! This module implements a compiler for compiling the rnix AST
-//! representation to Tvix bytecode.
-
-use crate::chunk::Chunk;
-use crate::errors::EvalResult;
-use crate::opcode::OpCode;
-use crate::value::{NixString, Value};
-
-use rnix;
-use rnix::types::{EntryHolder, TokenWrapper, TypedNode, Wrapper};
-
-struct Compiler {
-    chunk: Chunk,
-}
-
-impl Compiler {
-    fn compile(&mut self, node: rnix::SyntaxNode) -> EvalResult<()> {
-        match node.kind() {
-            // Root of a file contains no content, it's just a marker
-            // type.
-            rnix::SyntaxKind::NODE_ROOT => self.compile(node.first_child().expect("TODO")),
-
-            // Literals contain a single token comprising of the
-            // literal itself.
-            rnix::SyntaxKind::NODE_LITERAL => {
-                let value = rnix::types::Value::cast(node).unwrap();
-                self.compile_literal(value.to_value().expect("TODO"))
-            }
-
-            rnix::SyntaxKind::NODE_STRING => {
-                let op = rnix::types::Str::cast(node).unwrap();
-                self.compile_string(op)
-            }
-
-            // The interpolation node is just a wrapper around the
-            // inner value of a fragment, it only requires unwrapping.
-            rnix::SyntaxKind::NODE_STRING_INTERPOL => {
-                self.compile(node.first_child().expect("TODO (should not be possible)"))
-            }
-
-            rnix::SyntaxKind::NODE_BIN_OP => {
-                let op = rnix::types::BinOp::cast(node).expect("TODO (should not be possible)");
-                self.compile_binop(op)
-            }
-
-            rnix::SyntaxKind::NODE_UNARY_OP => {
-                let op = rnix::types::UnaryOp::cast(node).expect("TODO: (should not be possible)");
-                self.compile_unary_op(op)
-            }
-
-            rnix::SyntaxKind::NODE_PAREN => {
-                let node = rnix::types::Paren::cast(node).unwrap();
-                self.compile(node.inner().unwrap())
-            }
-
-            rnix::SyntaxKind::NODE_IDENT => {
-                let node = rnix::types::Ident::cast(node).unwrap();
-                self.compile_ident(node)
-            }
-
-            rnix::SyntaxKind::NODE_ATTR_SET => {
-                let node = rnix::types::AttrSet::cast(node).unwrap();
-                self.compile_attr_set(node)
-            }
-
-            rnix::SyntaxKind::NODE_LIST => {
-                let node = rnix::types::List::cast(node).unwrap();
-                self.compile_list(node)
-            }
-
-            kind => {
-                println!("visiting unsupported node: {:?}", kind);
-                Ok(())
-            }
-        }
-    }
-
-    fn compile_literal(&mut self, value: rnix::value::Value) -> EvalResult<()> {
-        match value {
-            rnix::NixValue::Float(f) => {
-                let idx = self.chunk.add_constant(Value::Float(f));
-                self.chunk.add_op(OpCode::OpConstant(idx));
-                Ok(())
-            }
-
-            rnix::NixValue::Integer(i) => {
-                let idx = self.chunk.add_constant(Value::Integer(i));
-                self.chunk.add_op(OpCode::OpConstant(idx));
-                Ok(())
-            }
-
-            rnix::NixValue::String(_) => todo!(),
-            rnix::NixValue::Path(_, _) => todo!(),
-        }
-    }
-
-    fn compile_string(&mut self, string: rnix::types::Str) -> EvalResult<()> {
-        let mut count = 0;
-
-        // The string parts are produced in literal order, however
-        // they need to be reversed on the stack in order to
-        // efficiently create the real string in case of
-        // interpolation.
-        for part in string.parts().into_iter().rev() {
-            count += 1;
-
-            match part {
-                // Interpolated expressions are compiled as normal and
-                // dealt with by the VM before being assembled into
-                // the final string.
-                rnix::StrPart::Ast(node) => self.compile(node)?,
-
-                rnix::StrPart::Literal(lit) => {
-                    let idx = self.chunk.add_constant(Value::String(NixString(lit)));
-                    self.chunk.add_op(OpCode::OpConstant(idx));
-                }
-            }
-        }
-
-        if count != 1 {
-            self.chunk.add_op(OpCode::OpInterpolate(count));
-        }
-
-        Ok(())
-    }
-
-    fn compile_binop(&mut self, op: rnix::types::BinOp) -> EvalResult<()> {
-        self.compile(op.lhs().unwrap())?;
-        self.compile(op.rhs().unwrap())?;
-
-        use rnix::types::BinOpKind;
-
-        let opcode = match op.operator().unwrap() {
-            BinOpKind::Add => OpCode::OpAdd,
-            BinOpKind::Sub => OpCode::OpSub,
-            BinOpKind::Mul => OpCode::OpMul,
-            BinOpKind::Div => OpCode::OpDiv,
-            BinOpKind::Equal => OpCode::OpEqual,
-            _ => todo!(),
-        };
-
-        self.chunk.add_op(opcode);
-        Ok(())
-    }
-
-    fn compile_unary_op(&mut self, op: rnix::types::UnaryOp) -> EvalResult<()> {
-        self.compile(op.value().unwrap())?;
-
-        use rnix::types::UnaryOpKind;
-        let opcode = match op.operator() {
-            UnaryOpKind::Invert => OpCode::OpInvert,
-            UnaryOpKind::Negate => OpCode::OpNegate,
-        };
-
-        self.chunk.add_op(opcode);
-        Ok(())
-    }
-
-    fn compile_ident(&mut self, node: rnix::types::Ident) -> EvalResult<()> {
-        match node.as_str() {
-            // TODO(tazjin): Nix technically allows code like
-            //
-            //   let null = 1; in null
-            //   => 1
-            //
-            // which we do *not* want to check at runtime. Once
-            // scoping is introduced, the compiler should carry some
-            // optimised information about any "weird" stuff that's
-            // happened to the scope (such as overrides of these
-            // literals, or builtins).
-            "true" => self.chunk.add_op(OpCode::OpTrue),
-            "false" => self.chunk.add_op(OpCode::OpFalse),
-            "null" => self.chunk.add_op(OpCode::OpNull),
-
-            _ => todo!("identifier access"),
-        };
-
-        Ok(())
-    }
-
-    // Compile attribute set literals into equivalent bytecode.
-    //
-    // This is complicated by a number of features specific to Nix
-    // attribute sets, most importantly:
-    //
-    // 1. Keys can be dynamically constructed through interpolation.
-    // 2. Keys can refer to nested attribute sets.
-    // 3. Attribute sets can (optionally) be recursive.
-    fn compile_attr_set(&mut self, node: rnix::types::AttrSet) -> EvalResult<()> {
-        let mut count = 0;
-
-        for kv in node.entries() {
-            count += 1;
-
-            // Because attribute set literals can contain nested keys,
-            // there is potentially more than one key fragment. If
-            // this is the case, a special operation to construct a
-            // runtime value representing the attribute path is
-            // emitted.
-            let mut key_count = 0;
-            for fragment in kv.key().unwrap().path() {
-                key_count += 1;
-
-                match fragment.kind() {
-                    rnix::SyntaxKind::NODE_IDENT => {
-                        let ident = rnix::types::Ident::cast(fragment).unwrap();
-
-                        // TODO(tazjin): intern!
-                        let idx = self
-                            .chunk
-                            .add_constant(Value::String(NixString(ident.as_str().to_string())));
-                        self.chunk.add_op(OpCode::OpConstant(idx));
-                    }
-
-                    // For all other expression types, we simply
-                    // compile them as normal. The operation should
-                    // result in a string value, which is checked at
-                    // runtime on construction.
-                    _ => self.compile(fragment)?,
-                }
-            }
-
-            // We're done with the key if there was only one fragment,
-            // otherwise we need to emit an instruction to construct
-            // the attribute path.
-            if key_count > 1 {
-                self.chunk.add_op(OpCode::OpAttrPath(2));
-            }
-
-            // The value is just compiled as normal so that its
-            // resulting value is on the stack when the attribute set
-            // is constructed at runtime.
-            self.compile(kv.value().unwrap())?;
-        }
-
-        self.chunk.add_op(OpCode::OpAttrs(count));
-        Ok(())
-    }
-
-    // Compile list literals into equivalent bytecode. List
-    // construction is fairly simple, composing of pushing code for
-    // each literal element and an instruction with the element count.
-    //
-    // The VM, after evaluating the code for each element, simply
-    // constructs the list from the given number of elements.
-    fn compile_list(&mut self, node: rnix::types::List) -> EvalResult<()> {
-        let mut count = 0;
-
-        for item in node.items() {
-            count += 1;
-            self.compile(item)?;
-        }
-
-        self.chunk.add_op(OpCode::OpList(count));
-        Ok(())
-    }
-}
-
-pub fn compile(ast: rnix::AST) -> EvalResult<Chunk> {
-    let mut c = Compiler {
-        chunk: Chunk::default(),
-    };
-
-    c.compile(ast.node())?;
-
-    Ok(c.chunk)
-}
diff --git a/tvix/eval/src/compiler/bindings.rs b/tvix/eval/src/compiler/bindings.rs
new file mode 100644
index 0000000000..634cc54022
--- /dev/null
+++ b/tvix/eval/src/compiler/bindings.rs
@@ -0,0 +1,826 @@
+//! This module implements compiler logic related to name/value binding
+//! definitions (that is, attribute sets and let-expressions).
+//!
+//! In the case of recursive scopes these cases share almost all of their
+//! (fairly complex) logic.
+
+use std::iter::Peekable;
+
+use rnix::ast::HasEntry;
+use rowan::ast::AstChildren;
+
+use super::*;
+
+type PeekableAttrs = Peekable<AstChildren<ast::Attr>>;
+
+/// What kind of bindings scope is being compiled?
+#[derive(Clone, Copy, PartialEq)]
+enum BindingsKind {
+    /// Standard `let ... in ...`-expression.
+    LetIn,
+
+    /// Non-recursive attribute set.
+    Attrs,
+
+    /// Recursive attribute set.
+    RecAttrs,
+}
+
+impl BindingsKind {
+    fn is_attrs(&self) -> bool {
+        matches!(self, BindingsKind::Attrs | BindingsKind::RecAttrs)
+    }
+}
+
+// Internal representation of an attribute set used for merging sets, or
+// inserting nested keys.
+#[derive(Clone)]
+struct AttributeSet {
+    /// Original span at which this set was first encountered.
+    span: Span,
+
+    /// Tracks the kind of set (rec or not).
+    kind: BindingsKind,
+
+    /// All inherited entries
+    inherits: Vec<ast::Inherit>,
+
+    /// All internal entries
+    entries: Vec<(Span, PeekableAttrs, ast::Expr)>,
+}
+
+impl ToSpan for AttributeSet {
+    fn span_for(&self, _: &codemap::File) -> Span {
+        self.span
+    }
+}
+
+impl AttributeSet {
+    fn from_ast(c: &Compiler, node: &ast::AttrSet) -> Self {
+        AttributeSet {
+            span: c.span_for(node),
+
+            // Kind of the attrs depends on the first time it is
+            // encountered. We actually believe this to be a Nix
+            // bug: https://github.com/NixOS/nix/issues/7111
+            kind: if node.rec_token().is_some() {
+                BindingsKind::RecAttrs
+            } else {
+                BindingsKind::Attrs
+            },
+
+            inherits: ast::HasEntry::inherits(node).collect(),
+
+            entries: ast::HasEntry::attrpath_values(node)
+                .map(|entry| {
+                    let span = c.span_for(&entry);
+                    (
+                        span,
+                        entry.attrpath().unwrap().attrs().peekable(),
+                        entry.value().unwrap(),
+                    )
+                })
+                .collect(),
+        }
+    }
+}
+
+// Data structures to track the bindings observed in the second pass, and
+// forward the information needed to compile their value.
+enum Binding {
+    InheritFrom {
+        namespace: ast::Expr,
+        name: SmolStr,
+        span: Span,
+    },
+
+    Plain {
+        expr: ast::Expr,
+    },
+
+    Set(AttributeSet),
+}
+
+impl Binding {
+    /// Merge the provided value into the current binding, or emit an
+    /// error if this turns out to be impossible.
+    fn merge(
+        &mut self,
+        c: &mut Compiler,
+        span: Span,
+        mut remaining_path: PeekableAttrs,
+        value: ast::Expr,
+    ) {
+        match self {
+            Binding::InheritFrom { name, ref span, .. } => {
+                c.emit_error(span, ErrorKind::UnmergeableInherit { name: name.clone() })
+            }
+
+            // If the value is not yet a nested binding, flip the representation
+            // and recurse.
+            Binding::Plain { expr } => match expr {
+                ast::Expr::AttrSet(existing) => {
+                    let nested = AttributeSet::from_ast(c, existing);
+                    *self = Binding::Set(nested);
+                    self.merge(c, span, remaining_path, value);
+                }
+
+                _ => c.emit_error(&value, ErrorKind::UnmergeableValue),
+            },
+
+            // If the value is nested further, it is simply inserted into the
+            // bindings with its full path and resolved recursively further
+            // down.
+            Binding::Set(existing) if remaining_path.peek().is_some() => {
+                existing.entries.push((span, remaining_path, value))
+            }
+
+            Binding::Set(existing) => {
+                if let ast::Expr::AttrSet(new) = value {
+                    existing.inherits.extend(ast::HasEntry::inherits(&new));
+                    existing
+                        .entries
+                        .extend(ast::HasEntry::attrpath_values(&new).map(|entry| {
+                            let span = c.span_for(&entry);
+                            (
+                                span,
+                                entry.attrpath().unwrap().attrs().peekable(),
+                                entry.value().unwrap(),
+                            )
+                        }));
+                } else {
+                    // This branch is unreachable because in cases where the
+                    // path is empty (i.e. there is no further nesting), the
+                    // previous try_merge function already verified that the
+                    // expression is an attribute set.
+
+                    // TODO(tazjin): Consider making this branch live by
+                    // shuffling that check around and emitting a static error
+                    // here instead of a runtime error.
+                    unreachable!()
+                }
+            }
+        }
+    }
+}
+
+enum KeySlot {
+    /// There is no key slot (`let`-expressions do not emit their key).
+    None { name: SmolStr },
+
+    /// The key is statically known and has a slot.
+    Static { slot: LocalIdx, name: SmolStr },
+
+    /// The key is dynamic, i.e. only known at runtime, and must be compiled
+    /// into its slot.
+    Dynamic { slot: LocalIdx, attr: ast::Attr },
+}
+
+struct TrackedBinding {
+    key_slot: KeySlot,
+    value_slot: LocalIdx,
+    binding: Binding,
+}
+
+impl TrackedBinding {
+    /// Does this binding match the given key?
+    ///
+    /// Used to determine which binding to merge another one into.
+    fn matches(&self, key: &str) -> bool {
+        match &self.key_slot {
+            KeySlot::None { name } => name == key,
+            KeySlot::Static { name, .. } => name == key,
+            KeySlot::Dynamic { .. } => false,
+        }
+    }
+}
+
+struct TrackedBindings {
+    bindings: Vec<TrackedBinding>,
+}
+
+impl TrackedBindings {
+    fn new() -> Self {
+        TrackedBindings { bindings: vec![] }
+    }
+
+    /// Attempt to merge an entry into an existing matching binding, assuming
+    /// that the provided binding is mergable (i.e. either a nested key or an
+    /// attribute set literal).
+    ///
+    /// Returns true if the binding was merged, false if it needs to be compiled
+    /// separately as a new binding.
+    fn try_merge(
+        &mut self,
+        c: &mut Compiler,
+        span: Span,
+        name: &ast::Attr,
+        mut remaining_path: PeekableAttrs,
+        value: ast::Expr,
+    ) -> bool {
+        // If the path has no more entries, and if the entry is not an
+        // attribute set literal, the entry can not be merged.
+        if remaining_path.peek().is_none() && !matches!(value, ast::Expr::AttrSet(_)) {
+            return false;
+        }
+
+        // If the first element of the path is not statically known, the entry
+        // can not be merged.
+        let name = match expr_static_attr_str(name) {
+            Some(name) => name,
+            None => return false,
+        };
+
+        // If there is no existing binding with this key, the entry can not be
+        // merged.
+        // TODO: benchmark whether using a map or something is useful over the
+        // `find` here
+        let binding = match self.bindings.iter_mut().find(|b| b.matches(&name)) {
+            Some(b) => b,
+            None => return false,
+        };
+
+        // No more excuses ... the binding can be merged!
+        binding.binding.merge(c, span, remaining_path, value);
+
+        true
+    }
+
+    /// Add a completely new binding to the tracked bindings.
+    fn track_new(&mut self, key_slot: KeySlot, value_slot: LocalIdx, binding: Binding) {
+        self.bindings.push(TrackedBinding {
+            key_slot,
+            value_slot,
+            binding,
+        });
+    }
+}
+
+/// Wrapper around the `ast::HasEntry` trait as that trait can not be
+/// implemented for custom types.
+trait HasEntryProxy {
+    fn inherits(&self) -> Box<dyn Iterator<Item = ast::Inherit>>;
+
+    fn attributes<'a>(
+        &self,
+        file: &'a codemap::File,
+    ) -> Box<dyn Iterator<Item = (Span, PeekableAttrs, ast::Expr)> + 'a>;
+}
+
+impl<N: HasEntry> HasEntryProxy for N {
+    fn inherits(&self) -> Box<dyn Iterator<Item = ast::Inherit>> {
+        Box::new(ast::HasEntry::inherits(self))
+    }
+
+    fn attributes<'a>(
+        &self,
+        file: &'a codemap::File,
+    ) -> Box<dyn Iterator<Item = (Span, PeekableAttrs, ast::Expr)> + 'a> {
+        Box::new(ast::HasEntry::attrpath_values(self).map(move |entry| {
+            (
+                entry.span_for(file),
+                entry.attrpath().unwrap().attrs().peekable(),
+                entry.value().unwrap(),
+            )
+        }))
+    }
+}
+
+impl HasEntryProxy for AttributeSet {
+    fn inherits(&self) -> Box<dyn Iterator<Item = ast::Inherit>> {
+        Box::new(self.inherits.clone().into_iter())
+    }
+
+    fn attributes<'a>(
+        &self,
+        _: &'a codemap::File,
+    ) -> Box<dyn Iterator<Item = (Span, PeekableAttrs, ast::Expr)> + 'a> {
+        Box::new(self.entries.clone().into_iter())
+    }
+}
+
+/// AST-traversing functions related to bindings.
+impl Compiler<'_, '_> {
+    /// Compile all inherits of a node with entries that do *not* have a
+    /// namespace to inherit from, and return the remaining ones that do.
+    fn compile_plain_inherits<N>(
+        &mut self,
+        slot: LocalIdx,
+        kind: BindingsKind,
+        count: &mut usize,
+        node: &N,
+    ) -> Vec<(ast::Expr, SmolStr, Span)>
+    where
+        N: ToSpan + HasEntryProxy,
+    {
+        // Pass over all inherits, resolving only those without namespaces.
+        // Since they always resolve in a higher scope, we can just compile and
+        // declare them immediately.
+        //
+        // Inherits with namespaces are returned to the caller.
+        let mut inherit_froms: Vec<(ast::Expr, SmolStr, Span)> = vec![];
+
+        for inherit in node.inherits() {
+            if inherit.attrs().peekable().peek().is_none() {
+                self.emit_warning(&inherit, WarningKind::EmptyInherit);
+                continue;
+            }
+
+            match inherit.from() {
+                // Within a `let` binding, inheriting from the outer scope is a
+                // no-op *if* there are no dynamic bindings.
+                None if !kind.is_attrs() && !self.has_dynamic_ancestor() => {
+                    self.emit_warning(&inherit, WarningKind::UselessInherit);
+                    continue;
+                }
+
+                None => {
+                    for attr in inherit.attrs() {
+                        let name = match expr_static_attr_str(&attr) {
+                            Some(name) => name,
+                            None => {
+                                self.emit_error(&attr, ErrorKind::DynamicKeyInScope("inherit"));
+                                continue;
+                            }
+                        };
+
+                        // If the identifier resolves statically in a `let`, it
+                        // has precedence over dynamic bindings, and the inherit
+                        // is useless.
+                        if kind == BindingsKind::LetIn
+                            && matches!(
+                                self.scope_mut().resolve_local(&name),
+                                LocalPosition::Known(_)
+                            )
+                        {
+                            self.emit_warning(&attr, WarningKind::UselessInherit);
+                            continue;
+                        }
+
+                        *count += 1;
+
+                        // Place key on the stack when compiling attribute sets.
+                        if kind.is_attrs() {
+                            self.emit_constant(name.as_str().into(), &attr);
+                            let span = self.span_for(&attr);
+                            self.scope_mut().declare_phantom(span, true);
+                        }
+
+                        // Place the value on the stack. Note that because plain
+                        // inherits are always in the outer scope, the slot of
+                        // *this* scope itself is used.
+                        self.compile_identifier_access(slot, &name, &attr);
+
+                        // In non-recursive attribute sets, the key slot must be
+                        // a phantom (i.e. the identifier can not be resolved in
+                        // this scope).
+                        let idx = if kind == BindingsKind::Attrs {
+                            let span = self.span_for(&attr);
+                            self.scope_mut().declare_phantom(span, false)
+                        } else {
+                            self.declare_local(&attr, name)
+                        };
+
+                        self.scope_mut().mark_initialised(idx);
+                    }
+                }
+
+                Some(from) => {
+                    for attr in inherit.attrs() {
+                        let name = match expr_static_attr_str(&attr) {
+                            Some(name) => name,
+                            None => {
+                                self.emit_error(&attr, ErrorKind::DynamicKeyInScope("inherit"));
+                                continue;
+                            }
+                        };
+
+                        *count += 1;
+                        inherit_froms.push((from.expr().unwrap(), name, self.span_for(&attr)));
+                    }
+                }
+            }
+        }
+
+        inherit_froms
+    }
+
+    /// Declare all namespaced inherits, that is inherits which are inheriting
+    /// values from an attribute set.
+    ///
+    /// This only ensures that the locals stack is aware of the inherits, it
+    /// does not yet emit bytecode that places them on the stack. This is up to
+    /// the owner of the `bindings` vector, which this function will populate.
+    fn declare_namespaced_inherits(
+        &mut self,
+        kind: BindingsKind,
+        inherit_froms: Vec<(ast::Expr, SmolStr, Span)>,
+        bindings: &mut TrackedBindings,
+    ) {
+        for (from, name, span) in inherit_froms {
+            let key_slot = if kind.is_attrs() {
+                // In an attribute set, the keys themselves are placed on the
+                // stack but their stack slot is inaccessible (it is only
+                // consumed by `OpAttrs`).
+                KeySlot::Static {
+                    slot: self.scope_mut().declare_phantom(span, false),
+                    name: name.clone(),
+                }
+            } else {
+                KeySlot::None { name: name.clone() }
+            };
+
+            let value_slot = match kind {
+                // In recursive scopes, the value needs to be accessible on the
+                // stack.
+                BindingsKind::LetIn | BindingsKind::RecAttrs => {
+                    self.declare_local(&span, name.clone())
+                }
+
+                // In non-recursive attribute sets, the value is inaccessible
+                // (only consumed by `OpAttrs`).
+                BindingsKind::Attrs => self.scope_mut().declare_phantom(span, false),
+            };
+
+            bindings.track_new(
+                key_slot,
+                value_slot,
+                Binding::InheritFrom {
+                    namespace: from,
+                    name,
+                    span,
+                },
+            );
+        }
+    }
+
+    /// Declare all regular bindings (i.e. `key = value;`) in a bindings scope,
+    /// but do not yet compile their values.
+    fn declare_bindings<N>(
+        &mut self,
+        kind: BindingsKind,
+        count: &mut usize,
+        bindings: &mut TrackedBindings,
+        node: &N,
+    ) where
+        N: ToSpan + HasEntryProxy,
+    {
+        for (span, mut path, value) in node.attributes(self.file) {
+            let key = path.next().unwrap();
+
+            if bindings.try_merge(self, span, &key, path.clone(), value.clone()) {
+                // Binding is nested, or already exists and was merged, move on.
+                continue;
+            }
+
+            *count += 1;
+
+            let key_span = self.span_for(&key);
+            let key_slot = match expr_static_attr_str(&key) {
+                Some(name) if kind.is_attrs() => KeySlot::Static {
+                    name,
+                    slot: self.scope_mut().declare_phantom(key_span, false),
+                },
+
+                Some(name) => KeySlot::None { name },
+
+                None if kind.is_attrs() => KeySlot::Dynamic {
+                    attr: key,
+                    slot: self.scope_mut().declare_phantom(key_span, false),
+                },
+
+                None => {
+                    self.emit_error(&key, ErrorKind::DynamicKeyInScope("let-expression"));
+                    continue;
+                }
+            };
+
+            let value_slot = match kind {
+                BindingsKind::LetIn | BindingsKind::RecAttrs => match &key_slot {
+                    // In recursive scopes, the value needs to be accessible on the
+                    // stack if it is statically known
+                    KeySlot::None { name } | KeySlot::Static { name, .. } => {
+                        self.declare_local(&key_span, name.as_str())
+                    }
+
+                    // Dynamic values are never resolvable (as their names are
+                    // of course only known at runtime).
+                    //
+                    // Note: This branch is unreachable in `let`-expressions.
+                    KeySlot::Dynamic { .. } => self.scope_mut().declare_phantom(key_span, false),
+                },
+
+                // In non-recursive attribute sets, the value is inaccessible
+                // (only consumed by `OpAttrs`).
+                BindingsKind::Attrs => self.scope_mut().declare_phantom(key_span, false),
+            };
+
+            let binding = if path.peek().is_some() {
+                Binding::Set(AttributeSet {
+                    span,
+                    kind: BindingsKind::Attrs,
+                    inherits: vec![],
+                    entries: vec![(span, path, value)],
+                })
+            } else {
+                Binding::Plain { expr: value }
+            };
+
+            bindings.track_new(key_slot, value_slot, binding);
+        }
+    }
+
+    /// Compile attribute set literals into equivalent bytecode.
+    ///
+    /// This is complicated by a number of features specific to Nix attribute
+    /// sets, most importantly:
+    ///
+    /// 1. Keys can be dynamically constructed through interpolation.
+    /// 2. Keys can refer to nested attribute sets.
+    /// 3. Attribute sets can (optionally) be recursive.
+    pub(super) fn compile_attr_set(&mut self, slot: LocalIdx, node: &ast::AttrSet) {
+        // Open a scope to track the positions of the temporaries used by the
+        // `OpAttrs` instruction.
+        self.scope_mut().begin_scope();
+
+        let kind = if node.rec_token().is_some() {
+            BindingsKind::RecAttrs
+        } else {
+            BindingsKind::Attrs
+        };
+
+        self.compile_bindings(slot, kind, node);
+
+        // Remove the temporary scope, but do not emit any additional cleanup
+        // (OpAttrs consumes all of these locals).
+        self.scope_mut().end_scope();
+    }
+
+    /// Actually binds all tracked bindings by emitting the bytecode that places
+    /// them in their stack slots.
+    fn bind_values(&mut self, bindings: TrackedBindings) {
+        let mut value_indices: Vec<LocalIdx> = vec![];
+
+        for binding in bindings.bindings.into_iter() {
+            value_indices.push(binding.value_slot);
+
+            match binding.key_slot {
+                KeySlot::None { .. } => {} // nothing to do here
+
+                KeySlot::Static { slot, name } => {
+                    let span = self.scope()[slot].span;
+                    self.emit_constant(name.as_str().into(), &span);
+                    self.scope_mut().mark_initialised(slot);
+                }
+
+                KeySlot::Dynamic { slot, attr } => {
+                    self.compile_attr(slot, &attr);
+                    self.scope_mut().mark_initialised(slot);
+                }
+            }
+
+            match binding.binding {
+                // This entry is an inherit (from) expr. The value is placed on
+                // the stack by selecting an attribute.
+                Binding::InheritFrom {
+                    namespace,
+                    name,
+                    span,
+                } => {
+                    // Create a thunk wrapping value (which may be one as well)
+                    // to avoid forcing the from expr too early.
+                    self.thunk(binding.value_slot, &namespace, |c, s| {
+                        c.compile(s, namespace.clone());
+                        c.emit_force(&namespace);
+
+                        c.emit_constant(name.as_str().into(), &span);
+                        c.push_op(OpCode::OpAttrsSelect, &span);
+                    })
+                }
+
+                // Binding is "just" a plain expression that needs to be
+                // compiled.
+                Binding::Plain { expr } => self.compile(binding.value_slot, expr),
+
+                // Binding is a merged or nested attribute set, and needs to be
+                // recursively compiled as another binding.
+                Binding::Set(set) => self.thunk(binding.value_slot, &set, |c, _| {
+                    c.scope_mut().begin_scope();
+                    c.compile_bindings(binding.value_slot, set.kind, &set);
+                    c.scope_mut().end_scope();
+                }),
+            }
+
+            // Any code after this point will observe the value in the right
+            // stack slot, so mark it as initialised.
+            self.scope_mut().mark_initialised(binding.value_slot);
+        }
+
+        // Final pass to emit finaliser instructions if necessary.
+        for idx in value_indices {
+            if self.scope()[idx].needs_finaliser {
+                let stack_idx = self.scope().stack_index(idx);
+                let span = self.scope()[idx].span;
+                self.push_op(OpCode::OpFinalise(stack_idx), &span);
+            }
+        }
+    }
+
+    fn compile_bindings<N>(&mut self, slot: LocalIdx, kind: BindingsKind, node: &N)
+    where
+        N: ToSpan + HasEntryProxy,
+    {
+        let mut count = 0;
+        self.scope_mut().begin_scope();
+
+        // Vector to track all observed bindings.
+        let mut bindings = TrackedBindings::new();
+
+        let inherit_froms = self.compile_plain_inherits(slot, kind, &mut count, node);
+        self.declare_namespaced_inherits(kind, inherit_froms, &mut bindings);
+        self.declare_bindings(kind, &mut count, &mut bindings, node);
+
+        // Check if we can bail out on empty bindings
+        if count == 0 {
+            // still need an attrset to exist, but it is empty.
+            if kind.is_attrs() {
+                self.emit_constant(Value::Attrs(Box::new(NixAttrs::empty())), node);
+                return;
+            }
+
+            self.emit_warning(node, WarningKind::EmptyLet);
+            return;
+        }
+
+        // Actually bind values and ensure they are on the stack.
+        self.bind_values(bindings);
+
+        if kind.is_attrs() {
+            self.push_op(OpCode::OpAttrs(Count(count)), node);
+        }
+
+        if count == 0 {
+            self.unthunk();
+        }
+    }
+
+    /// Compile a standard `let ...; in ...` expression.
+    ///
+    /// Unless in a non-standard scope, the encountered values are simply pushed
+    /// on the stack and their indices noted in the entries vector.
+    pub(super) fn compile_let_in(&mut self, slot: LocalIdx, node: &ast::LetIn) {
+        self.compile_bindings(slot, BindingsKind::LetIn, node);
+
+        // Deal with the body, then clean up the locals afterwards.
+        self.compile(slot, node.body().unwrap());
+        self.cleanup_scope(node);
+    }
+
+    pub(super) fn compile_legacy_let(&mut self, slot: LocalIdx, node: &ast::LegacyLet) {
+        self.emit_warning(node, WarningKind::DeprecatedLegacyLet);
+        self.scope_mut().begin_scope();
+        self.compile_bindings(slot, BindingsKind::RecAttrs, node);
+
+        // Remove the temporary scope, but do not emit any additional cleanup
+        // (OpAttrs consumes all of these locals).
+        self.scope_mut().end_scope();
+
+        self.emit_constant("body".into(), node);
+        self.push_op(OpCode::OpAttrsSelect, node);
+    }
+
+    /// Is the given identifier defined *by the user* in any current scope?
+    pub(super) fn is_user_defined(&mut self, ident: &str) -> bool {
+        matches!(
+            self.scope_mut().resolve_local(ident),
+            LocalPosition::Known(_) | LocalPosition::Recursive(_)
+        )
+    }
+
+    /// Resolve and compile access to an identifier in the scope.
+    fn compile_identifier_access<N: ToSpan + Clone>(
+        &mut self,
+        slot: LocalIdx,
+        ident: &str,
+        node: &N,
+    ) {
+        match self.scope_mut().resolve_local(ident) {
+            LocalPosition::Unknown => {
+                // Are we possibly dealing with an upvalue?
+                if let Some(idx) = self.resolve_upvalue(self.contexts.len() - 1, ident, node) {
+                    self.push_op(OpCode::OpGetUpvalue(idx), node);
+                    return;
+                }
+
+                // Globals are the "upmost upvalues": they behave
+                // exactly like a `let ... in` prepended to the
+                // program's text, and the global scope is nothing
+                // more than the parent scope of the root scope.
+                if let Some(global) = self.globals.get(ident) {
+                    self.emit_constant(global.clone(), &self.span_for(node));
+                    return;
+                }
+
+                // If there is a non-empty `with`-stack (or a parent context
+                // with one), emit a runtime dynamic resolution instruction.
+                //
+                // Since it is possible for users to e.g. assign a variable to a
+                // dynamic resolution without actually using it, this operation
+                // is wrapped in an extra thunk.
+                if self.has_dynamic_ancestor() {
+                    self.thunk(slot, node, |c, _| {
+                        c.context_mut().captures_with_stack = true;
+                        c.emit_constant(ident.into(), node);
+                        c.push_op(OpCode::OpResolveWith, node);
+                    });
+                    return;
+                }
+
+                // Otherwise, this variable is missing.
+                self.emit_error(node, ErrorKind::UnknownStaticVariable);
+            }
+
+            LocalPosition::Known(idx) => {
+                let stack_idx = self.scope().stack_index(idx);
+                self.push_op(OpCode::OpGetLocal(stack_idx), node);
+            }
+
+            // This identifier is referring to a value from the same scope which
+            // is not yet defined. This identifier access must be thunked.
+            LocalPosition::Recursive(idx) => self.thunk(slot, node, move |compiler, _| {
+                let upvalue_idx = compiler.add_upvalue(
+                    compiler.contexts.len() - 1,
+                    node,
+                    UpvalueKind::Local(idx),
+                );
+                compiler.push_op(OpCode::OpGetUpvalue(upvalue_idx), node);
+            }),
+        };
+    }
+
+    pub(super) fn compile_ident(&mut self, slot: LocalIdx, node: &ast::Ident) {
+        let ident = node.ident_token().unwrap();
+        self.compile_identifier_access(slot, ident.text(), node);
+    }
+}
+
+/// Private compiler helpers related to bindings.
+impl Compiler<'_, '_> {
+    fn resolve_upvalue<N: ToSpan>(
+        &mut self,
+        ctx_idx: usize,
+        name: &str,
+        node: &N,
+    ) -> Option<UpvalueIdx> {
+        if ctx_idx == 0 {
+            // There can not be any upvalue at the outermost context.
+            return None;
+        }
+
+        // Determine whether the upvalue is a local in the enclosing context.
+        match self.contexts[ctx_idx - 1].scope.resolve_local(name) {
+            // recursive upvalues are dealt with the same way as standard known
+            // ones, as thunks and closures are guaranteed to be placed on the
+            // stack (i.e. in the right position) *during* their runtime
+            // construction
+            LocalPosition::Known(idx) | LocalPosition::Recursive(idx) => {
+                return Some(self.add_upvalue(ctx_idx, node, UpvalueKind::Local(idx)))
+            }
+
+            LocalPosition::Unknown => { /* continue below */ }
+        };
+
+        // If the upvalue comes from even further up, we need to recurse to make
+        // sure that the upvalues are created at each level.
+        if let Some(idx) = self.resolve_upvalue(ctx_idx - 1, name, node) {
+            return Some(self.add_upvalue(ctx_idx, node, UpvalueKind::Upvalue(idx)));
+        }
+
+        None
+    }
+
+    fn add_upvalue<N: ToSpan>(
+        &mut self,
+        ctx_idx: usize,
+        node: &N,
+        kind: UpvalueKind,
+    ) -> UpvalueIdx {
+        // If there is already an upvalue closing over the specified index,
+        // retrieve that instead.
+        for (idx, existing) in self.contexts[ctx_idx].scope.upvalues.iter().enumerate() {
+            if existing.kind == kind {
+                return UpvalueIdx(idx);
+            }
+        }
+
+        let span = self.span_for(node);
+        self.contexts[ctx_idx]
+            .scope
+            .upvalues
+            .push(Upvalue { kind, span });
+
+        let idx = UpvalueIdx(self.contexts[ctx_idx].lambda.upvalue_count);
+        self.contexts[ctx_idx].lambda.upvalue_count += 1;
+        idx
+    }
+}
diff --git a/tvix/eval/src/compiler/import.rs b/tvix/eval/src/compiler/import.rs
new file mode 100644
index 0000000000..9036eec817
--- /dev/null
+++ b/tvix/eval/src/compiler/import.rs
@@ -0,0 +1,120 @@
+//! This module implements the Nix language's `import` feature, which
+//! is exposed as a builtin in the Nix language.
+//!
+//! This is not a typical builtin, as it needs access to internal
+//! compiler and VM state (such as the [`crate::SourceCode`]
+//! instance, or observers).
+
+use super::GlobalsMap;
+use genawaiter::rc::Gen;
+use std::rc::Weak;
+
+use crate::{
+    builtins::coerce_value_to_path,
+    generators::pin_generator,
+    observer::NoOpObserver,
+    value::{Builtin, Thunk},
+    vm::generators::{self, GenCo},
+    ErrorKind, SourceCode, Value,
+};
+
+async fn import_impl(
+    co: GenCo,
+    globals: Weak<GlobalsMap>,
+    source: SourceCode,
+    mut args: Vec<Value>,
+) -> Result<Value, ErrorKind> {
+    // TODO(sterni): canon_path()?
+    let mut path = match coerce_value_to_path(&co, args.pop().unwrap()).await? {
+        Err(cek) => return Ok(Value::Catchable(Box::new(cek))),
+        Ok(path) => path,
+    };
+
+    if path.is_dir() {
+        path.push("default.nix");
+    }
+
+    if let Some(cached) = generators::request_import_cache_lookup(&co, path.clone()).await {
+        return Ok(cached);
+    }
+
+    let mut reader = generators::request_open_file(&co, path.clone()).await;
+    // We read to a String instead of a Vec<u8> because rnix only supports
+    // string source files.
+    let mut contents = String::new();
+    reader.read_to_string(&mut contents)?;
+
+    let parsed = rnix::ast::Root::parse(&contents);
+    let errors = parsed.errors();
+    let file = source.add_file(path.to_string_lossy().to_string(), contents.to_owned());
+
+    if !errors.is_empty() {
+        return Err(ErrorKind::ImportParseError {
+            path,
+            file,
+            errors: errors.to_vec(),
+        });
+    }
+
+    let result = crate::compiler::compile(
+        &parsed.tree().expr().unwrap(),
+        Some(path.clone()),
+        // The VM must ensure that a strong reference to the globals outlives
+        // any self-references (which are weak) embedded within the globals. If
+        // the expect() below panics, it means that did not happen.
+        globals
+            .upgrade()
+            .expect("globals dropped while still in use"),
+        &source,
+        &file,
+        &mut NoOpObserver::default(),
+    )
+    .map_err(|err| ErrorKind::ImportCompilerError {
+        path: path.clone(),
+        errors: vec![err],
+    })?;
+
+    if !result.errors.is_empty() {
+        return Err(ErrorKind::ImportCompilerError {
+            path,
+            errors: result.errors,
+        });
+    }
+
+    for warning in result.warnings {
+        generators::emit_warning(&co, warning).await;
+    }
+
+    // Compilation succeeded, we can construct a thunk from whatever it spat
+    // out and return that.
+    let res = Value::Thunk(Thunk::new_suspended(
+        result.lambda,
+        generators::request_span(&co).await,
+    ));
+
+    generators::request_import_cache_put(&co, path, res.clone()).await;
+
+    Ok(res)
+}
+
+/// Constructs the `import` builtin. This builtin is special in that
+/// it needs to capture the [crate::SourceCode] structure to correctly
+/// track source code locations while invoking a compiler.
+// TODO: need to be able to pass through a CompilationObserver, too.
+// TODO: can the `SourceCode` come from the compiler?
+pub(super) fn builtins_import(globals: &Weak<GlobalsMap>, source: SourceCode) -> Builtin {
+    // This (very cheap, once-per-compiler-startup) clone exists
+    // solely in order to keep the borrow checker happy.  It
+    // resolves the tension between the requirements of
+    // Rc::new_cyclic() and Builtin::new()
+    let globals = globals.clone();
+
+    Builtin::new(
+        "import",
+        Some("Import the given file and return the Nix value it evaluates to"),
+        1,
+        move |args| {
+            Gen::new(|co| pin_generator(import_impl(co, globals.clone(), source.clone(), args)))
+        },
+    )
+}
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
new file mode 100644
index 0000000000..60c55dda27
--- /dev/null
+++ b/tvix/eval/src/compiler/mod.rs
@@ -0,0 +1,1684 @@
+//! This module implements a compiler for compiling the rnix AST
+//! representation to Tvix bytecode.
+//!
+//! A note on `unwrap()`: This module contains a lot of calls to
+//! `unwrap()` or `expect(...)` on data structures returned by `rnix`.
+//! The reason for this is that rnix uses the same data structures to
+//! represent broken and correct ASTs, so all typed AST variants have
+//! the ability to represent an incorrect node.
+//!
+//! However, at the time that the AST is passed to the compiler we
+//! have verified that `rnix` considers the code to be correct, so all
+//! variants are fulfilled. In cases where the invariant is guaranteed
+//! by the code in this module, `debug_assert!` has been used to catch
+//! mistakes early during development.
+
+mod bindings;
+mod import;
+mod optimiser;
+mod scope;
+
+use codemap::Span;
+use rnix::ast::{self, AstToken};
+use smol_str::SmolStr;
+use std::collections::{BTreeMap, HashMap};
+use std::path::{Path, PathBuf};
+use std::rc::{Rc, Weak};
+
+use crate::chunk::Chunk;
+use crate::errors::{CatchableErrorKind, Error, ErrorKind, EvalResult};
+use crate::observer::CompilerObserver;
+use crate::opcode::{CodeIdx, ConstantIdx, Count, JumpOffset, OpCode, UpvalueIdx};
+use crate::spans::LightSpan;
+use crate::spans::ToSpan;
+use crate::value::{Closure, Formals, Lambda, NixAttrs, Thunk, Value};
+use crate::warnings::{EvalWarning, WarningKind};
+use crate::CoercionKind;
+use crate::SourceCode;
+
+use self::scope::{LocalIdx, LocalPosition, Scope, Upvalue, UpvalueKind};
+
+/// Represents the result of compiling a piece of Nix code. If
+/// compilation was successful, the resulting bytecode can be passed
+/// to the VM.
+pub struct CompilationOutput {
+    pub lambda: Rc<Lambda>,
+    pub warnings: Vec<EvalWarning>,
+    pub errors: Vec<Error>,
+
+    // This field must outlive the rc::Weak reference which breaks the
+    // builtins -> import -> builtins reference cycle. For this
+    // reason, it must be passed to the VM.
+    pub globals: Rc<GlobalsMap>,
+}
+
+/// Represents the lambda currently being compiled.
+struct LambdaCtx {
+    lambda: Lambda,
+    scope: Scope,
+    captures_with_stack: bool,
+    unthunk: bool,
+}
+
+impl LambdaCtx {
+    fn new() -> Self {
+        LambdaCtx {
+            lambda: Lambda::default(),
+            scope: Default::default(),
+            captures_with_stack: false,
+            unthunk: false,
+        }
+    }
+
+    fn inherit(&self) -> Self {
+        LambdaCtx {
+            lambda: Lambda::default(),
+            scope: self.scope.inherit(),
+            captures_with_stack: false,
+            unthunk: false,
+        }
+    }
+}
+
+/// When compiling functions with an argument attribute set destructuring pattern,
+/// we need to do multiple passes over the declared formal arguments when setting
+/// up their local bindings (similarly to `let โ€ฆ in` expressions and recursive
+/// attribute sets. For this purpose, this struct is used to represent the two
+/// kinds of formal arguments:
+///
+/// - `TrackedFormal::NoDefault` is always required and causes an evaluation error
+///   if the corresponding attribute is missing in a function call.
+/// - `TrackedFormal::WithDefault` may be missing in the passed attribute setโ€”
+///   in which case a `default_expr` will be evaluated and placed in the formal
+///   argument's local variable slot.
+enum TrackedFormal {
+    NoDefault {
+        local_idx: LocalIdx,
+        pattern_entry: ast::PatEntry,
+    },
+    WithDefault {
+        local_idx: LocalIdx,
+        /// Extra phantom local used for coordinating runtime dispatching not observable to
+        /// the language user. Detailed description in `compile_param_pattern()`.
+        finalise_request_idx: LocalIdx,
+        default_expr: ast::Expr,
+        pattern_entry: ast::PatEntry,
+    },
+}
+
+impl TrackedFormal {
+    fn pattern_entry(&self) -> &ast::PatEntry {
+        match self {
+            TrackedFormal::NoDefault { pattern_entry, .. } => pattern_entry,
+            TrackedFormal::WithDefault { pattern_entry, .. } => pattern_entry,
+        }
+    }
+    fn local_idx(&self) -> LocalIdx {
+        match self {
+            TrackedFormal::NoDefault { local_idx, .. } => *local_idx,
+            TrackedFormal::WithDefault { local_idx, .. } => *local_idx,
+        }
+    }
+}
+
+/// The map of globally available functions and other values that
+/// should implicitly be resolvable in the global scope.
+pub(crate) type GlobalsMap = HashMap<&'static str, Value>;
+
+/// Set of builtins that (if they exist) should be made available in
+/// the global scope, meaning that they can be accessed not just
+/// through `builtins.<name>`, but directly as `<name>`. This is not
+/// configurable, it is based on what Nix 2.3 exposed.
+const GLOBAL_BUILTINS: &[&str] = &[
+    "abort",
+    "baseNameOf",
+    "derivation",
+    "derivationStrict",
+    "dirOf",
+    "fetchGit",
+    "fetchMercurial",
+    "fetchTarball",
+    "fromTOML",
+    "import",
+    "isNull",
+    "map",
+    "placeholder",
+    "removeAttrs",
+    "scopedImport",
+    "throw",
+    "toString",
+    "__curPos",
+];
+
+pub struct Compiler<'source, 'observer> {
+    contexts: Vec<LambdaCtx>,
+    warnings: Vec<EvalWarning>,
+    errors: Vec<Error>,
+    root_dir: PathBuf,
+
+    /// Carries all known global tokens; the full set of which is
+    /// created when the compiler is invoked.
+    ///
+    /// Each global has an associated token, which when encountered as
+    /// an identifier is resolved against the scope poisoning logic,
+    /// and a function that should emit code for the token.
+    globals: Rc<GlobalsMap>,
+
+    /// Reference to the struct holding all of the source code, which
+    /// is used for error creation.
+    source: &'source SourceCode,
+
+    /// File reference in the source map for the current file, which
+    /// is used for creating spans.
+    file: &'source codemap::File,
+
+    /// Carry an observer for the compilation process, which is called
+    /// whenever a chunk is emitted.
+    observer: &'observer mut dyn CompilerObserver,
+
+    /// Carry a count of nested scopes which have requested the
+    /// compiler not to emit anything. This used for compiling dead
+    /// code branches to catch errors & warnings in them.
+    dead_scope: usize,
+}
+
+impl Compiler<'_, '_> {
+    pub(super) fn span_for<S: ToSpan>(&self, to_span: &S) -> Span {
+        to_span.span_for(self.file)
+    }
+}
+
+/// Compiler construction
+impl<'source, 'observer> Compiler<'source, 'observer> {
+    pub(crate) fn new(
+        location: Option<PathBuf>,
+        globals: Rc<GlobalsMap>,
+        source: &'source SourceCode,
+        file: &'source codemap::File,
+        observer: &'observer mut dyn CompilerObserver,
+    ) -> EvalResult<Self> {
+        let mut root_dir = match location {
+            Some(dir) if cfg!(target_arch = "wasm32") || dir.is_absolute() => Ok(dir),
+            _ => {
+                let current_dir = std::env::current_dir().map_err(|e| {
+                    Error::new(
+                        ErrorKind::RelativePathResolution(format!(
+                            "could not determine current directory: {}",
+                            e
+                        )),
+                        file.span,
+                        source.clone(),
+                    )
+                })?;
+                if let Some(dir) = location {
+                    Ok(current_dir.join(dir))
+                } else {
+                    Ok(current_dir)
+                }
+            }
+        }?;
+
+        // If the path passed from the caller points to a file, the
+        // filename itself needs to be truncated as this must point to a
+        // directory.
+        if root_dir.is_file() {
+            root_dir.pop();
+        }
+
+        #[cfg(not(target_arch = "wasm32"))]
+        debug_assert!(root_dir.is_absolute());
+
+        Ok(Self {
+            root_dir,
+            source,
+            file,
+            observer,
+            globals,
+            contexts: vec![LambdaCtx::new()],
+            warnings: vec![],
+            errors: vec![],
+            dead_scope: 0,
+        })
+    }
+}
+
+// Helper functions for emitting code and metadata to the internal
+// structures of the compiler.
+impl Compiler<'_, '_> {
+    fn context(&self) -> &LambdaCtx {
+        &self.contexts[self.contexts.len() - 1]
+    }
+
+    fn context_mut(&mut self) -> &mut LambdaCtx {
+        let idx = self.contexts.len() - 1;
+        &mut self.contexts[idx]
+    }
+
+    fn chunk(&mut self) -> &mut Chunk {
+        &mut self.context_mut().lambda.chunk
+    }
+
+    fn scope(&self) -> &Scope {
+        &self.context().scope
+    }
+
+    fn scope_mut(&mut self) -> &mut Scope {
+        &mut self.context_mut().scope
+    }
+
+    /// Push a single instruction to the current bytecode chunk and
+    /// track the source span from which it was compiled.
+    fn push_op<T: ToSpan>(&mut self, data: OpCode, node: &T) -> CodeIdx {
+        if self.dead_scope > 0 {
+            return CodeIdx(0);
+        }
+
+        let span = self.span_for(node);
+        self.chunk().push_op(data, span)
+    }
+
+    /// Emit a single constant to the current bytecode chunk and track
+    /// the source span from which it was compiled.
+    pub(super) fn emit_constant<T: ToSpan>(&mut self, value: Value, node: &T) {
+        if self.dead_scope > 0 {
+            return;
+        }
+
+        let idx = self.chunk().push_constant(value);
+        self.push_op(OpCode::OpConstant(idx), node);
+    }
+}
+
+// Actual code-emitting AST traversal methods.
+impl Compiler<'_, '_> {
+    fn compile(&mut self, slot: LocalIdx, expr: ast::Expr) {
+        let expr = optimiser::optimise_expr(self, slot, expr);
+
+        match &expr {
+            ast::Expr::Literal(literal) => self.compile_literal(literal),
+            ast::Expr::Path(path) => self.compile_path(slot, path),
+            ast::Expr::Str(s) => self.compile_str(slot, s),
+
+            ast::Expr::UnaryOp(op) => self.thunk(slot, op, move |c, s| c.compile_unary_op(s, op)),
+
+            ast::Expr::BinOp(binop) => {
+                self.thunk(slot, binop, move |c, s| c.compile_binop(s, binop))
+            }
+
+            ast::Expr::HasAttr(has_attr) => {
+                self.thunk(slot, has_attr, move |c, s| c.compile_has_attr(s, has_attr))
+            }
+
+            ast::Expr::List(list) => self.thunk(slot, list, move |c, s| c.compile_list(s, list)),
+
+            ast::Expr::AttrSet(attrs) => {
+                self.thunk(slot, attrs, move |c, s| c.compile_attr_set(s, attrs))
+            }
+
+            ast::Expr::Select(select) => {
+                self.thunk(slot, select, move |c, s| c.compile_select(s, select))
+            }
+
+            ast::Expr::Assert(assert) => {
+                self.thunk(slot, assert, move |c, s| c.compile_assert(s, assert))
+            }
+            ast::Expr::IfElse(if_else) => {
+                self.thunk(slot, if_else, move |c, s| c.compile_if_else(s, if_else))
+            }
+
+            ast::Expr::LetIn(let_in) => {
+                self.thunk(slot, let_in, move |c, s| c.compile_let_in(s, let_in))
+            }
+
+            ast::Expr::Ident(ident) => self.compile_ident(slot, ident),
+            ast::Expr::With(with) => self.thunk(slot, with, |c, s| c.compile_with(s, with)),
+            ast::Expr::Lambda(lambda) => self.thunk(slot, lambda, move |c, s| {
+                c.compile_lambda_or_thunk(false, s, lambda, |c, s| c.compile_lambda(s, lambda))
+            }),
+            ast::Expr::Apply(apply) => {
+                self.thunk(slot, apply, move |c, s| c.compile_apply(s, apply))
+            }
+
+            // Parenthesized expressions are simply unwrapped, leaving
+            // their value on the stack.
+            ast::Expr::Paren(paren) => self.compile(slot, paren.expr().unwrap()),
+
+            ast::Expr::LegacyLet(legacy_let) => self.thunk(slot, legacy_let, move |c, s| {
+                c.compile_legacy_let(s, legacy_let)
+            }),
+
+            ast::Expr::Root(_) => unreachable!("there cannot be more than one root"),
+            ast::Expr::Error(_) => unreachable!("compile is only called on validated trees"),
+        }
+    }
+
+    /// Compiles an expression, but does not emit any code for it as
+    /// it is considered dead. This will still catch errors and
+    /// warnings in that expression.
+    ///
+    /// A warning about the that code being dead is assumed to already be
+    /// emitted by the caller of this.
+    fn compile_dead_code(&mut self, slot: LocalIdx, node: ast::Expr) {
+        self.dead_scope += 1;
+        self.compile(slot, node);
+        self.dead_scope -= 1;
+    }
+
+    fn compile_literal(&mut self, node: &ast::Literal) {
+        let value = match node.kind() {
+            ast::LiteralKind::Float(f) => Value::Float(f.value().unwrap()),
+            ast::LiteralKind::Integer(i) => match i.value() {
+                Ok(v) => Value::Integer(v),
+                Err(err) => return self.emit_error(node, err.into()),
+            },
+
+            ast::LiteralKind::Uri(u) => {
+                self.emit_warning(node, WarningKind::DeprecatedLiteralURL);
+                Value::from(u.syntax().text())
+            }
+        };
+
+        self.emit_constant(value, node);
+    }
+
+    fn compile_path(&mut self, slot: LocalIdx, node: &ast::Path) {
+        // TODO(tazjin): placeholder implementation while waiting for
+        // https://github.com/nix-community/rnix-parser/pull/96
+
+        let raw_path = node.to_string();
+        let path = if raw_path.starts_with('/') {
+            Path::new(&raw_path).to_owned()
+        } else if raw_path.starts_with('~') {
+            // We assume that home paths start with ~/ or fail to parse
+            // TODO: this should be checked using a parse-fail test.
+            debug_assert!(raw_path.len() > 2 && raw_path.starts_with("~/"));
+
+            let home_relative_path = &raw_path[2..(raw_path.len())];
+            self.emit_constant(
+                Value::UnresolvedPath(Box::new(home_relative_path.into())),
+                node,
+            );
+            self.push_op(OpCode::OpResolveHomePath, node);
+            return;
+        } else if raw_path.starts_with('<') {
+            // TODO: decide what to do with findFile
+            if raw_path.len() == 2 {
+                return self.emit_constant(
+                    Value::Catchable(Box::new(CatchableErrorKind::NixPathResolution(
+                        "Empty <> path not allowed".into(),
+                    ))),
+                    node,
+                );
+            }
+            let path = &raw_path[1..(raw_path.len() - 1)];
+            // Make a thunk to resolve the path (without using `findFile`, at least for now?)
+            return self.thunk(slot, node, move |c, _| {
+                c.emit_constant(Value::UnresolvedPath(Box::new(path.into())), node);
+                c.push_op(OpCode::OpFindFile, node);
+            });
+        } else {
+            let mut buf = self.root_dir.clone();
+            buf.push(&raw_path);
+            buf
+        };
+
+        // TODO: Use https://github.com/rust-lang/rfcs/issues/2208
+        // once it is available
+        let value = Value::Path(Box::new(crate::value::canon_path(path)));
+        self.emit_constant(value, node);
+    }
+
+    /// Helper that compiles the given string parts strictly. The caller
+    /// (`compile_str`) needs to figure out if the result of compiling this
+    /// needs to be thunked or not.
+    fn compile_str_parts(
+        &mut self,
+        slot: LocalIdx,
+        parent_node: &ast::Str,
+        parts: Vec<ast::InterpolPart<String>>,
+    ) {
+        // The string parts are produced in literal order, however
+        // they need to be reversed on the stack in order to
+        // efficiently create the real string in case of
+        // interpolation.
+        for part in parts.iter().rev() {
+            match part {
+                // Interpolated expressions are compiled as normal and
+                // dealt with by the VM before being assembled into
+                // the final string. We need to coerce them here,
+                // so OpInterpolate definitely has a string to consume.
+                ast::InterpolPart::Interpolation(ipol) => {
+                    self.compile(slot, ipol.expr().unwrap());
+                    // implicitly forces as well
+                    self.push_op(
+                        OpCode::OpCoerceToString(CoercionKind {
+                            strong: false,
+                            import_paths: true,
+                        }),
+                        ipol,
+                    );
+                }
+
+                ast::InterpolPart::Literal(lit) => {
+                    self.emit_constant(Value::from(lit.as_str()), parent_node);
+                }
+            }
+        }
+
+        if parts.len() != 1 {
+            self.push_op(OpCode::OpInterpolate(Count(parts.len())), parent_node);
+        }
+    }
+
+    fn compile_str(&mut self, slot: LocalIdx, node: &ast::Str) {
+        let parts = node.normalized_parts();
+
+        // We need to thunk string expressions if they are the result of
+        // interpolation. A string that only consists of a single part (`"${foo}"`)
+        // can't desugar to the enclosed expression (`foo`) because we need to
+        // coerce the result to a string value. This would require forcing the
+        // value of the inner expression, so we need to wrap it in another thunk.
+        if parts.len() != 1 || matches!(&parts[0], ast::InterpolPart::Interpolation(_)) {
+            self.thunk(slot, node, move |c, s| {
+                c.compile_str_parts(s, node, parts);
+            });
+        } else {
+            self.compile_str_parts(slot, node, parts);
+        }
+    }
+
+    fn compile_unary_op(&mut self, slot: LocalIdx, op: &ast::UnaryOp) {
+        self.compile(slot, op.expr().unwrap());
+        self.emit_force(op);
+
+        let opcode = match op.operator().unwrap() {
+            ast::UnaryOpKind::Invert => OpCode::OpInvert,
+            ast::UnaryOpKind::Negate => OpCode::OpNegate,
+        };
+
+        self.push_op(opcode, op);
+    }
+
+    fn compile_binop(&mut self, slot: LocalIdx, op: &ast::BinOp) {
+        use ast::BinOpKind;
+
+        // Short-circuiting and other strange operators, which are
+        // under the same node type as NODE_BIN_OP, but need to be
+        // handled separately (i.e. before compiling the expressions
+        // used for standard binary operators).
+
+        match op.operator().unwrap() {
+            BinOpKind::And => return self.compile_and(slot, op),
+            BinOpKind::Or => return self.compile_or(slot, op),
+            BinOpKind::Implication => return self.compile_implication(slot, op),
+            _ => {}
+        };
+
+        // For all other operators, the two values need to be left on
+        // the stack in the correct order before pushing the
+        // instruction for the operation itself.
+        self.compile(slot, op.lhs().unwrap());
+        self.emit_force(&op.lhs().unwrap());
+
+        self.compile(slot, op.rhs().unwrap());
+        self.emit_force(&op.rhs().unwrap());
+
+        match op.operator().unwrap() {
+            BinOpKind::Add => self.push_op(OpCode::OpAdd, op),
+            BinOpKind::Sub => self.push_op(OpCode::OpSub, op),
+            BinOpKind::Mul => self.push_op(OpCode::OpMul, op),
+            BinOpKind::Div => self.push_op(OpCode::OpDiv, op),
+            BinOpKind::Update => self.push_op(OpCode::OpAttrsUpdate, op),
+            BinOpKind::Equal => self.push_op(OpCode::OpEqual, op),
+            BinOpKind::Less => self.push_op(OpCode::OpLess, op),
+            BinOpKind::LessOrEq => self.push_op(OpCode::OpLessOrEq, op),
+            BinOpKind::More => self.push_op(OpCode::OpMore, op),
+            BinOpKind::MoreOrEq => self.push_op(OpCode::OpMoreOrEq, op),
+            BinOpKind::Concat => self.push_op(OpCode::OpConcat, op),
+
+            BinOpKind::NotEqual => {
+                self.push_op(OpCode::OpEqual, op);
+                self.push_op(OpCode::OpInvert, op)
+            }
+
+            // Handled by separate branch above.
+            BinOpKind::And | BinOpKind::Implication | BinOpKind::Or => {
+                unreachable!()
+            }
+        };
+    }
+
+    fn compile_and(&mut self, slot: LocalIdx, node: &ast::BinOp) {
+        debug_assert!(
+            matches!(node.operator(), Some(ast::BinOpKind::And)),
+            "compile_and called with wrong operator kind: {:?}",
+            node.operator(),
+        );
+
+        // Leave left-hand side value on the stack.
+        self.compile(slot, node.lhs().unwrap());
+        self.emit_force(&node.lhs().unwrap());
+
+        let throw_idx = self.push_op(OpCode::OpJumpIfCatchable(JumpOffset(0)), node);
+        // If this value is false, jump over the right-hand side - the
+        // whole expression is false.
+        let end_idx = self.push_op(OpCode::OpJumpIfFalse(JumpOffset(0)), node);
+
+        // Otherwise, remove the previous value and leave the
+        // right-hand side on the stack. Its result is now the value
+        // of the whole expression.
+        self.push_op(OpCode::OpPop, node);
+        self.compile(slot, node.rhs().unwrap());
+        self.emit_force(&node.rhs().unwrap());
+
+        self.patch_jump(end_idx);
+        self.push_op(OpCode::OpAssertBool, node);
+        self.patch_jump(throw_idx);
+    }
+
+    fn compile_or(&mut self, slot: LocalIdx, node: &ast::BinOp) {
+        debug_assert!(
+            matches!(node.operator(), Some(ast::BinOpKind::Or)),
+            "compile_or called with wrong operator kind: {:?}",
+            node.operator(),
+        );
+
+        // Leave left-hand side value on the stack
+        self.compile(slot, node.lhs().unwrap());
+        self.emit_force(&node.lhs().unwrap());
+
+        let throw_idx = self.push_op(OpCode::OpJumpIfCatchable(JumpOffset(0)), node);
+        // Opposite of above: If this value is **true**, we can
+        // short-circuit the right-hand side.
+        let end_idx = self.push_op(OpCode::OpJumpIfTrue(JumpOffset(0)), node);
+        self.push_op(OpCode::OpPop, node);
+        self.compile(slot, node.rhs().unwrap());
+        self.emit_force(&node.rhs().unwrap());
+
+        self.patch_jump(end_idx);
+        self.push_op(OpCode::OpAssertBool, node);
+        self.patch_jump(throw_idx);
+    }
+
+    fn compile_implication(&mut self, slot: LocalIdx, node: &ast::BinOp) {
+        debug_assert!(
+            matches!(node.operator(), Some(ast::BinOpKind::Implication)),
+            "compile_implication called with wrong operator kind: {:?}",
+            node.operator(),
+        );
+
+        // Leave left-hand side value on the stack and invert it.
+        self.compile(slot, node.lhs().unwrap());
+        self.emit_force(&node.lhs().unwrap());
+        let throw_idx = self.push_op(OpCode::OpJumpIfCatchable(JumpOffset(0)), node);
+        self.push_op(OpCode::OpInvert, node);
+
+        // Exactly as `||` (because `a -> b` = `!a || b`).
+        let end_idx = self.push_op(OpCode::OpJumpIfTrue(JumpOffset(0)), node);
+        self.push_op(OpCode::OpPop, node);
+        self.compile(slot, node.rhs().unwrap());
+        self.emit_force(&node.rhs().unwrap());
+
+        self.patch_jump(end_idx);
+        self.push_op(OpCode::OpAssertBool, node);
+        self.patch_jump(throw_idx);
+    }
+
+    /// Compile list literals into equivalent bytecode. List
+    /// construction is fairly simple, consisting of pushing code for
+    /// each literal element and an instruction with the element
+    /// count.
+    ///
+    /// The VM, after evaluating the code for each element, simply
+    /// constructs the list from the given number of elements.
+    fn compile_list(&mut self, slot: LocalIdx, node: &ast::List) {
+        let mut count = 0;
+
+        // Open a temporary scope to correctly account for stack items
+        // that exist during the construction.
+        self.scope_mut().begin_scope();
+
+        for item in node.items() {
+            // Start tracing new stack slots from the second list
+            // element onwards. The first list element is located in
+            // the stack slot of the list itself.
+            let item_slot = match count {
+                0 => slot,
+                _ => {
+                    let item_span = self.span_for(&item);
+                    self.scope_mut().declare_phantom(item_span, false)
+                }
+            };
+
+            count += 1;
+            self.compile(item_slot, item);
+            self.scope_mut().mark_initialised(item_slot);
+        }
+
+        if count == 0 {
+            self.unthunk();
+        }
+
+        self.push_op(OpCode::OpList(Count(count)), node);
+        self.scope_mut().end_scope();
+    }
+
+    fn compile_attr(&mut self, slot: LocalIdx, node: &ast::Attr) {
+        match node {
+            ast::Attr::Dynamic(dynamic) => {
+                self.compile(slot, dynamic.expr().unwrap());
+                self.emit_force(&dynamic.expr().unwrap());
+            }
+
+            ast::Attr::Str(s) => {
+                self.compile_str(slot, s);
+                self.emit_force(s);
+            }
+
+            ast::Attr::Ident(ident) => self.emit_literal_ident(ident),
+        }
+    }
+
+    fn compile_has_attr(&mut self, slot: LocalIdx, node: &ast::HasAttr) {
+        // Put the attribute set on the stack.
+        self.compile(slot, node.expr().unwrap());
+        self.emit_force(node);
+
+        // Push all path fragments with an operation for fetching the
+        // next nested element, for all fragments except the last one.
+        for (count, fragment) in node.attrpath().unwrap().attrs().enumerate() {
+            if count > 0 {
+                self.push_op(OpCode::OpAttrsTrySelect, &fragment);
+                self.emit_force(&fragment);
+            }
+
+            self.compile_attr(slot, &fragment);
+        }
+
+        // After the last fragment, emit the actual instruction that
+        // leaves a boolean on the stack.
+        self.push_op(OpCode::OpHasAttr, node);
+    }
+
+    /// When compiling select or select_or expressions, an optimisation is
+    /// possible of compiling the set emitted a constant attribute set by
+    /// immediately replacing it with the actual value.
+    ///
+    /// We take care not to emit an error here, as that would interfere with
+    /// thunking behaviour (there can be perfectly valid Nix code that accesses
+    /// a statically known attribute set that is lacking a key, because that
+    /// thunk is never evaluated). If anything is missing, just inform the
+    /// caller that the optimisation did not take place and move on. We may want
+    /// to emit warnings here in the future.
+    fn optimise_select(&mut self, path: &ast::Attrpath) -> bool {
+        // If compiling the set emitted a constant attribute set, the
+        // associated constant can immediately be replaced with the
+        // actual value.
+        //
+        // We take care not to emit an error here, as that would
+        // interfere with thunking behaviour (there can be perfectly
+        // valid Nix code that accesses a statically known attribute
+        // set that is lacking a key, because that thunk is never
+        // evaluated). If anything is missing, just move on. We may
+        // want to emit warnings here in the future.
+        if let Some(OpCode::OpConstant(ConstantIdx(idx))) = self.chunk().code.last().cloned() {
+            let constant = &mut self.chunk().constants[idx];
+            if let Value::Attrs(attrs) = constant {
+                let mut path_iter = path.attrs();
+
+                // Only do this optimisation if there is a *single*
+                // element in the attribute path. It is extremely
+                // unlikely that we'd have a static nested set.
+                if let (Some(attr), None) = (path_iter.next(), path_iter.next()) {
+                    // Only do this optimisation for statically known attrs.
+                    if let Some(ident) = expr_static_attr_str(&attr) {
+                        if let Some(selected_value) = attrs.select(ident.as_bytes()) {
+                            *constant = selected_value.clone();
+
+                            // If this worked, we can unthunk the current thunk.
+                            self.unthunk();
+
+                            return true;
+                        }
+                    }
+                }
+            }
+        }
+
+        false
+    }
+
+    fn compile_select(&mut self, slot: LocalIdx, node: &ast::Select) {
+        let set = node.expr().unwrap();
+        let path = node.attrpath().unwrap();
+
+        if node.or_token().is_some() {
+            return self.compile_select_or(slot, set, path, node.default_expr().unwrap());
+        }
+
+        // Push the set onto the stack
+        self.compile(slot, set.clone());
+        if self.optimise_select(&path) {
+            return;
+        }
+
+        // Compile each key fragment and emit access instructions.
+        //
+        // TODO: multi-select instruction to avoid re-pushing attrs on
+        // nested selects.
+        for fragment in path.attrs() {
+            // Force the current set value.
+            self.emit_force(&set);
+
+            self.compile_attr(slot, &fragment);
+            self.push_op(OpCode::OpAttrsSelect, &fragment);
+        }
+    }
+
+    /// Compile an `or` expression into a chunk of conditional jumps.
+    ///
+    /// If at any point during attribute set traversal a key is
+    /// missing, the `OpAttrOrNotFound` instruction will leave a
+    /// special sentinel value on the stack.
+    ///
+    /// After each access, a conditional jump evaluates the top of the
+    /// stack and short-circuits to the default value if it sees the
+    /// sentinel.
+    ///
+    /// Code like `{ a.b = 1; }.a.c or 42` yields this bytecode and
+    /// runtime stack:
+    ///
+    /// ```notrust
+    ///            Bytecode                     Runtime stack
+    ///  โ”Œโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”   โ”Œโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”
+    ///  โ”‚    ...                     โ”‚   โ”‚ ...                     โ”‚
+    ///  โ”‚ 5  OP_ATTRS(1)             โ”‚ โ†’ โ”‚ 5  [ { a.b = 1; }     ] โ”‚
+    ///  โ”‚ 6  OP_CONSTANT("a")        โ”‚ โ†’ โ”‚ 6  [ { a.b = 1; } "a" ] โ”‚
+    ///  โ”‚ 7  OP_ATTR_OR_NOT_FOUND    โ”‚ โ†’ โ”‚ 7  [ { b = 1; }       ] โ”‚
+    ///  โ”‚ 8  JUMP_IF_NOT_FOUND(13)   โ”‚ โ†’ โ”‚ 8  [ { b = 1; }       ] โ”‚
+    ///  โ”‚ 9  OP_CONSTANT("C")        โ”‚ โ†’ โ”‚ 9  [ { b = 1; } "c"   ] โ”‚
+    ///  โ”‚ 10 OP_ATTR_OR_NOT_FOUND    โ”‚ โ†’ โ”‚ 10 [ NOT_FOUND        ] โ”‚
+    ///  โ”‚ 11 JUMP_IF_NOT_FOUND(13)   โ”‚ โ†’ โ”‚ 11 [                  ] โ”‚
+    ///  โ”‚ 12 JUMP(14)                โ”‚   โ”‚ ..     jumped over      โ”‚
+    ///  โ”‚ 13 CONSTANT(42)            โ”‚ โ†’ โ”‚ 12 [ 42 ]               โ”‚
+    ///  โ”‚ 14 ...                     โ”‚   โ”‚ ..   ....               โ”‚
+    ///  โ””โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”˜   โ””โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”˜
+    /// ```
+    fn compile_select_or(
+        &mut self,
+        slot: LocalIdx,
+        set: ast::Expr,
+        path: ast::Attrpath,
+        default: ast::Expr,
+    ) {
+        self.compile(slot, set);
+        if self.optimise_select(&path) {
+            return;
+        }
+
+        let mut jumps = vec![];
+
+        for fragment in path.attrs() {
+            self.emit_force(&fragment);
+            self.compile_attr(slot, &fragment.clone());
+            self.push_op(OpCode::OpAttrsTrySelect, &fragment);
+            jumps.push(self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), &fragment));
+        }
+
+        let final_jump = self.push_op(OpCode::OpJump(JumpOffset(0)), &path);
+
+        for jump in jumps {
+            self.patch_jump(jump);
+        }
+
+        // Compile the default value expression and patch the final
+        // jump to point *beyond* it.
+        self.compile(slot, default);
+        self.patch_jump(final_jump);
+    }
+
+    /// Compile `assert` expressions using jumping instructions in the VM.
+    ///
+    /// ```notrust
+    ///                        โ”Œโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”
+    ///                        โ”‚ 0  [ conditional ]  โ”‚
+    ///                        โ”‚ 1   JUMP_IF_FALSE  โ†’โ”ผโ”€โ”
+    ///                        โ”‚ 2  [  main body  ]  โ”‚ โ”‚ Jump to else body if
+    ///                       โ”Œโ”ผโ”€3โ”€โ†     JUMP        โ”‚ โ”‚ condition is false.
+    ///  Jump over else body  โ”‚โ”‚ 4   OP_ASSERT_FAIL โ†โ”ผโ”€โ”˜
+    ///  if condition is true.โ””โ”ผโ”€5โ”€โ†’     ...         โ”‚
+    ///                        โ””โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”˜
+    /// ```
+    fn compile_assert(&mut self, slot: LocalIdx, node: &ast::Assert) {
+        // Compile the assertion condition to leave its value on the stack.
+        self.compile(slot, node.condition().unwrap());
+        self.emit_force(&node.condition().unwrap());
+        let throw_idx = self.push_op(OpCode::OpJumpIfCatchable(JumpOffset(0)), node);
+        let then_idx = self.push_op(OpCode::OpJumpIfFalse(JumpOffset(0)), node);
+
+        self.push_op(OpCode::OpPop, node);
+        self.compile(slot, node.body().unwrap());
+
+        let else_idx = self.push_op(OpCode::OpJump(JumpOffset(0)), node);
+
+        self.patch_jump(then_idx);
+        self.push_op(OpCode::OpPop, node);
+        self.push_op(OpCode::OpAssertFail, &node.condition().unwrap());
+
+        self.patch_jump(else_idx);
+        self.patch_jump(throw_idx);
+    }
+
+    /// Compile conditional expressions using jumping instructions in the VM.
+    ///
+    /// ```notrust
+    ///                        โ”Œโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”
+    ///                        โ”‚ 0  [ conditional ] โ”‚
+    ///                        โ”‚ 1   JUMP_IF_FALSE โ†’โ”ผโ”€โ”
+    ///                        โ”‚ 2  [  main body  ] โ”‚ โ”‚ Jump to else body if
+    ///                       โ”Œโ”ผโ”€3โ”€โ†     JUMP       โ”‚ โ”‚ condition is false.
+    ///  Jump over else body  โ”‚โ”‚ 4  [  else body  ]โ†โ”ผโ”€โ”˜
+    ///  if condition is true.โ””โ”ผโ”€5โ”€โ†’     ...        โ”‚
+    ///                        โ””โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”˜
+    /// ```
+    fn compile_if_else(&mut self, slot: LocalIdx, node: &ast::IfElse) {
+        self.compile(slot, node.condition().unwrap());
+        self.emit_force(&node.condition().unwrap());
+
+        let throw_idx = self.push_op(
+            OpCode::OpJumpIfCatchable(JumpOffset(0)),
+            &node.condition().unwrap(),
+        );
+        let then_idx = self.push_op(
+            OpCode::OpJumpIfFalse(JumpOffset(0)),
+            &node.condition().unwrap(),
+        );
+
+        self.push_op(OpCode::OpPop, node); // discard condition value
+        self.compile(slot, node.body().unwrap());
+
+        let else_idx = self.push_op(OpCode::OpJump(JumpOffset(0)), node);
+
+        self.patch_jump(then_idx); // patch jump *to* else_body
+        self.push_op(OpCode::OpPop, node); // discard condition value
+        self.compile(slot, node.else_body().unwrap());
+
+        self.patch_jump(else_idx); // patch jump *over* else body
+        self.patch_jump(throw_idx); // patch jump *over* else body
+    }
+
+    /// Compile `with` expressions by emitting instructions that
+    /// pop/remove the indices of attribute sets that are implicitly
+    /// in scope through `with` on the "with-stack".
+    fn compile_with(&mut self, slot: LocalIdx, node: &ast::With) {
+        self.scope_mut().begin_scope();
+        // TODO: Detect if the namespace is just an identifier, and
+        // resolve that directly (thus avoiding duplication on the
+        // stack).
+        self.compile(slot, node.namespace().unwrap());
+
+        let span = self.span_for(&node.namespace().unwrap());
+
+        // The attribute set from which `with` inherits values
+        // occupies a slot on the stack, but this stack slot is not
+        // directly accessible. As it must be accounted for to
+        // calculate correct offsets, what we call a "phantom" local
+        // is declared here.
+        let local_idx = self.scope_mut().declare_phantom(span, true);
+        let with_idx = self.scope().stack_index(local_idx);
+
+        self.scope_mut().push_with();
+
+        self.push_op(OpCode::OpPushWith(with_idx), &node.namespace().unwrap());
+
+        self.compile(slot, node.body().unwrap());
+
+        self.push_op(OpCode::OpPopWith, node);
+        self.scope_mut().pop_with();
+        self.cleanup_scope(node);
+    }
+
+    /// Compiles pattern function arguments, such as `{ a, b }: ...`.
+    ///
+    /// These patterns are treated as a special case of locals binding
+    /// where the attribute set itself is placed on the first stack
+    /// slot of the call frame (either as a phantom, or named in case
+    /// of an `@` binding), and the function call sets up the rest of
+    /// the stack as if the parameters were rewritten into a `let`
+    /// binding.
+    ///
+    /// For example:
+    ///
+    /// ```nix
+    /// ({ a, b ? 2, c ? a * b, ... }@args: <body>)  { a = 10; }
+    /// ```
+    ///
+    /// would be compiled similarly to a binding such as
+    ///
+    /// ```nix
+    /// let args = { a = 10; };
+    /// in let a = args.a;
+    ///        b = args.a or 2;
+    ///        c = args.c or a * b;
+    ///    in <body>
+    /// ```
+    ///
+    /// However, there are two properties of pattern function arguments that can
+    /// not be compiled by desugaring in this way:
+    ///
+    /// 1. Bindings have to fail if too many arguments are provided. This is
+    ///    done by emitting a special instruction that checks the set of keys
+    ///    from a constant containing the expected keys.
+    /// 2. Formal arguments with a default expression are (as an optimization and
+    ///    because it is simpler) not wrapped in another thunk, instead compiled
+    ///    and accessed separately. This means that the default expression may
+    ///    never make it into the local's stack slot if the argument is provided
+    ///    by the caller. We need to take this into account and skip any
+    ///    operations specific to the expression like thunk finalisation in such
+    ///    cases.
+    fn compile_param_pattern(&mut self, pattern: &ast::Pattern) -> (Formals, CodeIdx) {
+        let span = self.span_for(pattern);
+
+        let (set_idx, pat_bind_name) = match pattern.pat_bind() {
+            Some(name) => {
+                let pat_bind_name = name.ident().unwrap().to_string();
+                (
+                    self.declare_local(&name, pat_bind_name.clone()),
+                    Some(pat_bind_name),
+                )
+            }
+            None => (self.scope_mut().declare_phantom(span, true), None),
+        };
+
+        // At call time, the attribute set is already at the top of the stack.
+        self.scope_mut().mark_initialised(set_idx);
+        self.emit_force(pattern);
+        let throw_idx = self.push_op(OpCode::OpJumpIfCatchable(JumpOffset(0)), pattern);
+        // Evaluation fails on a type error, even if the argument(s) are unused.
+        self.push_op(OpCode::OpAssertAttrs, pattern);
+
+        let ellipsis = pattern.ellipsis_token().is_some();
+        if !ellipsis {
+            self.push_op(OpCode::OpValidateClosedFormals, pattern);
+        }
+
+        // Similar to `let ... in ...`, we now do multiple passes over
+        // the bindings to first declare them, then populate them, and
+        // then finalise any necessary recursion into the scope.
+        let mut entries: Vec<TrackedFormal> = vec![];
+        let mut arguments = BTreeMap::default();
+
+        for entry in pattern.pat_entries() {
+            let ident = entry.ident().unwrap();
+            let idx = self.declare_local(&ident, ident.to_string());
+
+            arguments.insert(ident.into(), entry.default().is_some());
+
+            if let Some(default_expr) = entry.default() {
+                entries.push(TrackedFormal::WithDefault {
+                    local_idx: idx,
+                    // This phantom is used to track at runtime (!) whether we need to
+                    // finalise the local's stack slot or not. The relevant instructions are
+                    // emitted in the second pass where the mechanism is explained as well.
+                    finalise_request_idx: {
+                        let span = self.span_for(&default_expr);
+                        self.scope_mut().declare_phantom(span, false)
+                    },
+                    default_expr,
+                    pattern_entry: entry,
+                });
+            } else {
+                entries.push(TrackedFormal::NoDefault {
+                    local_idx: idx,
+                    pattern_entry: entry,
+                });
+            }
+        }
+
+        // For each of the bindings, push the set on the stack and
+        // attempt to select from it.
+        let stack_idx = self.scope().stack_index(set_idx);
+        for tracked_formal in entries.iter() {
+            self.push_op(OpCode::OpGetLocal(stack_idx), pattern);
+            self.emit_literal_ident(&tracked_formal.pattern_entry().ident().unwrap());
+
+            let idx = tracked_formal.local_idx();
+
+            // Use the same mechanism as `compile_select_or` if a
+            // default value was provided, or simply select otherwise.
+            match tracked_formal {
+                TrackedFormal::WithDefault {
+                    default_expr,
+                    pattern_entry,
+                    ..
+                } => {
+                    // The tricky bit about compiling a formal argument with a default value
+                    // is that the default may be a thunk that may depend on the value of
+                    // other formal arguments, i.e. may need to be finalised. This
+                    // finalisation can only happen if we are actually using the default
+                    // valueโ€”otherwise OpFinalise will crash on an already finalised (or
+                    // non-thunk) value.
+                    //
+                    // Thus we use an additional local to track whether we wound up
+                    // defaulting or not. `FinaliseRequest(false)` indicates that we should
+                    // not finalise, as we did not default.
+                    //
+                    // We are being wasteful with VM stack space in case of default
+                    // expressions that don't end up needing to be finalised. Unfortunately
+                    // we only know better after compiling the default expression, so
+                    // avoiding unnecessary locals would mean we'd need to modify the chunk
+                    // after the fact.
+                    self.push_op(OpCode::OpAttrsTrySelect, &pattern_entry.ident().unwrap());
+                    let jump_to_default =
+                        self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), default_expr);
+
+                    self.emit_constant(Value::FinaliseRequest(false), default_expr);
+
+                    let jump_over_default =
+                        self.push_op(OpCode::OpJump(JumpOffset(0)), default_expr);
+
+                    self.patch_jump(jump_to_default);
+
+                    // Does not need to thunked since compile() already does so when necessary
+                    self.compile(idx, default_expr.clone());
+
+                    self.emit_constant(Value::FinaliseRequest(true), default_expr);
+
+                    self.patch_jump(jump_over_default);
+                }
+                TrackedFormal::NoDefault { pattern_entry, .. } => {
+                    self.push_op(OpCode::OpAttrsSelect, &pattern_entry.ident().unwrap());
+                }
+            }
+
+            self.scope_mut().mark_initialised(idx);
+            if let TrackedFormal::WithDefault {
+                finalise_request_idx,
+                ..
+            } = tracked_formal
+            {
+                self.scope_mut().mark_initialised(*finalise_request_idx);
+            }
+        }
+
+        for tracked_formal in entries.iter() {
+            if self.scope()[tracked_formal.local_idx()].needs_finaliser {
+                let stack_idx = self.scope().stack_index(tracked_formal.local_idx());
+                match tracked_formal {
+                    TrackedFormal::NoDefault { .. } =>
+                        panic!("Tvix bug: local for pattern formal needs finaliser, but has no default expr"),
+                    TrackedFormal::WithDefault { finalise_request_idx, .. } => {
+                        let finalise_request_stack_idx = self.scope().stack_index(*finalise_request_idx);
+
+                        // TODO(sterni): better spans
+                        self.push_op(
+                            OpCode::OpGetLocal(finalise_request_stack_idx),
+                            pattern
+                        );
+                        let jump_over_finalise =
+                            self.push_op(
+                                OpCode::OpJumpIfNoFinaliseRequest(
+                                    JumpOffset(0)),
+                                pattern
+                            );
+                        self.push_op(
+                            OpCode::OpFinalise(stack_idx),
+                            pattern,
+                        );
+                        self.patch_jump(jump_over_finalise);
+                        // Get rid of finaliser request value on the stack
+                        self.push_op(OpCode::OpPop, pattern);
+                    }
+                }
+            }
+        }
+
+        (
+            (Formals {
+                arguments,
+                ellipsis,
+                span,
+                name: pat_bind_name,
+            }),
+            throw_idx,
+        )
+    }
+
+    fn compile_lambda(&mut self, slot: LocalIdx, node: &ast::Lambda) -> Option<CodeIdx> {
+        // Compile the function itself, recording its formal arguments (if any)
+        // for later use
+        let formals = match node.param().unwrap() {
+            ast::Param::Pattern(pat) => Some(self.compile_param_pattern(&pat)),
+
+            ast::Param::IdentParam(param) => {
+                let name = param
+                    .ident()
+                    .unwrap()
+                    .ident_token()
+                    .unwrap()
+                    .text()
+                    .to_string();
+
+                let idx = self.declare_local(&param, &name);
+                self.scope_mut().mark_initialised(idx);
+                None
+            }
+        };
+
+        self.compile(slot, node.body().unwrap());
+        if let Some((formals, throw_idx)) = formals {
+            self.context_mut().lambda.formals = Some(formals);
+            Some(throw_idx)
+        } else {
+            self.context_mut().lambda.formals = None;
+            None
+        }
+    }
+
+    fn thunk<N, F>(&mut self, outer_slot: LocalIdx, node: &N, content: F)
+    where
+        N: ToSpan,
+        F: FnOnce(&mut Compiler, LocalIdx),
+    {
+        self.compile_lambda_or_thunk(true, outer_slot, node, |comp, idx| {
+            content(comp, idx);
+            None
+        })
+    }
+
+    /// Mark the current thunk as redundant, i.e. possible to merge directly
+    /// into its parent lambda context without affecting runtime behaviour.
+    fn unthunk(&mut self) {
+        self.context_mut().unthunk = true;
+    }
+
+    /// Compile an expression into a runtime closure or thunk
+    fn compile_lambda_or_thunk<N, F>(
+        &mut self,
+        is_suspended_thunk: bool,
+        outer_slot: LocalIdx,
+        node: &N,
+        content: F,
+    ) where
+        N: ToSpan,
+        F: FnOnce(&mut Compiler, LocalIdx) -> Option<CodeIdx>,
+    {
+        let name = self.scope()[outer_slot].name();
+        self.new_context();
+
+        // Set the (optional) name of the current slot on the lambda that is
+        // being compiled.
+        self.context_mut().lambda.name = name;
+
+        let span = self.span_for(node);
+        let slot = self.scope_mut().declare_phantom(span, false);
+        self.scope_mut().begin_scope();
+
+        let throw_idx = content(self, slot);
+        self.cleanup_scope(node);
+        if let Some(throw_idx) = throw_idx {
+            self.patch_jump(throw_idx);
+        }
+
+        // TODO: determine and insert enclosing name, if available.
+
+        // Pop the lambda context back off, and emit the finished
+        // lambda as a constant.
+        let mut compiled = self.contexts.pop().unwrap();
+
+        // The compiler might have decided to unthunk, i.e. raise the compiled
+        // code to the parent context. In that case we do so and return right
+        // away.
+        if compiled.unthunk && is_suspended_thunk {
+            self.chunk().extend(compiled.lambda.chunk);
+            return;
+        }
+
+        // Emit an instruction to inform the VM that the chunk has ended.
+        compiled
+            .lambda
+            .chunk
+            .push_op(OpCode::OpReturn, self.span_for(node));
+
+        // Capturing the with stack counts as an upvalue, as it is
+        // emitted as an upvalue data instruction.
+        if compiled.captures_with_stack {
+            compiled.lambda.upvalue_count += 1;
+        }
+
+        let lambda = Rc::new(compiled.lambda);
+        if is_suspended_thunk {
+            self.observer.observe_compiled_thunk(&lambda);
+        } else {
+            self.observer.observe_compiled_lambda(&lambda);
+        }
+
+        // If no upvalues are captured, emit directly and move on.
+        if lambda.upvalue_count == 0 {
+            self.emit_constant(
+                if is_suspended_thunk {
+                    Value::Thunk(Thunk::new_suspended(lambda, LightSpan::new_actual(span)))
+                } else {
+                    Value::Closure(Rc::new(Closure::new(lambda)))
+                },
+                node,
+            );
+            return;
+        }
+
+        // Otherwise, we need to emit the variable number of
+        // operands that allow the runtime to close over the
+        // upvalues and leave a blueprint in the constant index from
+        // which the result can be constructed.
+        let blueprint_idx = self.chunk().push_constant(Value::Blueprint(lambda));
+
+        let code_idx = self.push_op(
+            if is_suspended_thunk {
+                OpCode::OpThunkSuspended(blueprint_idx)
+            } else {
+                OpCode::OpThunkClosure(blueprint_idx)
+            },
+            node,
+        );
+
+        self.emit_upvalue_data(
+            outer_slot,
+            node,
+            compiled.scope.upvalues,
+            compiled.captures_with_stack,
+        );
+
+        if !is_suspended_thunk && !self.scope()[outer_slot].needs_finaliser {
+            if !self.scope()[outer_slot].must_thunk {
+                // The closure has upvalues, but is not recursive.  Therefore no thunk is required,
+                // which saves us the overhead of Rc<RefCell<>>
+                self.chunk()[code_idx] = OpCode::OpClosure(blueprint_idx);
+            } else {
+                // This case occurs when a closure has upvalue-references to itself but does not need a
+                // finaliser.  Since no OpFinalise will be emitted later on we synthesize one here.
+                // It is needed here only to set [`Closure::is_finalised`] which is used for sanity checks.
+                #[cfg(debug_assertions)]
+                self.push_op(
+                    OpCode::OpFinalise(self.scope().stack_index(outer_slot)),
+                    &self.span_for(node),
+                );
+            }
+        }
+    }
+
+    fn compile_apply(&mut self, slot: LocalIdx, node: &ast::Apply) {
+        // To call a function, we leave its arguments on the stack,
+        // followed by the function expression itself, and then emit a
+        // call instruction. This way, the stack is perfectly laid out
+        // to enter the function call straight away.
+        self.compile(slot, node.argument().unwrap());
+        self.compile(slot, node.lambda().unwrap());
+        self.emit_force(&node.lambda().unwrap());
+        self.push_op(OpCode::OpCall, node);
+    }
+
+    /// Emit the data instructions that the runtime needs to correctly
+    /// assemble the upvalues struct.
+    fn emit_upvalue_data<T: ToSpan>(
+        &mut self,
+        slot: LocalIdx,
+        node: &T,
+        upvalues: Vec<Upvalue>,
+        capture_with: bool,
+    ) {
+        for upvalue in upvalues {
+            match upvalue.kind {
+                UpvalueKind::Local(idx) => {
+                    let target = &self.scope()[idx];
+                    let stack_idx = self.scope().stack_index(idx);
+
+                    // If the target is not yet initialised, we need to defer
+                    // the local access
+                    if !target.initialised {
+                        self.push_op(OpCode::DataDeferredLocal(stack_idx), &upvalue.span);
+                        self.scope_mut().mark_needs_finaliser(slot);
+                    } else {
+                        // a self-reference
+                        if slot == idx {
+                            self.scope_mut().mark_must_thunk(slot);
+                        }
+                        self.push_op(OpCode::DataStackIdx(stack_idx), &upvalue.span);
+                    }
+                }
+
+                UpvalueKind::Upvalue(idx) => {
+                    self.push_op(OpCode::DataUpvalueIdx(idx), &upvalue.span);
+                }
+            };
+        }
+
+        if capture_with {
+            // TODO(tazjin): probably better to emit span for the ident that caused this
+            self.push_op(OpCode::DataCaptureWith, node);
+        }
+    }
+
+    /// Emit the literal string value of an identifier. Required for
+    /// several operations related to attribute sets, where
+    /// identifiers are used as string keys.
+    fn emit_literal_ident(&mut self, ident: &ast::Ident) {
+        self.emit_constant(Value::String(ident.clone().into()), ident);
+    }
+
+    /// Patch the jump instruction at the given index, setting its
+    /// jump offset from the placeholder to the current code position.
+    ///
+    /// This is required because the actual target offset of jumps is
+    /// not known at the time when the jump operation itself is
+    /// emitted.
+    fn patch_jump(&mut self, idx: CodeIdx) {
+        let offset = JumpOffset(self.chunk().code.len() - 1 - idx.0);
+
+        match &mut self.chunk().code[idx.0] {
+            OpCode::OpJump(n)
+            | OpCode::OpJumpIfFalse(n)
+            | OpCode::OpJumpIfTrue(n)
+            | OpCode::OpJumpIfCatchable(n)
+            | OpCode::OpJumpIfNotFound(n)
+            | OpCode::OpJumpIfNoFinaliseRequest(n) => {
+                *n = offset;
+            }
+
+            op => panic!("attempted to patch unsupported op: {:?}", op),
+        }
+    }
+
+    /// Decrease scope depth of the current function and emit
+    /// instructions to clean up the stack at runtime.
+    fn cleanup_scope<N: ToSpan>(&mut self, node: &N) {
+        // When ending a scope, all corresponding locals need to be
+        // removed, but the value of the body needs to remain on the
+        // stack. This is implemented by a separate instruction.
+        let (popcount, unused_spans) = self.scope_mut().end_scope();
+
+        for span in &unused_spans {
+            self.emit_warning(span, WarningKind::UnusedBinding);
+        }
+
+        if popcount > 0 {
+            self.push_op(OpCode::OpCloseScope(Count(popcount)), node);
+        }
+    }
+
+    /// Open a new lambda context within which to compile a function,
+    /// closure or thunk.
+    fn new_context(&mut self) {
+        self.contexts.push(self.context().inherit());
+    }
+
+    /// Declare a local variable known in the scope that is being
+    /// compiled by pushing it to the locals. This is used to
+    /// determine the stack offset of variables.
+    fn declare_local<S: Into<String>, N: ToSpan>(&mut self, node: &N, name: S) -> LocalIdx {
+        let name = name.into();
+        let depth = self.scope().scope_depth();
+
+        // Do this little dance to turn name:&'a str into the same
+        // string with &'static lifetime, as required by WarningKind
+        if let Some((global_ident, _)) = self.globals.get_key_value(name.as_str()) {
+            self.emit_warning(node, WarningKind::ShadowedGlobal(global_ident));
+        }
+
+        let span = self.span_for(node);
+        let (idx, shadowed) = self.scope_mut().declare_local(name, span);
+
+        if let Some(shadow_idx) = shadowed {
+            let other = &self.scope()[shadow_idx];
+            if other.depth == depth {
+                self.emit_error(node, ErrorKind::VariableAlreadyDefined(other.span));
+            }
+        }
+
+        idx
+    }
+
+    /// Determine whether the current lambda context has any ancestors
+    /// that use dynamic scope resolution, and mark contexts as
+    /// needing to capture their enclosing `with`-stack in their
+    /// upvalues.
+    fn has_dynamic_ancestor(&mut self) -> bool {
+        let mut ancestor_has_with = false;
+
+        for ctx in self.contexts.iter_mut() {
+            if ancestor_has_with {
+                // If the ancestor has an active with stack, mark this
+                // lambda context as needing to capture it.
+                ctx.captures_with_stack = true;
+            } else {
+                // otherwise, check this context and move on
+                ancestor_has_with = ctx.scope.has_with();
+            }
+        }
+
+        ancestor_has_with
+    }
+
+    fn emit_force<N: ToSpan>(&mut self, node: &N) {
+        if let Some(&OpCode::OpConstant(c)) = self.chunk().last_op() {
+            if !self.chunk().get_constant(c).unwrap().is_thunk() {
+                // Optimization: Don't emit a force op for non-thunk constants, since they don't
+                // need one!
+                // TODO: this is probably doable for more ops (?)
+                return;
+            }
+        }
+
+        self.push_op(OpCode::OpForce, node);
+    }
+
+    fn emit_warning<N: ToSpan>(&mut self, node: &N, kind: WarningKind) {
+        let span = self.span_for(node);
+        self.warnings.push(EvalWarning { kind, span })
+    }
+
+    fn emit_error<N: ToSpan>(&mut self, node: &N, kind: ErrorKind) {
+        let span = self.span_for(node);
+        self.errors
+            .push(Error::new(kind, span, self.source.clone()))
+    }
+}
+
+/// Convert a non-dynamic string expression to a string if possible.
+fn expr_static_str(node: &ast::Str) -> Option<SmolStr> {
+    let mut parts = node.normalized_parts();
+
+    if parts.len() != 1 {
+        return None;
+    }
+
+    if let Some(ast::InterpolPart::Literal(lit)) = parts.pop() {
+        return Some(SmolStr::new(lit));
+    }
+
+    None
+}
+
+/// Convert the provided `ast::Attr` into a statically known string if
+/// possible.
+fn expr_static_attr_str(node: &ast::Attr) -> Option<SmolStr> {
+    match node {
+        ast::Attr::Ident(ident) => Some(ident.ident_token().unwrap().text().into()),
+        ast::Attr::Str(s) => expr_static_str(s),
+
+        // The dynamic node type is just a wrapper. C++ Nix does not care
+        // about the dynamic wrapper when determining whether the node
+        // itself is dynamic, it depends solely on the expression inside
+        // (i.e. `let ${"a"} = 1; in a` is valid).
+        ast::Attr::Dynamic(ref dynamic) => match dynamic.expr().unwrap() {
+            ast::Expr::Str(s) => expr_static_str(&s),
+            _ => None,
+        },
+    }
+}
+
+/// Create a delayed source-only builtin compilation, for a builtin
+/// which is written in Nix code.
+///
+/// **Important:** tvix *panics* if a builtin with invalid source code
+/// is supplied. This is because there is no user-friendly way to
+/// thread the errors out of this function right now.
+fn compile_src_builtin(
+    name: &'static str,
+    code: &str,
+    source: SourceCode,
+    weak: &Weak<GlobalsMap>,
+) -> Value {
+    use std::fmt::Write;
+
+    let parsed = rnix::ast::Root::parse(code);
+
+    if !parsed.errors().is_empty() {
+        let mut out = format!("BUG: code for source-builtin '{}' had parser errors", name);
+        for error in parsed.errors() {
+            writeln!(out, "{}", error).unwrap();
+        }
+
+        panic!("{}", out);
+    }
+
+    let file = source.add_file(format!("<src-builtins/{}.nix>", name), code.to_string());
+    let weak = weak.clone();
+
+    Value::Thunk(Thunk::new_suspended_native(Box::new(move || {
+        let result = compile(
+            &parsed.tree().expr().unwrap(),
+            None,
+            weak.upgrade().unwrap(),
+            &source,
+            &file,
+            &mut crate::observer::NoOpObserver {},
+        )
+        .map_err(|e| ErrorKind::NativeError {
+            gen_type: "derivation",
+            err: Box::new(e),
+        })?;
+
+        if !result.errors.is_empty() {
+            return Err(ErrorKind::ImportCompilerError {
+                path: format!("src-builtins/{}.nix", name).into(),
+                errors: result.errors,
+            });
+        }
+
+        Ok(Value::Thunk(Thunk::new_suspended(
+            result.lambda,
+            LightSpan::Actual { span: file.span },
+        )))
+    })))
+}
+
+/// Prepare the full set of globals available in evaluated code. These
+/// are constructed from the set of builtins supplied by the caller,
+/// which are made available globally under the `builtins` identifier.
+///
+/// A subset of builtins (specified by [`GLOBAL_BUILTINS`]) is
+/// available globally *iff* they are set.
+///
+/// Optionally adds the `import` feature if desired by the caller.
+pub fn prepare_globals(
+    builtins: Vec<(&'static str, Value)>,
+    src_builtins: Vec<(&'static str, &'static str)>,
+    source: SourceCode,
+    enable_import: bool,
+) -> Rc<GlobalsMap> {
+    Rc::new_cyclic(Box::new(move |weak: &Weak<GlobalsMap>| {
+        // First step is to construct the builtins themselves as
+        // `NixAttrs`.
+        let mut builtins: GlobalsMap = HashMap::from_iter(builtins);
+
+        // At this point, optionally insert `import` if enabled. To
+        // "tie the knot" of `import` needing the full set of globals
+        // to instantiate its compiler, the `Weak` reference is passed
+        // here.
+        if enable_import {
+            let import = Value::Builtin(import::builtins_import(weak, source.clone()));
+            builtins.insert("import", import);
+        }
+
+        // Next, the actual map of globals which the compiler will use
+        // to resolve identifiers is constructed.
+        let mut globals: GlobalsMap = HashMap::new();
+
+        // builtins contain themselves (`builtins.builtins`), which we
+        // can resolve by manually constructing a suspended thunk that
+        // dereferences the same weak pointer as above.
+        let weak_globals = weak.clone();
+        builtins.insert(
+            "builtins",
+            Value::Thunk(Thunk::new_suspended_native(Box::new(move || {
+                Ok(weak_globals
+                    .upgrade()
+                    .unwrap()
+                    .get("builtins")
+                    .cloned()
+                    .unwrap())
+            }))),
+        );
+
+        // Insert top-level static value builtins.
+        globals.insert("true", Value::Bool(true));
+        globals.insert("false", Value::Bool(false));
+        globals.insert("null", Value::Null);
+
+        // If "source builtins" were supplied, compile them and insert
+        // them.
+        builtins.extend(src_builtins.into_iter().map(move |(name, code)| {
+            let compiled = compile_src_builtin(name, code, source.clone(), weak);
+            (name, compiled)
+        }));
+
+        // Construct the actual `builtins` attribute set and insert it
+        // in the global scope.
+        globals.insert(
+            "builtins",
+            Value::attrs(NixAttrs::from_iter(builtins.clone())),
+        );
+
+        // Finally, the builtins that should be globally available are
+        // "elevated" to the outer scope.
+        for global in GLOBAL_BUILTINS {
+            if let Some(builtin) = builtins.get(global).cloned() {
+                globals.insert(global, builtin);
+            }
+        }
+
+        globals
+    }))
+}
+
+pub fn compile(
+    expr: &ast::Expr,
+    location: Option<PathBuf>,
+    globals: Rc<GlobalsMap>,
+    source: &SourceCode,
+    file: &codemap::File,
+    observer: &mut dyn CompilerObserver,
+) -> EvalResult<CompilationOutput> {
+    let mut c = Compiler::new(location, globals.clone(), source, file, observer)?;
+
+    let root_span = c.span_for(expr);
+    let root_slot = c.scope_mut().declare_phantom(root_span, false);
+    c.compile(root_slot, expr.clone());
+
+    // The final operation of any top-level Nix program must always be
+    // `OpForce`. A thunk should not be returned to the user in an
+    // unevaluated state (though in practice, a value *containing* a
+    // thunk might be returned).
+    c.emit_force(expr);
+    c.push_op(OpCode::OpReturn, &root_span);
+
+    let lambda = Rc::new(c.contexts.pop().unwrap().lambda);
+    c.observer.observe_compiled_toplevel(&lambda);
+
+    Ok(CompilationOutput {
+        lambda,
+        warnings: c.warnings,
+        errors: c.errors,
+        globals,
+    })
+}
diff --git a/tvix/eval/src/compiler/optimiser.rs b/tvix/eval/src/compiler/optimiser.rs
new file mode 100644
index 0000000000..48960d355c
--- /dev/null
+++ b/tvix/eval/src/compiler/optimiser.rs
@@ -0,0 +1,125 @@
+//! Helper functions for extending the compiler with more linter-like
+//! functionality while compiling (i.e. smarter warnings).
+
+use super::*;
+
+use ast::Expr;
+
+/// Optimise the given expression where possible.
+pub(super) fn optimise_expr(c: &mut Compiler, slot: LocalIdx, expr: ast::Expr) -> ast::Expr {
+    match expr {
+        Expr::BinOp(_) => optimise_bin_op(c, slot, expr),
+        _ => expr,
+    }
+}
+
+enum LitBool {
+    Expr(Expr),
+    True(Expr),
+    False(Expr),
+}
+
+/// Is this a literal boolean, or something else?
+fn is_lit_bool(expr: ast::Expr) -> LitBool {
+    if let ast::Expr::Ident(ident) = &expr {
+        match ident.ident_token().unwrap().text() {
+            "true" => LitBool::True(expr),
+            "false" => LitBool::False(expr),
+            _ => LitBool::Expr(expr),
+        }
+    } else {
+        LitBool::Expr(expr)
+    }
+}
+
+/// Detect useless binary operations (i.e. useless bool comparisons).
+fn optimise_bin_op(c: &mut Compiler, slot: LocalIdx, expr: ast::Expr) -> ast::Expr {
+    use ast::BinOpKind;
+
+    // bail out of this check if the user has overridden either `true`
+    // or `false` identifiers. Note that they will have received a
+    // separate warning about this for shadowing the global(s).
+    if c.is_user_defined("true") || c.is_user_defined("false") {
+        return expr;
+    }
+
+    if let Expr::BinOp(op) = &expr {
+        let lhs = is_lit_bool(op.lhs().unwrap());
+        let rhs = is_lit_bool(op.rhs().unwrap());
+
+        match (op.operator().unwrap(), lhs, rhs) {
+            // useless `false` arm in `||` expression
+            (BinOpKind::Or, LitBool::False(f), LitBool::Expr(other))
+            | (BinOpKind::Or, LitBool::Expr(other), LitBool::False(f)) => {
+                c.emit_warning(
+                    &f,
+                    WarningKind::UselessBoolOperation(
+                        "this `false` has no effect on the result of the comparison",
+                    ),
+                );
+
+                return other;
+            }
+
+            // useless `true` arm in `&&` expression
+            (BinOpKind::And, LitBool::True(t), LitBool::Expr(other))
+            | (BinOpKind::And, LitBool::Expr(other), LitBool::True(t)) => {
+                c.emit_warning(
+                    &t,
+                    WarningKind::UselessBoolOperation(
+                        "this `true` has no effect on the result of the comparison",
+                    ),
+                );
+
+                return other;
+            }
+
+            // useless `||` expression (one arm is `true`), return
+            // `true` directly (and warn about dead code on the right)
+            (BinOpKind::Or, LitBool::True(t), LitBool::Expr(other)) => {
+                c.emit_warning(
+                    op,
+                    WarningKind::UselessBoolOperation("this expression is always true"),
+                );
+
+                c.compile_dead_code(slot, other);
+
+                return t;
+            }
+
+            (BinOpKind::Or, _, LitBool::True(t)) | (BinOpKind::Or, LitBool::True(t), _) => {
+                c.emit_warning(
+                    op,
+                    WarningKind::UselessBoolOperation("this expression is always true"),
+                );
+
+                return t;
+            }
+
+            // useless `&&` expression (one arm is `false), same as above
+            (BinOpKind::And, LitBool::False(f), LitBool::Expr(other)) => {
+                c.emit_warning(
+                    op,
+                    WarningKind::UselessBoolOperation("this expression is always false"),
+                );
+
+                c.compile_dead_code(slot, other);
+
+                return f;
+            }
+
+            (BinOpKind::And, _, LitBool::False(f)) | (BinOpKind::Or, LitBool::False(f), _) => {
+                c.emit_warning(
+                    op,
+                    WarningKind::UselessBoolOperation("this expression is always false"),
+                );
+
+                return f;
+            }
+
+            _ => { /* nothing to optimise */ }
+        }
+    }
+
+    expr
+}
diff --git a/tvix/eval/src/compiler/scope.rs b/tvix/eval/src/compiler/scope.rs
new file mode 100644
index 0000000000..892727c107
--- /dev/null
+++ b/tvix/eval/src/compiler/scope.rs
@@ -0,0 +1,378 @@
+//! This module implements the scope-tracking logic of the Tvix
+//! compiler.
+//!
+//! Scoping in Nix is fairly complicated, there are features like
+//! mutually recursive bindings, `with`, upvalue capturing, and so
+//! on that introduce a fair bit of complexity.
+//!
+//! Tvix attempts to do as much of the heavy lifting of this at
+//! compile time, and leave the runtime to mostly deal with known
+//! stack indices. To do this, the compiler simulates where locals
+//! will be at runtime using the data structures implemented here.
+
+use std::{
+    collections::{hash_map, HashMap},
+    ops::Index,
+};
+
+use smol_str::SmolStr;
+
+use crate::opcode::{StackIdx, UpvalueIdx};
+
+#[derive(Debug)]
+enum LocalName {
+    /// Normally declared local with a statically known name.
+    Ident(String),
+
+    /// Phantom stack value (e.g. attribute set used for `with`) that
+    /// must be accounted for to calculate correct stack offsets.
+    Phantom,
+}
+
+/// Represents a single local already known to the compiler.
+#[derive(Debug)]
+pub struct Local {
+    /// Identifier of this local. This is always a statically known
+    /// value (Nix does not allow dynamic identifier names in locals),
+    /// or a "phantom" value not accessible by users.
+    name: LocalName,
+
+    /// Source span at which this local was declared.
+    pub span: codemap::Span,
+
+    /// Scope depth of this local.
+    pub depth: usize,
+
+    /// Is this local initialised?
+    pub initialised: bool,
+
+    /// Is this local known to have been used at all?
+    pub used: bool,
+
+    /// Does this local need to be finalised after the enclosing scope
+    /// is completely constructed?
+    pub needs_finaliser: bool,
+
+    /// Does this local's upvalues contain a reference to itself?
+    pub must_thunk: bool,
+}
+
+impl Local {
+    /// Retrieve the name of the given local (if available).
+    pub fn name(&self) -> Option<SmolStr> {
+        match &self.name {
+            LocalName::Phantom => None,
+            LocalName::Ident(name) => Some(SmolStr::new(name)),
+        }
+    }
+
+    /// Is this local intentionally ignored? (i.e. name starts with `_`)
+    pub fn is_ignored(&self) -> bool {
+        match &self.name {
+            LocalName::Ident(name) => name.starts_with('_'),
+            LocalName::Phantom => false,
+        }
+    }
+}
+
+/// Represents the current position of an identifier as resolved in a scope.
+pub enum LocalPosition {
+    /// Local is not known in this scope.
+    Unknown,
+
+    /// Local is known at the given local index.
+    Known(LocalIdx),
+
+    /// Local is known, but is being accessed recursively within its
+    /// own initialisation. Depending on context, this is either an
+    /// error or forcing a closure/thunk.
+    Recursive(LocalIdx),
+}
+
+/// Represents the different ways in which upvalues can be captured in
+/// closures or thunks.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum UpvalueKind {
+    /// This upvalue captures a local from the stack.
+    Local(LocalIdx),
+
+    /// This upvalue captures an enclosing upvalue.
+    Upvalue(UpvalueIdx),
+}
+
+#[derive(Clone, Debug)]
+pub struct Upvalue {
+    pub kind: UpvalueKind,
+    pub span: codemap::Span,
+}
+
+/// The index of a local in the scope's local array at compile time.
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd)]
+pub struct LocalIdx(usize);
+
+/// Helper struct for indexing over `Scope::locals` by name.
+#[derive(Debug)]
+enum ByName {
+    Single(LocalIdx),
+    Shadowed(Vec<LocalIdx>),
+}
+
+impl ByName {
+    /// Add an additional index for this name.
+    fn add_idx(&mut self, new: LocalIdx) {
+        match self {
+            ByName::Shadowed(indices) => indices.push(new),
+            ByName::Single(idx) => {
+                *self = ByName::Shadowed(vec![*idx, new]);
+            }
+        }
+    }
+
+    /// Remove the most recent index for this name, unless it is a
+    /// single. Returns `true` if an entry was removed.
+    fn remove_idx(&mut self) -> bool {
+        match self {
+            ByName::Single(_) => false,
+            ByName::Shadowed(indices) => match indices[..] {
+                [fst, _snd] => {
+                    *self = ByName::Single(fst);
+                    true
+                }
+                _ => {
+                    indices.pop();
+                    true
+                }
+            },
+        }
+    }
+
+    /// Return the most recent index.
+    pub fn index(&self) -> LocalIdx {
+        match self {
+            ByName::Single(idx) => *idx,
+            ByName::Shadowed(vec) => *vec.last().unwrap(),
+        }
+    }
+}
+
+/// Represents a scope known during compilation, which can be resolved
+/// directly to stack indices.
+#[derive(Debug, Default)]
+pub struct Scope {
+    locals: Vec<Local>,
+    pub upvalues: Vec<Upvalue>,
+
+    /// Secondary by-name index over locals.
+    by_name: HashMap<String, ByName>,
+
+    /// How many scopes "deep" are these locals?
+    scope_depth: usize,
+
+    /// Current size of the `with`-stack at runtime.
+    with_stack_size: usize,
+}
+
+impl Index<LocalIdx> for Scope {
+    type Output = Local;
+
+    fn index(&self, index: LocalIdx) -> &Self::Output {
+        &self.locals[index.0]
+    }
+}
+
+impl Scope {
+    /// Inherit scope details from a parent scope (required for
+    /// correctly nesting scopes in lambdas and thunks when special
+    /// scope features like dynamic resolution are present).
+    pub fn inherit(&self) -> Self {
+        Self {
+            scope_depth: self.scope_depth + 1,
+            with_stack_size: self.with_stack_size,
+            ..Default::default()
+        }
+    }
+
+    /// Increase the `with`-stack size of this scope.
+    pub fn push_with(&mut self) {
+        self.with_stack_size += 1;
+    }
+
+    /// Decrease the `with`-stack size of this scope.
+    pub fn pop_with(&mut self) {
+        self.with_stack_size -= 1;
+    }
+
+    /// Does this scope currently require dynamic runtime resolution
+    /// of identifiers that could not be found?
+    pub fn has_with(&self) -> bool {
+        self.with_stack_size > 0
+    }
+
+    /// Resolve the stack index of a statically known local.
+    pub fn resolve_local(&mut self, name: &str) -> LocalPosition {
+        if let Some(by_name) = self.by_name.get(name) {
+            let idx = by_name.index();
+            let local = self
+                .locals
+                .get_mut(idx.0)
+                .expect("invalid compiler state: indexed local missing");
+
+            local.used = true;
+
+            // This local is still being initialised, meaning that
+            // we know its final runtime stack position, but it is
+            // not yet on the stack.
+            if !local.initialised {
+                return LocalPosition::Recursive(idx);
+            }
+
+            return LocalPosition::Known(idx);
+        }
+
+        LocalPosition::Unknown
+    }
+
+    /// Declare a local variable that occupies a stack slot and should
+    /// be accounted for, but is not directly accessible by users
+    /// (e.g. attribute sets used for `with`).
+    pub fn declare_phantom(&mut self, span: codemap::Span, initialised: bool) -> LocalIdx {
+        let idx = self.locals.len();
+        self.locals.push(Local {
+            initialised,
+            span,
+            name: LocalName::Phantom,
+            depth: self.scope_depth,
+            needs_finaliser: false,
+            must_thunk: false,
+            used: true,
+        });
+
+        LocalIdx(idx)
+    }
+
+    /// Declare an uninitialised, named local variable.
+    ///
+    /// Returns the `LocalIdx` of the new local, and optionally the
+    /// index of a previous local shadowed by this one.
+    pub fn declare_local(
+        &mut self,
+        name: String,
+        span: codemap::Span,
+    ) -> (LocalIdx, Option<LocalIdx>) {
+        let idx = LocalIdx(self.locals.len());
+        self.locals.push(Local {
+            name: LocalName::Ident(name.clone()),
+            span,
+            depth: self.scope_depth,
+            initialised: false,
+            needs_finaliser: false,
+            must_thunk: false,
+            used: false,
+        });
+
+        let mut shadowed = None;
+        match self.by_name.entry(name) {
+            hash_map::Entry::Occupied(mut entry) => {
+                let existing = entry.get_mut();
+                shadowed = Some(existing.index());
+                existing.add_idx(idx);
+            }
+            hash_map::Entry::Vacant(entry) => {
+                entry.insert(ByName::Single(idx));
+            }
+        }
+
+        (idx, shadowed)
+    }
+
+    /// Mark local as initialised after compiling its expression.
+    pub fn mark_initialised(&mut self, idx: LocalIdx) {
+        self.locals[idx.0].initialised = true;
+    }
+
+    /// Mark local as needing a finaliser.
+    pub fn mark_needs_finaliser(&mut self, idx: LocalIdx) {
+        self.locals[idx.0].needs_finaliser = true;
+    }
+
+    /// Mark local as must be wrapped in a thunk.  This happens if
+    /// the local has a reference to itself in its upvalues.
+    pub fn mark_must_thunk(&mut self, idx: LocalIdx) {
+        self.locals[idx.0].must_thunk = true;
+    }
+
+    /// Compute the runtime stack index for a given local by
+    /// accounting for uninitialised variables at scopes below this
+    /// one.
+    pub fn stack_index(&self, idx: LocalIdx) -> StackIdx {
+        let uninitialised_count = self.locals[..(idx.0)]
+            .iter()
+            .filter(|l| !l.initialised && self[idx].depth > l.depth)
+            .count();
+
+        StackIdx(idx.0 - uninitialised_count)
+    }
+
+    /// Increase the current scope depth (e.g. within a new bindings
+    /// block, or `with`-scope).
+    pub fn begin_scope(&mut self) {
+        self.scope_depth += 1;
+    }
+
+    /// Decrease the scope depth and remove all locals still tracked
+    /// for the current scope.
+    ///
+    /// Returns the count of locals that were dropped while marked as
+    /// initialised (used by the compiler to determine whether to emit
+    /// scope cleanup operations), as well as the spans of the
+    /// definitions of unused locals (used by the compiler to emit
+    /// unused binding warnings).
+    pub fn end_scope(&mut self) -> (usize, Vec<codemap::Span>) {
+        debug_assert!(self.scope_depth != 0, "can not end top scope");
+
+        let mut pops = 0;
+        let mut unused_spans = vec![];
+
+        // TL;DR - iterate from the back while things belonging to the
+        // ended scope still exist.
+        while self.locals.last().unwrap().depth == self.scope_depth {
+            if let Some(local) = self.locals.pop() {
+                // pop the local from the stack if it was actually
+                // initialised
+                if local.initialised {
+                    pops += 1;
+                }
+
+                // analyse whether the local was accessed during its
+                // lifetime, and emit a warning otherwise (unless the
+                // user explicitly chose to ignore it by prefixing the
+                // identifier with `_`)
+                if !local.used && !local.is_ignored() {
+                    unused_spans.push(local.span);
+                }
+
+                // remove the by-name index if this was a named local
+                if let LocalName::Ident(name) = local.name {
+                    if let hash_map::Entry::Occupied(mut entry) = self.by_name.entry(name) {
+                        // If no removal occured through `remove_idx`
+                        // (i.e. there was no shadowing going on),
+                        // nuke the whole entry.
+                        if !entry.get_mut().remove_idx() {
+                            entry.remove();
+                        }
+                    }
+                }
+            }
+        }
+
+        self.scope_depth -= 1;
+
+        (pops, unused_spans)
+    }
+
+    /// Access the current scope depth.
+    pub fn scope_depth(&self) -> usize {
+        self.scope_depth
+    }
+}
diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index fb9f3b6ec5..652252dadf 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -1,25 +1,1109 @@
-use std::fmt::Display;
+use std::error;
+use std::io;
+use std::path::PathBuf;
+use std::rc::Rc;
+use std::str::Utf8Error;
+use std::string::FromUtf8Error;
+use std::sync::Arc;
+use std::{fmt::Debug, fmt::Display, num::ParseIntError};
+
+use codemap::{File, Span};
+use codemap_diagnostic::{ColorConfig, Diagnostic, Emitter, Level, SpanLabel, SpanStyle};
+use smol_str::SmolStr;
+use xml::writer::Error as XmlError;
+
+use crate::spans::ToSpan;
+use crate::value::{CoercionKind, NixString};
+use crate::{SourceCode, Value};
+
+/// "CatchableErrorKind" errors -- those which can be detected by
+/// `builtins.tryEval`.
+///
+/// Note: this type is deliberately *not* incorporated as a variant
+/// of ErrorKind, because then Result<Value,ErrorKind> would have
+/// redundant representations for catchable errors, which would make
+/// it too easy to handle errors incorrectly:
+///
+///   - Ok(Value::Catchable(cek))
+///   - Err(ErrorKind::ThisVariantDoesNotExist(cek))
+///
+/// Because CatchableErrorKind is not a variant of ErrorKind, you
+/// will often see functions which return a type like:
+///
+///   Result<Result<T,CatchableErrorKind>,ErrorKind>
+///
+/// ... where T is any type other than Value.  This is unfortunate,
+/// because Rust's magic `?`-syntax does not work on nested Result
+/// values like this.
+// TODO(amjoseph): investigate result<T,Either<CatchableErrorKind,ErrorKind>>
+#[derive(Clone, Debug)]
+pub enum CatchableErrorKind {
+    Throw(Box<str>),
+    AssertionFailed,
+    UnimplementedFeature(Box<str>),
+    /// Resolving a user-supplied angle brackets path literal failed in some way.
+    NixPathResolution(Box<str>),
+}
+
+impl Display for CatchableErrorKind {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            CatchableErrorKind::Throw(s) => write!(f, "error thrown: {}", s),
+            CatchableErrorKind::AssertionFailed => write!(f, "assertion failed"),
+            CatchableErrorKind::UnimplementedFeature(s) => {
+                write!(f, "feature {} is not implemented yet", s)
+            }
+            CatchableErrorKind::NixPathResolution(s) => {
+                write!(f, "Nix path entry could not be resolved: {}", s)
+            }
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub enum ErrorKind {
+    /// These are user-generated errors through builtins.
+    Abort(String),
+
+    DivisionByZero,
 
-#[derive(Debug)]
-pub enum Error {
     DuplicateAttrsKey {
         key: String,
     },
 
-    InvalidKeyType {
-        given: &'static str,
+    /// Attempted to specify an invalid key type (e.g. integer) in a
+    /// dynamic attribute name.
+    InvalidAttributeName(Value),
+
+    AttributeNotFound {
+        name: String,
     },
 
+    /// Attempted to index into a list beyond its boundaries.
+    IndexOutOfBounds {
+        index: i64,
+    },
+
+    /// Attempted to call `builtins.tail` on an empty list.
+    TailEmptyList,
+
     TypeError {
         expected: &'static str,
         actual: &'static str,
     },
+
+    Incomparable {
+        lhs: &'static str,
+        rhs: &'static str,
+    },
+
+    /// Resolving a user-supplied relative or home-relative path literal failed in some way.
+    RelativePathResolution(String),
+
+    /// Dynamic keys are not allowed in some scopes.
+    DynamicKeyInScope(&'static str),
+
+    /// Unknown variable in statically known scope.
+    UnknownStaticVariable,
+
+    /// Unknown variable in dynamic scope (with, rec, ...).
+    UnknownDynamicVariable(String),
+
+    /// User is defining the same variable twice at the same depth.
+    VariableAlreadyDefined(Span),
+
+    /// Attempt to call something that is not callable.
+    NotCallable(&'static str),
+
+    /// Infinite recursion encountered while forcing thunks.
+    InfiniteRecursion {
+        first_force: Span,
+        suspended_at: Option<Span>,
+        content_span: Option<Span>,
+    },
+
+    ParseErrors(Vec<rnix::parser::ParseError>),
+
+    /// An error occured while executing some native code (e.g. a
+    /// builtin), and needs to be chained up.
+    NativeError {
+        gen_type: &'static str,
+        err: Box<Error>,
+    },
+
+    /// An error occured while executing Tvix bytecode, but needs to
+    /// be chained up.
+    BytecodeError(Box<Error>),
+
+    /// Given type can't be coerced to a string in the respective context
+    NotCoercibleToString {
+        from: &'static str,
+        kind: CoercionKind,
+    },
+
+    /// The given string doesn't represent an absolute path
+    NotAnAbsolutePath(PathBuf),
+
+    /// An error occurred when parsing an integer
+    ParseIntError(ParseIntError),
+
+    // Errors specific to nested attribute sets and merges thereof.
+    /// Nested attributes can not be merged with an inherited value.
+    UnmergeableInherit {
+        name: SmolStr,
+    },
+
+    /// Nested attributes can not be merged with values that are not
+    /// literal attribute sets.
+    UnmergeableValue,
+
+    /// Parse errors occured while importing a file.
+    ImportParseError {
+        path: PathBuf,
+        file: Arc<File>,
+        errors: Vec<rnix::parser::ParseError>,
+    },
+
+    /// Compilation errors occured while importing a file.
+    ImportCompilerError {
+        path: PathBuf,
+        errors: Vec<Error>,
+    },
+
+    /// I/O errors
+    IO {
+        path: Option<PathBuf>,
+        error: Rc<io::Error>,
+    },
+
+    /// Errors parsing JSON, or serializing as JSON.
+    JsonError(String),
+
+    /// Nix value that can not be serialised to JSON.
+    NotSerialisableToJson(&'static str),
+
+    /// Errors converting TOML to a value
+    FromTomlError(String),
+
+    /// An unexpected argument was supplied to a function that takes formal parameters
+    UnexpectedArgument {
+        arg: NixString,
+        formals_span: Span,
+    },
+
+    /// Invalid UTF-8 was encoutered somewhere
+    Utf8,
+
+    /// Errors while serialising to XML.
+    Xml(Rc<XmlError>),
+
+    /// Variant for errors that bubble up to eval from other Tvix
+    /// components.
+    TvixError(Rc<dyn error::Error>),
+
+    /// Variant for code paths that are known bugs in Tvix (usually
+    /// issues with the compiler/VM interaction).
+    TvixBug {
+        msg: &'static str,
+        metadata: Option<Rc<dyn Debug>>,
+    },
+
+    /// Tvix internal warning for features triggered by users that are
+    /// not actually implemented yet, and without which eval can not
+    /// proceed.
+    NotImplemented(&'static str),
+
+    /// Internal variant which should disappear during error construction.
+    WithContext {
+        context: String,
+        underlying: Box<ErrorKind>,
+    },
+
+    /// Unexpected context string
+    UnexpectedContext,
+
+    /// Top-level evaluation result was a catchable Nix error, and
+    /// should fail the evaluation.
+    ///
+    /// This variant **must** only be used at the top-level of
+    /// tvix-eval when returning a result to the user, never inside of
+    /// eval code.
+    CatchableError(CatchableErrorKind),
+
+    /// Invalid hash type specified, must be one of "md5", "sha1", "sha256"
+    /// or "sha512"
+    UnknownHashType(String),
+}
+
+impl error::Error for Error {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        match &self.kind {
+            ErrorKind::NativeError { err, .. } | ErrorKind::BytecodeError(err) => err.source(),
+            ErrorKind::ParseErrors(err) => err.first().map(|e| e as &dyn error::Error),
+            ErrorKind::ParseIntError(err) => Some(err),
+            ErrorKind::ImportParseError { errors, .. } => {
+                errors.first().map(|e| e as &dyn error::Error)
+            }
+            ErrorKind::ImportCompilerError { errors, .. } => {
+                errors.first().map(|e| e as &dyn error::Error)
+            }
+            ErrorKind::IO { error, .. } => Some(error.as_ref()),
+            ErrorKind::Xml(error) => Some(error.as_ref()),
+            ErrorKind::TvixError(error) => Some(error.as_ref()),
+            _ => None,
+        }
+    }
+}
+
+impl From<ParseIntError> for ErrorKind {
+    fn from(e: ParseIntError) -> Self {
+        Self::ParseIntError(e)
+    }
+}
+
+impl From<Utf8Error> for ErrorKind {
+    fn from(_: Utf8Error) -> Self {
+        Self::NotImplemented("FromUtf8Error not handled: https://b.tvl.fyi/issues/189")
+    }
+}
+
+impl From<FromUtf8Error> for ErrorKind {
+    fn from(_: FromUtf8Error) -> Self {
+        Self::NotImplemented("FromUtf8Error not handled: https://b.tvl.fyi/issues/189")
+    }
+}
+
+impl From<bstr::Utf8Error> for ErrorKind {
+    fn from(_: bstr::Utf8Error) -> Self {
+        Self::Utf8
+    }
+}
+
+impl From<bstr::FromUtf8Error> for ErrorKind {
+    fn from(_value: bstr::FromUtf8Error) -> Self {
+        Self::Utf8
+    }
+}
+
+impl From<XmlError> for ErrorKind {
+    fn from(err: XmlError) -> Self {
+        Self::Xml(Rc::new(err))
+    }
+}
+
+impl From<io::Error> for ErrorKind {
+    fn from(e: io::Error) -> Self {
+        ErrorKind::IO {
+            path: None,
+            error: Rc::new(e),
+        }
+    }
+}
+
+impl From<serde_json::Error> for ErrorKind {
+    fn from(err: serde_json::Error) -> Self {
+        // Can't just put the `serde_json::Error` in the ErrorKind since it doesn't impl `Clone`
+        Self::JsonError(err.to_string())
+    }
+}
+
+impl From<toml::de::Error> for ErrorKind {
+    fn from(err: toml::de::Error) -> Self {
+        Self::FromTomlError(format!("error in TOML serialization: {err}"))
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct Error {
+    pub kind: ErrorKind,
+    pub span: Span,
+    pub contexts: Vec<String>,
+    pub source: SourceCode,
+}
+
+impl Error {
+    pub fn new(mut kind: ErrorKind, span: Span, source: SourceCode) -> Self {
+        let mut contexts = vec![];
+        while let ErrorKind::WithContext {
+            context,
+            underlying,
+        } = kind
+        {
+            kind = *underlying;
+            contexts.push(context);
+        }
+
+        Error {
+            kind,
+            span,
+            contexts,
+            source,
+        }
+    }
+}
+
+impl Display for ErrorKind {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match &self {
+            ErrorKind::Abort(msg) => write!(f, "evaluation aborted: {}", msg),
+
+            ErrorKind::DivisionByZero => write!(f, "division by zero"),
+
+            ErrorKind::DuplicateAttrsKey { key } => {
+                write!(f, "attribute key '{}' already defined", key)
+            }
+
+            ErrorKind::InvalidAttributeName(val) => write!(
+                f,
+                "found attribute name '{}' of type '{}', but attribute names must be strings",
+                val,
+                val.type_of()
+            ),
+
+            ErrorKind::AttributeNotFound { name } => write!(
+                f,
+                "attribute with name '{}' could not be found in the set",
+                name
+            ),
+
+            ErrorKind::IndexOutOfBounds { index } => {
+                write!(f, "list index '{}' is out of bounds", index)
+            }
+
+            ErrorKind::TailEmptyList => write!(f, "'tail' called on an empty list"),
+
+            ErrorKind::TypeError { expected, actual } => write!(
+                f,
+                "expected value of type '{}', but found a '{}'",
+                expected, actual
+            ),
+
+            ErrorKind::Incomparable { lhs, rhs } => {
+                write!(f, "can not compare a {} with a {}", lhs, rhs)
+            }
+
+            ErrorKind::RelativePathResolution(err) => {
+                write!(f, "could not resolve path: {}", err)
+            }
+
+            ErrorKind::DynamicKeyInScope(scope) => {
+                write!(f, "dynamically evaluated keys are not allowed in {}", scope)
+            }
+
+            ErrorKind::UnknownStaticVariable => write!(f, "variable not found"),
+
+            ErrorKind::UnknownDynamicVariable(name) => write!(
+                f,
+                r#"variable '{}' could not be found
+
+Note that this occured within a `with`-expression. The problem may be related
+to a missing value in the attribute set(s) included via `with`."#,
+                name
+            ),
+
+            ErrorKind::VariableAlreadyDefined(_) => write!(f, "variable has already been defined"),
+
+            ErrorKind::NotCallable(other_type) => {
+                write!(
+                    f,
+                    "only functions and builtins can be called, but this is a '{}'",
+                    other_type
+                )
+            }
+
+            ErrorKind::InfiniteRecursion { .. } => write!(f, "infinite recursion encountered"),
+
+            // Errors themselves ignored here & handled in Self::spans instead
+            ErrorKind::ParseErrors(_) => write!(f, "failed to parse Nix code:"),
+
+            ErrorKind::NativeError { gen_type, .. } => {
+                write!(f, "while evaluating this as native code ({})", gen_type)
+            }
+
+            ErrorKind::BytecodeError(_) => write!(f, "while evaluating this Nix code"),
+
+            ErrorKind::NotCoercibleToString { kind, from } => {
+                let kindly = if kind.strong { "strongly" } else { "weakly" };
+
+                let hint = if *from == "set" {
+                    ", missing a `__toString` or `outPath` attribute"
+                } else {
+                    ""
+                };
+
+                write!(f, "cannot ({kindly}) coerce {from} to a string{hint}")
+            }
+
+            ErrorKind::NotAnAbsolutePath(given) => {
+                write!(
+                    f,
+                    "string '{}' does not represent an absolute path",
+                    given.to_string_lossy()
+                )
+            }
+
+            ErrorKind::ParseIntError(err) => {
+                write!(f, "invalid integer: {}", err)
+            }
+
+            ErrorKind::UnmergeableInherit { name } => {
+                write!(
+                    f,
+                    "cannot merge a nested attribute set into the inherited entry '{}'",
+                    name
+                )
+            }
+
+            ErrorKind::UnmergeableValue => {
+                write!(
+                    f,
+                    "nested attribute sets or keys can only be merged with literal attribute sets"
+                )
+            }
+
+            // Errors themselves ignored here & handled in Self::spans instead
+            ErrorKind::ImportParseError { path, .. } => {
+                write!(
+                    f,
+                    "parse errors occured while importing '{}'",
+                    path.to_string_lossy()
+                )
+            }
+
+            ErrorKind::ImportCompilerError { path, .. } => {
+                writeln!(
+                    f,
+                    "compiler errors occured while importing '{}'",
+                    path.to_string_lossy()
+                )
+            }
+
+            ErrorKind::IO { path, error } => {
+                write!(f, "I/O error: ")?;
+                if let Some(path) = path {
+                    write!(f, "{}: ", path.display())?;
+                }
+                write!(f, "{error}")
+            }
+
+            ErrorKind::JsonError(msg) => {
+                write!(f, "Error converting JSON to a Nix value or back: {msg}")
+            }
+
+            ErrorKind::NotSerialisableToJson(_type) => {
+                write!(f, "a {} cannot be converted to JSON", _type)
+            }
+
+            ErrorKind::FromTomlError(msg) => {
+                write!(f, "Error converting TOML to a Nix value: {msg}")
+            }
+
+            ErrorKind::UnexpectedArgument { arg, .. } => {
+                write!(f, "Unexpected argument `{arg}` supplied to function",)
+            }
+
+            ErrorKind::Utf8 => {
+                write!(f, "Invalid UTF-8 in string")
+            }
+
+            ErrorKind::Xml(error) => write!(f, "failed to serialise to XML: {error}"),
+
+            ErrorKind::TvixError(inner_error) => {
+                write!(f, "{inner_error}")
+            }
+
+            ErrorKind::TvixBug { msg, metadata } => {
+                write!(f, "Tvix bug: {}", msg)?;
+
+                if let Some(metadata) = metadata {
+                    write!(f, "; metadata: {:?}", metadata)?;
+                }
+
+                Ok(())
+            }
+
+            ErrorKind::NotImplemented(feature) => {
+                write!(f, "feature not yet implemented in Tvix: {}", feature)
+            }
+
+            ErrorKind::WithContext { .. } => {
+                panic!("internal ErrorKind::WithContext variant leaked")
+            }
+
+            ErrorKind::UnexpectedContext => {
+                write!(f, "unexpected context string")
+            }
+
+            ErrorKind::CatchableError(inner) => {
+                write!(f, "{}", inner)
+            }
+
+            ErrorKind::UnknownHashType(hash_type) => {
+                write!(f, "unknown hash type '{}'", hash_type)
+            }
+        }
+    }
 }
 
 impl Display for Error {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        writeln!(f, "{:?}", self)
+        write!(f, "{}", self.kind)
     }
 }
 
 pub type EvalResult<T> = Result<T, Error>;
+
+/// Human-readable names for rnix syntaxes.
+fn name_for_syntax(syntax: &rnix::SyntaxKind) -> &'static str {
+    match syntax {
+        rnix::SyntaxKind::TOKEN_COMMENT => "a comment",
+        rnix::SyntaxKind::TOKEN_WHITESPACE => "whitespace",
+        rnix::SyntaxKind::TOKEN_ASSERT => "`assert`-keyword",
+        rnix::SyntaxKind::TOKEN_ELSE => "`else`-keyword",
+        rnix::SyntaxKind::TOKEN_IN => "`in`-keyword",
+        rnix::SyntaxKind::TOKEN_IF => "`if`-keyword",
+        rnix::SyntaxKind::TOKEN_INHERIT => "`inherit`-keyword",
+        rnix::SyntaxKind::TOKEN_LET => "`let`-keyword",
+        rnix::SyntaxKind::TOKEN_OR => "`or`-keyword",
+        rnix::SyntaxKind::TOKEN_REC => "`rec`-keyword",
+        rnix::SyntaxKind::TOKEN_THEN => "`then`-keyword",
+        rnix::SyntaxKind::TOKEN_WITH => "`with`-keyword",
+        rnix::SyntaxKind::TOKEN_L_BRACE => "{",
+        rnix::SyntaxKind::TOKEN_R_BRACE => "}",
+        rnix::SyntaxKind::TOKEN_L_BRACK => "[",
+        rnix::SyntaxKind::TOKEN_R_BRACK => "]",
+        rnix::SyntaxKind::TOKEN_ASSIGN => "=",
+        rnix::SyntaxKind::TOKEN_AT => "@",
+        rnix::SyntaxKind::TOKEN_COLON => ":",
+        rnix::SyntaxKind::TOKEN_COMMA => "`,`",
+        rnix::SyntaxKind::TOKEN_DOT => ".",
+        rnix::SyntaxKind::TOKEN_ELLIPSIS => "...",
+        rnix::SyntaxKind::TOKEN_QUESTION => "?",
+        rnix::SyntaxKind::TOKEN_SEMICOLON => ";",
+        rnix::SyntaxKind::TOKEN_L_PAREN => "(",
+        rnix::SyntaxKind::TOKEN_R_PAREN => ")",
+        rnix::SyntaxKind::TOKEN_CONCAT => "++",
+        rnix::SyntaxKind::TOKEN_INVERT => "!",
+        rnix::SyntaxKind::TOKEN_UPDATE => "//",
+        rnix::SyntaxKind::TOKEN_ADD => "+",
+        rnix::SyntaxKind::TOKEN_SUB => "-",
+        rnix::SyntaxKind::TOKEN_MUL => "*",
+        rnix::SyntaxKind::TOKEN_DIV => "/",
+        rnix::SyntaxKind::TOKEN_AND_AND => "&&",
+        rnix::SyntaxKind::TOKEN_EQUAL => "==",
+        rnix::SyntaxKind::TOKEN_IMPLICATION => "->",
+        rnix::SyntaxKind::TOKEN_LESS => "<",
+        rnix::SyntaxKind::TOKEN_LESS_OR_EQ => "<=",
+        rnix::SyntaxKind::TOKEN_MORE => ">",
+        rnix::SyntaxKind::TOKEN_MORE_OR_EQ => ">=",
+        rnix::SyntaxKind::TOKEN_NOT_EQUAL => "!=",
+        rnix::SyntaxKind::TOKEN_OR_OR => "||",
+        rnix::SyntaxKind::TOKEN_FLOAT => "a float",
+        rnix::SyntaxKind::TOKEN_IDENT => "an identifier",
+        rnix::SyntaxKind::TOKEN_INTEGER => "an integer",
+        rnix::SyntaxKind::TOKEN_INTERPOL_END => "}",
+        rnix::SyntaxKind::TOKEN_INTERPOL_START => "${",
+        rnix::SyntaxKind::TOKEN_PATH => "a path",
+        rnix::SyntaxKind::TOKEN_URI => "a literal URI",
+        rnix::SyntaxKind::TOKEN_STRING_CONTENT => "content of a string",
+        rnix::SyntaxKind::TOKEN_STRING_END => "\"",
+        rnix::SyntaxKind::TOKEN_STRING_START => "\"",
+
+        rnix::SyntaxKind::NODE_APPLY => "a function application",
+        rnix::SyntaxKind::NODE_ASSERT => "an assertion",
+        rnix::SyntaxKind::NODE_ATTRPATH => "an attribute path",
+        rnix::SyntaxKind::NODE_DYNAMIC => "a dynamic identifier",
+
+        rnix::SyntaxKind::NODE_IDENT => "an identifier",
+        rnix::SyntaxKind::NODE_IF_ELSE => "an `if`-expression",
+        rnix::SyntaxKind::NODE_SELECT => "a `select`-expression",
+        rnix::SyntaxKind::NODE_INHERIT => "inherited values",
+        rnix::SyntaxKind::NODE_INHERIT_FROM => "inherited values",
+        rnix::SyntaxKind::NODE_STRING => "a string",
+        rnix::SyntaxKind::NODE_INTERPOL => "an interpolation",
+        rnix::SyntaxKind::NODE_LAMBDA => "a function",
+        rnix::SyntaxKind::NODE_IDENT_PARAM => "a function parameter",
+        rnix::SyntaxKind::NODE_LEGACY_LET => "a legacy `let`-expression",
+        rnix::SyntaxKind::NODE_LET_IN => "a `let`-expression",
+        rnix::SyntaxKind::NODE_LIST => "a list",
+        rnix::SyntaxKind::NODE_BIN_OP => "a binary operator",
+        rnix::SyntaxKind::NODE_PAREN => "a parenthesised expression",
+        rnix::SyntaxKind::NODE_PATTERN => "a function argument pattern",
+        rnix::SyntaxKind::NODE_PAT_BIND => "an argument pattern binding",
+        rnix::SyntaxKind::NODE_PAT_ENTRY => "an argument pattern entry",
+        rnix::SyntaxKind::NODE_ROOT => "a Nix expression",
+        rnix::SyntaxKind::NODE_ATTR_SET => "an attribute set",
+        rnix::SyntaxKind::NODE_ATTRPATH_VALUE => "an attribute set entry",
+        rnix::SyntaxKind::NODE_UNARY_OP => "a unary operator",
+        rnix::SyntaxKind::NODE_LITERAL => "a literal value",
+        rnix::SyntaxKind::NODE_WITH => "a `with`-expression",
+        rnix::SyntaxKind::NODE_PATH => "a path",
+        rnix::SyntaxKind::NODE_HAS_ATTR => "`?`-operator",
+
+        // TODO(tazjin): unsure what these variants are, lets crash!
+        rnix::SyntaxKind::NODE_ERROR => todo!("NODE_ERROR found, tell tazjin!"),
+        rnix::SyntaxKind::TOKEN_ERROR => todo!("TOKEN_ERROR found, tell tazjin!"),
+        _ => todo!(),
+    }
+}
+
+/// Construct the string representation for a list of expected parser tokens.
+fn expected_syntax(one_of: &[rnix::SyntaxKind]) -> String {
+    match one_of.len() {
+        0 => "nothing".into(),
+        1 => format!("'{}'", name_for_syntax(&one_of[0])),
+        _ => {
+            let mut out: String = "one of: ".into();
+            let end = one_of.len() - 1;
+
+            for (idx, item) in one_of.iter().enumerate() {
+                if idx != 0 {
+                    out.push_str(", ");
+                } else if idx == end {
+                    out.push_str(", or ");
+                };
+
+                out.push_str(name_for_syntax(item));
+            }
+
+            out
+        }
+    }
+}
+
+/// Process a list of parse errors into a set of span labels, annotating parse
+/// errors.
+fn spans_for_parse_errors(file: &File, errors: &[rnix::parser::ParseError]) -> Vec<SpanLabel> {
+    // rnix has a tendency to emit some identical errors more than once, but
+    // they do not enhance the user experience necessarily, so we filter them
+    // out
+    let mut had_eof = false;
+
+    errors
+        .iter()
+        .enumerate()
+        .filter_map(|(idx, err)| {
+            let (span, label): (Span, String) = match err {
+                rnix::parser::ParseError::Unexpected(range) => (
+                    range.span_for(file),
+                    "found an unexpected syntax element here".into(),
+                ),
+
+                rnix::parser::ParseError::UnexpectedExtra(range) => (
+                    range.span_for(file),
+                    "found unexpected extra elements at the root of the expression".into(),
+                ),
+
+                rnix::parser::ParseError::UnexpectedWanted(found, range, wanted) => {
+                    let span = range.span_for(file);
+                    (
+                        span,
+                        format!(
+                            "found '{}', but expected {}",
+                            name_for_syntax(found),
+                            expected_syntax(wanted),
+                        ),
+                    )
+                }
+
+                rnix::parser::ParseError::UnexpectedEOF => {
+                    if had_eof {
+                        return None;
+                    }
+
+                    had_eof = true;
+
+                    (
+                        file.span,
+                        "code ended unexpectedly while the parser still expected more".into(),
+                    )
+                }
+
+                rnix::parser::ParseError::UnexpectedEOFWanted(wanted) => {
+                    had_eof = true;
+
+                    (
+                        file.span,
+                        format!(
+                            "code ended unexpectedly, but wanted {}",
+                            expected_syntax(wanted)
+                        ),
+                    )
+                }
+
+                rnix::parser::ParseError::DuplicatedArgs(range, name) => (
+                    range.span_for(file),
+                    format!(
+                        "the function argument pattern '{}' was bound more than once",
+                        name
+                    ),
+                ),
+
+                rnix::parser::ParseError::RecursionLimitExceeded => (
+                    file.span,
+                    "this code exceeds the parser's recursion limit, please report a Tvix bug"
+                        .to_string(),
+                ),
+
+                // TODO: can rnix even still throw this? it's semantic!
+                rnix::parser::ParseError::UnexpectedDoubleBind(range) => (
+                    range.span_for(file),
+                    "this pattern was bound more than once".into(),
+                ),
+
+                // The error enum is marked as `#[non_exhaustive]` in rnix,
+                // which disables the compiler error for missing a variant. This
+                // feature makes it possible for users to miss critical updates
+                // of enum variants for a more exciting runtime experience.
+                new => todo!("new parse error variant: {}", new),
+            };
+
+            Some(SpanLabel {
+                span,
+                label: Some(label),
+                style: if idx == 0 {
+                    SpanStyle::Primary
+                } else {
+                    SpanStyle::Secondary
+                },
+            })
+        })
+        .collect()
+}
+
+impl Error {
+    pub fn fancy_format_str(&self) -> String {
+        let mut out = vec![];
+        Emitter::vec(&mut out, Some(&*self.source.codemap())).emit(&self.diagnostics());
+        String::from_utf8_lossy(&out).to_string()
+    }
+
+    /// Render a fancy, human-readable output of this error and print
+    /// it to stderr.
+    pub fn fancy_format_stderr(&self) {
+        Emitter::stderr(ColorConfig::Auto, Some(&*self.source.codemap())).emit(&self.diagnostics());
+    }
+
+    /// Create the optional span label displayed as an annotation on
+    /// the underlined span of the error.
+    fn span_label(&self) -> Option<String> {
+        let label = match &self.kind {
+            ErrorKind::DuplicateAttrsKey { .. } => "in this attribute set",
+            ErrorKind::InvalidAttributeName(_) => "in this attribute set",
+            ErrorKind::RelativePathResolution(_) => "in this path literal",
+            ErrorKind::UnexpectedArgument { .. } => "in this function call",
+            ErrorKind::UnexpectedContext => "in this string",
+
+            // The spans for some errors don't have any more descriptive stuff
+            // in them, or we don't utilise it yet.
+            ErrorKind::Abort(_)
+            | ErrorKind::AttributeNotFound { .. }
+            | ErrorKind::IndexOutOfBounds { .. }
+            | ErrorKind::TailEmptyList
+            | ErrorKind::TypeError { .. }
+            | ErrorKind::Incomparable { .. }
+            | ErrorKind::DivisionByZero
+            | ErrorKind::DynamicKeyInScope(_)
+            | ErrorKind::UnknownStaticVariable
+            | ErrorKind::UnknownDynamicVariable(_)
+            | ErrorKind::VariableAlreadyDefined(_)
+            | ErrorKind::NotCallable(_)
+            | ErrorKind::InfiniteRecursion { .. }
+            | ErrorKind::ParseErrors(_)
+            | ErrorKind::NativeError { .. }
+            | ErrorKind::BytecodeError(_)
+            | ErrorKind::NotCoercibleToString { .. }
+            | ErrorKind::NotAnAbsolutePath(_)
+            | ErrorKind::ParseIntError(_)
+            | ErrorKind::UnmergeableInherit { .. }
+            | ErrorKind::UnmergeableValue
+            | ErrorKind::ImportParseError { .. }
+            | ErrorKind::ImportCompilerError { .. }
+            | ErrorKind::IO { .. }
+            | ErrorKind::JsonError(_)
+            | ErrorKind::NotSerialisableToJson(_)
+            | ErrorKind::FromTomlError(_)
+            | ErrorKind::Xml(_)
+            | ErrorKind::Utf8
+            | ErrorKind::TvixError(_)
+            | ErrorKind::TvixBug { .. }
+            | ErrorKind::NotImplemented(_)
+            | ErrorKind::WithContext { .. }
+            | ErrorKind::UnknownHashType(_)
+            | ErrorKind::CatchableError(_) => return None,
+        };
+
+        Some(label.into())
+    }
+
+    /// Return the unique error code for this variant which can be
+    /// used to refer users to documentation.
+    fn code(&self) -> &'static str {
+        match self.kind {
+            ErrorKind::CatchableError(CatchableErrorKind::Throw(_)) => "E001",
+            ErrorKind::Abort(_) => "E002",
+            ErrorKind::CatchableError(CatchableErrorKind::AssertionFailed) => "E003",
+            ErrorKind::InvalidAttributeName { .. } => "E004",
+            ErrorKind::AttributeNotFound { .. } => "E005",
+            ErrorKind::TypeError { .. } => "E006",
+            ErrorKind::Incomparable { .. } => "E007",
+            ErrorKind::CatchableError(CatchableErrorKind::NixPathResolution(_)) => "E008",
+            ErrorKind::DynamicKeyInScope(_) => "E009",
+            ErrorKind::UnknownStaticVariable => "E010",
+            ErrorKind::UnknownDynamicVariable(_) => "E011",
+            ErrorKind::VariableAlreadyDefined(_) => "E012",
+            ErrorKind::NotCallable(_) => "E013",
+            ErrorKind::InfiniteRecursion { .. } => "E014",
+            ErrorKind::ParseErrors(_) => "E015",
+            ErrorKind::DuplicateAttrsKey { .. } => "E016",
+            ErrorKind::NotCoercibleToString { .. } => "E018",
+            ErrorKind::IndexOutOfBounds { .. } => "E019",
+            ErrorKind::NotAnAbsolutePath(_) => "E020",
+            ErrorKind::ParseIntError(_) => "E021",
+            ErrorKind::TailEmptyList { .. } => "E023",
+            ErrorKind::UnmergeableInherit { .. } => "E024",
+            ErrorKind::UnmergeableValue => "E025",
+            ErrorKind::ImportParseError { .. } => "E027",
+            ErrorKind::ImportCompilerError { .. } => "E028",
+            ErrorKind::IO { .. } => "E029",
+            ErrorKind::JsonError { .. } => "E030",
+            ErrorKind::UnexpectedArgument { .. } => "E031",
+            ErrorKind::RelativePathResolution(_) => "E032",
+            ErrorKind::DivisionByZero => "E033",
+            ErrorKind::Xml(_) => "E034",
+            ErrorKind::FromTomlError(_) => "E035",
+            ErrorKind::NotSerialisableToJson(_) => "E036",
+            ErrorKind::UnexpectedContext => "E037",
+            ErrorKind::Utf8 => "E038",
+            ErrorKind::UnknownHashType(_) => "E039",
+
+            // Special error code for errors from other Tvix
+            // components. We may want to introduce a code namespacing
+            // system to have these errors pass codes through.
+            ErrorKind::TvixError(_) => "E997",
+
+            // Special error code that is not part of the normal
+            // ordering.
+            ErrorKind::TvixBug { .. } => "E998",
+
+            // Placeholder error while Tvix is under construction.
+            ErrorKind::CatchableError(CatchableErrorKind::UnimplementedFeature(_))
+            | ErrorKind::NotImplemented(_) => "E999",
+
+            // Chained errors should yield the code of the innermost
+            // error.
+            ErrorKind::NativeError { ref err, .. } | ErrorKind::BytecodeError(ref err) => {
+                err.code()
+            }
+
+            ErrorKind::WithContext { .. } => {
+                panic!("internal ErrorKind::WithContext variant leaked")
+            }
+        }
+    }
+
+    fn spans(&self) -> Vec<SpanLabel> {
+        let mut spans = match &self.kind {
+            ErrorKind::ImportParseError { errors, file, .. } => {
+                spans_for_parse_errors(file, errors)
+            }
+
+            ErrorKind::ParseErrors(errors) => {
+                let file = self.source.get_file(self.span);
+                spans_for_parse_errors(&file, errors)
+            }
+
+            ErrorKind::UnexpectedArgument { formals_span, .. } => {
+                vec![
+                    SpanLabel {
+                        label: self.span_label(),
+                        span: self.span,
+                        style: SpanStyle::Primary,
+                    },
+                    SpanLabel {
+                        label: Some("the accepted arguments".into()),
+                        span: *formals_span,
+                        style: SpanStyle::Secondary,
+                    },
+                ]
+            }
+
+            ErrorKind::InfiniteRecursion {
+                first_force,
+                suspended_at,
+                content_span,
+            } => {
+                let mut spans = vec![];
+
+                if let Some(content_span) = content_span {
+                    spans.push(SpanLabel {
+                        label: Some("this lazily-evaluated code".into()),
+                        span: *content_span,
+                        style: SpanStyle::Secondary,
+                    })
+                }
+
+                if let Some(suspended_at) = suspended_at {
+                    spans.push(SpanLabel {
+                        label: Some("which was instantiated here".into()),
+                        span: *suspended_at,
+                        style: SpanStyle::Secondary,
+                    })
+                }
+
+                spans.push(SpanLabel {
+                    label: Some("was first requested to be evaluated here".into()),
+                    span: *first_force,
+                    style: SpanStyle::Secondary,
+                });
+
+                spans.push(SpanLabel {
+                    label: Some("but then requested again here during its own evaluation".into()),
+                    span: self.span,
+                    style: SpanStyle::Primary,
+                });
+
+                spans
+            }
+
+            // All other errors pretty much have the same shape.
+            _ => {
+                vec![SpanLabel {
+                    label: self.span_label(),
+                    span: self.span,
+                    style: SpanStyle::Primary,
+                }]
+            }
+        };
+
+        for ctx in &self.contexts {
+            spans.push(SpanLabel {
+                label: Some(format!("while {}", ctx)),
+                span: self.span,
+                style: SpanStyle::Secondary,
+            });
+        }
+
+        spans
+    }
+
+    /// Create the primary diagnostic for a given error.
+    fn diagnostic(&self) -> Diagnostic {
+        Diagnostic {
+            level: Level::Error,
+            message: self.to_string(),
+            spans: self.spans(),
+            code: Some(self.code().into()),
+        }
+    }
+
+    /// Return the primary diagnostic and all further associated diagnostics (if
+    /// any) of an error.
+    fn diagnostics(&self) -> Vec<Diagnostic> {
+        match &self.kind {
+            ErrorKind::ImportCompilerError { errors, .. } => {
+                let mut out = vec![self.diagnostic()];
+                out.extend(errors.iter().map(|e| e.diagnostic()));
+                out
+            }
+
+            // When encountering either of these error kinds, we are dealing
+            // with the top of an error chain.
+            //
+            // An error chain creates a list of diagnostics which provide trace
+            // information.
+            //
+            // We don't know how deep this chain is, so we avoid recursing in
+            // this function while unrolling the chain.
+            ErrorKind::NativeError { err: next, .. } | ErrorKind::BytecodeError(next) => {
+                // Accumulated diagnostics to return.
+                let mut diagnostics: Vec<Diagnostic> = vec![];
+
+                // The next (inner) error to add to the diagnostics, after this
+                // one.
+                let mut next = *next.clone();
+
+                // Diagnostic message for *this* error.
+                let mut this_message = self.to_string();
+
+                // Primary span for *this* error.
+                let mut this_span = self.span;
+
+                // Diagnostic spans for *this* error.
+                let mut this_spans = self.spans();
+
+                loop {
+                    if is_new_span(
+                        this_span,
+                        diagnostics.last().and_then(|last| last.spans.last()),
+                    ) {
+                        diagnostics.push(Diagnostic {
+                            level: Level::Note,
+                            message: this_message,
+                            spans: this_spans,
+                            code: None, // only the top-level error has one
+                        });
+                    }
+
+                    this_message = next.to_string();
+                    this_span = next.span;
+                    this_spans = next.spans();
+
+                    match next.kind {
+                        ErrorKind::NativeError { err: inner, .. }
+                        | ErrorKind::BytecodeError(inner) => {
+                            next = *inner;
+                            continue;
+                        }
+                        _ => {
+                            diagnostics.extend(next.diagnostics());
+                            break;
+                        }
+                    }
+                }
+
+                diagnostics
+            }
+
+            _ => vec![self.diagnostic()],
+        }
+    }
+}
+
+// Check if this error is in a different span from its immediate ancestor.
+fn is_new_span(this_span: Span, parent: Option<&SpanLabel>) -> bool {
+    match parent {
+        None => true,
+        Some(parent) => parent.span != this_span,
+    }
+}
+
+// Convenience methods to add context on other types.
+pub trait AddContext {
+    /// Add context to the error-carrying type.
+    fn context<S: Into<String>>(self, ctx: S) -> Self;
+}
+
+impl AddContext for ErrorKind {
+    fn context<S: Into<String>>(self, ctx: S) -> Self {
+        ErrorKind::WithContext {
+            context: ctx.into(),
+            underlying: Box::new(self),
+        }
+    }
+}
+
+impl<T> AddContext for Result<T, ErrorKind> {
+    fn context<S: Into<String>>(self, ctx: S) -> Self {
+        self.map_err(|kind| kind.context(ctx))
+    }
+}
+
+impl<T> AddContext for Result<T, Error> {
+    fn context<S: Into<String>>(self, ctx: S) -> Self {
+        self.map_err(|err| Error {
+            kind: err.kind.context(ctx),
+            ..err
+        })
+    }
+}
diff --git a/tvix/eval/src/eval.rs b/tvix/eval/src/eval.rs
deleted file mode 100644
index 370aad494d..0000000000
--- a/tvix/eval/src/eval.rs
+++ /dev/null
@@ -1,20 +0,0 @@
-use rnix::{self, types::TypedNode};
-
-use crate::errors::EvalResult;
-
-pub fn interpret(code: String) -> EvalResult<String> {
-    let ast = rnix::parse(&code);
-
-    let errors = ast.errors();
-    if !errors.is_empty() {
-        todo!()
-    }
-
-    println!("{}", ast.root().dump());
-
-    let code = crate::compiler::compile(ast)?;
-    println!("code: {:?}", code);
-
-    let value = crate::vm::run_chunk(code)?;
-    Ok(format!("value: {} :: {}", value, value.type_of()))
-}
diff --git a/tvix/eval/src/io.rs b/tvix/eval/src/io.rs
new file mode 100644
index 0000000000..f775077af8
--- /dev/null
+++ b/tvix/eval/src/io.rs
@@ -0,0 +1,164 @@
+//! Interface for injecting I/O-related functionality into tvix-eval.
+//!
+//! The Nix language contains several builtins (e.g. `builtins.readDir`), as
+//! well as language feature (e.g. string-"coercion" of paths) that interact
+//! with the filesystem.
+//!
+//! The language evaluator implemented by this crate does not depend on any
+//! particular filesystem interaction model. Instead, this module provides a
+//! trait that can be implemented by tvix-eval callers to provide the
+//! functionality they desire.
+//!
+//! In theory this can be used to implement "mocked" filesystem interactions, or
+//! interaction with remote filesystems, etc.
+//!
+//! In the context of Nix builds, callers also use this interface to determine
+//! how store paths are opened and so on.
+
+use std::{
+    fs::File,
+    io,
+    path::{Path, PathBuf},
+};
+
+#[cfg(target_family = "unix")]
+use std::os::unix::ffi::OsStringExt;
+
+/// Types of files as represented by `builtins.readDir` in Nix.
+#[derive(Debug)]
+pub enum FileType {
+    Directory,
+    Regular,
+    Symlink,
+    Unknown,
+}
+
+/// Represents all possible filesystem interactions that exist in the Nix
+/// language, and that need to be executed somehow.
+///
+/// This trait is specifically *only* concerned with what is visible on the
+/// level of the language. All internal implementation details are not part of
+/// this trait.
+pub trait EvalIO {
+    /// Verify whether the file at the specified path exists.
+    ///
+    /// This is used for the following language evaluation cases:
+    ///
+    /// * checking whether a file added to the `NIX_PATH` actually exists when
+    ///   it is referenced in `<...>` brackets.
+    /// * `builtins.pathExists :: path -> bool`
+    fn path_exists(&self, path: &Path) -> io::Result<bool>;
+
+    /// Open the file at the specified path to a `io::Read`.
+    fn open(&self, path: &Path) -> io::Result<Box<dyn io::Read>>;
+
+    /// Read the directory at the specified path and return the names
+    /// of its entries associated with their [`FileType`].
+    ///
+    /// This is used for the following language evaluation cases:
+    ///
+    /// * `builtins.readDir :: path -> attrs<filename, filetype>`
+    fn read_dir(&self, path: &Path) -> io::Result<Vec<(bytes::Bytes, FileType)>>;
+
+    /// Import the given path. What this means depends on the implementation,
+    /// for example for a `std::io`-based implementation this might be a no-op,
+    /// while for a Tvix store this might be a copy of the given files to the
+    /// store.
+    ///
+    /// This is used for the following language evaluation cases:
+    ///
+    /// * string coercion of path literals (e.g. `/foo/bar`), which are expected
+    ///   to return a path
+    /// * `builtins.toJSON` on a path literal, also expected to return a path
+    fn import_path(&self, path: &Path) -> io::Result<PathBuf>;
+
+    /// Returns the root of the store directory, if such a thing
+    /// exists in the evaluation context.
+    ///
+    /// This is used for the following language evaluation cases:
+    ///
+    /// * `builtins.storeDir :: string`
+    fn store_dir(&self) -> Option<String> {
+        None
+    }
+}
+
+/// Implementation of [`EvalIO`] that simply uses the equivalent
+/// standard library functions, i.e. does local file-IO.
+#[cfg(feature = "impure")]
+pub struct StdIO;
+
+// TODO: we might want to make this whole impl to be target_family = "unix".
+#[cfg(feature = "impure")]
+impl EvalIO for StdIO {
+    fn path_exists(&self, path: &Path) -> io::Result<bool> {
+        path.try_exists()
+    }
+
+    fn open(&self, path: &Path) -> io::Result<Box<dyn io::Read>> {
+        Ok(Box::new(File::open(path)?))
+    }
+
+    fn read_dir(&self, path: &Path) -> io::Result<Vec<(bytes::Bytes, FileType)>> {
+        let mut result = vec![];
+
+        for entry in path.read_dir()? {
+            let entry = entry?;
+            let file_type = entry.metadata()?.file_type();
+
+            let val = if file_type.is_dir() {
+                FileType::Directory
+            } else if file_type.is_file() {
+                FileType::Regular
+            } else if file_type.is_symlink() {
+                FileType::Symlink
+            } else {
+                FileType::Unknown
+            };
+
+            result.push((entry.file_name().into_vec().into(), val))
+        }
+
+        Ok(result)
+    }
+
+    // this is a no-op for `std::io`, as the user can already refer to
+    // the path directly
+    fn import_path(&self, path: &Path) -> io::Result<PathBuf> {
+        Ok(path.to_path_buf())
+    }
+}
+
+/// Dummy implementation of [`EvalIO`], can be used in contexts where
+/// IO is not available but code should "pretend" that it is.
+pub struct DummyIO;
+
+impl EvalIO for DummyIO {
+    fn path_exists(&self, _: &Path) -> io::Result<bool> {
+        Err(io::Error::new(
+            io::ErrorKind::Unsupported,
+            "I/O methods are not implemented in DummyIO",
+        ))
+    }
+
+    fn open(&self, _: &Path) -> io::Result<Box<dyn io::Read>> {
+        Err(io::Error::new(
+            io::ErrorKind::Unsupported,
+            "I/O methods are not implemented in DummyIO",
+        ))
+    }
+
+    fn read_dir(&self, _: &Path) -> io::Result<Vec<(bytes::Bytes, FileType)>> {
+        Err(io::Error::new(
+            io::ErrorKind::Unsupported,
+            "I/O methods are not implemented in DummyIO",
+        ))
+    }
+
+    fn import_path(&self, _: &Path) -> io::Result<PathBuf> {
+        Err(io::Error::new(
+            io::ErrorKind::Unsupported,
+            "I/O methods are not implemented in DummyIO",
+        ))
+    }
+}
diff --git a/tvix/eval/src/lib.rs b/tvix/eval/src/lib.rs
new file mode 100644
index 0000000000..845964cb7e
--- /dev/null
+++ b/tvix/eval/src/lib.rs
@@ -0,0 +1,394 @@
+//! `tvix-eval` implements the evaluation of the Nix programming language in
+//! Tvix.
+//!
+//! It is designed to allow users to use Nix as a versatile language for
+//! different use-cases.
+//!
+//! This module exports the high-level functions and types needed for evaluating
+//! Nix code and interacting with the language's data structures.
+//!
+//! Nix has several language features that make use of impurities (such as
+//! reading from the NIX_PATH environment variable, or interacting with files).
+//! These features are optional and the API of this crate exposes functionality
+//! for controlling how they work.
+
+pub mod builtins;
+mod chunk;
+mod compiler;
+mod errors;
+mod io;
+pub mod observer;
+mod opcode;
+mod pretty_ast;
+mod source;
+mod spans;
+mod systems;
+mod upvalues;
+mod value;
+mod vm;
+mod warnings;
+
+mod nix_search_path;
+#[cfg(test)]
+mod properties;
+#[cfg(test)]
+mod test_utils;
+#[cfg(test)]
+mod tests;
+
+use std::path::PathBuf;
+use std::rc::Rc;
+use std::str::FromStr;
+use std::sync::Arc;
+
+use crate::compiler::GlobalsMap;
+use crate::observer::{CompilerObserver, RuntimeObserver};
+use crate::value::Lambda;
+use crate::vm::run_lambda;
+
+// Re-export the public interface used by other crates.
+pub use crate::compiler::{compile, prepare_globals, CompilationOutput};
+pub use crate::errors::{AddContext, CatchableErrorKind, Error, ErrorKind, EvalResult};
+pub use crate::io::{DummyIO, EvalIO, FileType};
+pub use crate::pretty_ast::pretty_print_expr;
+pub use crate::source::SourceCode;
+pub use crate::value::{NixContext, NixContextElement};
+pub use crate::vm::generators;
+pub use crate::warnings::{EvalWarning, WarningKind};
+pub use builtin_macros;
+
+pub use crate::value::{Builtin, CoercionKind, NixAttrs, NixList, NixString, Value};
+
+#[cfg(feature = "impure")]
+pub use crate::io::StdIO;
+
+/// An `Evaluation` represents how a piece of Nix code is evaluated. It can be
+/// instantiated and configured directly, or it can be accessed through the
+/// various simplified helper methods available below.
+///
+/// Public fields are intended to be set by the caller. Setting all
+/// fields is optional.
+pub struct Evaluation<'co, 'ro, IO> {
+    /// Source code map used for error reporting.
+    source_map: SourceCode,
+
+    /// Set of all builtins that should be available during the
+    /// evaluation.
+    ///
+    /// This defaults to all pure builtins. Users might want to add
+    /// the set of impure builtins, or other custom builtins.
+    pub builtins: Vec<(&'static str, Value)>,
+
+    /// Set of builtins that are implemented in Nix itself and should
+    /// be compiled and inserted in the builtins set.
+    pub src_builtins: Vec<(&'static str, &'static str)>,
+
+    /// Implementation of file-IO to use during evaluation, e.g. for
+    /// impure builtins.
+    ///
+    /// Defaults to [`DummyIO`] if not set explicitly.
+    pub io_handle: IO,
+
+    /// Determines whether the `import` builtin should be made
+    /// available. Note that this depends on the `io_handle` being
+    /// able to read the files specified as arguments to `import`.
+    pub enable_import: bool,
+
+    /// Determines whether the returned value should be strictly
+    /// evaluated, that is whether its list and attribute set elements
+    /// should be forced recursively.
+    pub strict: bool,
+
+    /// (optional) Nix search path, e.g. the value of `NIX_PATH` used
+    /// for resolving items on the search path (such as `<nixpkgs>`).
+    pub nix_path: Option<String>,
+
+    /// (optional) compiler observer for reporting on compilation
+    /// details, like the emitted bytecode.
+    pub compiler_observer: Option<&'co mut dyn CompilerObserver>,
+
+    /// (optional) runtime observer, for reporting on execution steps
+    /// of Nix code.
+    pub runtime_observer: Option<&'ro mut dyn RuntimeObserver>,
+}
+
+/// Result of evaluating a piece of Nix code. If evaluation succeeded, a value
+/// will be present (and potentially some warnings!). If evaluation failed,
+/// errors will be present.
+#[derive(Debug, Default)]
+pub struct EvaluationResult {
+    /// Nix value that the code evaluated to.
+    pub value: Option<Value>,
+
+    /// Errors that occured during evaluation (if any).
+    pub errors: Vec<Error>,
+
+    /// Warnings that occured during evaluation. Warnings are not critical, but
+    /// should be addressed either to modernise code or improve performance.
+    pub warnings: Vec<EvalWarning>,
+
+    /// AST node that was parsed from the code (on success only).
+    pub expr: Option<rnix::ast::Expr>,
+}
+
+impl<'co, 'ro, IO> Evaluation<'co, 'ro, IO>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
+    /// Initialize an `Evaluation`.
+    pub fn new(io_handle: IO, enable_import: bool) -> Self {
+        let mut builtins = builtins::pure_builtins();
+        builtins.extend(builtins::placeholders()); // these are temporary
+
+        Self {
+            source_map: SourceCode::default(),
+            enable_import,
+            io_handle,
+            builtins,
+            src_builtins: vec![],
+            strict: false,
+            nix_path: None,
+            compiler_observer: None,
+            runtime_observer: None,
+        }
+    }
+}
+
+impl<'co, 'ro> Evaluation<'co, 'ro, Box<dyn EvalIO>> {
+    /// Initialize an `Evaluation`, without the import statement available, and
+    /// all IO operations stubbed out.
+    pub fn new_pure() -> Self {
+        Self::new(Box::new(DummyIO) as Box<dyn EvalIO>, false)
+    }
+
+    #[cfg(feature = "impure")]
+    /// Configure an `Evaluation` to have impure features available
+    /// with the given I/O implementation.
+    ///
+    /// If no I/O implementation is supplied, [`StdIO`] is used by
+    /// default.
+    pub fn enable_impure(&mut self, io: Option<Box<dyn EvalIO>>) {
+        self.io_handle = io.unwrap_or_else(|| Box::new(StdIO) as Box<dyn EvalIO>);
+        self.enable_import = true;
+        self.builtins.extend(builtins::impure_builtins());
+
+        // Make `NIX_PATH` resolutions work by default, unless the
+        // user already overrode this with something else.
+        if self.nix_path.is_none() {
+            self.nix_path = std::env::var("NIX_PATH").ok();
+        }
+    }
+
+    #[cfg(feature = "impure")]
+    /// Initialise an `Evaluation`, with all impure features turned on by default.
+    pub fn new_impure() -> Self {
+        let mut eval = Self::new_pure();
+        eval.enable_impure(None);
+        eval
+    }
+}
+
+impl<'co, 'ro, IO> Evaluation<'co, 'ro, IO>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
+    /// Clone the reference to the contained source code map. This is used after
+    /// an evaluation for pretty error printing.
+    pub fn source_map(&self) -> SourceCode {
+        self.source_map.clone()
+    }
+
+    /// Only compile the provided source code, at an optional location of the
+    /// source code (i.e. path to the file it was read from; used for error
+    /// reporting, and for resolving relative paths in impure functions)
+    /// This does not *run* the code, it only provides analysis (errors and
+    /// warnings) of the compiler.
+    pub fn compile_only(
+        mut self,
+        code: impl AsRef<str>,
+        location: Option<PathBuf>,
+    ) -> EvaluationResult {
+        let mut result = EvaluationResult::default();
+        let source = self.source_map();
+
+        let location_str = location
+            .as_ref()
+            .map(|p| p.to_string_lossy().to_string())
+            .unwrap_or_else(|| "[code]".into());
+
+        let file = source.add_file(location_str, code.as_ref().to_string());
+
+        let mut noop_observer = observer::NoOpObserver::default();
+        let compiler_observer = self.compiler_observer.take().unwrap_or(&mut noop_observer);
+
+        parse_compile_internal(
+            &mut result,
+            code.as_ref(),
+            file,
+            location,
+            source,
+            self.builtins,
+            self.src_builtins,
+            self.enable_import,
+            compiler_observer,
+        );
+
+        result
+    }
+
+    /// Evaluate the provided source code, at an optional location of the source
+    /// code (i.e. path to the file it was read from; used for error reporting,
+    /// and for resolving relative paths in impure functions)
+    pub fn evaluate(
+        mut self,
+        code: impl AsRef<str>,
+        location: Option<PathBuf>,
+    ) -> EvaluationResult {
+        let mut result = EvaluationResult::default();
+        let source = self.source_map();
+
+        let location_str = location
+            .as_ref()
+            .map(|p| p.to_string_lossy().to_string())
+            .unwrap_or_else(|| "[code]".into());
+
+        let file = source.add_file(location_str, code.as_ref().to_string());
+
+        let mut noop_observer = observer::NoOpObserver::default();
+        let compiler_observer = self.compiler_observer.take().unwrap_or(&mut noop_observer);
+
+        // Insert a storeDir builtin *iff* a store directory is present.
+        if let Some(store_dir) = self.io_handle.as_ref().store_dir() {
+            self.builtins.push(("storeDir", store_dir.into()));
+        }
+
+        let (lambda, globals) = match parse_compile_internal(
+            &mut result,
+            code.as_ref(),
+            file.clone(),
+            location,
+            source.clone(),
+            self.builtins,
+            self.src_builtins,
+            self.enable_import,
+            compiler_observer,
+        ) {
+            None => return result,
+            Some(cr) => cr,
+        };
+
+        // If bytecode was returned, there were no errors and the
+        // code is safe to execute.
+
+        let nix_path = self
+            .nix_path
+            .as_ref()
+            .and_then(|s| match nix_search_path::NixSearchPath::from_str(s) {
+                Ok(path) => Some(path),
+                Err(err) => {
+                    result.warnings.push(EvalWarning {
+                        kind: WarningKind::InvalidNixPath(err.to_string()),
+                        span: file.span,
+                    });
+                    None
+                }
+            })
+            .unwrap_or_default();
+
+        let runtime_observer = self.runtime_observer.take().unwrap_or(&mut noop_observer);
+
+        let vm_result = run_lambda(
+            nix_path,
+            self.io_handle,
+            runtime_observer,
+            source.clone(),
+            globals,
+            lambda,
+            self.strict,
+        );
+
+        match vm_result {
+            Ok(mut runtime_result) => {
+                result.warnings.append(&mut runtime_result.warnings);
+                if let Value::Catchable(inner) = runtime_result.value {
+                    result.errors.push(Error::new(
+                        ErrorKind::CatchableError(*inner),
+                        file.span,
+                        source,
+                    ));
+                    return result;
+                }
+
+                result.value = Some(runtime_result.value);
+            }
+            Err(err) => {
+                result.errors.push(err);
+            }
+        }
+
+        result
+    }
+}
+
+/// Internal helper function for common parsing & compilation logic
+/// between the public functions.
+#[allow(clippy::too_many_arguments)] // internal API, no point making an indirection type
+fn parse_compile_internal(
+    result: &mut EvaluationResult,
+    code: &str,
+    file: Arc<codemap::File>,
+    location: Option<PathBuf>,
+    source: SourceCode,
+    builtins: Vec<(&'static str, Value)>,
+    src_builtins: Vec<(&'static str, &'static str)>,
+    enable_import: bool,
+    compiler_observer: &mut dyn CompilerObserver,
+) -> Option<(Rc<Lambda>, Rc<GlobalsMap>)> {
+    let parsed = rnix::ast::Root::parse(code);
+    let parse_errors = parsed.errors();
+
+    if !parse_errors.is_empty() {
+        result.errors.push(Error::new(
+            ErrorKind::ParseErrors(parse_errors.to_vec()),
+            file.span,
+            source,
+        ));
+        return None;
+    }
+
+    // At this point we know that the code is free of parse errors and
+    // we can continue to compile it. The expression is persisted in
+    // the result, in case the caller needs it for something.
+    result.expr = parsed.tree().expr();
+
+    let builtins =
+        crate::compiler::prepare_globals(builtins, src_builtins, source.clone(), enable_import);
+
+    let compiler_result = match compiler::compile(
+        result.expr.as_ref().unwrap(),
+        location,
+        builtins,
+        &source,
+        &file,
+        compiler_observer,
+    ) {
+        Ok(result) => result,
+        Err(err) => {
+            result.errors.push(err);
+            return None;
+        }
+    };
+
+    result.warnings = compiler_result.warnings;
+    result.errors.extend(compiler_result.errors);
+
+    // Short-circuit if errors exist at this point (do not pass broken
+    // bytecode to the runtime).
+    if !result.errors.is_empty() {
+        return None;
+    }
+
+    // Return the lambda (for execution) and the globals map (to
+    // ensure the invariant that the globals outlive the runtime).
+    Some((compiler_result.lambda, compiler_result.globals))
+}
diff --git a/tvix/eval/src/main.rs b/tvix/eval/src/main.rs
deleted file mode 100644
index 4cfa0a137a..0000000000
--- a/tvix/eval/src/main.rs
+++ /dev/null
@@ -1,54 +0,0 @@
-use std::{
-    env, fs,
-    io::{self, Write},
-    mem, process,
-};
-
-mod chunk;
-mod compiler;
-mod errors;
-mod eval;
-mod opcode;
-mod value;
-mod vm;
-
-fn main() {
-    let mut args = env::args();
-    if args.len() > 2 {
-        println!("Usage: tvix-eval [script]");
-        process::exit(1);
-    }
-
-    if let Some(file) = args.nth(1) {
-        run_file(&file);
-    } else {
-        run_prompt();
-    }
-}
-
-fn run_file(file: &str) {
-    let contents = fs::read_to_string(file).expect("failed to read the input file");
-
-    run(contents);
-}
-
-fn run_prompt() {
-    let mut line = String::new();
-
-    loop {
-        print!("> ");
-        io::stdout().flush().unwrap();
-        io::stdin()
-            .read_line(&mut line)
-            .expect("failed to read user input");
-        run(mem::take(&mut line));
-        line.clear();
-    }
-}
-
-fn run(code: String) {
-    match eval::interpret(code) {
-        Ok(result) => println!("=> {}", result),
-        Err(err) => eprintln!("{}", err),
-    }
-}
diff --git a/tvix/eval/src/nix_search_path.rs b/tvix/eval/src/nix_search_path.rs
new file mode 100644
index 0000000000..566ca12238
--- /dev/null
+++ b/tvix/eval/src/nix_search_path.rs
@@ -0,0 +1,256 @@
+use path_clean::PathClean;
+use std::convert::Infallible;
+use std::path::{Path, PathBuf};
+use std::str::FromStr;
+
+use crate::errors::{CatchableErrorKind, ErrorKind};
+use crate::EvalIO;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+enum NixSearchPathEntry {
+    /// Resolve subdirectories of this path within `<...>` brackets. This
+    /// corresponds to bare paths within the `NIX_PATH` environment variable
+    ///
+    /// For example, with `NixSearchPathEntry::Path("/example")` and the following
+    /// directory structure:
+    ///
+    /// ```notrust
+    /// example
+    /// โ””โ”€โ”€ subdir
+    ///     โ””โ”€โ”€ grandchild
+    /// ```
+    ///
+    /// A Nix path literal `<subdir>` would resolve to `/example/subdir`, and a
+    /// Nix path literal `<subdir/grandchild>` would resolve to
+    /// `/example/subdir/grandchild`
+    Path(PathBuf),
+
+    /// Resolve paths starting with `prefix` as subdirectories of `path`. This
+    /// corresponds to `prefix=path` within the `NIX_PATH` environment variable.
+    ///
+    /// For example, with `NixSearchPathEntry::Prefix { prefix: "prefix", path:
+    /// "/example" }` and the following directory structure:
+    ///
+    /// ```notrust
+    /// example
+    /// โ””โ”€โ”€ subdir
+    ///     โ””โ”€โ”€ grandchild
+    /// ```
+    ///
+    /// A Nix path literal `<prefix/subdir>` would resolve to `/example/subdir`,
+    /// and a Nix path literal `<prefix/subdir/grandchild>` would resolve to
+    /// `/example/subdir/grandchild`
+    Prefix { prefix: PathBuf, path: PathBuf },
+}
+
+fn canonicalise(path: PathBuf) -> Result<PathBuf, ErrorKind> {
+    let absolute = if path.is_absolute() {
+        path
+    } else {
+        // TODO(tazjin): probably panics in wasm?
+        std::env::current_dir()
+            .map_err(|e| ErrorKind::IO {
+                path: Some(path.clone()),
+                error: e.into(),
+            })?
+            .join(path)
+    }
+    .clean();
+
+    Ok(absolute)
+}
+
+impl NixSearchPathEntry {
+    /// Determine whether this path entry matches the given lookup path.
+    ///
+    /// For bare paths, an entry is considered to match if a matching
+    /// file exists under it.
+    ///
+    /// For prefixed path, an entry matches if the prefix does.
+    // TODO(tazjin): verify these rules in the C++ impl, seems fishy.
+    fn resolve<IO>(&self, io: IO, lookup_path: &Path) -> Result<Option<PathBuf>, ErrorKind>
+    where
+        IO: AsRef<dyn EvalIO>,
+    {
+        let path = match self {
+            NixSearchPathEntry::Path(parent) => canonicalise(parent.join(lookup_path))?,
+
+            NixSearchPathEntry::Prefix { prefix, path } => {
+                if let Ok(child_path) = lookup_path.strip_prefix(prefix) {
+                    canonicalise(path.join(child_path))?
+                } else {
+                    return Ok(None);
+                }
+            }
+        };
+
+        if io.as_ref().path_exists(&path).map_err(|e| ErrorKind::IO {
+            path: Some(path.clone()),
+            error: e.into(),
+        })? {
+            Ok(Some(path))
+        } else {
+            Ok(None)
+        }
+    }
+}
+
+impl FromStr for NixSearchPathEntry {
+    type Err = Infallible;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s.split_once('=') {
+            Some((prefix, path)) => Ok(Self::Prefix {
+                prefix: prefix.into(),
+                path: path.into(),
+            }),
+            None => Ok(Self::Path(s.into())),
+        }
+    }
+}
+
+/// Struct implementing the format and path resolution rules of the `NIX_PATH`
+/// environment variable.
+///
+/// This struct can be constructed by parsing a string using the [`FromStr`]
+/// impl, or via [`str::parse`]. Nix `<...>` paths can then be resolved using
+/// [`NixSearchPath::resolve`].
+#[derive(Default, Debug, Clone, PartialEq, Eq)]
+pub struct NixSearchPath {
+    entries: Vec<NixSearchPathEntry>,
+}
+
+impl NixSearchPath {
+    /// Attempt to resolve the given `path` within this [`NixSearchPath`] using the
+    /// path resolution rules for `<...>`-style paths
+    pub fn resolve<P, IO>(
+        &self,
+        io: IO,
+        path: P,
+    ) -> Result<Result<PathBuf, CatchableErrorKind>, ErrorKind>
+    where
+        P: AsRef<Path>,
+        IO: AsRef<dyn EvalIO>,
+    {
+        let path = path.as_ref();
+        for entry in &self.entries {
+            if let Some(p) = entry.resolve(&io, path)? {
+                return Ok(Ok(p));
+            }
+        }
+        Ok(Err(CatchableErrorKind::NixPathResolution(
+            format!(
+                "path '{}' was not found in the Nix search path",
+                path.display()
+            )
+            .into_boxed_str(),
+        )))
+    }
+}
+
+impl FromStr for NixSearchPath {
+    type Err = Infallible;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let entries = s
+            .split(':')
+            .map(|s| s.parse())
+            .collect::<Result<Vec<_>, _>>()?;
+        Ok(NixSearchPath { entries })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    mod parse {
+        use super::*;
+
+        #[test]
+        fn bare_paths() {
+            assert_eq!(
+                NixSearchPath::from_str("/foo/bar:/baz").unwrap(),
+                NixSearchPath {
+                    entries: vec![
+                        NixSearchPathEntry::Path("/foo/bar".into()),
+                        NixSearchPathEntry::Path("/baz".into())
+                    ],
+                }
+            );
+        }
+
+        #[test]
+        fn mixed_prefix_and_paths() {
+            assert_eq!(
+                NixSearchPath::from_str("nixpkgs=/my/nixpkgs:/etc/nixos").unwrap(),
+                NixSearchPath {
+                    entries: vec![
+                        NixSearchPathEntry::Prefix {
+                            prefix: "nixpkgs".into(),
+                            path: "/my/nixpkgs".into()
+                        },
+                        NixSearchPathEntry::Path("/etc/nixos".into())
+                    ],
+                }
+            );
+        }
+    }
+
+    mod resolve {
+        use crate::StdIO;
+        use path_clean::PathClean;
+        use std::env::current_dir;
+
+        use super::*;
+
+        #[test]
+        fn simple_dir() {
+            let nix_search_path = NixSearchPath::from_str("./.").unwrap();
+            let io = Box::new(StdIO {}) as Box<dyn EvalIO>;
+            let res = nix_search_path.resolve(&io, "src").unwrap();
+            assert_eq!(
+                res.unwrap().to_path_buf(),
+                current_dir().unwrap().join("src").clean()
+            );
+        }
+
+        #[test]
+        fn failed_resolution() {
+            let nix_search_path = NixSearchPath::from_str("./.").unwrap();
+            let io = Box::new(StdIO {}) as Box<dyn EvalIO>;
+            let err = nix_search_path.resolve(&io, "nope").unwrap();
+            assert!(
+                matches!(err, Err(CatchableErrorKind::NixPathResolution(..))),
+                "err = {err:?}"
+            );
+        }
+
+        #[test]
+        fn second_in_path() {
+            let nix_search_path = NixSearchPath::from_str("./.:/").unwrap();
+            let io = Box::new(StdIO {}) as Box<dyn EvalIO>;
+            let res = nix_search_path.resolve(&io, "etc").unwrap();
+            assert_eq!(res.unwrap().to_path_buf(), Path::new("/etc"));
+        }
+
+        #[test]
+        fn prefix() {
+            let nix_search_path = NixSearchPath::from_str("/:tvix=.").unwrap();
+            let io = Box::new(StdIO {}) as Box<dyn EvalIO>;
+            let res = nix_search_path.resolve(&io, "tvix/src").unwrap();
+            assert_eq!(
+                res.unwrap().to_path_buf(),
+                current_dir().unwrap().join("src").clean()
+            );
+        }
+
+        #[test]
+        fn matching_prefix() {
+            let nix_search_path = NixSearchPath::from_str("/:tvix=.").unwrap();
+            let io = Box::new(StdIO {}) as Box<dyn EvalIO>;
+            let res = nix_search_path.resolve(&io, "tvix").unwrap();
+            assert_eq!(res.unwrap().to_path_buf(), current_dir().unwrap().clean());
+        }
+    }
+}
diff --git a/tvix/eval/src/observer.rs b/tvix/eval/src/observer.rs
new file mode 100644
index 0000000000..f5de399315
--- /dev/null
+++ b/tvix/eval/src/observer.rs
@@ -0,0 +1,318 @@
+//! Implements traits for things that wish to observe internal state
+//! changes of tvix-eval.
+//!
+//! This can be used to gain insights from compilation, to trace the
+//! runtime, and so on.
+//!
+//! All methods are optional, that is, observers can implement only
+/// what they are interested in observing.
+use std::io::Write;
+use std::rc::Rc;
+use std::time::Instant;
+use tabwriter::TabWriter;
+
+use crate::chunk::Chunk;
+use crate::generators::VMRequest;
+use crate::opcode::{CodeIdx, OpCode};
+use crate::value::Lambda;
+use crate::SourceCode;
+use crate::Value;
+
+/// Implemented by types that wish to observe internal happenings of
+/// the Tvix compiler.
+pub trait CompilerObserver {
+    /// Called when the compiler finishes compilation of the top-level
+    /// of an expression (usually the root Nix expression of a file).
+    fn observe_compiled_toplevel(&mut self, _: &Rc<Lambda>) {}
+
+    /// Called when the compiler finishes compilation of a
+    /// user-defined function.
+    ///
+    /// Note that in Nix there are only single argument functions, so
+    /// in an expression like `a: b: c: ...` this method will be
+    /// called three times.
+    fn observe_compiled_lambda(&mut self, _: &Rc<Lambda>) {}
+
+    /// Called when the compiler finishes compilation of a thunk.
+    fn observe_compiled_thunk(&mut self, _: &Rc<Lambda>) {}
+}
+
+/// Implemented by types that wish to observe internal happenings of
+/// the Tvix virtual machine at runtime.
+pub trait RuntimeObserver {
+    /// Called when the runtime enters a new call frame.
+    fn observe_enter_call_frame(&mut self, _arg_count: usize, _: &Rc<Lambda>, _call_depth: usize) {}
+
+    /// Called when the runtime exits a call frame.
+    fn observe_exit_call_frame(&mut self, _frame_at: usize, _stack: &[Value]) {}
+
+    /// Called when the runtime suspends a call frame.
+    fn observe_suspend_call_frame(&mut self, _frame_at: usize, _stack: &[Value]) {}
+
+    /// Called when the runtime enters a generator frame.
+    fn observe_enter_generator(&mut self, _frame_at: usize, _name: &str, _stack: &[Value]) {}
+
+    /// Called when the runtime exits a generator frame.
+    fn observe_exit_generator(&mut self, _frame_at: usize, _name: &str, _stack: &[Value]) {}
+
+    /// Called when the runtime suspends a generator frame.
+    fn observe_suspend_generator(&mut self, _frame_at: usize, _name: &str, _stack: &[Value]) {}
+
+    /// Called when a generator requests an action from the VM.
+    fn observe_generator_request(&mut self, _name: &str, _msg: &VMRequest) {}
+
+    /// Called when the runtime replaces the current call frame for a
+    /// tail call.
+    fn observe_tail_call(&mut self, _frame_at: usize, _: &Rc<Lambda>) {}
+
+    /// Called when the runtime enters a builtin.
+    fn observe_enter_builtin(&mut self, _name: &'static str) {}
+
+    /// Called when the runtime exits a builtin.
+    fn observe_exit_builtin(&mut self, _name: &'static str, _stack: &[Value]) {}
+
+    /// Called when the runtime *begins* executing an instruction. The
+    /// provided stack is the state at the beginning of the operation.
+    fn observe_execute_op(&mut self, _ip: CodeIdx, _: &OpCode, _: &[Value]) {}
+}
+
+#[derive(Default)]
+pub struct NoOpObserver {}
+
+impl CompilerObserver for NoOpObserver {}
+impl RuntimeObserver for NoOpObserver {}
+
+/// An observer that prints disassembled chunk information to its
+/// internal writer whenwever the compiler emits a toplevel function,
+/// closure or thunk.
+pub struct DisassemblingObserver<W: Write> {
+    source: SourceCode,
+    writer: TabWriter<W>,
+}
+
+impl<W: Write> DisassemblingObserver<W> {
+    pub fn new(source: SourceCode, writer: W) -> Self {
+        Self {
+            source,
+            writer: TabWriter::new(writer),
+        }
+    }
+
+    fn lambda_header(&mut self, kind: &str, lambda: &Rc<Lambda>) {
+        let _ = writeln!(
+            &mut self.writer,
+            "=== compiled {} @ {:p} ({} ops) ===",
+            kind,
+            *lambda,
+            lambda.chunk.code.len()
+        );
+    }
+
+    fn disassemble_chunk(&mut self, chunk: &Chunk) {
+        // calculate width of the widest address in the chunk
+        let width = format!("{:#x}", chunk.code.len() - 1).len();
+
+        for (idx, _) in chunk.code.iter().enumerate() {
+            let _ = chunk.disassemble_op(&mut self.writer, &self.source, width, CodeIdx(idx));
+        }
+    }
+}
+
+impl<W: Write> CompilerObserver for DisassemblingObserver<W> {
+    fn observe_compiled_toplevel(&mut self, lambda: &Rc<Lambda>) {
+        self.lambda_header("toplevel", lambda);
+        self.disassemble_chunk(&lambda.chunk);
+        let _ = self.writer.flush();
+    }
+
+    fn observe_compiled_lambda(&mut self, lambda: &Rc<Lambda>) {
+        self.lambda_header("lambda", lambda);
+        self.disassemble_chunk(&lambda.chunk);
+        let _ = self.writer.flush();
+    }
+
+    fn observe_compiled_thunk(&mut self, lambda: &Rc<Lambda>) {
+        self.lambda_header("thunk", lambda);
+        self.disassemble_chunk(&lambda.chunk);
+        let _ = self.writer.flush();
+    }
+}
+
+/// An observer that collects a textual representation of an entire
+/// runtime execution.
+pub struct TracingObserver<W: Write> {
+    // If timing is enabled, contains the timestamp of the last-emitted trace event
+    last_event: Option<Instant>,
+    writer: TabWriter<W>,
+}
+
+impl<W: Write> TracingObserver<W> {
+    pub fn new(writer: W) -> Self {
+        Self {
+            last_event: None,
+            writer: TabWriter::new(writer),
+        }
+    }
+
+    /// Write the time of each runtime event, relative to when this method is called
+    pub fn enable_timing(&mut self) {
+        self.last_event = Some(Instant::now());
+    }
+
+    fn maybe_write_time(&mut self) {
+        if let Some(last_event) = &mut self.last_event {
+            let _ = write!(&mut self.writer, "+{}ns\t", last_event.elapsed().as_nanos());
+            *last_event = Instant::now();
+        }
+    }
+
+    fn write_value(&mut self, val: &Value) {
+        let _ = match val {
+            // Potentially large types which we only want to print
+            // the type of (and avoid recursing).
+            Value::List(l) => write!(&mut self.writer, "list[{}] ", l.len()),
+            Value::Attrs(a) => write!(&mut self.writer, "attrs[{}] ", a.len()),
+            Value::Thunk(t) if t.is_evaluated() => {
+                self.write_value(&t.value());
+                Ok(())
+            }
+
+            // For other value types, defer to the standard value printer.
+            _ => write!(&mut self.writer, "{} ", val),
+        };
+    }
+
+    fn write_stack(&mut self, stack: &[Value]) {
+        let _ = write!(&mut self.writer, "[ ");
+
+        // Print out a maximum of 6 values from the top of the stack,
+        // before abbreviating it to `...`.
+        for (i, val) in stack.iter().rev().enumerate() {
+            if i == 6 {
+                let _ = write!(&mut self.writer, "...");
+                break;
+            }
+
+            self.write_value(val);
+        }
+
+        let _ = writeln!(&mut self.writer, "]");
+    }
+}
+
+impl<W: Write> RuntimeObserver for TracingObserver<W> {
+    fn observe_enter_call_frame(
+        &mut self,
+        arg_count: usize,
+        lambda: &Rc<Lambda>,
+        call_depth: usize,
+    ) {
+        self.maybe_write_time();
+
+        let _ = write!(&mut self.writer, "=== entering ");
+
+        let _ = if arg_count == 0 {
+            write!(&mut self.writer, "thunk ")
+        } else {
+            write!(&mut self.writer, "closure ")
+        };
+
+        if let Some(name) = &lambda.name {
+            let _ = write!(&mut self.writer, "'{}' ", name);
+        }
+
+        let _ = writeln!(
+            &mut self.writer,
+            "in frame[{}] @ {:p} ===",
+            call_depth, *lambda
+        );
+    }
+
+    /// Called when the runtime exits a call frame.
+    fn observe_exit_call_frame(&mut self, frame_at: usize, stack: &[Value]) {
+        self.maybe_write_time();
+        let _ = write!(&mut self.writer, "=== exiting frame {} ===\t ", frame_at);
+        self.write_stack(stack);
+    }
+
+    fn observe_suspend_call_frame(&mut self, frame_at: usize, stack: &[Value]) {
+        self.maybe_write_time();
+        let _ = write!(&mut self.writer, "=== suspending frame {} ===\t", frame_at);
+
+        self.write_stack(stack);
+    }
+
+    fn observe_enter_generator(&mut self, frame_at: usize, name: &str, stack: &[Value]) {
+        self.maybe_write_time();
+        let _ = write!(
+            &mut self.writer,
+            "=== entering generator frame '{}' [{}] ===\t",
+            name, frame_at,
+        );
+
+        self.write_stack(stack);
+    }
+
+    fn observe_exit_generator(&mut self, frame_at: usize, name: &str, stack: &[Value]) {
+        self.maybe_write_time();
+        let _ = write!(
+            &mut self.writer,
+            "=== exiting generator '{}' [{}] ===\t",
+            name, frame_at
+        );
+
+        self.write_stack(stack);
+    }
+
+    fn observe_suspend_generator(&mut self, frame_at: usize, name: &str, stack: &[Value]) {
+        self.maybe_write_time();
+        let _ = write!(
+            &mut self.writer,
+            "=== suspending generator '{}' [{}] ===\t",
+            name, frame_at
+        );
+
+        self.write_stack(stack);
+    }
+
+    fn observe_generator_request(&mut self, name: &str, msg: &VMRequest) {
+        self.maybe_write_time();
+        let _ = writeln!(
+            &mut self.writer,
+            "=== generator '{}' requested {} ===",
+            name, msg
+        );
+    }
+
+    fn observe_enter_builtin(&mut self, name: &'static str) {
+        self.maybe_write_time();
+        let _ = writeln!(&mut self.writer, "=== entering builtin {} ===", name);
+    }
+
+    fn observe_exit_builtin(&mut self, name: &'static str, stack: &[Value]) {
+        self.maybe_write_time();
+        let _ = write!(&mut self.writer, "=== exiting builtin {} ===\t", name);
+        self.write_stack(stack);
+    }
+
+    fn observe_tail_call(&mut self, frame_at: usize, lambda: &Rc<Lambda>) {
+        self.maybe_write_time();
+        let _ = writeln!(
+            &mut self.writer,
+            "=== tail-calling {:p} in frame[{}] ===",
+            *lambda, frame_at
+        );
+    }
+
+    fn observe_execute_op(&mut self, ip: CodeIdx, op: &OpCode, stack: &[Value]) {
+        self.maybe_write_time();
+        let _ = write!(&mut self.writer, "{:04} {:?}\t", ip.0, op);
+        self.write_stack(stack);
+    }
+}
+
+impl<W: Write> Drop for TracingObserver<W> {
+    fn drop(&mut self) {
+        let _ = self.writer.flush();
+    }
+}
diff --git a/tvix/eval/src/opcode.rs b/tvix/eval/src/opcode.rs
index 622a02ac85..f89c1c12e7 100644
--- a/tvix/eval/src/opcode.rs
+++ b/tvix/eval/src/opcode.rs
@@ -1,42 +1,284 @@
 //! This module implements the instruction set running on the abstract
 //! machine implemented by tvix.
 
-#[derive(Clone, Copy, Debug)]
+use std::ops::{AddAssign, Sub};
+
+/// Index of a constant in the current code chunk.
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct ConstantIdx(pub usize);
 
+/// Index of an instruction in the current code chunk.
+#[repr(transparent)]
 #[derive(Clone, Copy, Debug)]
 pub struct CodeIdx(pub usize);
 
-#[derive(Clone, Copy, Debug)]
+impl AddAssign<usize> for CodeIdx {
+    fn add_assign(&mut self, rhs: usize) {
+        *self = CodeIdx(self.0 + rhs)
+    }
+}
+
+impl Sub<usize> for CodeIdx {
+    type Output = Self;
+
+    fn sub(self, rhs: usize) -> Self::Output {
+        CodeIdx(self.0 - rhs)
+    }
+}
+
+/// Index of a value in the runtime stack.  This is an offset
+/// *relative to* the VM value stack_base of the CallFrame
+/// containing the opcode which contains this StackIdx.
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd)]
+pub struct StackIdx(pub usize);
+
+/// Index of an upvalue within a closure's bound-variable upvalue
+/// list.  This is an absolute index into the Upvalues of the
+/// CallFrame containing the opcode which contains this UpvalueIdx.
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct UpvalueIdx(pub usize);
+
+/// Offset by which an instruction pointer should change in a jump.
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct JumpOffset(pub usize);
+
+/// Provided count for an instruction (could represent e.g. a number
+/// of elements).
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Count(pub usize);
+
+/// All variants of this enum carry a bounded amount of data to
+/// ensure that no heap allocations are needed for an Opcode.
+///
+/// In documentation comments, stack positions are referred to by
+/// indices written in `{}` as such, where required:
+///
+/// ```notrust
+///                             --- top of the stack
+///                            /
+///                           v
+///       [ ... | 3 | 2 | 1 | 0 ]
+///                   ^
+///                  /
+/// 2 values deep ---
+/// ```
+///
+/// Unless otherwise specified, operations leave their result at the
+/// top of the stack.
+#[warn(variant_size_differences)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum OpCode {
-    // Push a constant onto the stack.
+    /// Push a constant onto the stack.
     OpConstant(ConstantIdx),
 
-    // Push a literal value.
-    OpNull,
-    OpTrue,
-    OpFalse,
-
     // Unary operators
+    /// Discard a value from the stack.
+    OpPop,
+
+    /// Invert the boolean at the top of the stack.
     OpInvert,
+
+    // Binary operators
+    /// Invert the sign of the number at the top of the stack.
     OpNegate,
 
-    // Arithmetic binary operators
+    /// Sum up the two numbers at the top of the stack.
     OpAdd,
+
+    /// Subtract the number at {1} from the number at {2}.
     OpSub,
+
+    /// Multiply the two numbers at the top of the stack.
     OpMul,
+
+    /// Divide the two numbers at the top of the stack.
     OpDiv,
 
-    // Logical binary operators
+    // Comparison operators
+    /// Check the two values at the top of the stack for Nix-equality.
     OpEqual,
 
+    /// Check whether the value at {2} is less than {1}.
+    OpLess,
+
+    /// Check whether the value at {2} is less than or equal to {1}.
+    OpLessOrEq,
+
+    /// Check whether the value at {2} is greater than {1}.
+    OpMore,
+
+    /// Check whether the value at {2} is greater than or equal to {1}.
+    OpMoreOrEq,
+
+    // Logical operators & generic jumps
+    /// Jump forward in the bytecode specified by the number of
+    /// instructions in its usize operand.
+    OpJump(JumpOffset),
+
+    /// Jump forward in the bytecode specified by the number of
+    /// instructions in its usize operand, *if* the value at the top
+    /// of the stack is `true`.
+    OpJumpIfTrue(JumpOffset),
+
+    /// Jump forward in the bytecode specified by the number of
+    /// instructions in its usize operand, *if* the value at the top
+    /// of the stack is `false`.
+    OpJumpIfFalse(JumpOffset),
+
+    /// Pop one stack item and jump forward in the bytecode
+    /// specified by the number of instructions in its usize
+    /// operand, *if* the value at the top of the stack is a
+    /// Value::Catchable.
+    OpJumpIfCatchable(JumpOffset),
+
+    /// Jump forward in the bytecode specified by the number of
+    /// instructions in its usize operand, *if* the value at the top
+    /// of the stack is the internal value representing a missing
+    /// attribute set key.
+    OpJumpIfNotFound(JumpOffset),
+
+    /// Jump forward in the bytecode specified by the number of
+    /// instructions in its usize operand, *if* the value at the top
+    /// of the stack is *not* the internal value requesting a
+    /// stack value finalisation.
+    OpJumpIfNoFinaliseRequest(JumpOffset),
+
     // Attribute sets
-    OpAttrs(usize),
-    OpAttrPath(usize),
+    /// Construct an attribute set from the given number of key-value pairs on the top of the stack
+    ///
+    /// Note that this takes the count of *pairs*, not the number of *stack values* - the actual
+    /// number of values popped off the stack will be twice the argument to this op
+    OpAttrs(Count),
+    /// Merge the attribute set at {2} into the attribute set at {1},
+    /// and leave the new set at the top of the stack.
+    OpAttrsUpdate,
+
+    /// Select the attribute with the name at {1} from the set at {2}.
+    OpAttrsSelect,
+
+    /// Select the attribute with the name at {1} from the set at {2}, but leave
+    /// a `Value::AttrNotFound` in the stack instead of failing if it is
+    /// missing.
+    OpAttrsTrySelect,
+
+    /// Check for the presence of the attribute with the name at {1} in the set
+    /// at {2}.
+    OpHasAttr,
+
+    /// Throw an error if the attribute set at the top of the stack has any attributes
+    /// other than those listed in the formals of the current lambda
+    ///
+    /// Panics if the current frame is not a lambda with formals
+    OpValidateClosedFormals,
+
+    // `with`-handling
+    /// Push a value onto the runtime `with`-stack to enable dynamic identifier
+    /// resolution. The absolute stack index of the value is supplied as a usize
+    /// operand.
+    OpPushWith(StackIdx),
+
+    /// Pop the last runtime `with`-stack element.
+    OpPopWith,
+
+    /// Dynamically resolve an identifier with the name at {1} from the runtime
+    /// `with`-stack.
+    OpResolveWith,
 
     // Lists
-    OpList(usize),
+    /// Construct a list from the given number of values at the top of the
+    /// stack.
+    OpList(Count),
+
+    /// Concatenate the lists at {2} and {1}.
+    OpConcat,
 
     // Strings
-    OpInterpolate(usize),
+    /// Interpolate the given number of string fragments into a single string.
+    OpInterpolate(Count),
+
+    /// Force the Value on the stack and coerce it to a string
+    OpCoerceToString(crate::CoercionKind),
+
+    // Paths
+    /// Attempt to resolve the Value on the stack using the configured [`NixSearchPath`][]
+    ///
+    /// [`NixSearchPath`]: crate::nix_search_path::NixSearchPath
+    OpFindFile,
+
+    /// Attempt to resolve a path literal relative to the home dir
+    OpResolveHomePath,
+
+    // Type assertion operators
+    /// Assert that the value at {1} is a boolean, and fail with a runtime error
+    /// otherwise.
+    OpAssertBool,
+    OpAssertAttrs,
+
+    /// Access local identifiers with statically known positions.
+    OpGetLocal(StackIdx),
+
+    /// Close scopes while leaving their expression value around.
+    OpCloseScope(Count), // number of locals to pop
+
+    /// Return an error indicating that an `assert` failed
+    OpAssertFail,
+
+    // Lambdas & closures
+    /// Call the value at {1} in a new VM callframe
+    OpCall,
+
+    /// Retrieve the upvalue at the given index from the closure or thunk
+    /// currently under evaluation.
+    OpGetUpvalue(UpvalueIdx),
+
+    /// Construct a closure which has upvalues but no self-references
+    OpClosure(ConstantIdx),
+
+    /// Construct a closure which has self-references (direct or via upvalues)
+    OpThunkClosure(ConstantIdx),
+
+    /// Construct a suspended thunk, used to delay a computation for laziness.
+    OpThunkSuspended(ConstantIdx),
+
+    /// Force the value at {1} until it is a `Thunk::Evaluated`.
+    OpForce,
+
+    /// Finalise initialisation of the upvalues of the value in the given stack
+    /// index (which must be a Value::Thunk) after the scope is fully bound.
+    OpFinalise(StackIdx),
+
+    /// Final instruction emitted in a chunk. Does not have an
+    /// inherent effect, but can simplify VM logic as a marker in some
+    /// cases.
+    ///
+    /// Can be thought of as "returning" the value to the parent
+    /// frame, hence the name.
+    OpReturn,
+
+    // [`OpClosure`], [`OpThunkSuspended`], and [`OpThunkClosure`] have a
+    // variable number of arguments to the instruction, which is
+    // represented here by making their data part of the opcodes.
+    // Each of these two opcodes has a `ConstantIdx`, which must
+    // reference a `Value::Blueprint(Lambda)`.  The `upvalue_count`
+    // field in that `Lambda` indicates the number of arguments it
+    // takes, and the opcode must be followed by exactly this number
+    // of `Data*` opcodes.  The VM skips over these by advancing the
+    // instruction pointer.
+    //
+    // It is illegal for a `Data*` opcode to appear anywhere else.
+    /// Populate a static upvalue by copying from the stack immediately.
+    DataStackIdx(StackIdx),
+    /// Populate a static upvalue of a thunk by copying it the stack, but do
+    /// when the thunk is finalised (by OpFinalise) rather than immediately.
+    DataDeferredLocal(StackIdx),
+    /// Populate a static upvalue by copying it from the upvalues of an
+    /// enclosing scope.
+    DataUpvalueIdx(UpvalueIdx),
+    /// Populate dynamic upvalues by saving a copy of the with-stack.
+    DataCaptureWith,
 }
diff --git a/tvix/eval/src/pretty_ast.rs b/tvix/eval/src/pretty_ast.rs
new file mode 100644
index 0000000000..5ac115e21c
--- /dev/null
+++ b/tvix/eval/src/pretty_ast.rs
@@ -0,0 +1,468 @@
+//! Pretty-printed format for the rnix AST representation.
+//!
+//! The AST is serialised into a JSON structure that can then be
+//! printed in either minimised or well-formatted style.
+
+use rnix::ast::{self, AstToken, HasEntry};
+use serde::{ser::SerializeMap, Serialize, Serializer};
+
+pub fn pretty_print_expr(expr: &ast::Expr) -> String {
+    serde_json::ser::to_string_pretty(&SerializeAST(expr))
+        .expect("serializing AST should always succeed")
+}
+
+#[repr(transparent)]
+struct SerializeAST<S>(S);
+
+impl<'a> Serialize for SerializeAST<&'a ast::Apply> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "apply")?;
+        map.serialize_entry("fn", &SerializeAST(&self.0.lambda().unwrap()))?;
+        map.serialize_entry("arg", &SerializeAST(&self.0.argument().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Assert> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "assert")?;
+        map.serialize_entry("condition", &SerializeAST(&self.0.condition().unwrap()))?;
+        map.serialize_entry("body", &SerializeAST(&self.0.body().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Error> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "error")?;
+        map.serialize_entry("node", &self.0.to_string())?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::IfElse> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(4))?;
+        map.serialize_entry("kind", "if_else")?;
+        map.serialize_entry("condition", &SerializeAST(&self.0.condition().unwrap()))?;
+        map.serialize_entry("then_body", &SerializeAST(&self.0.body().unwrap()))?;
+        map.serialize_entry("else_body", &SerializeAST(&self.0.else_body().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Select> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let size = match self.0.default_expr() {
+            Some(_) => 4,
+            None => 3,
+        };
+
+        let mut map = serializer.serialize_map(Some(size))?;
+        map.serialize_entry("kind", "select")?;
+        map.serialize_entry("set", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.serialize_entry("path", &SerializeAST(self.0.attrpath().unwrap()))?;
+
+        if let Some(default) = self.0.default_expr() {
+            map.serialize_entry("default", &SerializeAST(&default))?;
+        }
+
+        map.end()
+    }
+}
+
+impl Serialize for SerializeAST<ast::InterpolPart<String>> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match &self.0 {
+            ast::InterpolPart::Literal(s) => Serialize::serialize(s, serializer),
+            ast::InterpolPart::Interpolation(node) => {
+                Serialize::serialize(&SerializeAST(&node.expr().unwrap()), serializer)
+            }
+        }
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Str> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "string")?;
+
+        map.serialize_entry(
+            "parts",
+            &self
+                .0
+                .normalized_parts()
+                .into_iter()
+                .map(SerializeAST)
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl Serialize for SerializeAST<ast::InterpolPart<ast::PathContent>> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match &self.0 {
+            ast::InterpolPart::Literal(p) => Serialize::serialize(p.syntax().text(), serializer),
+            ast::InterpolPart::Interpolation(node) => {
+                Serialize::serialize(&SerializeAST(&node.expr().unwrap()), serializer)
+            }
+        }
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Path> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "path")?;
+
+        map.serialize_entry(
+            "parts",
+            &self.0.parts().map(SerializeAST).collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Literal> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "literal")?;
+
+        match self.0.kind() {
+            ast::LiteralKind::Float(val) => map.serialize_entry("float", &val.value().unwrap()),
+            ast::LiteralKind::Integer(val) => map.serialize_entry("int", &val.value().unwrap()),
+            ast::LiteralKind::Uri(val) => map.serialize_entry("uri", val.syntax().text()),
+        }?;
+
+        map.end()
+    }
+}
+
+impl Serialize for SerializeAST<ast::PatEntry> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(None)?;
+        map.serialize_entry("ident", &SerializeAST(&self.0.ident().unwrap()))?;
+
+        if let Some(default) = self.0.default() {
+            map.serialize_entry("default", &SerializeAST(&default))?;
+        }
+
+        map.end()
+    }
+}
+
+impl Serialize for SerializeAST<ast::Param> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match &self.0 {
+            ast::Param::Pattern(pat) => {
+                let mut map = serializer.serialize_map(None)?;
+                map.serialize_entry("kind", "formals")?;
+
+                map.serialize_entry(
+                    "entries",
+                    &pat.pat_entries().map(SerializeAST).collect::<Vec<_>>(),
+                )?;
+
+                if let Some(bind) = pat.pat_bind() {
+                    map.serialize_entry("bind", &SerializeAST(&bind.ident().unwrap()))?;
+                }
+
+                map.serialize_entry("ellipsis", &pat.ellipsis_token().is_some())?;
+
+                map.end()
+            }
+
+            ast::Param::IdentParam(node) => {
+                Serialize::serialize(&SerializeAST(&node.ident().unwrap()), serializer)
+            }
+        }
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Lambda> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "lambda")?;
+        map.serialize_entry("param", &SerializeAST(self.0.param().unwrap()))?;
+        map.serialize_entry("body", &SerializeAST(self.0.body().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::LegacyLet> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "legacy_let")?;
+
+        map.serialize_entry(
+            "entries",
+            &self
+                .0
+                .attrpath_values()
+                .map(SerializeAST)
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.serialize_entry(
+            "inherits",
+            &self.0.inherits().map(SerializeAST).collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::LetIn> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "let")?;
+
+        map.serialize_entry(
+            "entries",
+            &self
+                .0
+                .attrpath_values()
+                .map(SerializeAST)
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.serialize_entry(
+            "inherits",
+            &self.0.inherits().map(SerializeAST).collect::<Vec<_>>(),
+        )?;
+
+        map.serialize_entry("body", &SerializeAST(&self.0.body().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::List> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let list = self.0.items().map(SerializeAST).collect::<Vec<_>>();
+
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "list")?;
+        map.serialize_entry("items", &list)?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::BinOp> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(4))?;
+        map.serialize_entry("kind", "binary_op")?;
+
+        map.serialize_entry(
+            "operator",
+            match self.0.operator().unwrap() {
+                ast::BinOpKind::Concat => "concat",
+                ast::BinOpKind::Update => "update",
+                ast::BinOpKind::Add => "add",
+                ast::BinOpKind::Sub => "sub",
+                ast::BinOpKind::Mul => "mul",
+                ast::BinOpKind::Div => "div",
+                ast::BinOpKind::And => "and",
+                ast::BinOpKind::Equal => "equal",
+                ast::BinOpKind::Implication => "implication",
+                ast::BinOpKind::Less => "less",
+                ast::BinOpKind::LessOrEq => "less_or_eq",
+                ast::BinOpKind::More => "more",
+                ast::BinOpKind::MoreOrEq => "more_or_eq",
+                ast::BinOpKind::NotEqual => "not_equal",
+                ast::BinOpKind::Or => "or",
+            },
+        )?;
+
+        map.serialize_entry("lhs", &SerializeAST(&self.0.lhs().unwrap()))?;
+        map.serialize_entry("rhs", &SerializeAST(&self.0.rhs().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Paren> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "paren")?;
+        map.serialize_entry("expr", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Root> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "root")?;
+        map.serialize_entry("expr", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.end()
+    }
+}
+
+impl Serialize for SerializeAST<ast::AttrpathValue> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("name", &SerializeAST(self.0.attrpath().unwrap()))?;
+        map.serialize_entry("value", &SerializeAST(self.0.value().unwrap()))?;
+        map.end()
+    }
+}
+
+impl Serialize for SerializeAST<ast::Inherit> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(None)?;
+
+        if let Some(from) = self.0.from() {
+            map.serialize_entry("namespace", &SerializeAST(&from.expr().unwrap()))?;
+        }
+
+        map.serialize_entry(
+            "names",
+            &self.0.attrs().map(SerializeAST).collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::AttrSet> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(None)?;
+        map.serialize_entry("kind", "attrset")?;
+        map.serialize_entry("recursive", &self.0.rec_token().is_some())?;
+
+        map.serialize_entry(
+            "entries",
+            &self
+                .0
+                .attrpath_values()
+                .map(SerializeAST)
+                .collect::<Vec<_>>(),
+        )?;
+
+        map.serialize_entry(
+            "inherits",
+            &self.0.inherits().map(SerializeAST).collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::UnaryOp> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "unary_op")?;
+
+        map.serialize_entry(
+            "operator",
+            match self.0.operator().unwrap() {
+                ast::UnaryOpKind::Invert => "invert",
+                ast::UnaryOpKind::Negate => "negate",
+            },
+        )?;
+
+        map.serialize_entry("expr", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Ident> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "ident")?;
+        map.serialize_entry("ident", self.0.ident_token().unwrap().text())?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::With> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "with")?;
+        map.serialize_entry("with", &SerializeAST(&self.0.namespace().unwrap()))?;
+        map.serialize_entry("body", &SerializeAST(&self.0.body().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Dynamic> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "dynamic")?;
+        map.serialize_entry("expr", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.end()
+    }
+}
+
+impl Serialize for SerializeAST<ast::Attr> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match &self.0 {
+            ast::Attr::Ident(ident) => Serialize::serialize(&SerializeAST(ident), serializer),
+            ast::Attr::Dynamic(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Attr::Str(node) => Serialize::serialize(&SerializeAST(node), serializer),
+        }
+    }
+}
+
+impl Serialize for SerializeAST<ast::Attrpath> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(2))?;
+        map.serialize_entry("kind", "attrpath")?;
+
+        map.serialize_entry(
+            "path",
+            &self.0.attrs().map(SerializeAST).collect::<Vec<_>>(),
+        )?;
+
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::HasAttr> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut map = serializer.serialize_map(Some(3))?;
+        map.serialize_entry("kind", "has_attr")?;
+        map.serialize_entry("expr", &SerializeAST(&self.0.expr().unwrap()))?;
+        map.serialize_entry("attrpath", &SerializeAST(self.0.attrpath().unwrap()))?;
+        map.end()
+    }
+}
+
+impl<'a> Serialize for SerializeAST<&'a ast::Expr> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        match self.0 {
+            ast::Expr::Apply(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Assert(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Error(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::IfElse(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Select(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Str(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Path(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Literal(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Lambda(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::LegacyLet(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::LetIn(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::List(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::BinOp(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Paren(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Root(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::AttrSet(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::UnaryOp(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::Ident(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::With(node) => Serialize::serialize(&SerializeAST(node), serializer),
+            ast::Expr::HasAttr(node) => Serialize::serialize(&SerializeAST(node), serializer),
+        }
+    }
+}
+
+impl Serialize for SerializeAST<ast::Expr> {
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        SerializeAST(&self.0).serialize(serializer)
+    }
+}
diff --git a/tvix/eval/src/properties.rs b/tvix/eval/src/properties.rs
new file mode 100644
index 0000000000..45c1cdfce9
--- /dev/null
+++ b/tvix/eval/src/properties.rs
@@ -0,0 +1,164 @@
+//! Macros that generate proptest test suites checking laws of stdlib traits
+
+/// Generate a suite of tests to check the laws of the [`Eq`] impl for the given type
+macro_rules! eq_laws {
+    ($ty: ty) => {
+        eq_laws!(
+            #[strategy(::proptest::arbitrary::any::<$ty>())]
+            $ty,
+            Default::default()
+        );
+    };
+    ($ty: ty, $config: expr) => {
+        eq_laws!(
+            #[strategy(::proptest::arbitrary::any::<$ty>())]
+            $ty,
+            $config
+        );
+    };
+    (#[$meta: meta] $ty: ty, $config: expr) => {
+        #[allow(clippy::eq_op)]
+        mod eq {
+            use test_strategy::proptest;
+
+            use super::*;
+
+            #[proptest($config)]
+            fn reflexive(#[$meta] x: $ty) {
+                assert!(x == x);
+            }
+
+            #[proptest($config)]
+            fn symmetric(#[$meta] x: $ty, #[$meta] y: $ty) {
+                assert_eq!(x == y, y == x);
+            }
+
+            #[proptest($config)]
+            fn transitive(#[$meta] x: $ty, #[$meta] y: $ty, #[$meta] z: $ty) {
+                if x == y && y == z {
+                    assert!(x == z);
+                }
+            }
+        }
+    };
+}
+
+/// Generate a suite of tests to check the laws of the [`Ord`] impl for the given type
+macro_rules! ord_laws {
+    ($ty: ty) => {
+        ord_laws!(
+            #[strategy(::proptest::arbitrary::any::<$ty>())]
+            $ty,
+            Default::default()
+        );
+    };
+    ($ty: ty, $config: expr) => {
+        ord_laws!(
+            #[strategy(::proptest::arbitrary::any::<$ty>())]
+            $ty,
+            $config
+        );
+    };
+    (#[$meta: meta] $ty: ty, $config: expr) => {
+        mod ord {
+            use test_strategy::proptest;
+
+            use super::*;
+
+            #[proptest($config)]
+            fn partial_cmp_matches_cmp(#[$meta] x: $ty, #[$meta] y: $ty) {
+                assert_eq!(x.partial_cmp(&y), Some(x.cmp(&y)));
+            }
+
+            #[proptest($config)]
+            fn dual(#[$meta] x: $ty, #[$meta] y: $ty) {
+                if x < y {
+                    assert!(y > x);
+                }
+                if y < x {
+                    assert!(x > y);
+                }
+            }
+
+            #[proptest($config)]
+            fn le_transitive(#[$meta] x: $ty, #[$meta] y: $ty, #[$meta] z: $ty) {
+                if x < y && y < z {
+                    assert!(x < z)
+                }
+            }
+
+            #[proptest($config)]
+            fn gt_transitive(#[$meta] x: $ty, #[$meta] y: $ty, #[$meta] z: $ty) {
+                if x > y && y > z {
+                    assert!(x > z)
+                }
+            }
+
+            #[proptest($config)]
+            fn trichotomy(#[$meta] x: $ty, #[$meta] y: $ty) {
+                let less = x < y;
+                let greater = x > y;
+                let eq = x == y;
+
+                if less {
+                    assert!(!greater);
+                    assert!(!eq);
+                }
+
+                if greater {
+                    assert!(!less);
+                    assert!(!eq);
+                }
+
+                if eq {
+                    assert!(!less);
+                    assert!(!greater);
+                }
+            }
+        }
+    };
+}
+
+/// Generate a test to check the laws of the [`Hash`] impl for the given type
+macro_rules! hash_laws {
+    ($ty: ty) => {
+        hash_laws!(
+            #[strategy(::proptest::arbitrary::any::<$ty>())]
+            $ty,
+            Default::default()
+        );
+    };
+    ($ty: ty, $config: expr) => {
+        hash_laws!(
+            #[strategy(::proptest::arbitrary::any::<$ty>())]
+            $ty,
+            $config
+        );
+    };
+    (#[$meta: meta] $ty: ty, $config: expr) => {
+        mod hash {
+            use test_strategy::proptest;
+
+            use super::*;
+
+            #[proptest($config)]
+            fn matches_eq(#[$meta] x: $ty, #[$meta] y: $ty) {
+                let hash = |x: &$ty| {
+                    use std::hash::Hasher;
+
+                    let mut hasher = ::std::collections::hash_map::DefaultHasher::new();
+                    x.hash(&mut hasher);
+                    hasher.finish()
+                };
+
+                if x == y {
+                    assert_eq!(hash(&x), hash(&y));
+                }
+            }
+        }
+    };
+}
+
+pub(crate) use eq_laws;
+pub(crate) use hash_laws;
+pub(crate) use ord_laws;
diff --git a/tvix/eval/src/source.rs b/tvix/eval/src/source.rs
new file mode 100644
index 0000000000..5a7f10abb8
--- /dev/null
+++ b/tvix/eval/src/source.rs
@@ -0,0 +1,65 @@
+//! This module contains utilities for dealing with the codemap that
+//! needs to be carried across different compiler instantiations in an
+//! evaluation.
+//!
+//! The data type `SourceCode` should be carried through all relevant
+//! places instead of copying the codemap structures directly.
+
+use std::{
+    cell::{Ref, RefCell, RefMut},
+    rc::Rc,
+    sync::Arc,
+};
+
+use codemap::{CodeMap, Span};
+
+/// Tracks all source code in a Tvix evaluation for accurate error
+/// reporting.
+#[derive(Clone, Debug)]
+pub struct SourceCode(Rc<RefCell<CodeMap>>);
+
+impl SourceCode {
+    /// Access a read-only reference to the codemap.
+    pub fn codemap(&self) -> Ref<CodeMap> {
+        self.0.borrow()
+    }
+
+    /// Access a writable reference to the codemap.
+    fn codemap_mut(&self) -> RefMut<CodeMap> {
+        self.0.borrow_mut()
+    }
+
+    /// Add a file to the codemap. The returned Arc is managed by the
+    /// codemap internally and can be used like a normal reference.
+    pub fn add_file(&self, name: String, code: String) -> Arc<codemap::File> {
+        self.codemap_mut().add_file(name, code)
+    }
+
+    /// Retrieve the line number of the given span. If it spans
+    /// multiple lines, the first line will be returned.
+    pub fn get_line(&self, span: Span) -> usize {
+        // lines are 0-indexed in the codemap, but users probably want
+        // real line numbers
+        self.codemap().look_up_span(span).begin.line + 1
+    }
+
+    /// Returns the literal source slice of the given span.
+    pub fn source_slice(&self, span: Span) -> Ref<str> {
+        Ref::map(self.codemap(), |c| {
+            c.find_file(span.low()).source_slice(span)
+        })
+    }
+
+    /// Returns the reference to the file structure that a given span
+    /// is in.
+    pub fn get_file(&self, span: Span) -> Arc<codemap::File> {
+        self.codemap().look_up_span(span).file
+    }
+}
+
+impl Default for SourceCode {
+    /// Create a new SourceCode instance.
+    fn default() -> Self {
+        Self(Rc::new(RefCell::new(CodeMap::new())))
+    }
+}
diff --git a/tvix/eval/src/spans.rs b/tvix/eval/src/spans.rs
new file mode 100644
index 0000000000..f422093b0d
--- /dev/null
+++ b/tvix/eval/src/spans.rs
@@ -0,0 +1,109 @@
+//! Utilities for dealing with span tracking in the compiler and in
+//! error reporting.
+
+use codemap::{File, Span};
+use rnix::ast;
+use rowan::ast::AstNode;
+
+/// Helper struct to carry information required for making a span, but
+/// without actually performing the (expensive) span lookup.
+///
+/// This is used for tracking spans across thunk boundaries, as they
+/// are frequently instantiated but spans are only used in error or
+/// warning cases.
+#[derive(Clone, Debug)]
+pub enum LightSpan {
+    /// The span has already been computed and can just be used right
+    /// away.
+    Actual { span: Span },
+}
+
+impl LightSpan {
+    pub fn new_actual(span: Span) -> Self {
+        Self::Actual { span }
+    }
+
+    pub fn span(&self) -> Span {
+        match self {
+            LightSpan::Actual { span } => *span,
+        }
+    }
+}
+
+impl From<Span> for LightSpan {
+    fn from(span: Span) -> Self {
+        LightSpan::Actual { span }
+    }
+}
+
+/// Trait implemented by all types from which we can retrieve a span.
+pub trait ToSpan {
+    fn span_for(&self, file: &File) -> Span;
+}
+
+impl ToSpan for Span {
+    fn span_for(&self, _: &File) -> Span {
+        *self
+    }
+}
+
+impl ToSpan for rnix::TextRange {
+    fn span_for(&self, file: &File) -> Span {
+        file.span
+            .subspan(u32::from(self.start()) as u64, u32::from(self.end()) as u64)
+    }
+}
+
+impl ToSpan for rnix::SyntaxToken {
+    fn span_for(&self, file: &File) -> Span {
+        self.text_range().span_for(file)
+    }
+}
+
+impl ToSpan for rnix::SyntaxNode {
+    fn span_for(&self, file: &File) -> Span {
+        self.text_range().span_for(file)
+    }
+}
+
+/// Generates a `ToSpan` implementation for a type implementing
+/// `rowan::AstNode`. This is impossible to do as a blanket
+/// implementation because `rustc` forbids these implementations for
+/// traits from third-party crates due to a belief that semantic
+/// versioning truly could work (it doesn't).
+macro_rules! expr_to_span {
+    ( $type:path ) => {
+        impl ToSpan for $type {
+            fn span_for(&self, file: &File) -> Span {
+                self.syntax().span_for(file)
+            }
+        }
+    };
+}
+
+expr_to_span!(ast::Expr);
+expr_to_span!(ast::Apply);
+expr_to_span!(ast::Assert);
+expr_to_span!(ast::Attr);
+expr_to_span!(ast::AttrSet);
+expr_to_span!(ast::Attrpath);
+expr_to_span!(ast::AttrpathValue);
+expr_to_span!(ast::BinOp);
+expr_to_span!(ast::HasAttr);
+expr_to_span!(ast::Ident);
+expr_to_span!(ast::IdentParam);
+expr_to_span!(ast::IfElse);
+expr_to_span!(ast::Inherit);
+expr_to_span!(ast::Interpol);
+expr_to_span!(ast::Lambda);
+expr_to_span!(ast::LegacyLet);
+expr_to_span!(ast::LetIn);
+expr_to_span!(ast::List);
+expr_to_span!(ast::Literal);
+expr_to_span!(ast::PatBind);
+expr_to_span!(ast::Path);
+expr_to_span!(ast::Pattern);
+expr_to_span!(ast::Select);
+expr_to_span!(ast::Str);
+expr_to_span!(ast::UnaryOp);
+expr_to_span!(ast::With);
diff --git a/tvix/eval/src/systems.rs b/tvix/eval/src/systems.rs
new file mode 100644
index 0000000000..16386cb9e0
--- /dev/null
+++ b/tvix/eval/src/systems.rs
@@ -0,0 +1,351 @@
+/// true iff the argument is recognized by cppnix as the second
+/// coordinate of a "nix double"
+fn is_second_coordinate(x: &str) -> bool {
+    matches!(x, "linux" | "darwin" | "netbsd" | "openbsd" | "freebsd")
+}
+
+/// This function takes an llvm triple (which may have three or four
+/// components, separated by dashes) and returns the "best"
+/// approximation as a nix double, where "best" is currently defined
+/// as "however cppnix handles it".
+pub fn llvm_triple_to_nix_double(llvm_triple: &str) -> String {
+    let parts: Vec<&str> = llvm_triple.split('-').collect();
+    let cpu = match parts[0] {
+        "armv6" => "armv6l", // cppnix appends an "l" to armv6
+        "armv7" => "armv7l", // cppnix appends an "l" to armv7
+        x => match x.as_bytes() {
+            [b'i', _, b'8', b'6'] => "i686", // cppnix glob-matches against i*86
+            _ => x,
+        },
+    };
+    let os = match parts[1..] {
+        [_vendor, kernel, _environment] if is_second_coordinate(kernel) => kernel,
+        [_vendor, kernel] if is_second_coordinate(kernel) => kernel,
+        [kernel, _environment] if is_second_coordinate(kernel) => kernel,
+
+        // Rustc uses wasm32-unknown-unknown, which is rejected by
+        // config.sub, for wasm-in-the-browser environments.  Rustc
+        // should be using wasm32-unknown-none, which config.sub
+        // accepts.  Hopefully the rustc people will change their
+        // triple before stabilising this triple.  In the meantime,
+        // we fix it here in order to unbreak tvixbolt.
+        //
+        // https://doc.rust-lang.org/beta/nightly-rustc/rustc_target/spec/wasm32_unknown_unknown/index.html
+        ["unknown", "unknown"] if cpu == "wasm32" => "none",
+
+        _ => panic!("unrecognized triple {llvm_triple}"),
+    };
+    format!("{cpu}-{os}")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_systems() {
+        assert_eq!(
+            llvm_triple_to_nix_double("aarch64-unknown-linux-gnu"),
+            "aarch64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("i686-unknown-linux-gnu"),
+            "i686-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("x86_64-apple-darwin"),
+            "x86_64-darwin"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("x86_64-unknown-linux-gnu"),
+            "x86_64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("aarch64-apple-darwin"),
+            "aarch64-darwin"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("aarch64-unknown-linux-musl"),
+            "aarch64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("arm-unknown-linux-gnueabi"),
+            "arm-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("arm-unknown-linux-gnueabihf"),
+            "arm-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv7-unknown-linux-gnueabihf"),
+            "armv7l-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mips-unknown-linux-gnu"),
+            "mips-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mips64-unknown-linux-gnuabi64"),
+            "mips64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mips64-unknown-linux-gnuabin32"),
+            "mips64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mips64el-unknown-linux-gnuabi64"),
+            "mips64el-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mips64el-unknown-linux-gnuabin32"),
+            "mips64el-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mipsel-unknown-linux-gnu"),
+            "mipsel-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("powerpc-unknown-linux-gnu"),
+            "powerpc-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("powerpc64-unknown-linux-gnu"),
+            "powerpc64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("powerpc64le-unknown-linux-gnu"),
+            "powerpc64le-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("s390x-unknown-linux-gnu"),
+            "s390x-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("x86_64-unknown-linux-musl"),
+            "x86_64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("x86_64-unknown-netbsd"),
+            "x86_64-netbsd"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("aarch64-linux-android"),
+            "aarch64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("arm-linux-androideabi"),
+            "arm-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("arm-unknown-linux-musleabi"),
+            "arm-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("arm-unknown-linux-musleabihf"),
+            "arm-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv5te-unknown-linux-gnueabi"),
+            "armv5te-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv5te-unknown-linux-musleabi"),
+            "armv5te-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv7-linux-androideabi"),
+            "armv7l-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv7-unknown-linux-gnueabi"),
+            "armv7l-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv7-unknown-linux-musleabi"),
+            "armv7l-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv7-unknown-linux-musleabihf"),
+            "armv7l-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("i586-unknown-linux-gnu"),
+            "i686-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("i586-unknown-linux-musl"),
+            "i686-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("i686-linux-android"),
+            "i686-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("i686-unknown-linux-musl"),
+            "i686-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mips-unknown-linux-musl"),
+            "mips-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mips64-unknown-linux-muslabi64"),
+            "mips64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mips64el-unknown-linux-muslabi64"),
+            "mips64el-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mipsel-unknown-linux-musl"),
+            "mipsel-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("sparc64-unknown-linux-gnu"),
+            "sparc64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("thumbv7neon-linux-androideabi"),
+            "thumbv7neon-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("thumbv7neon-unknown-linux-gnueabihf"),
+            "thumbv7neon-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("x86_64-linux-android"),
+            "x86_64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("x86_64-unknown-linux-gnux32"),
+            "x86_64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("aarch64-unknown-linux-gnu_ilp32"),
+            "aarch64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("aarch64-unknown-netbsd"),
+            "aarch64-netbsd"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("aarch64_be-unknown-linux-gnu_ilp32"),
+            "aarch64_be-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("aarch64_be-unknown-linux-gnu"),
+            "aarch64_be-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armeb-unknown-linux-gnueabi"),
+            "armeb-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv4t-unknown-linux-gnueabi"),
+            "armv4t-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv6-unknown-netbsd-eabihf"),
+            "armv6l-netbsd"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv7-unknown-linux-uclibceabi"),
+            "armv7l-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv7-unknown-linux-uclibceabihf"),
+            "armv7l-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("armv7-unknown-netbsd-eabihf"),
+            "armv7l-netbsd"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("hexagon-unknown-linux-musl"),
+            "hexagon-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("i686-unknown-netbsd"),
+            "i686-netbsd"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("m68k-unknown-linux-gnu"),
+            "m68k-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mips-unknown-linux-uclibc"),
+            "mips-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mips64-openwrt-linux-musl"),
+            "mips64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mipsel-unknown-linux-uclibc"),
+            "mipsel-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mipsisa32r6-unknown-linux-gnu"),
+            "mipsisa32r6-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mipsisa32r6el-unknown-linux-gnu"),
+            "mipsisa32r6el-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mipsisa64r6-unknown-linux-gnuabi64"),
+            "mipsisa64r6-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("mipsisa64r6el-unknown-linux-gnuabi64"),
+            "mipsisa64r6el-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("powerpc-unknown-linux-gnuspe"),
+            "powerpc-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("powerpc-unknown-linux-musl"),
+            "powerpc-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("powerpc-unknown-netbsd"),
+            "powerpc-netbsd"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("powerpc64-unknown-linux-musl"),
+            "powerpc64-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("powerpc64le-unknown-linux-musl"),
+            "powerpc64le-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("riscv32gc-unknown-linux-gnu"),
+            "riscv32gc-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("riscv32gc-unknown-linux-musl"),
+            "riscv32gc-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("riscv64gc-unknown-linux-musl"),
+            "riscv64gc-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("s390x-unknown-linux-musl"),
+            "s390x-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("sparc-unknown-linux-gnu"),
+            "sparc-linux"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("sparc64-unknown-netbsd"),
+            "sparc64-netbsd"
+        );
+        assert_eq!(
+            llvm_triple_to_nix_double("thumbv7neon-unknown-linux-musleabihf"),
+            "thumbv7neon-linux"
+        );
+    }
+}
diff --git a/tvix/eval/src/test_utils.rs b/tvix/eval/src/test_utils.rs
new file mode 100644
index 0000000000..a7d1c3f968
--- /dev/null
+++ b/tvix/eval/src/test_utils.rs
@@ -0,0 +1,8 @@
+use codemap::CodeMap;
+
+/// Create a dummy [`codemap::Span`] for use in tests
+pub(crate) fn dummy_span() -> codemap::Span {
+    let mut codemap = CodeMap::new();
+    let file = codemap.add_file("<dummy>".to_owned(), "<dummy>".to_owned());
+    file.span
+}
diff --git a/tvix/eval/src/tests/mod.rs b/tvix/eval/src/tests/mod.rs
new file mode 100644
index 0000000000..5a7708e298
--- /dev/null
+++ b/tvix/eval/src/tests/mod.rs
@@ -0,0 +1,203 @@
+use crate::{value::Value, EvalIO};
+use builtin_macros::builtins;
+use pretty_assertions::assert_eq;
+use rstest::rstest;
+use std::path::PathBuf;
+
+/// Module for one-off tests which do not follow the rest of the
+/// test layout.
+mod one_offs;
+
+#[builtins]
+mod mock_builtins {
+    //! Builtins which are required by language tests, but should not
+    //! actually exist in //tvix/eval.
+    use crate as tvix_eval;
+    use crate::generators::GenCo;
+    use crate::*;
+    use genawaiter::rc::Gen;
+
+    #[builtin("derivation")]
+    async fn builtin_derivation(co: GenCo, input: Value) -> Result<Value, ErrorKind> {
+        let input = input.to_attrs()?;
+        let attrs = input.update(NixAttrs::from_iter(
+            [
+                (
+                    "outPath",
+                    "/nix/store/00000000000000000000000000000000-mock",
+                ),
+                (
+                    "drvPath",
+                    "/nix/store/00000000000000000000000000000000-mock.drv",
+                ),
+                ("type", "derivation"),
+            ]
+            .into_iter(),
+        ));
+
+        Ok(Value::Attrs(Box::new(attrs)))
+    }
+}
+
+fn eval_test(code_path: PathBuf, expect_success: bool) {
+    std::env::set_var("TEST_VAR", "foo"); // for eval-okay-getenv.nix
+
+    eprintln!("path: {}", code_path.display());
+    assert_eq!(
+        code_path.extension().unwrap(),
+        "nix",
+        "test files always end in .nix"
+    );
+
+    let code = std::fs::read_to_string(&code_path).expect("should be able to read test code");
+
+    let mut eval = crate::Evaluation::new_impure();
+    eval.strict = true;
+    eval.builtins.extend(mock_builtins::builtins());
+
+    let result = eval.evaluate(code, Some(code_path.clone()));
+    let failed = match result.value {
+        Some(Value::Catchable(_)) => true,
+        _ => !result.errors.is_empty(),
+    };
+    if expect_success && failed {
+        panic!(
+            "{}: evaluation of eval-okay test should succeed, but failed with {:?}",
+            code_path.display(),
+            result.errors,
+        );
+    }
+
+    if !expect_success && failed {
+        return;
+    }
+    // !expect_success can also mean the output differs, so don't panic if the
+    // evaluation didn't fail.
+
+    let value = result.value.unwrap();
+    let result_str = value.to_string();
+
+    let exp_path = code_path.with_extension("exp");
+    if exp_path.exists() {
+        // If there's an .exp file provided alongside, compare it with the
+        // output of the NixValue .to_string() method.
+        let exp_str = std::fs::read_to_string(&exp_path).expect("unable to read .exp file");
+
+        if expect_success {
+            assert_eq!(
+                result_str,
+                exp_str.trim(),
+                "{}: result value representation (left) must match expectation (right)",
+                code_path.display()
+            );
+        } else {
+            assert_ne!(
+                result_str,
+                exp_str.trim(),
+                "{}: test passed unexpectedly!  consider moving it out of notyetpassing",
+                code_path.display()
+            );
+
+            // Early return here, we don't compare .xml outputs if this is a !
+            // expect_success test.
+            return;
+        }
+    }
+
+    let exp_xml_path = code_path.with_extension("exp.xml");
+    if exp_xml_path.exists() {
+        // If there's an XML file provided alongside, compare it with the
+        // output produced when serializing the Value as XML.
+        let exp_xml_str = std::fs::read_to_string(exp_xml_path).expect("unable to read .xml file");
+
+        let mut xml_actual_buf = Vec::new();
+        crate::builtins::value_to_xml(&mut xml_actual_buf, &value).expect("value_to_xml failed");
+
+        assert_eq!(
+            String::from_utf8(xml_actual_buf).expect("to_xml produced invalid utf-8"),
+            exp_xml_str,
+            "{}: result value representation (left) must match expectation (right)",
+            code_path.display()
+        );
+    }
+}
+
+// identity-* tests contain Nix code snippets which should evaluate to
+// themselves exactly (i.e. literals).
+#[rstest]
+fn identity(#[files("src/tests/tvix_tests/identity-*.nix")] code_path: PathBuf) {
+    let code = std::fs::read_to_string(code_path).expect("should be able to read test code");
+
+    let mut eval = crate::Evaluation::new(Box::new(crate::StdIO) as Box<dyn EvalIO>, false);
+    eval.strict = true;
+
+    let result = eval.evaluate(&code, None);
+    assert!(
+        result.errors.is_empty(),
+        "evaluation of identity test failed: {:?}",
+        result.errors
+    );
+
+    let result_str = result.value.unwrap().to_string();
+
+    assert_eq!(
+        result_str,
+        code.trim(),
+        "result value representation (left) must match expectation (right)"
+    )
+}
+
+// eval-okay-* tests contain a snippet of Nix code, and an expectation
+// of the produced string output of the evaluator.
+//
+// These evaluations are always supposed to succeed, i.e. all snippets
+// are guaranteed to be valid Nix code.
+#[rstest]
+fn eval_okay(#[files("src/tests/tvix_tests/eval-okay-*.nix")] code_path: PathBuf) {
+    eval_test(code_path, true)
+}
+
+// eval-okay-* tests from the original Nix test suite.
+#[cfg(feature = "nix_tests")]
+#[rstest]
+fn nix_eval_okay(#[files("src/tests/nix_tests/eval-okay-*.nix")] code_path: PathBuf) {
+    eval_test(code_path, true)
+}
+
+// eval-okay-* tests from the original Nix test suite which do not yet pass for tvix
+//
+// Eventually there will be none of these left, and this function
+// will disappear :)
+//
+// Please don't submit failing tests unless they're in
+// notyetpassing; this makes the test suite much more useful for
+// regression testing, since there should always be zero non-ignored
+// failing tests.
+#[rstest]
+fn nix_eval_okay_currently_failing(
+    #[files("src/tests/nix_tests/notyetpassing/eval-okay-*.nix")] code_path: PathBuf,
+) {
+    eval_test(code_path, false)
+}
+
+#[rstest]
+fn eval_okay_currently_failing(
+    #[files("src/tests/tvix_tests/notyetpassing/eval-okay-*.nix")] code_path: PathBuf,
+) {
+    eval_test(code_path, false)
+}
+
+// eval-fail-* tests contain a snippet of Nix code, which is
+// expected to fail evaluation.  The exact type of failure
+// (assertion, parse error, etc) is not currently checked.
+#[rstest]
+fn eval_fail(#[files("src/tests/tvix_tests/eval-fail-*.nix")] code_path: PathBuf) {
+    eval_test(code_path, false)
+}
+
+// eval-fail-* tests from the original Nix test suite.
+#[cfg(feature = "nix_tests")]
+#[rstest]
+fn nix_eval_fail(#[files("src/tests/nix_tests/eval-fail-*.nix")] code_path: PathBuf) {
+    eval_test(code_path, false)
+}
diff --git a/tvix/eval/src/tests/nix_tests/README.md b/tvix/eval/src/tests/nix_tests/README.md
new file mode 100644
index 0000000000..357f3547da
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/README.md
@@ -0,0 +1,8 @@
+These test definitions are taken from the Nix 2.3 code base, they can
+be found upstream at:
+
+  https://github.com/NixOS/nix/tree/2.3.16/tests/lang
+
+These tests follow the licensing directions of Nix 2.3 itself:
+
+  https://github.com/NixOS/nix/blob/2.3.16/COPYING
diff --git a/tvix/eval/src/tests/nix_tests/binary-data b/tvix/eval/src/tests/nix_tests/binary-data
new file mode 100644
index 0000000000..06d7405020
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/binary-data
Binary files differdiff --git a/tvix/eval/src/tests/nix_tests/data b/tvix/eval/src/tests/nix_tests/data
new file mode 100644
index 0000000000..257cc5642c
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/data
@@ -0,0 +1 @@
+foo
diff --git a/tvix/eval/src/tests/nix_tests/dir1/a.nix b/tvix/eval/src/tests/nix_tests/dir1/a.nix
new file mode 100644
index 0000000000..231f150c57
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/dir1/a.nix
@@ -0,0 +1 @@
+"a"
diff --git a/tvix/eval/src/tests/nix_tests/dir2/a.nix b/tvix/eval/src/tests/nix_tests/dir2/a.nix
new file mode 100644
index 0000000000..170df520ab
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/dir2/a.nix
@@ -0,0 +1 @@
+"X"
diff --git a/tvix/eval/src/tests/nix_tests/dir2/b.nix b/tvix/eval/src/tests/nix_tests/dir2/b.nix
new file mode 100644
index 0000000000..19010cc35c
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/dir2/b.nix
@@ -0,0 +1 @@
+"b"
diff --git a/tvix/eval/src/tests/nix_tests/dir3/a.nix b/tvix/eval/src/tests/nix_tests/dir3/a.nix
new file mode 100644
index 0000000000..170df520ab
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/dir3/a.nix
@@ -0,0 +1 @@
+"X"
diff --git a/tvix/eval/src/tests/nix_tests/dir3/b.nix b/tvix/eval/src/tests/nix_tests/dir3/b.nix
new file mode 100644
index 0000000000..170df520ab
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/dir3/b.nix
@@ -0,0 +1 @@
+"X"
diff --git a/tvix/eval/src/tests/nix_tests/dir3/c.nix b/tvix/eval/src/tests/nix_tests/dir3/c.nix
new file mode 100644
index 0000000000..cdf158597e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/dir3/c.nix
@@ -0,0 +1 @@
+"c"
diff --git a/tvix/eval/src/tests/nix_tests/dir4/a.nix b/tvix/eval/src/tests/nix_tests/dir4/a.nix
new file mode 100644
index 0000000000..170df520ab
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/dir4/a.nix
@@ -0,0 +1 @@
+"X"
diff --git a/tvix/eval/src/tests/nix_tests/dir4/c.nix b/tvix/eval/src/tests/nix_tests/dir4/c.nix
new file mode 100644
index 0000000000..170df520ab
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/dir4/c.nix
@@ -0,0 +1 @@
+"X"
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-abort.nix b/tvix/eval/src/tests/nix_tests/eval-fail-abort.nix
new file mode 100644
index 0000000000..75c51bceb5
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-abort.nix
@@ -0,0 +1 @@
+if true then abort "this should fail" else 1
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-assert.nix b/tvix/eval/src/tests/nix_tests/eval-fail-assert.nix
new file mode 100644
index 0000000000..3b7a1e8bf0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-assert.nix
@@ -0,0 +1,5 @@
+let {
+  x = arg: assert arg == "y"; 123;
+
+  body = x "x";
+}
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-bad-antiquote-1.nix b/tvix/eval/src/tests/nix_tests/eval-fail-bad-antiquote-1.nix
new file mode 100644
index 0000000000..ffe9c983c2
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-bad-antiquote-1.nix
@@ -0,0 +1 @@
+"${x: x}"
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-bad-antiquote-3.nix b/tvix/eval/src/tests/nix_tests/eval-fail-bad-antiquote-3.nix
new file mode 100644
index 0000000000..65b9d4f505
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-bad-antiquote-3.nix
@@ -0,0 +1 @@
+''${x: x}''
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-blackhole.nix b/tvix/eval/src/tests/nix_tests/eval-fail-blackhole.nix
new file mode 100644
index 0000000000..81133b511c
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-blackhole.nix
@@ -0,0 +1,5 @@
+let {
+  body = x;
+  x = y;
+  y = x;
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-deepseq.nix b/tvix/eval/src/tests/nix_tests/eval-fail-deepseq.nix
new file mode 100644
index 0000000000..9baa49b063
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-deepseq.nix
@@ -0,0 +1 @@
+builtins.deepSeq { x = abort "foo"; } 456
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-foldlStrict-strict-op-application.nix b/tvix/eval/src/tests/nix_tests/eval-fail-foldlStrict-strict-op-application.nix
new file mode 100644
index 0000000000..1620cc76ee
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-foldlStrict-strict-op-application.nix
@@ -0,0 +1,5 @@
+# Tests that the result of applying op is forced even if the value is never used
+builtins.foldl'
+  (_: f: f null)
+  null
+  [ (_: throw "Not the final value, but is still forced!") (_: 23) ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-hashfile-missing.nix b/tvix/eval/src/tests/nix_tests/eval-fail-hashfile-missing.nix
new file mode 100644
index 0000000000..ce098b8238
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-hashfile-missing.nix
@@ -0,0 +1,5 @@
+let
+  paths = [ ./this-file-is-definitely-not-there-7392097 "/and/neither/is/this/37293620" ];
+in
+  toString (builtins.concatLists (map (hash: map (builtins.hashFile hash) paths) ["md5" "sha1" "sha256" "sha512"]))
+
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-missing-arg.nix b/tvix/eval/src/tests/nix_tests/eval-fail-missing-arg.nix
new file mode 100644
index 0000000000..c4be9797c5
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-missing-arg.nix
@@ -0,0 +1 @@
+({x, y, z}: x + y + z) {x = "foo"; z = "bar";}
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-path-slash.nix b/tvix/eval/src/tests/nix_tests/eval-fail-path-slash.nix
new file mode 100644
index 0000000000..8c2e104c78
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-path-slash.nix
@@ -0,0 +1,6 @@
+# Trailing slashes in paths are not allowed.
+# This restriction could be lifted sometime,
+# for example if we make '/' a path concatenation operator.
+# See https://github.com/NixOS/nix/issues/1138
+# and https://nixos.org/nix-dev/2016-June/020829.html
+/nix/store/
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-remove.nix b/tvix/eval/src/tests/nix_tests/eval-fail-remove.nix
new file mode 100644
index 0000000000..539e0eb0a6
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-remove.nix
@@ -0,0 +1,5 @@
+let {
+  attrs = {x = 123; y = 456;};
+
+  body = (removeAttrs attrs ["x"]).x;
+}
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-seq.nix b/tvix/eval/src/tests/nix_tests/eval-fail-seq.nix
new file mode 100644
index 0000000000..cddbbfd326
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-seq.nix
@@ -0,0 +1 @@
+builtins.seq (abort "foo") 2
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-substring.nix b/tvix/eval/src/tests/nix_tests/eval-fail-substring.nix
new file mode 100644
index 0000000000..f37c2bc0a1
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-substring.nix
@@ -0,0 +1 @@
+builtins.substring (builtins.sub 0 1) 1 "x"
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-to-path.nix b/tvix/eval/src/tests/nix_tests/eval-fail-to-path.nix
new file mode 100644
index 0000000000..5e322bc313
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-to-path.nix
@@ -0,0 +1 @@
+builtins.toPath "foo/bar"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-any-all.exp b/tvix/eval/src/tests/nix_tests/eval-okay-any-all.exp
new file mode 100644
index 0000000000..eb273f45b2
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-any-all.exp
@@ -0,0 +1 @@
+[ false false true true true true false true ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-any-all.nix b/tvix/eval/src/tests/nix_tests/eval-okay-any-all.nix
new file mode 100644
index 0000000000..a3f26ea2aa
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-any-all.nix
@@ -0,0 +1,11 @@
+with builtins;
+
+[ (any (x: x == 1) [])
+  (any (x: x == 1) [2 3 4])
+  (any (x: x == 1) [1 2 3 4])
+  (any (x: x == 1) [4 3 2 1])
+  (all (x: x == 1) [])
+  (all (x: x == 1) [1])
+  (all (x: x == 1) [1 2 3])
+  (all (x: x == 1) [1 1 1])
+]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-arithmetic.exp b/tvix/eval/src/tests/nix_tests/eval-okay-arithmetic.exp
new file mode 100644
index 0000000000..5c54d10b7b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-arithmetic.exp
@@ -0,0 +1 @@
+2216
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-arithmetic.nix b/tvix/eval/src/tests/nix_tests/eval-okay-arithmetic.nix
new file mode 100644
index 0000000000..7e9e6a0b66
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-arithmetic.nix
@@ -0,0 +1,59 @@
+with import ./lib.nix;
+
+let {
+
+  /* Supposedly tail recursive version:
+
+  range_ = accum: first: last:
+    if first == last then ([first] ++ accum)
+    else range_ ([first] ++ accum) (builtins.add first 1) last;
+
+  range = range_ [];
+  */
+
+  x = 12;
+
+  err = abort "urgh";
+
+  body = sum
+    [ (sum (range 1 50))
+      (123 + 456)
+      (0 + -10 + -(-11) + -x)
+      (10 - 7 - -2)
+      (10 - (6 - -1))
+      (10 - 1 + 2)
+      (3 * 4 * 5)
+      (56088 / 123 / 2)
+      (3 + 4 * const 5 0 - 6 / id 2)
+
+      (builtins.bitAnd 12 10) # 0b1100 & 0b1010 =  8
+      (builtins.bitOr  12 10) # 0b1100 | 0b1010 = 14
+      (builtins.bitXor 12 10) # 0b1100 ^ 0b1010 =  6
+
+      (if 3 < 7 then 1 else err)
+      (if 7 < 3 then err else 1)
+      (if 3 < 3 then err else 1)
+
+      (if 3 <= 7 then 1 else err)
+      (if 7 <= 3 then err else 1)
+      (if 3 <= 3 then 1 else err)
+
+      (if 3 > 7 then err else 1)
+      (if 7 > 3 then 1 else err)
+      (if 3 > 3 then err else 1)
+
+      (if 3 >= 7 then err else 1)
+      (if 7 >= 3 then 1 else err)
+      (if 3 >= 3 then 1 else err)
+
+      (if 2 > 1 == 1 < 2 then 1 else err)
+      (if 1 + 2 * 3 >= 7 then 1 else err)
+      (if 1 + 2 * 3 < 7 then err else 1)
+
+      # Not integer, but so what.
+      (if "aa" < "ab" then 1 else err)
+      (if "aa" < "aa" then err else 1)
+      (if "foo" < "foobar" then 1 else err)
+    ];
+
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrnames.exp b/tvix/eval/src/tests/nix_tests/eval-okay-attrnames.exp
new file mode 100644
index 0000000000..b4aa387e07
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrnames.exp
@@ -0,0 +1 @@
+"newxfoonewxy"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrnames.nix b/tvix/eval/src/tests/nix_tests/eval-okay-attrnames.nix
new file mode 100644
index 0000000000..e5b26e9f2e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrnames.nix
@@ -0,0 +1,11 @@
+with import ./lib.nix;
+
+let
+
+  attrs = {y = "y"; x = "x"; foo = "foo";} // rec {x = "newx"; bar = x;};
+
+  names = builtins.attrNames attrs;
+
+  values = map (name: builtins.getAttr name attrs) names;
+
+in assert values == builtins.attrValues attrs; concat values
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrs.exp b/tvix/eval/src/tests/nix_tests/eval-okay-attrs.exp
new file mode 100644
index 0000000000..45b0f829eb
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrs.exp
@@ -0,0 +1 @@
+987
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrs.nix b/tvix/eval/src/tests/nix_tests/eval-okay-attrs.nix
new file mode 100644
index 0000000000..810b31a5da
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrs.nix
@@ -0,0 +1,5 @@
+let {
+  as = { x = 123; y = 456; } // { z = 789; } // { z = 987; };
+
+  body = if as ? a then as.a else assert as ? z; as.z;
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrs2.exp b/tvix/eval/src/tests/nix_tests/eval-okay-attrs2.exp
new file mode 100644
index 0000000000..45b0f829eb
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrs2.exp
@@ -0,0 +1 @@
+987
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrs2.nix b/tvix/eval/src/tests/nix_tests/eval-okay-attrs2.nix
new file mode 100644
index 0000000000..9e06b83ac1
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrs2.nix
@@ -0,0 +1,10 @@
+let {
+  as = { x = 123; y = 456; } // { z = 789; } // { z = 987; };
+
+  A = "a";
+  Z = "z";
+
+  body = if builtins.hasAttr A as
+         then builtins.getAttr A as
+         else assert builtins.hasAttr Z as; builtins.getAttr Z as;
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrs3.exp b/tvix/eval/src/tests/nix_tests/eval-okay-attrs3.exp
new file mode 100644
index 0000000000..19de4fdf79
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrs3.exp
@@ -0,0 +1 @@
+"foo 22 80 itchyxac"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrs3.nix b/tvix/eval/src/tests/nix_tests/eval-okay-attrs3.nix
new file mode 100644
index 0000000000..f29de11fe6
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrs3.nix
@@ -0,0 +1,22 @@
+let
+
+  config = 
+    {
+      services.sshd.enable = true;
+      services.sshd.port = 22;
+      services.httpd.port = 80;
+      hostName = "itchy";
+      a.b.c.d.e.f.g.h.i.j.k.l.m.n.o.p.q.r.s.t.u.v.w.x.y.z = "x";
+      foo = {
+        a = "a";
+        b.c = "c";
+      };
+    };
+
+in
+  if config.services.sshd.enable
+  then "foo ${toString config.services.sshd.port} ${toString config.services.httpd.port} ${config.hostName}"
+       + "${config.a.b.c.d.e.f.g.h.i.j.k.l.m.n.o.p.q.r.s.t.u.v.w.x.y.z}"
+       + "${config.foo.a}"
+       + "${config.foo.b.c}"
+  else "bar"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrs4.exp b/tvix/eval/src/tests/nix_tests/eval-okay-attrs4.exp
new file mode 100644
index 0000000000..1851731442
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrs4.exp
@@ -0,0 +1 @@
+[ true false true false false true false false ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrs4.nix b/tvix/eval/src/tests/nix_tests/eval-okay-attrs4.nix
new file mode 100644
index 0000000000..43ec81210f
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrs4.nix
@@ -0,0 +1,7 @@
+let
+
+  as = { x.y.z = 123; a.b.c = 456; };
+
+  bs = null;
+
+in [ (as ? x) (as ? y) (as ? x.y.z) (as ? x.y.z.a) (as ? x.y.a) (as ? a.b.c) (bs ? x) (bs ? x.y.z) ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrs5.exp b/tvix/eval/src/tests/nix_tests/eval-okay-attrs5.exp
new file mode 100644
index 0000000000..ce0430d780
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrs5.exp
@@ -0,0 +1 @@
+[ 123 "foo" 456 456 "foo" "xyzzy" "xyzzy" true ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-attrs5.nix b/tvix/eval/src/tests/nix_tests/eval-okay-attrs5.nix
new file mode 100644
index 0000000000..a4584cd3b3
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-attrs5.nix
@@ -0,0 +1,21 @@
+with import ./lib.nix;
+
+let
+
+  as = { x.y.z = 123; a.b.c = 456; };
+
+  bs = { f-o-o.bar = "foo"; };
+
+  or = x: y: x || y;
+  
+in
+  [ as.x.y.z
+    as.foo or "foo"
+    as.x.y.bla or as.a.b.c
+    as.a.b.c or as.x.y.z
+    as.x.y.bla or bs.f-o-o.bar or "xyzzy"
+    as.x.y.bla or bs.bar.foo or "xyzzy"
+    (123).bla or null.foo or "xyzzy"
+    # Backwards compatibility test.
+    (fold or [] [true false false])
+  ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-1.exp b/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-1.exp
new file mode 100644
index 0000000000..3e754364cc
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-1.exp
@@ -0,0 +1 @@
+"a\nb"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-1.nix b/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-1.nix
new file mode 100644
index 0000000000..7fef3dddd4
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-1.nix
@@ -0,0 +1,2 @@
+"a\
+b"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-2.exp b/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-2.exp
new file mode 100644
index 0000000000..3e754364cc
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-2.exp
@@ -0,0 +1 @@
+"a\nb"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-2.nix b/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-2.nix
new file mode 100644
index 0000000000..35ddf495c6
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-backslash-newline-2.nix
@@ -0,0 +1,2 @@
+''a''\
+b''
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-builtins-add.exp b/tvix/eval/src/tests/nix_tests/eval-okay-builtins-add.exp
new file mode 100644
index 0000000000..0350b518a7
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-builtins-add.exp
@@ -0,0 +1 @@
+[ 5 4 "int" "tt" "float" 4 ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-builtins-add.nix b/tvix/eval/src/tests/nix_tests/eval-okay-builtins-add.nix
new file mode 100644
index 0000000000..c841816222
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-builtins-add.nix
@@ -0,0 +1,8 @@
+[
+(builtins.add 2 3)
+(builtins.add 2 2)
+(builtins.typeOf (builtins.add 2  2))
+("t" + "t")
+(builtins.typeOf (builtins.add 2.0 2))
+(builtins.add 2.0 2)
+]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-builtins.exp b/tvix/eval/src/tests/nix_tests/eval-okay-builtins.exp
new file mode 100644
index 0000000000..0661686d61
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-builtins.exp
@@ -0,0 +1 @@
+/foo
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-builtins.nix b/tvix/eval/src/tests/nix_tests/eval-okay-builtins.nix
new file mode 100644
index 0000000000..e9d65e88a8
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-builtins.nix
@@ -0,0 +1,12 @@
+assert builtins ? currentSystem;
+assert !builtins ? __currentSystem;
+
+let {
+
+  x = if builtins ? dirOf then builtins.dirOf /foo/bar else "";
+
+  y = if builtins ? fnord then builtins.fnord "foo" else "";
+
+  body = x + y;
+  
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-callable-attrs.exp b/tvix/eval/src/tests/nix_tests/eval-okay-callable-attrs.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-callable-attrs.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-callable-attrs.nix b/tvix/eval/src/tests/nix_tests/eval-okay-callable-attrs.nix
new file mode 100644
index 0000000000..310a030df0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-callable-attrs.nix
@@ -0,0 +1 @@
+({ __functor = self: x: self.foo && x; foo = false; } // { foo = true; }) true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-catattrs.exp b/tvix/eval/src/tests/nix_tests/eval-okay-catattrs.exp
new file mode 100644
index 0000000000..b4a1e66d6b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-catattrs.exp
@@ -0,0 +1 @@
+[ 1 2 ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-catattrs.nix b/tvix/eval/src/tests/nix_tests/eval-okay-catattrs.nix
new file mode 100644
index 0000000000..2c3dc10da5
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-catattrs.nix
@@ -0,0 +1 @@
+builtins.catAttrs "a" [ { a = 1; } { b = 0; } { a = 2; } ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-closure.exp b/tvix/eval/src/tests/nix_tests/eval-okay-closure.exp
new file mode 100644
index 0000000000..e7dbf97816
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-closure.exp
@@ -0,0 +1 @@
+[ { foo = true; key = -13; } { foo = true; key = -12; } { foo = true; key = -11; } { foo = true; key = -9; } { foo = true; key = -8; } { foo = true; key = -7; } { foo = true; key = -5; } { foo = true; key = -4; } { foo = true; key = -3; } { key = -1; } { foo = true; key = 0; } { foo = true; key = 1; } { foo = true; key = 2; } { foo = true; key = 4; } { foo = true; key = 5; } { foo = true; key = 6; } { key = 8; } { foo = true; key = 9; } { foo = true; key = 10; } { foo = true; key = 13; } { foo = true; key = 14; } { foo = true; key = 15; } { key = 17; } { foo = true; key = 18; } { foo = true; key = 19; } { foo = true; key = 22; } { foo = true; key = 23; } { key = 26; } { foo = true; key = 27; } { foo = true; key = 28; } { foo = true; key = 31; } { foo = true; key = 32; } { key = 35; } { foo = true; key = 36; } { foo = true; key = 40; } { foo = true; key = 41; } { key = 44; } { foo = true; key = 45; } { foo = true; key = 49; } { key = 53; } { foo = true; key = 54; } { foo = true; key = 58; } { key = 62; } { foo = true; key = 67; } { key = 71; } { key = 80; } ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-closure.exp.xml b/tvix/eval/src/tests/nix_tests/eval-okay-closure.exp.xml
new file mode 100644
index 0000000000..dffc03a998
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-closure.exp.xml
@@ -0,0 +1,343 @@
+<?xml version='1.0' encoding='utf-8'?>
+<expr>
+  <list>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="-13" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="-12" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="-11" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="-9" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="-8" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="-7" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="-5" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="-4" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="-3" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="key">
+        <int value="-1" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="0" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="1" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="2" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="4" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="5" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="6" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="key">
+        <int value="8" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="9" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="10" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="13" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="14" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="15" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="key">
+        <int value="17" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="18" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="19" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="22" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="23" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="key">
+        <int value="26" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="27" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="28" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="31" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="32" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="key">
+        <int value="35" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="36" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="40" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="41" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="key">
+        <int value="44" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="45" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="49" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="key">
+        <int value="53" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="54" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="58" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="key">
+        <int value="62" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="foo">
+        <bool value="true" />
+      </attr>
+      <attr name="key">
+        <int value="67" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="key">
+        <int value="71" />
+      </attr>
+    </attrs>
+    <attrs>
+      <attr name="key">
+        <int value="80" />
+      </attr>
+    </attrs>
+  </list>
+</expr>
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-closure.nix b/tvix/eval/src/tests/nix_tests/eval-okay-closure.nix
new file mode 100644
index 0000000000..cccd4dc357
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-closure.nix
@@ -0,0 +1,13 @@
+let
+
+  closure = builtins.genericClosure {
+    startSet = [{key = 80;}];
+    operator = {key, foo ? false}:
+      if builtins.lessThan key 0
+      then []
+      else [{key = builtins.sub key 9;} {key = builtins.sub key 13; foo = true;}];
+  };
+
+  sort = (import ./lib.nix).sortBy (a: b: builtins.lessThan a.key b.key);
+
+in sort closure
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-comments.exp b/tvix/eval/src/tests/nix_tests/eval-okay-comments.exp
new file mode 100644
index 0000000000..7182dc2f9b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-comments.exp
@@ -0,0 +1 @@
+"abcdefghijklmnopqrstuvwxyz"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-comments.nix b/tvix/eval/src/tests/nix_tests/eval-okay-comments.nix
new file mode 100644
index 0000000000..cb2cce2180
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-comments.nix
@@ -0,0 +1,59 @@
+# A simple comment
+"a"+ # And another
+## A double comment
+"b"+  ## And another
+# Nested # comments #
+"c"+   # and # some # other #
+# An empty line, following here:
+
+"d"+      # and a comment not starting the line !
+
+"e"+
+/* multiline comments */
+"f" +
+/* multiline
+   comments,
+   on
+   multiple
+   lines
+*/
+"g" +
+# Small, tricky comments
+/**/ "h"+ /*/*/ "i"+ /***/ "j"+ /* /*/ "k"+ /*/* /*/ "l"+
+# Comments with an even number of ending '*' used to fail:
+"m"+
+/* */ /* **/ /* ***/ /* ****/ "n"+
+/* */ /** */ /*** */ /**** */ "o"+
+/** **/ /*** ***/ /**** ****/ "p"+
+/* * ** *** **** ***** */     "q"+
+# Random comments
+/* ***** ////// * / * / /* */ "r"+
+# Mixed comments
+/* # */
+"s"+
+# /* #
+"t"+
+# /* # */
+"u"+
+# /*********/
+"v"+
+## */*
+"w"+
+/*
+ * Multiline, decorated comments
+ * # This ain't a nest'd comm'nt
+ */
+"x"+
+''${/** with **/"y"
+  # real
+  /* comments
+     inside ! # */
+
+  # (and empty lines)
+
+}''+          /* And a multiline comment,
+                 on the same line,
+                 after some spaces
+*/             # followed by a one-line comment
+"z"
+/* EOF */
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-concat.exp b/tvix/eval/src/tests/nix_tests/eval-okay-concat.exp
new file mode 100644
index 0000000000..bb4bbd5774
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-concat.exp
@@ -0,0 +1 @@
+[ 1 2 3 4 5 6 7 8 9 ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-concat.nix b/tvix/eval/src/tests/nix_tests/eval-okay-concat.nix
new file mode 100644
index 0000000000..d158a9bf05
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-concat.nix
@@ -0,0 +1 @@
+[1 2 3] ++ [4 5 6] ++ [7 8 9]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-concatmap.exp b/tvix/eval/src/tests/nix_tests/eval-okay-concatmap.exp
new file mode 100644
index 0000000000..3b8be7739d
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-concatmap.exp
@@ -0,0 +1 @@
+[ [ 1 3 5 7 9 ] [ "a" "z" "b" "z" ] ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-concatmap.nix b/tvix/eval/src/tests/nix_tests/eval-okay-concatmap.nix
new file mode 100644
index 0000000000..97da5d37a4
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-concatmap.nix
@@ -0,0 +1,5 @@
+with import ./lib.nix;
+
+[ (builtins.concatMap (x: if x / 2 * 2 == x then [] else [ x ]) (range 0 10))
+  (builtins.concatMap (x: [x] ++ ["z"]) ["a" "b"])
+]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-concatstringssep.exp b/tvix/eval/src/tests/nix_tests/eval-okay-concatstringssep.exp
new file mode 100644
index 0000000000..93987647ff
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-concatstringssep.exp
@@ -0,0 +1 @@
+[ "" "foobarxyzzy" "foo, bar, xyzzy" "foo" "" ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-concatstringssep.nix b/tvix/eval/src/tests/nix_tests/eval-okay-concatstringssep.nix
new file mode 100644
index 0000000000..adc4c41bd5
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-concatstringssep.nix
@@ -0,0 +1,8 @@
+with builtins;
+
+[ (concatStringsSep "" [])
+  (concatStringsSep "" ["foo" "bar" "xyzzy"])
+  (concatStringsSep ", " ["foo" "bar" "xyzzy"])
+  (concatStringsSep ", " ["foo"])
+  (concatStringsSep ", " [])
+]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-deepseq.exp b/tvix/eval/src/tests/nix_tests/eval-okay-deepseq.exp
new file mode 100644
index 0000000000..8d38505c16
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-deepseq.exp
@@ -0,0 +1 @@
+456
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-deepseq.nix b/tvix/eval/src/tests/nix_tests/eval-okay-deepseq.nix
new file mode 100644
index 0000000000..53aa4b1dc2
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-deepseq.nix
@@ -0,0 +1 @@
+builtins.deepSeq (let as = { x = 123; y = as; }; in as) 456
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-delayed-with-inherit.exp b/tvix/eval/src/tests/nix_tests/eval-okay-delayed-with-inherit.exp
new file mode 100644
index 0000000000..eaacb55c1a
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-delayed-with-inherit.exp
@@ -0,0 +1 @@
+"b-overridden"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-delayed-with-inherit.nix b/tvix/eval/src/tests/nix_tests/eval-okay-delayed-with-inherit.nix
new file mode 100644
index 0000000000..84b388c271
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-delayed-with-inherit.nix
@@ -0,0 +1,24 @@
+let
+  pkgs_ = with pkgs; {
+    a = derivation {
+      name = "a";
+      system = builtins.currentSystem;
+      builder = "/bin/sh";
+      args = [ "-c" "touch $out" ];
+      inherit b;
+    };
+
+    inherit b;
+  };
+
+  packageOverrides = p: {
+    b = derivation {
+      name = "b-overridden";
+      system = builtins.currentSystem;
+      builder = "/bin/sh";
+      args = [ "-c" "touch $out" ];
+    };
+  };
+
+  pkgs = pkgs_ // (packageOverrides pkgs_);
+in pkgs.a.b.name
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.exp b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.nix b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.nix
new file mode 100644
index 0000000000..6d57bf8549
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.nix
@@ -0,0 +1 @@
+{ a."${"b"}" = true; a."${"c"}" = false; }.a.b
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-bare.exp b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-bare.exp
new file mode 100644
index 0000000000..df8750afc0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-bare.exp
@@ -0,0 +1 @@
+{ binds = true; hasAttrs = true; multiAttrs = true; recBinds = true; selectAttrs = true; selectOrAttrs = true; }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-bare.nix b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-bare.nix
new file mode 100644
index 0000000000..0dbe15e638
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-bare.nix
@@ -0,0 +1,17 @@
+let
+  aString = "a";
+
+  bString = "b";
+in {
+  hasAttrs = { a.b = null; } ? ${aString}.b;
+
+  selectAttrs = { a.b = true; }.a.${bString};
+
+  selectOrAttrs = { }.${aString} or true;
+
+  binds = { ${aString}."${bString}c" = true; }.a.bc;
+
+  recBinds = rec { ${bString} = a; a = true; }.b;
+
+  multiAttrs = { ${aString} = true; ${bString} = false; }.a;
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.exp b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.exp
new file mode 100644
index 0000000000..df8750afc0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.exp
@@ -0,0 +1 @@
+{ binds = true; hasAttrs = true; multiAttrs = true; recBinds = true; selectAttrs = true; selectOrAttrs = true; }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.nix b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.nix
new file mode 100644
index 0000000000..ee02ac7e65
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.nix
@@ -0,0 +1,17 @@
+let
+  aString = "a";
+
+  bString = "b";
+in {
+  hasAttrs = { a.b = null; } ? "${aString}".b;
+
+  selectAttrs = { a.b = true; }.a."${bString}";
+
+  selectOrAttrs = { }."${aString}" or true;
+
+  binds = { "${aString}"."${bString}c" = true; }.a.bc;
+
+  recBinds = rec { "${bString}" = a; a = true; }.b;
+
+  multiAttrs = { "${aString}" = true; "${bString}" = false; }.a;
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-elem.exp b/tvix/eval/src/tests/nix_tests/eval-okay-elem.exp
new file mode 100644
index 0000000000..3cf6c0e962
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-elem.exp
@@ -0,0 +1 @@
+[ true false 30 ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-elem.nix b/tvix/eval/src/tests/nix_tests/eval-okay-elem.nix
new file mode 100644
index 0000000000..71ea7a4ed0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-elem.nix
@@ -0,0 +1,6 @@
+with import ./lib.nix;
+
+let xs = range 10 40; in
+
+[ (builtins.elem 23 xs) (builtins.elem 42 xs) (builtins.elemAt xs 20) ]
+
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.exp b/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.exp
new file mode 100644
index 0000000000..cb5537d5d7
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.exp
@@ -0,0 +1 @@
+"ab"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.nix b/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.nix
new file mode 100644
index 0000000000..78c133afdd
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.nix
@@ -0,0 +1 @@
+({}: {x,y,}: "${x}${y}") {} {x = "a"; y = "b";}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-eq.exp b/tvix/eval/src/tests/nix_tests/eval-okay-eq.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-eq.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-eq.nix b/tvix/eval/src/tests/nix_tests/eval-okay-eq.nix
new file mode 100644
index 0000000000..73d200b381
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-eq.nix
@@ -0,0 +1,3 @@
+["foobar" (rec {x = 1; y = x;})]
+==
+[("foo" + "bar") ({x = 1; y = 1;})]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-filter.exp b/tvix/eval/src/tests/nix_tests/eval-okay-filter.exp
new file mode 100644
index 0000000000..355d51c27d
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-filter.exp
@@ -0,0 +1 @@
+[ 0 2 4 6 8 10 100 102 104 106 108 110 ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-filter.nix b/tvix/eval/src/tests/nix_tests/eval-okay-filter.nix
new file mode 100644
index 0000000000..85109b0d0e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-filter.nix
@@ -0,0 +1,5 @@
+with import ./lib.nix;
+
+builtins.filter
+  (x: x / 2 * 2 == x)
+  (builtins.concatLists [ (range 0 10) (range 100 110) ])
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-flatten.exp b/tvix/eval/src/tests/nix_tests/eval-okay-flatten.exp
new file mode 100644
index 0000000000..b979b2b8b9
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-flatten.exp
@@ -0,0 +1 @@
+"1234567"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-flatten.nix b/tvix/eval/src/tests/nix_tests/eval-okay-flatten.nix
new file mode 100644
index 0000000000..fe911e9683
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-flatten.nix
@@ -0,0 +1,8 @@
+with import ./lib.nix;
+
+let {
+
+  l = ["1" "2" ["3" ["4"] ["5" "6"]] "7"];
+
+  body = concat (flatten l);
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-float.exp b/tvix/eval/src/tests/nix_tests/eval-okay-float.exp
new file mode 100644
index 0000000000..3c50a8adce
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-float.exp
@@ -0,0 +1 @@
+[ 3.4 3.5 2.5 1.5 ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-float.nix b/tvix/eval/src/tests/nix_tests/eval-okay-float.nix
new file mode 100644
index 0000000000..b2702c7b16
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-float.nix
@@ -0,0 +1,6 @@
+[
+  (1.1 + 2.3)
+  (builtins.add (0.5 + 0.5) (2.0 + 0.5))
+  ((0.5 + 0.5) * (2.0 + 0.5))
+  ((1.5 + 1.5) / (0.5 * 4.0))
+]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-elements.exp b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-elements.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-elements.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-elements.nix b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-elements.nix
new file mode 100644
index 0000000000..c666e07f3a
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-elements.nix
@@ -0,0 +1,9 @@
+# Tests that the rhs argument of op is not forced unconditionally
+let
+  lst = builtins.foldl'
+    (acc: x: acc ++ [ x ])
+    [ ]
+    [ 42 (throw "this shouldn't be evaluated") ];
+in
+
+builtins.head lst
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.exp b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.nix b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.nix
new file mode 100644
index 0000000000..abcd5366ab
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.nix
@@ -0,0 +1,6 @@
+# Checks that the nul value for the accumulator is not forced unconditionally.
+# Some languages provide a foldl' that is strict in this argument, but Nix does not.
+builtins.foldl'
+  (_: x: x)
+  (throw "This is never forced")
+  [ "but the results of applying op are" 42 ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict.exp b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict.exp
new file mode 100644
index 0000000000..837e12b406
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict.exp
@@ -0,0 +1 @@
+500500
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict.nix b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict.nix
new file mode 100644
index 0000000000..3b87188d24
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict.nix
@@ -0,0 +1,3 @@
+with import ./lib.nix;
+
+builtins.foldl' (x: y: x + y) 0 (range 1 1000)
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-fromTOML.exp b/tvix/eval/src/tests/nix_tests/eval-okay-fromTOML.exp
new file mode 100644
index 0000000000..d0dd3af2c8
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-fromTOML.exp
@@ -0,0 +1 @@
+[ { clients = { data = [ [ "gamma" "delta" ] [ 1 2 ] ]; hosts = [ "alpha" "omega" ]; }; database = { connection_max = 5000; enabled = true; ports = [ 8001 8001 8002 ]; server = "192.168.1.1"; }; owner = { name = "Tom Preston-Werner"; }; servers = { alpha = { dc = "eqdc10"; ip = "10.0.0.1"; }; beta = { dc = "eqdc10"; ip = "10.0.0.2"; }; }; title = "TOML Example"; } { "1234" = "value"; "127.0.0.1" = "value"; a = { b = { c = { }; }; }; arr1 = [ 1 2 3 ]; arr2 = [ "red" "yellow" "green" ]; arr3 = [ [ 1 2 ] [ 3 4 5 ] ]; arr4 = [ "all" "strings" "are the same" "type" ]; arr5 = [ [ 1 2 ] [ "a" "b" "c" ] ]; arr7 = [ 1 2 3 ]; arr8 = [ 1 2 ]; bare-key = "value"; bare_key = "value"; bin1 = 214; bool1 = true; bool2 = false; "character encoding" = "value"; d = { e = { f = { }; }; }; dog = { "tater.man" = { type = { name = "pug"; }; }; }; flt1 = 1; flt2 = 3.1415; flt3 = -0.01; flt4 = 5e+22; flt5 = 1e+06; flt6 = -0.02; flt7 = 6.626e-34; flt8 = 9.22462e+06; fruit = [ { name = "apple"; physical = { color = "red"; shape = "round"; }; variety = [ { name = "red delicious"; } { name = "granny smith"; } ]; } { name = "banana"; variety = [ { name = "plantain"; } ]; } ]; g = { h = { i = { }; }; }; hex1 = 3735928559; hex2 = 3735928559; hex3 = 3735928559; int1 = 99; int2 = 42; int3 = 0; int4 = -17; int5 = 1000; int6 = 5349221; int7 = 12345; j = { "สž" = { l = { }; }; }; key = "value"; key2 = "value"; name = "Orange"; oct1 = 342391; oct2 = 493; physical = { color = "orange"; shape = "round"; }; products = [ { name = "Hammer"; sku = 738594937; } { } { color = "gray"; name = "Nail"; sku = 284758393; } ]; "quoted \"value\"" = "value"; site = { "google.com" = true; }; str = "I'm a string. \"You can quote me\". Name\tJosรฉ\nLocation\tSF."; table-1 = { key1 = "some string"; key2 = 123; }; table-2 = { key1 = "another string"; key2 = 456; }; x = { y = { z = { w = { animal = { type = { name = "pug"; }; }; name = { first = "Tom"; last = "Preston-Werner"; }; point = { x = 1; y = 2; }; }; }; }; }; "สŽวสž" = "value"; } { metadata = { "checksum aho-corasick 0.6.4 (registry+https://github.com/rust-lang/crates.io-index)" = "d6531d44de723825aa81398a6415283229725a00fa30713812ab9323faa82fc4"; "checksum ansi_term 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ee49baf6cb617b853aa8d93bf420db2383fab46d314482ca2803b40d5fde979b"; "checksum ansi_term 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)" = "23ac7c30002a5accbf7e8987d0632fa6de155b7c3d39d0067317a391e00a2ef6"; "checksum arrayvec 0.4.7 (registry+https://github.com/rust-lang/crates.io-index)" = "a1e964f9e24d588183fcb43503abda40d288c8657dfc27311516ce2f05675aef"; }; package = [ { dependencies = [ "memchr 2.0.1 (registry+https://github.com/rust-lang/crates.io-index)" ]; name = "aho-corasick"; source = "registry+https://github.com/rust-lang/crates.io-index"; version = "0.6.4"; } { name = "ansi_term"; source = "registry+https://github.com/rust-lang/crates.io-index"; version = "0.9.0"; } { dependencies = [ "libc 0.2.42 (registry+https://github.com/rust-lang/crates.io-index)" "termion 1.5.1 (registry+https://github.com/rust-lang/crates.io-index)" "winapi 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)" ]; name = "atty"; source = "registry+https://github.com/rust-lang/crates.io-index"; version = "0.2.10"; } ]; } { a = [ [ { b = true; } ] ]; c = [ [ { d = true; } ] ]; e = [ [ 123 ] ]; } ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-fromTOML.nix b/tvix/eval/src/tests/nix_tests/eval-okay-fromTOML.nix
new file mode 100644
index 0000000000..9639326899
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-fromTOML.nix
@@ -0,0 +1,208 @@
+[
+
+  (builtins.fromTOML ''
+    # This is a TOML document.
+
+    title = "TOML Example"
+
+    [owner]
+    name = "Tom Preston-Werner"
+    #dob = 1979-05-27T07:32:00-08:00 # First class dates
+
+    [database]
+    server = "192.168.1.1"
+    ports = [ 8001, 8001, 8002 ]
+    connection_max = 5000
+    enabled = true
+
+    [servers]
+
+      # Indentation (tabs and/or spaces) is allowed but not required
+      [servers.alpha]
+      ip = "10.0.0.1"
+      dc = "eqdc10"
+
+      [servers.beta]
+      ip = "10.0.0.2"
+      dc = "eqdc10"
+
+    [clients]
+    data = [ ["gamma", "delta"], [1, 2] ]
+
+    # Line breaks are OK when inside arrays
+    hosts = [
+      "alpha",
+      "omega"
+    ]
+  '')
+
+  (builtins.fromTOML ''
+    key = "value"
+    bare_key = "value"
+    bare-key = "value"
+    1234 = "value"
+
+    "127.0.0.1" = "value"
+    "character encoding" = "value"
+    "สŽวสž" = "value"
+    'key2' = "value"
+    'quoted "value"' = "value"
+
+    name = "Orange"
+
+    physical.color = "orange"
+    physical.shape = "round"
+    site."google.com" = true
+
+    # This is legal according to the spec, but cpptoml doesn't handle it.
+    #a.b.c = 1
+    #a.d = 2
+
+    str = "I'm a string. \"You can quote me\". Name\tJos\u00E9\nLocation\tSF."
+
+    int1 = +99
+    int2 = 42
+    int3 = 0
+    int4 = -17
+    int5 = 1_000
+    int6 = 5_349_221
+    int7 = 1_2_3_4_5
+
+    hex1 = 0xDEADBEEF
+    hex2 = 0xdeadbeef
+    hex3 = 0xdead_beef
+
+    oct1 = 0o01234567
+    oct2 = 0o755
+
+    bin1 = 0b11010110
+
+    flt1 = +1.0
+    flt2 = 3.1415
+    flt3 = -0.01
+    flt4 = 5e+22
+    flt5 = 1e6
+    flt6 = -2E-2
+    flt7 = 6.626e-34
+    flt8 = 9_224_617.445_991_228_313
+
+    bool1 = true
+    bool2 = false
+
+    # FIXME: not supported because Nix doesn't have a date/time type.
+    #odt1 = 1979-05-27T07:32:00Z
+    #odt2 = 1979-05-27T00:32:00-07:00
+    #odt3 = 1979-05-27T00:32:00.999999-07:00
+    #odt4 = 1979-05-27 07:32:00Z
+    #ldt1 = 1979-05-27T07:32:00
+    #ldt2 = 1979-05-27T00:32:00.999999
+    #ld1 = 1979-05-27
+    #lt1 = 07:32:00
+    #lt2 = 00:32:00.999999
+
+    arr1 = [ 1, 2, 3 ]
+    arr2 = [ "red", "yellow", "green" ]
+    arr3 = [ [ 1, 2 ], [3, 4, 5] ]
+    arr4 = [ "all", 'strings', """are the same""", ''''type'''']
+    arr5 = [ [ 1, 2 ], ["a", "b", "c"] ]
+
+    arr7 = [
+      1, 2, 3
+    ]
+
+    arr8 = [
+      1,
+      2, # this is ok
+    ]
+
+    [table-1]
+    key1 = "some string"
+    key2 = 123
+
+
+    [table-2]
+    key1 = "another string"
+    key2 = 456
+
+    [dog."tater.man"]
+    type.name = "pug"
+
+    [a.b.c]
+    [ d.e.f ]
+    [ g .  h  . i ]
+    [ j . "สž" . 'l' ]
+    [x.y.z.w]
+
+    name = { first = "Tom", last = "Preston-Werner" }
+    point = { x = 1, y = 2 }
+    animal = { type.name = "pug" }
+
+    [[products]]
+    name = "Hammer"
+    sku = 738594937
+
+    [[products]]
+
+    [[products]]
+    name = "Nail"
+    sku = 284758393
+    color = "gray"
+
+    [[fruit]]
+      name = "apple"
+
+      [fruit.physical]
+        color = "red"
+        shape = "round"
+
+      [[fruit.variety]]
+        name = "red delicious"
+
+      [[fruit.variety]]
+        name = "granny smith"
+
+    [[fruit]]
+      name = "banana"
+
+      [[fruit.variety]]
+        name = "plantain"
+  '')
+
+  (builtins.fromTOML ''
+    [[package]]
+    name = "aho-corasick"
+    version = "0.6.4"
+    source = "registry+https://github.com/rust-lang/crates.io-index"
+    dependencies = [
+     "memchr 2.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
+    ]
+
+    [[package]]
+    name = "ansi_term"
+    version = "0.9.0"
+    source = "registry+https://github.com/rust-lang/crates.io-index"
+
+    [[package]]
+    name = "atty"
+    version = "0.2.10"
+    source = "registry+https://github.com/rust-lang/crates.io-index"
+    dependencies = [
+     "libc 0.2.42 (registry+https://github.com/rust-lang/crates.io-index)",
+     "termion 1.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
+     "winapi 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)",
+    ]
+
+    [metadata]
+    "checksum aho-corasick 0.6.4 (registry+https://github.com/rust-lang/crates.io-index)" = "d6531d44de723825aa81398a6415283229725a00fa30713812ab9323faa82fc4"
+    "checksum ansi_term 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ee49baf6cb617b853aa8d93bf420db2383fab46d314482ca2803b40d5fde979b"
+    "checksum ansi_term 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)" = "23ac7c30002a5accbf7e8987d0632fa6de155b7c3d39d0067317a391e00a2ef6"
+    "checksum arrayvec 0.4.7 (registry+https://github.com/rust-lang/crates.io-index)" = "a1e964f9e24d588183fcb43503abda40d288c8657dfc27311516ce2f05675aef"
+  '')
+
+  (builtins.fromTOML ''
+    a = [[{ b = true }]]
+    c = [ [ { d = true } ] ]
+    e = [[123]]
+  '')
+
+]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-fromjson-escapes.exp b/tvix/eval/src/tests/nix_tests/eval-okay-fromjson-escapes.exp
new file mode 100644
index 0000000000..add5505a82
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-fromjson-escapes.exp
@@ -0,0 +1 @@
+"quote \" reverse solidus \\ solidus / backspace  formfeed  newline \n carriage return \r horizontal tab \t 1 char unicode encoded backspace  1 char unicode encoded e with accent รฉ 2 char unicode encoded s with caron ลก 3 char unicode encoded rightwards arrow โ†’"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-fromjson-escapes.nix b/tvix/eval/src/tests/nix_tests/eval-okay-fromjson-escapes.nix
new file mode 100644
index 0000000000..f007135077
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-fromjson-escapes.nix
@@ -0,0 +1,3 @@
+# This string contains all supported escapes in a JSON string, per json.org
+# \b and \f are not supported by Nix
+builtins.fromJSON ''"quote \" reverse solidus \\ solidus \/ backspace \b formfeed \f newline \n carriage return \r horizontal tab \t 1 char unicode encoded backspace \u0008 1 char unicode encoded e with accent \u00e9 2 char unicode encoded s with caron \u0161 3 char unicode encoded rightwards arrow \u2192"''
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-fromjson.exp b/tvix/eval/src/tests/nix_tests/eval-okay-fromjson.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-fromjson.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-fromjson.nix b/tvix/eval/src/tests/nix_tests/eval-okay-fromjson.nix
new file mode 100644
index 0000000000..e1c0f86cc4
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-fromjson.nix
@@ -0,0 +1,35 @@
+builtins.fromJSON
+  ''
+    {
+      "Video": {
+          "Title":  "The Penguin Chronicles",
+          "Width":  1920,
+          "Height": 1080,
+          "EmbeddedData": [3.14159, 23493,null, true  ,false, -10],
+          "Thumb": {
+              "Url":    "http://www.example.com/video/5678931",
+              "Width":  200,
+              "Height": 250
+          },
+          "Subtitle" : false,
+          "Latitude":  46.2051,
+          "Longitude": 6.0723
+        }
+    }
+  ''
+==
+  { Video =
+    { Title = "The Penguin Chronicles";
+      Width = 1920;
+      Height = 1080;
+      EmbeddedData = [ 3.14159 23493 null true false (0-10) ];
+      Thumb =
+        { Url = "http://www.example.com/video/5678931";
+          Width = 200;
+          Height = 250;
+        };
+      Subtitle = false;
+      Latitude = 46.2051;
+      Longitude = 6.0723;
+    };
+  }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-functionargs.exp b/tvix/eval/src/tests/nix_tests/eval-okay-functionargs.exp
new file mode 100644
index 0000000000..c1c9f8ffaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-functionargs.exp
@@ -0,0 +1 @@
+[ "stdenv" "fetchurl" "aterm-stdenv" "aterm-stdenv2" "libX11" "libXv" "mplayer-stdenv2.libXv-libX11" "mplayer-stdenv2.libXv-libX11_2" "nix-stdenv-aterm-stdenv" "nix-stdenv2-aterm2-stdenv2" ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-functionargs.exp.xml b/tvix/eval/src/tests/nix_tests/eval-okay-functionargs.exp.xml
new file mode 100644
index 0000000000..651f54c363
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-functionargs.exp.xml
@@ -0,0 +1,15 @@
+<?xml version='1.0' encoding='utf-8'?>
+<expr>
+  <list>
+    <string value="stdenv" />
+    <string value="fetchurl" />
+    <string value="aterm-stdenv" />
+    <string value="aterm-stdenv2" />
+    <string value="libX11" />
+    <string value="libXv" />
+    <string value="mplayer-stdenv2.libXv-libX11" />
+    <string value="mplayer-stdenv2.libXv-libX11_2" />
+    <string value="nix-stdenv-aterm-stdenv" />
+    <string value="nix-stdenv2-aterm2-stdenv2" />
+  </list>
+</expr>
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-functionargs.nix b/tvix/eval/src/tests/nix_tests/eval-okay-functionargs.nix
new file mode 100644
index 0000000000..68dca62ee1
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-functionargs.nix
@@ -0,0 +1,80 @@
+let
+
+  stdenvFun = { }: { name = "stdenv"; };
+  stdenv2Fun = { }: { name = "stdenv2"; };
+  fetchurlFun = { stdenv }: assert stdenv.name == "stdenv"; { name = "fetchurl"; };
+  atermFun = { stdenv, fetchurl }: { name = "aterm-${stdenv.name}"; };
+  aterm2Fun = { stdenv, fetchurl }: { name = "aterm2-${stdenv.name}"; };
+  nixFun = { stdenv, fetchurl, aterm }: { name = "nix-${stdenv.name}-${aterm.name}"; };
+  
+  mplayerFun =
+    { stdenv, fetchurl, enableX11 ? false, xorg ? null, enableFoo ? true, foo ? null  }:
+    assert stdenv.name == "stdenv2";
+    assert enableX11 -> xorg.libXv.name == "libXv";
+    assert enableFoo -> foo != null;
+    { name = "mplayer-${stdenv.name}.${xorg.libXv.name}-${xorg.libX11.name}"; };
+
+  makeOverridable = f: origArgs: f origArgs //
+    { override = newArgs:
+        makeOverridable f (origArgs // (if builtins.isFunction newArgs then newArgs origArgs else newArgs));
+    };
+    
+  callPackage_ = pkgs: f: args:
+    makeOverridable f ((builtins.intersectAttrs (builtins.functionArgs f) pkgs) // args);
+
+  allPackages =
+    { overrides ? (pkgs: pkgsPrev: { }) }:
+    let
+      callPackage = callPackage_ pkgs;
+      pkgs = pkgsStd // (overrides pkgs pkgsStd);
+      pkgsStd = {
+        inherit pkgs;
+        stdenv = callPackage stdenvFun { };
+        stdenv2 = callPackage stdenv2Fun { };
+        fetchurl = callPackage fetchurlFun { };
+        aterm = callPackage atermFun { };
+        xorg = callPackage xorgFun { };
+        mplayer = callPackage mplayerFun { stdenv = pkgs.stdenv2; enableFoo = false; };
+        nix = callPackage nixFun { };
+      };
+    in pkgs;
+
+  libX11Fun = { stdenv, fetchurl }: { name = "libX11"; };
+  libX11_2Fun = { stdenv, fetchurl }: { name = "libX11_2"; };
+  libXvFun = { stdenv, fetchurl, libX11 }: { name = "libXv"; };
+  
+  xorgFun =
+    { pkgs }:
+    let callPackage = callPackage_ (pkgs // pkgs.xorg); in
+    {
+      libX11 = callPackage libX11Fun { };
+      libXv = callPackage libXvFun { };
+    };
+
+in
+
+let
+
+  pkgs = allPackages { };
+  
+  pkgs2 = allPackages {
+    overrides = pkgs: pkgsPrev: {
+      stdenv = pkgs.stdenv2;
+      nix = pkgsPrev.nix.override { aterm = aterm2Fun { inherit (pkgs) stdenv fetchurl; }; };
+      xorg = pkgsPrev.xorg // { libX11 = libX11_2Fun { inherit (pkgs) stdenv fetchurl; }; };
+    };
+  };
+  
+in
+
+  [ pkgs.stdenv.name
+    pkgs.fetchurl.name
+    pkgs.aterm.name
+    pkgs2.aterm.name
+    pkgs.xorg.libX11.name
+    pkgs.xorg.libXv.name
+    pkgs.mplayer.name
+    pkgs2.mplayer.name
+    pkgs.nix.name
+    pkgs2.nix.name
+  ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-getenv.exp b/tvix/eval/src/tests/nix_tests/eval-okay-getenv.exp
new file mode 100644
index 0000000000..14e24d4190
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-getenv.exp
@@ -0,0 +1 @@
+"foobar"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-getenv.nix b/tvix/eval/src/tests/nix_tests/eval-okay-getenv.nix
new file mode 100644
index 0000000000..4cfec5f553
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-getenv.nix
@@ -0,0 +1 @@
+builtins.getEnv "TEST_VAR" + (if builtins.getEnv "NO_SUCH_VAR" == "" then "bar" else "bla")
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-groupBy.exp b/tvix/eval/src/tests/nix_tests/eval-okay-groupBy.exp
new file mode 100644
index 0000000000..bfca5652a5
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-groupBy.exp
@@ -0,0 +1 @@
+{ "1" = [ 9 ]; "2" = [ 8 ]; "3" = [ 13 29 ]; "4" = [ 3 4 10 11 17 18 ]; "5" = [ 0 23 26 28 ]; "6" = [ 1 12 21 27 30 ]; "7" = [ 7 22 ]; "8" = [ 14 ]; "9" = [ 19 ]; b = [ 16 25 ]; c = [ 24 ]; d = [ 2 ]; e = [ 5 6 15 31 ]; f = [ 20 ]; }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-groupBy.nix b/tvix/eval/src/tests/nix_tests/eval-okay-groupBy.nix
new file mode 100644
index 0000000000..862d89dbd6
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-groupBy.nix
@@ -0,0 +1,5 @@
+with import ./lib.nix;
+
+builtins.groupBy (n:
+  builtins.substring 0 1 (builtins.hashString "sha256" (toString n))
+) (range 0 31)
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-hash.exp b/tvix/eval/src/tests/nix_tests/eval-okay-hash.exp
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-hash.exp
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-hashfile.exp b/tvix/eval/src/tests/nix_tests/eval-okay-hashfile.exp
new file mode 100644
index 0000000000..ff1e8293ef
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-hashfile.exp
@@ -0,0 +1 @@
+[ "d3b07384d113edec49eaa6238ad5ff00" "0f343b0931126a20f133d67c2b018a3b" "f1d2d2f924e986ac86fdf7b36c94bcdf32beec15" "60cacbf3d72e1e7834203da608037b1bf83b40e8" "b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c" "5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef" "0cf9180a764aba863a67b6d72f0918bc131c6772642cb2dce5a34f0a702f9470ddc2bf125c12198b1995c233c34b4afd346c54a2334c350a948a51b6e8b4e6b6" "8efb4f73c5655351c444eb109230c556d39e2c7624e9c11abc9e3fb4b9b9254218cc5085b454a9698d085cfa92198491f07a723be4574adc70617b73eb0b6461" ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-hashfile.nix b/tvix/eval/src/tests/nix_tests/eval-okay-hashfile.nix
new file mode 100644
index 0000000000..aff5a18568
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-hashfile.nix
@@ -0,0 +1,4 @@
+let
+  paths = [ ./data ./binary-data ];
+in
+  builtins.concatLists (map (hash: map (builtins.hashFile hash) paths) ["md5" "sha1" "sha256" "sha512"])
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-hashstring.exp b/tvix/eval/src/tests/nix_tests/eval-okay-hashstring.exp
new file mode 100644
index 0000000000..d720a082dd
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-hashstring.exp
@@ -0,0 +1 @@
+[ "d41d8cd98f00b204e9800998ecf8427e" "6c69ee7f211c640419d5366cc076ae46" "bb3438fbabd460ea6dbd27d153e2233b" "da39a3ee5e6b4b0d3255bfef95601890afd80709" "cd54e8568c1b37cf1e5badb0779bcbf382212189" "6d12e10b1d331dad210e47fd25d4f260802b7e77" "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855" "900a4469df00ccbfd0c145c6d1e4b7953dd0afafadd7534e3a4019e8d38fc663" "ad0387b3bd8652f730ca46d25f9c170af0fd589f42e7f23f5a9e6412d97d7e56" "cf83e1357eefb8bdf1542850d66d8007d620e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0ff8318d2877eec2f63b931bd47417a81a538327af927da3e" "9d0886f8c6b389398a16257bc79780fab9831c7fc11c8ab07fa732cb7b348feade382f92617c9c5305fefba0af02ab5fd39a587d330997ff5bd0db19f7666653" "21644b72aa259e5a588cd3afbafb1d4310f4889680f6c83b9d531596a5a284f34dbebff409d23bcc86aee6bad10c891606f075c6f4755cb536da27db5693f3a7" ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-hashstring.nix b/tvix/eval/src/tests/nix_tests/eval-okay-hashstring.nix
new file mode 100644
index 0000000000..b0f62b245c
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-hashstring.nix
@@ -0,0 +1,4 @@
+let
+  strings = [ "" "text 1" "text 2" ];
+in
+  builtins.concatLists (map (hash: map (builtins.hashString hash) strings) ["md5" "sha1" "sha256" "sha512"])
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-if.exp b/tvix/eval/src/tests/nix_tests/eval-okay-if.exp
new file mode 100644
index 0000000000..00750edc07
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-if.exp
@@ -0,0 +1 @@
+3
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-if.nix b/tvix/eval/src/tests/nix_tests/eval-okay-if.nix
new file mode 100644
index 0000000000..23e4c74d50
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-if.nix
@@ -0,0 +1 @@
+if "foo" != "f" + "oo" then 1 else if false then 2 else 3
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-ind-string.exp b/tvix/eval/src/tests/nix_tests/eval-okay-ind-string.exp
new file mode 100644
index 0000000000..7862331fa5
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-ind-string.exp
@@ -0,0 +1 @@
+"This is an indented multi-line string\nliteral.  An amount of whitespace at\nthe start of each line matching the minimum\nindentation of all lines in the string\nliteral together will be removed.  Thus,\nin this case four spaces will be\nstripped from each line, even though\n  THIS LINE is indented six spaces.\n\nAlso, empty lines don't count in the\ndetermination of the indentation level (the\nprevious empty line has indentation 0, but\nit doesn't matter).\nIf the string starts with whitespace\n  followed by a newline, it's stripped, but\n  that's not the case here. Two spaces are\n  stripped because of the \"  \" at the start. \nThis line is indented\na bit further.\nAnti-quotations, like so, are\nalso allowed.\n  The \\ is not special here.\n' can be followed by any character except another ', e.g. 'x'.\nLikewise for $, e.g. $$ or $varName.\nBut ' followed by ' is special, as is $ followed by {.\nIf you want them, use anti-quotations: '', \${.\n   Tabs are not interpreted as whitespace (since we can't guess\n   what tab settings are intended), so don't use them.\n\tThis line starts with a space and a tab, so only one\n   space will be stripped from each line.\nAlso note that if the last line (just before the closing ' ')\nconsists only of whitespace, it's ignored.  But here there is\nsome non-whitespace stuff, so the line isn't removed. \nThis shows a hacky way to preserve an empty line after the start.\nBut there's no reason to do so: you could just repeat the empty\nline.\n  Similarly you can force an indentation level,\n  in this case to 2 spaces.  This works because the anti-quote\n  is significant (not whitespace).\nstart on network-interfaces\n\nstart script\n\n  rm -f /var/run/opengl-driver\n  ln -sf 123 /var/run/opengl-driver\n\n  rm -f /var/log/slim.log\n   \nend script\n\nenv SLIM_CFGFILE=abc\nenv SLIM_THEMESDIR=def\nenv FONTCONFIG_FILE=/etc/fonts/fonts.conf  \t\t\t\t# !!! cleanup\nenv XKB_BINDIR=foo/bin         \t\t\t\t# Needed for the Xkb extension.\nenv LD_LIBRARY_PATH=libX11/lib:libXext/lib:/usr/lib/          # related to xorg-sys-opengl - needed to load libglx for (AI)GLX support (for compiz)\n\nenv XORG_DRI_DRIVER_PATH=nvidiaDrivers/X11R6/lib/modules/drivers/ \n\nexec slim/bin/slim\nEscaping of ' followed by ': ''\nEscaping of $ followed by {: \${\nAnd finally to interpret \\n etc. as in a string: \n, \r, \t.\nfoo\n'bla'\nbar\ncut -d $'\\t' -f 1\nending dollar $$\n"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-ind-string.nix b/tvix/eval/src/tests/nix_tests/eval-okay-ind-string.nix
new file mode 100644
index 0000000000..95d59b5083
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-ind-string.nix
@@ -0,0 +1,128 @@
+let
+
+  s1 = ''
+    This is an indented multi-line string
+    literal.  An amount of whitespace at
+    the start of each line matching the minimum
+    indentation of all lines in the string
+    literal together will be removed.  Thus,
+    in this case four spaces will be
+    stripped from each line, even though
+      THIS LINE is indented six spaces.
+
+    Also, empty lines don't count in the
+    determination of the indentation level (the
+    previous empty line has indentation 0, but
+    it doesn't matter).
+  '';
+
+  s2 = ''  If the string starts with whitespace
+    followed by a newline, it's stripped, but
+    that's not the case here. Two spaces are
+    stripped because of the "  " at the start. 
+  '';
+
+  s3 = ''
+      This line is indented
+      a bit further.
+        ''; # indentation of last line doesn't count if it's empty
+
+  s4 = ''
+    Anti-quotations, like ${if true then "so" else "not so"}, are
+    also allowed.
+  '';
+
+  s5 = ''
+      The \ is not special here.
+    ' can be followed by any character except another ', e.g. 'x'.
+    Likewise for $, e.g. $$ or $varName.
+    But ' followed by ' is special, as is $ followed by {.
+    If you want them, use anti-quotations: ${"''"}, ${"\${"}.
+  '';
+
+  s6 = ''  
+    Tabs are not interpreted as whitespace (since we can't guess
+    what tab settings are intended), so don't use them.
+ 	This line starts with a space and a tab, so only one
+    space will be stripped from each line.
+  '';
+
+  s7 = ''
+    Also note that if the last line (just before the closing ' ')
+    consists only of whitespace, it's ignored.  But here there is
+    some non-whitespace stuff, so the line isn't removed. '';
+
+  s8 = ''    ${""}
+    This shows a hacky way to preserve an empty line after the start.
+    But there's no reason to do so: you could just repeat the empty
+    line.
+  '';
+
+  s9 = ''
+  ${""}  Similarly you can force an indentation level,
+    in this case to 2 spaces.  This works because the anti-quote
+    is significant (not whitespace).
+  '';
+
+  s10 = ''
+  '';
+
+  s11 = '''';
+
+  s12 = ''   '';
+
+  s13 = ''
+    start on network-interfaces
+
+    start script
+    
+      rm -f /var/run/opengl-driver
+      ${if true
+        then "ln -sf 123 /var/run/opengl-driver"
+        else if true
+        then "ln -sf 456 /var/run/opengl-driver"
+        else ""
+      }
+
+      rm -f /var/log/slim.log
+       
+    end script
+
+    env SLIM_CFGFILE=${"abc"}
+    env SLIM_THEMESDIR=${"def"}
+    env FONTCONFIG_FILE=/etc/fonts/fonts.conf  				# !!! cleanup
+    env XKB_BINDIR=${"foo"}/bin         				# Needed for the Xkb extension.
+    env LD_LIBRARY_PATH=${"libX11"}/lib:${"libXext"}/lib:/usr/lib/          # related to xorg-sys-opengl - needed to load libglx for (AI)GLX support (for compiz)
+
+    ${if true
+      then "env XORG_DRI_DRIVER_PATH=${"nvidiaDrivers"}/X11R6/lib/modules/drivers/"
+    else if true
+      then "env XORG_DRI_DRIVER_PATH=${"mesa"}/lib/modules/dri"
+      else ""
+    } 
+
+    exec ${"slim"}/bin/slim
+  '';
+
+  s14 = ''
+    Escaping of ' followed by ': '''
+    Escaping of $ followed by {: ''${
+    And finally to interpret \n etc. as in a string: ''\n, ''\r, ''\t.
+  '';
+
+  # Regression test: string interpolation in '${x}' should work, but didn't.
+  s15 = let x = "bla"; in ''
+    foo
+    '${x}'
+    bar
+  '';
+
+  # Regression test: accept $'.
+  s16 = ''
+    cut -d $'\t' -f 1
+  '';
+
+  # Accept dollars at end of strings 
+  s17 = ''ending dollar $'' + ''$'' + "\n";
+
+in s1 + s2 + s3 + s4 + s5 + s6 + s7 + s8 + s9 + s10 + s11 + s12 + s13 + s14 + s15 + s16 + s17
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-intersectAttrs.exp b/tvix/eval/src/tests/nix_tests/eval-okay-intersectAttrs.exp
new file mode 100644
index 0000000000..50445bc0ee
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-intersectAttrs.exp
@@ -0,0 +1 @@
+[ { } { a = 1; } { a = 1; } { a = "a"; } { m = 1; } { m = "m"; } { n = 1; } { n = "n"; } { n = 1; p = 2; } { n = "n"; p = "p"; } { n = 1; p = 2; } { n = "n"; p = "p"; } { a = "a"; b = "b"; c = "c"; d = "d"; e = "e"; f = "f"; g = "g"; h = "h"; i = "i"; j = "j"; k = "k"; l = "l"; m = "m"; n = "n"; o = "o"; p = "p"; q = "q"; r = "r"; s = "s"; t = "t"; u = "u"; v = "v"; w = "w"; x = "x"; y = "y"; z = "z"; } true ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-intersectAttrs.nix b/tvix/eval/src/tests/nix_tests/eval-okay-intersectAttrs.nix
new file mode 100644
index 0000000000..39d49938cc
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-intersectAttrs.nix
@@ -0,0 +1,50 @@
+let
+  alphabet =
+  { a = "a";
+    b = "b";
+    c = "c";
+    d = "d";
+    e = "e";
+    f = "f";
+    g = "g";
+    h = "h";
+    i = "i";
+    j = "j";
+    k = "k";
+    l = "l";
+    m = "m";
+    n = "n";
+    o = "o";
+    p = "p";
+    q = "q";
+    r = "r";
+    s = "s";
+    t = "t";
+    u = "u";
+    v = "v";
+    w = "w";
+    x = "x";
+    y = "y";
+    z = "z";
+  };
+  foo = {
+    inherit (alphabet) f o b a r z q u x;
+    aa = throw "aa";
+  };
+  alphabetFail = builtins.mapAttrs throw alphabet;
+in
+[ (builtins.intersectAttrs { a = abort "l1"; } { b = abort "r1"; })
+  (builtins.intersectAttrs { a = abort "l2"; } { a = 1; })
+  (builtins.intersectAttrs alphabetFail { a = 1; })
+  (builtins.intersectAttrs  { a = abort "laa"; } alphabet)
+  (builtins.intersectAttrs alphabetFail { m = 1; })
+  (builtins.intersectAttrs  { m = abort "lam"; } alphabet)
+  (builtins.intersectAttrs alphabetFail { n = 1; })
+  (builtins.intersectAttrs  { n = abort "lan"; } alphabet)
+  (builtins.intersectAttrs alphabetFail { n = 1; p = 2; })
+  (builtins.intersectAttrs  { n = abort "lan2"; p = abort "lap"; } alphabet)
+  (builtins.intersectAttrs alphabetFail { n = 1; p = 2; })
+  (builtins.intersectAttrs  { n = abort "lan2"; p = abort "lap"; } alphabet)
+  (builtins.intersectAttrs alphabetFail alphabet)
+  (builtins.intersectAttrs alphabet foo == builtins.intersectAttrs foo alphabet)
+]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-let.exp b/tvix/eval/src/tests/nix_tests/eval-okay-let.exp
new file mode 100644
index 0000000000..14e24d4190
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-let.exp
@@ -0,0 +1 @@
+"foobar"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-let.nix b/tvix/eval/src/tests/nix_tests/eval-okay-let.nix
new file mode 100644
index 0000000000..fe118c5282
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-let.nix
@@ -0,0 +1,5 @@
+let {
+  x = "foo";
+  y = "bar";
+  body = x + y;
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-list.exp b/tvix/eval/src/tests/nix_tests/eval-okay-list.exp
new file mode 100644
index 0000000000..f784f26d83
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-list.exp
@@ -0,0 +1 @@
+"foobarblatest"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-list.nix b/tvix/eval/src/tests/nix_tests/eval-okay-list.nix
new file mode 100644
index 0000000000..d433bcf908
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-list.nix
@@ -0,0 +1,7 @@
+with import ./lib.nix;
+
+let {
+
+  body = concat ["foo" "bar" "bla" "test"];
+    
+}
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-listtoattrs.exp b/tvix/eval/src/tests/nix_tests/eval-okay-listtoattrs.exp
new file mode 100644
index 0000000000..74abef7bc6
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-listtoattrs.exp
@@ -0,0 +1 @@
+"AAbar"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-listtoattrs.nix b/tvix/eval/src/tests/nix_tests/eval-okay-listtoattrs.nix
new file mode 100644
index 0000000000..4186e029b5
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-listtoattrs.nix
@@ -0,0 +1,11 @@
+# this test shows how to use listToAttrs and that evaluation is still lazy (throw isn't called)
+with import ./lib.nix;
+
+let 
+  asi = name: value : { inherit name value; };
+  list = [ ( asi "a" "A" ) ( asi "b" "B" ) ];
+  a = builtins.listToAttrs list;
+  b = builtins.listToAttrs ( list ++ list );
+  r = builtins.listToAttrs [ (asi "result" [ a b ]) ( asi "throw" (throw "this should not be thrown")) ];
+  x = builtins.listToAttrs [ (asi "foo" "bar") (asi "foo" "bla") ];
+in concat (map (x: x.a) r.result) + x.foo
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-logic.exp b/tvix/eval/src/tests/nix_tests/eval-okay-logic.exp
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-logic.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-logic.nix b/tvix/eval/src/tests/nix_tests/eval-okay-logic.nix
new file mode 100644
index 0000000000..fbb1279440
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-logic.nix
@@ -0,0 +1 @@
+assert !false && (true || false) -> true; 1
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-map.exp b/tvix/eval/src/tests/nix_tests/eval-okay-map.exp
new file mode 100644
index 0000000000..dbb64f717b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-map.exp
@@ -0,0 +1 @@
+"foobarblabarxyzzybar"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-map.nix b/tvix/eval/src/tests/nix_tests/eval-okay-map.nix
new file mode 100644
index 0000000000..a76c1d8114
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-map.nix
@@ -0,0 +1,3 @@
+with import ./lib.nix;
+
+concat (map (x: x + "bar") [ "foo" "bla" "xyzzy" ])
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-mapattrs.exp b/tvix/eval/src/tests/nix_tests/eval-okay-mapattrs.exp
new file mode 100644
index 0000000000..3f113f17ba
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-mapattrs.exp
@@ -0,0 +1 @@
+{ x = "x-foo"; y = "y-bar"; }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-mapattrs.nix b/tvix/eval/src/tests/nix_tests/eval-okay-mapattrs.nix
new file mode 100644
index 0000000000..f075b6275e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-mapattrs.nix
@@ -0,0 +1,3 @@
+with import ./lib.nix;
+
+builtins.mapAttrs (name: value: name + "-" + value) { x = "foo"; y = "bar"; }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-nested-with.exp b/tvix/eval/src/tests/nix_tests/eval-okay-nested-with.exp
new file mode 100644
index 0000000000..0cfbf08886
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-nested-with.exp
@@ -0,0 +1 @@
+2
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-nested-with.nix b/tvix/eval/src/tests/nix_tests/eval-okay-nested-with.nix
new file mode 100644
index 0000000000..ba9d79aa79
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-nested-with.nix
@@ -0,0 +1,3 @@
+with { x = 1; };
+with { x = 2; };
+x
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-new-let.exp b/tvix/eval/src/tests/nix_tests/eval-okay-new-let.exp
new file mode 100644
index 0000000000..f98b388071
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-new-let.exp
@@ -0,0 +1 @@
+"xyzzyfoobar"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-new-let.nix b/tvix/eval/src/tests/nix_tests/eval-okay-new-let.nix
new file mode 100644
index 0000000000..7381231415
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-new-let.nix
@@ -0,0 +1,14 @@
+let
+
+  f = z: 
+
+    let
+      x = "foo";
+      y = "bar";
+      body = 1; # compat test
+    in
+      z + x + y;
+
+  arg = "xyzzy";
+
+in f arg
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-null-dynamic-attrs.exp b/tvix/eval/src/tests/nix_tests/eval-okay-null-dynamic-attrs.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-null-dynamic-attrs.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-null-dynamic-attrs.nix b/tvix/eval/src/tests/nix_tests/eval-okay-null-dynamic-attrs.nix
new file mode 100644
index 0000000000..b060c0bc98
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-null-dynamic-attrs.nix
@@ -0,0 +1 @@
+{ ${null} = true; } == {}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-partition.exp b/tvix/eval/src/tests/nix_tests/eval-okay-partition.exp
new file mode 100644
index 0000000000..cd8b8b020c
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-partition.exp
@@ -0,0 +1 @@
+{ right = [ 0 2 4 6 8 10 100 102 104 106 108 110 ]; wrong = [ 1 3 5 7 9 101 103 105 107 109 ]; }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-partition.nix b/tvix/eval/src/tests/nix_tests/eval-okay-partition.nix
new file mode 100644
index 0000000000..846d2ce494
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-partition.nix
@@ -0,0 +1,5 @@
+with import ./lib.nix;
+
+builtins.partition
+  (x: x / 2 * 2 == x)
+  (builtins.concatLists [ (range 0 10) (range 100 110) ])
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-path.exp b/tvix/eval/src/tests/nix_tests/eval-okay-path.exp
new file mode 100644
index 0000000000..3ce7f82830
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-path.exp
@@ -0,0 +1 @@
+"/nix/store/ya937r4ydw0l6kayq8jkyqaips9c75jm-output"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-pathexists.exp b/tvix/eval/src/tests/nix_tests/eval-okay-pathexists.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-pathexists.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-pathexists.nix b/tvix/eval/src/tests/nix_tests/eval-okay-pathexists.nix
new file mode 100644
index 0000000000..50c28ee0cd
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-pathexists.nix
@@ -0,0 +1,5 @@
+builtins.pathExists (builtins.toPath ./lib.nix)
+&& builtins.pathExists (builtins.toPath (builtins.toString ./lib.nix))
+&& !builtins.pathExists (builtins.toPath (builtins.toString ./bla.nix))
+&& builtins.pathExists ./lib.nix
+&& !builtins.pathExists ./bla.nix
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-patterns.exp b/tvix/eval/src/tests/nix_tests/eval-okay-patterns.exp
new file mode 100644
index 0000000000..a4304010fe
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-patterns.exp
@@ -0,0 +1 @@
+"abcxyzDDDDEFijk"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-patterns.nix b/tvix/eval/src/tests/nix_tests/eval-okay-patterns.nix
new file mode 100644
index 0000000000..96fd25a015
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-patterns.nix
@@ -0,0 +1,16 @@
+let
+
+  f = args@{x, y, z}: x + args.y + z;
+
+  g = {x, y, z}@args: f args;
+
+  h = {x ? "d", y ? x, z ? args.x}@args: x + y + z;
+
+  j = {x, y, z, ...}: x + y + z;
+
+in
+  f {x = "a"; y = "b"; z = "c";} +
+  g {x = "x"; y = "y"; z = "z";} +
+  h {x = "D";} +
+  h {x = "D"; y = "E"; z = "F";} +
+  j {x = "i"; y = "j"; z = "k"; bla = "bla"; foo = "bar";}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-readfile.exp b/tvix/eval/src/tests/nix_tests/eval-okay-readfile.exp
new file mode 100644
index 0000000000..a2c87d0c43
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-readfile.exp
@@ -0,0 +1 @@
+"builtins.readFile ./eval-okay-readfile.nix\n"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-readfile.nix b/tvix/eval/src/tests/nix_tests/eval-okay-readfile.nix
new file mode 100644
index 0000000000..82f7cb1743
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-readfile.nix
@@ -0,0 +1 @@
+builtins.readFile ./eval-okay-readfile.nix
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-redefine-builtin.exp b/tvix/eval/src/tests/nix_tests/eval-okay-redefine-builtin.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-redefine-builtin.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-redefine-builtin.nix b/tvix/eval/src/tests/nix_tests/eval-okay-redefine-builtin.nix
new file mode 100644
index 0000000000..df9fc3f37d
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-redefine-builtin.nix
@@ -0,0 +1,3 @@
+let
+  throw = abort "Error!";
+in (builtins.tryEval <foobaz>).success
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-regex-match.exp b/tvix/eval/src/tests/nix_tests/eval-okay-regex-match.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-regex-match.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-regex-match.nix b/tvix/eval/src/tests/nix_tests/eval-okay-regex-match.nix
new file mode 100644
index 0000000000..273e259071
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-regex-match.nix
@@ -0,0 +1,29 @@
+with builtins;
+
+let
+
+  matches = pat: s: match pat s != null;
+
+  splitFN = match "((.*)/)?([^/]*)\\.(nix|cc)";
+
+in
+
+assert  matches "foobar" "foobar";
+assert  matches "fo*" "f";
+assert !matches "fo+" "f";
+assert  matches "fo*" "fo";
+assert  matches "fo*" "foo";
+assert  matches "fo+" "foo";
+assert  matches "fo{1,2}" "foo";
+assert !matches "fo{1,2}" "fooo";
+assert !matches "fo*" "foobar";
+assert  matches "[[:space:]]+([^[:space:]]+)[[:space:]]+" "  foo   ";
+assert !matches "[[:space:]]+([[:upper:]]+)[[:space:]]+" "  foo   ";
+
+assert match "(.*)\\.nix" "foobar.nix" == [ "foobar" ];
+assert match "[[:space:]]+([[:upper:]]+)[[:space:]]+" "  FOO   " == [ "FOO" ];
+
+assert splitFN "/path/to/foobar.nix" == [ "/path/to/" "/path/to" "foobar" "nix" ];
+assert splitFN "foobar.cc" == [ null null "foobar" "cc" ];
+
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-regex-split.exp b/tvix/eval/src/tests/nix_tests/eval-okay-regex-split.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-regex-split.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-regex-split.nix b/tvix/eval/src/tests/nix_tests/eval-okay-regex-split.nix
new file mode 100644
index 0000000000..0073e05778
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-regex-split.nix
@@ -0,0 +1,48 @@
+with builtins;
+
+# Non capturing regex returns empty lists
+assert  split "foobar" "foobar"  == ["" [] ""];
+assert  split "fo*" "f"          == ["" [] ""];
+assert  split "fo+" "f"          == ["f"];
+assert  split "fo*" "fo"         == ["" [] ""];
+assert  split "fo*" "foo"        == ["" [] ""];
+assert  split "fo+" "foo"        == ["" [] ""];
+assert  split "fo{1,2}" "foo"    == ["" [] ""];
+assert  split "fo{1,2}" "fooo"   == ["" [] "o"];
+assert  split "fo*" "foobar"     == ["" [] "bar"];
+
+# Capturing regex returns a list of sub-matches
+assert  split "(fo*)" "f"        == ["" ["f"] ""];
+assert  split "(fo+)" "f"        == ["f"];
+assert  split "(fo*)" "fo"       == ["" ["fo"] ""];
+assert  split "(f)(o*)" "f"      == ["" ["f" ""] ""];
+assert  split "(f)(o*)" "foo"    == ["" ["f" "oo"] ""];
+assert  split "(fo+)" "foo"      == ["" ["foo"] ""];
+assert  split "(fo{1,2})" "foo"  == ["" ["foo"] ""];
+assert  split "(fo{1,2})" "fooo" == ["" ["foo"] "o"];
+assert  split "(fo*)" "foobar"   == ["" ["foo"] "bar"];
+
+# Matches are greedy.
+assert  split "(o+)" "oooofoooo" == ["" ["oooo"] "f" ["oooo"] ""];
+
+# Matches multiple times.
+assert  split "(b)" "foobarbaz"  == ["foo" ["b"] "ar" ["b"] "az"];
+
+# Split large strings containing newlines. null are inserted when a
+# pattern within the current did not match anything.
+assert  split "[[:space:]]+|([',.!?])" ''
+  Nix Rocks!
+  That's why I use it.
+''  == [
+  "Nix" [ null ] "Rocks" ["!"] "" [ null ]
+  "That" ["'"] "s" [ null ] "why" [ null ] "I" [ null ] "use" [ null ] "it" ["."] "" [ null ]
+  ""
+];
+
+# Documentation examples
+assert  split  "(a)b" "abc"      == [ "" [ "a" ] "c" ];
+assert  split  "([ac])" "abc"    == [ "" [ "a" ] "b" [ "c" ] "" ];
+assert  split  "(a)|(c)" "abc"   == [ "" [ "a" null ] "b" [ null "c" ] "" ];
+assert  split  "([[:upper:]]+)" "  FOO   " == [ "  " [ "FOO" ] "   " ];
+
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220122.exp b/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220122.exp
new file mode 100644
index 0000000000..00750edc07
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220122.exp
@@ -0,0 +1 @@
+3
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220122.nix b/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220122.nix
new file mode 100644
index 0000000000..694e9a13b7
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220122.nix
@@ -0,0 +1 @@
+((_: _) 1) + ((__: __) 2)
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220125.exp b/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220125.exp
new file mode 100644
index 0000000000..00750edc07
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220125.exp
@@ -0,0 +1 @@
+3
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220125.nix b/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220125.nix
new file mode 100644
index 0000000000..4855023739
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-regression-20220125.nix
@@ -0,0 +1,2 @@
+((__curPosFoo: __curPosFoo) 1) + ((__curPosBar: __curPosBar) 2)
+
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-remove.exp b/tvix/eval/src/tests/nix_tests/eval-okay-remove.exp
new file mode 100644
index 0000000000..8d38505c16
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-remove.exp
@@ -0,0 +1 @@
+456
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-remove.nix b/tvix/eval/src/tests/nix_tests/eval-okay-remove.nix
new file mode 100644
index 0000000000..4ad5ba897f
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-remove.nix
@@ -0,0 +1,5 @@
+let {
+  attrs = {x = 123; y = 456;};
+
+  body = (removeAttrs attrs ["x"]).y;
+}
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-1.exp b/tvix/eval/src/tests/nix_tests/eval-okay-scope-1.exp
new file mode 100644
index 0000000000..00750edc07
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-1.exp
@@ -0,0 +1 @@
+3
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-1.nix b/tvix/eval/src/tests/nix_tests/eval-okay-scope-1.nix
new file mode 100644
index 0000000000..fa38a7174e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-1.nix
@@ -0,0 +1,6 @@
+(({x}: x:
+
+  { x = 1;
+    y = x;
+  }
+) {x = 2;} 3).y
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-2.exp b/tvix/eval/src/tests/nix_tests/eval-okay-scope-2.exp
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-2.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-2.nix b/tvix/eval/src/tests/nix_tests/eval-okay-scope-2.nix
new file mode 100644
index 0000000000..eb8b02bc49
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-2.nix
@@ -0,0 +1,6 @@
+((x: {x}:
+  rec {
+    x = 1;
+    y = x;
+  }
+) 2 {x = 3;}).y
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-3.exp b/tvix/eval/src/tests/nix_tests/eval-okay-scope-3.exp
new file mode 100644
index 0000000000..b8626c4cff
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-3.exp
@@ -0,0 +1 @@
+4
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-3.nix b/tvix/eval/src/tests/nix_tests/eval-okay-scope-3.nix
new file mode 100644
index 0000000000..10d6bc04d8
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-3.nix
@@ -0,0 +1,6 @@
+((x: as: {x}:
+  rec {
+    inherit (as) x;
+    y = x;
+  }
+) 2 {x = 4;} {x = 3;}).y
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-4.exp b/tvix/eval/src/tests/nix_tests/eval-okay-scope-4.exp
new file mode 100644
index 0000000000..00ff03a46c
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-4.exp
@@ -0,0 +1 @@
+"ccdd"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-4.nix b/tvix/eval/src/tests/nix_tests/eval-okay-scope-4.nix
new file mode 100644
index 0000000000..dc8243bc85
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-4.nix
@@ -0,0 +1,10 @@
+let {
+
+  x = "a";
+  y = "b";
+
+  f = {x ? y, y ? x}: x + y;
+
+  body = f {x = "c";} + f {y = "d";};
+
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-6.exp b/tvix/eval/src/tests/nix_tests/eval-okay-scope-6.exp
new file mode 100644
index 0000000000..00ff03a46c
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-6.exp
@@ -0,0 +1 @@
+"ccdd"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-6.nix b/tvix/eval/src/tests/nix_tests/eval-okay-scope-6.nix
new file mode 100644
index 0000000000..0995d4e7e7
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-6.nix
@@ -0,0 +1,7 @@
+let {
+
+  f = {x ? y, y ? x}: x + y;
+
+  body = f {x = "c";} + f {y = "d";};
+
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-7.exp b/tvix/eval/src/tests/nix_tests/eval-okay-scope-7.exp
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-7.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-scope-7.nix b/tvix/eval/src/tests/nix_tests/eval-okay-scope-7.nix
new file mode 100644
index 0000000000..4da02968f6
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-scope-7.nix
@@ -0,0 +1,6 @@
+rec {
+  inherit (x) y;
+  x = {
+    y = 1;
+  };
+}.y
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-seq.exp b/tvix/eval/src/tests/nix_tests/eval-okay-seq.exp
new file mode 100644
index 0000000000..0cfbf08886
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-seq.exp
@@ -0,0 +1 @@
+2
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-seq.nix b/tvix/eval/src/tests/nix_tests/eval-okay-seq.nix
new file mode 100644
index 0000000000..0a9a21c03b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-seq.nix
@@ -0,0 +1 @@
+builtins.seq 1 2
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-sort.exp b/tvix/eval/src/tests/nix_tests/eval-okay-sort.exp
new file mode 100644
index 0000000000..899119e20e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-sort.exp
@@ -0,0 +1 @@
+[ [ 42 77 147 249 483 526 ] [ 526 483 249 147 77 42 ] [ "bar" "fnord" "foo" "xyzzy" ] [ { key = 1; value = "foo"; } { key = 1; value = "fnord"; } { key = 2; value = "bar"; } ] [ [ ] [ ] [ 1 ] [ 1 4 ] [ 1 5 ] [ 1 6 ] [ 2 ] [ 2 3 ] [ 3 ] [ 3 ] ] ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-sort.nix b/tvix/eval/src/tests/nix_tests/eval-okay-sort.nix
new file mode 100644
index 0000000000..50aa78e403
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-sort.nix
@@ -0,0 +1,20 @@
+with builtins;
+
+[ (sort lessThan [ 483 249 526 147 42 77 ])
+  (sort (x: y: y < x) [ 483 249 526 147 42 77 ])
+  (sort lessThan [ "foo" "bar" "xyzzy" "fnord" ])
+  (sort (x: y: x.key < y.key)
+    [ { key = 1; value = "foo"; } { key = 2; value = "bar"; } { key = 1; value = "fnord"; } ])
+  (sort lessThan [
+    [ 1 6 ]
+    [ ]
+    [ 2 3 ]
+    [ 3 ]
+    [ 1 5 ]
+    [ 2 ]
+    [ 1 ]
+    [ ]
+    [ 1 4 ]
+    [ 3 ]
+  ])
+]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-splitversion.exp b/tvix/eval/src/tests/nix_tests/eval-okay-splitversion.exp
new file mode 100644
index 0000000000..153ceb8186
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-splitversion.exp
@@ -0,0 +1 @@
+[ "1" "2" "3" ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-splitversion.nix b/tvix/eval/src/tests/nix_tests/eval-okay-splitversion.nix
new file mode 100644
index 0000000000..9e5c99d2e7
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-splitversion.nix
@@ -0,0 +1 @@
+builtins.splitVersion "1.2.3"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-string.exp b/tvix/eval/src/tests/nix_tests/eval-okay-string.exp
new file mode 100644
index 0000000000..63f650f73a
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-string.exp
@@ -0,0 +1 @@
+"foobar/a/b/c/d/foo/xyzzy/foo.txt/../foo/x/yescape: \"quote\" \n \\end\nof\nlinefoobarblaatfoo$bar$\"$\"$"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-string.nix b/tvix/eval/src/tests/nix_tests/eval-okay-string.nix
new file mode 100644
index 0000000000..47cc989ad4
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-string.nix
@@ -0,0 +1,12 @@
+"foo" + "bar"
+  + toString (/a/b + /c/d)
+  + toString (/foo/bar + "/../xyzzy/." + "/foo.txt")
+  + ("/../foo" + toString /x/y)
+  + "escape: \"quote\" \n \\"
+  + "end
+of
+line"
+  + "foo${if true then "b${"a" + "r"}" else "xyzzy"}blaat"
+  + "foo$bar"
+  + "$\"$\""
+  + "$"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-strings-as-attrs-names.exp b/tvix/eval/src/tests/nix_tests/eval-okay-strings-as-attrs-names.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-strings-as-attrs-names.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-strings-as-attrs-names.nix b/tvix/eval/src/tests/nix_tests/eval-okay-strings-as-attrs-names.nix
new file mode 100644
index 0000000000..5e40928dbe
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-strings-as-attrs-names.nix
@@ -0,0 +1,20 @@
+let
+
+  attr = {
+    "key 1" = "test";
+    "key 2" = "caseok";
+  };
+
+  t1 = builtins.getAttr "key 1" attr;
+  t2 = attr."key 2";
+  t3 = attr ? "key 1";
+  t4 = builtins.attrNames { inherit (attr) "key 1"; };
+
+  # This is permitted, but there is currently no way to reference this
+  # variable.
+  "foo bar" = 1;
+
+in t1 == "test"
+   && t2 == "caseok"
+   && t3 == true
+   && t4 == ["key 1"]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-substring.exp b/tvix/eval/src/tests/nix_tests/eval-okay-substring.exp
new file mode 100644
index 0000000000..6aace04b0f
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-substring.exp
@@ -0,0 +1 @@
+"ooxfoobarybarzobaabbc"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-substring.nix b/tvix/eval/src/tests/nix_tests/eval-okay-substring.nix
new file mode 100644
index 0000000000..424af00d9b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-substring.nix
@@ -0,0 +1,21 @@
+with builtins;
+
+let
+
+  s = "foobar";
+
+in
+
+substring 1 2 s
++ "x"
++ substring 0 (stringLength s) s
++ "y"
++ substring 3 100 s
++ "z"
++ substring 2 (sub (stringLength s) 3) s
++ "a"
++ substring 3 0 s
++ "b"
++ substring 3 1 s
++ "c"
++ substring 5 10 "perl"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-tail-call-1.exp b/tvix/eval/src/tests/nix_tests/eval-okay-tail-call-1.exp
new file mode 100644
index 0000000000..f7393e847d
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-tail-call-1.exp
@@ -0,0 +1 @@
+100000
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-tail-call-1.nix b/tvix/eval/src/tests/nix_tests/eval-okay-tail-call-1.nix
new file mode 100644
index 0000000000..a3962ce3fd
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-tail-call-1.nix
@@ -0,0 +1,3 @@
+let
+  f = n: if n == 100000 then n else f (n + 1);
+in f 0
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-tojson.exp b/tvix/eval/src/tests/nix_tests/eval-okay-tojson.exp
new file mode 100644
index 0000000000..e92aae3235
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-tojson.exp
@@ -0,0 +1 @@
+"{\"a\":123,\"b\":-456,\"c\":\"foo\",\"d\":\"foo\\n\\\"bar\\\"\",\"e\":true,\"f\":false,\"g\":[1,2,3],\"h\":[\"a\",[\"b\",{\"foo\\nbar\":{}}]],\"i\":3,\"j\":1.44,\"k\":\"foo\"}"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-tojson.nix b/tvix/eval/src/tests/nix_tests/eval-okay-tojson.nix
new file mode 100644
index 0000000000..ce67943bea
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-tojson.nix
@@ -0,0 +1,13 @@
+builtins.toJSON
+  { a = 123;
+    b = -456;
+    c = "foo";
+    d = "foo\n\"bar\"";
+    e = true;
+    f = false;
+    g = [ 1 2 3 ];
+    h = [ "a" [ "b" { "foo\nbar" = {}; } ] ];
+    i = 1 + 2;
+    j = 1.44;
+    k = { __toString = self: self.a; a = "foo"; };
+  }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-toxml.exp b/tvix/eval/src/tests/nix_tests/eval-okay-toxml.exp
new file mode 100644
index 0000000000..828220890e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-toxml.exp
@@ -0,0 +1 @@
+"<?xml version='1.0' encoding='utf-8'?>\n<expr>\n  <attrs>\n    <attr name=\"a\">\n      <string value=\"s\" />\n    </attr>\n  </attrs>\n</expr>\n"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-toxml.nix b/tvix/eval/src/tests/nix_tests/eval-okay-toxml.nix
new file mode 100644
index 0000000000..068c97a6c1
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-toxml.nix
@@ -0,0 +1,3 @@
+# Make sure the expected XML output is produced; in particular, make sure it
+# doesn't contain source location information.
+builtins.toXML { a = "s"; }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-toxml2.exp b/tvix/eval/src/tests/nix_tests/eval-okay-toxml2.exp
new file mode 100644
index 0000000000..634a841eb1
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-toxml2.exp
@@ -0,0 +1 @@
+"<?xml version='1.0' encoding='utf-8'?>\n<expr>\n  <list>\n    <string value=\"ab\" />\n    <int value=\"10\" />\n    <attrs>\n      <attr name=\"x\">\n        <string value=\"x\" />\n      </attr>\n      <attr name=\"y\">\n        <string value=\"x\" />\n      </attr>\n    </attrs>\n  </list>\n</expr>\n"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-toxml2.nix b/tvix/eval/src/tests/nix_tests/eval-okay-toxml2.nix
new file mode 100644
index 0000000000..ff1791b30e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-toxml2.nix
@@ -0,0 +1 @@
+builtins.toXML [("a" + "b") 10 (rec {x = "x"; y = x;})]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-tryeval.exp b/tvix/eval/src/tests/nix_tests/eval-okay-tryeval.exp
new file mode 100644
index 0000000000..2b2e6fa711
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-tryeval.exp
@@ -0,0 +1 @@
+{ x = { success = true; value = "x"; }; y = { success = false; value = false; }; z = { success = false; value = false; }; }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-tryeval.nix b/tvix/eval/src/tests/nix_tests/eval-okay-tryeval.nix
new file mode 100644
index 0000000000..629bc440a8
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-tryeval.nix
@@ -0,0 +1,5 @@
+{
+  x = builtins.tryEval "x";
+  y = builtins.tryEval (assert false; "y");
+  z = builtins.tryEval (throw "bla");
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-types.exp b/tvix/eval/src/tests/nix_tests/eval-okay-types.exp
new file mode 100644
index 0000000000..92a1532993
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-types.exp
@@ -0,0 +1 @@
+[ true false true false true false true false true true true true true true true true true true true false true true true false "int" "bool" "string" "null" "set" "list" "lambda" "lambda" "lambda" "lambda" ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-types.nix b/tvix/eval/src/tests/nix_tests/eval-okay-types.nix
new file mode 100644
index 0000000000..9b58be5d1d
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-types.nix
@@ -0,0 +1,37 @@
+with builtins;
+
+[ (isNull null)
+  (isNull (x: x))
+  (isFunction (x: x))
+  (isFunction "fnord")
+  (isString ("foo" + "bar"))
+  (isString [ "x" ])
+  (isInt (1 + 2))
+  (isInt { x = 123; })
+  (isInt (1 / 2))
+  (isInt (1 + 1))
+  (isInt (1 / 2))
+  (isInt (1 * 2))
+  (isInt (1 - 2))
+  (isFloat (1.2))
+  (isFloat (1 + 1.0))
+  (isFloat (1 / 2.0))
+  (isFloat (1 * 2.0))
+  (isFloat (1 - 2.0))
+  (isBool (true && false))
+  (isBool null)
+  (isPath /nix/store)
+  (isPath ./.)
+  (isAttrs { x = 123; })
+  (isAttrs null)
+  (typeOf (3 * 4))
+  (typeOf true)
+  (typeOf "xyzzy")
+  (typeOf null)
+  (typeOf { x = 456; })
+  (typeOf [ 1 2 3 ])
+  (typeOf (x: x))
+  (typeOf ((x: y: x) 1))
+  (typeOf map)
+  (typeOf (map (x: x)))
+]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-versions.exp b/tvix/eval/src/tests/nix_tests/eval-okay-versions.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-versions.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-versions.nix b/tvix/eval/src/tests/nix_tests/eval-okay-versions.nix
new file mode 100644
index 0000000000..e9111f5f43
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-versions.nix
@@ -0,0 +1,43 @@
+let
+
+  name1 = "hello-1.0.2";
+  name2 = "hello";
+  name3 = "915resolution-0.5.2";
+  name4 = "xf86-video-i810-1.7.4";
+  name5 = "name-that-ends-with-dash--1.0";
+
+  eq = 0;
+  lt = builtins.sub 0 1;
+  gt = 1;
+
+  versionTest = v1: v2: expected:
+    let d1 = builtins.compareVersions v1 v2;
+        d2 = builtins.compareVersions v2 v1;
+    in d1 == builtins.sub 0 d2 && d1 == expected;
+
+  tests = [
+    ((builtins.parseDrvName name1).name == "hello")
+    ((builtins.parseDrvName name1).version == "1.0.2")
+    ((builtins.parseDrvName name2).name == "hello")
+    ((builtins.parseDrvName name2).version == "")
+    ((builtins.parseDrvName name3).name == "915resolution")
+    ((builtins.parseDrvName name3).version == "0.5.2")
+    ((builtins.parseDrvName name4).name == "xf86-video-i810")
+    ((builtins.parseDrvName name4).version == "1.7.4")
+    ((builtins.parseDrvName name5).name == "name-that-ends-with-dash")
+    ((builtins.parseDrvName name5).version == "-1.0")
+    (versionTest "1.0" "2.3" lt)
+    (versionTest "2.1" "2.3" lt)
+    (versionTest "2.3" "2.3" eq)
+    (versionTest "2.5" "2.3" gt)
+    (versionTest "3.1" "2.3" gt)
+    (versionTest "2.3.1" "2.3" gt)
+    (versionTest "2.3.1" "2.3a" gt)
+    (versionTest "2.3pre1" "2.3" lt)
+    (versionTest "2.3pre3" "2.3pre12" lt)
+    (versionTest "2.3a" "2.3c" lt)
+    (versionTest "2.3pre1" "2.3c" lt)
+    (versionTest "2.3pre1" "2.3q" lt)
+  ];
+
+in (import ./lib.nix).and tests
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-with.exp b/tvix/eval/src/tests/nix_tests/eval-okay-with.exp
new file mode 100644
index 0000000000..378c8dc804
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-with.exp
@@ -0,0 +1 @@
+"xyzzybarxyzzybar"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-with.nix b/tvix/eval/src/tests/nix_tests/eval-okay-with.nix
new file mode 100644
index 0000000000..033e8d3aba
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-with.nix
@@ -0,0 +1,19 @@
+let {
+
+  a = "xyzzy";
+
+  as = {
+    a = "foo";
+    b = "bar";
+  };
+
+  bs = {
+    a = "bar";
+  };
+
+  x = with as; a + b;
+
+  y = with as; with bs; a + b;
+
+  body = x + y;
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-xml.exp.xml b/tvix/eval/src/tests/nix_tests/eval-okay-xml.exp.xml
new file mode 100644
index 0000000000..20099326cc
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-xml.exp.xml
@@ -0,0 +1,52 @@
+<?xml version='1.0' encoding='utf-8'?>
+<expr>
+  <attrs>
+    <attr name="a">
+      <string value="foo" />
+    </attr>
+    <attr name="at">
+      <function>
+        <attrspat name="args">
+          <attr name="x" />
+          <attr name="y" />
+          <attr name="z" />
+        </attrspat>
+      </function>
+    </attr>
+    <attr name="b">
+      <string value="bar" />
+    </attr>
+    <attr name="c">
+      <string value="foobar" />
+    </attr>
+    <attr name="ellipsis">
+      <function>
+        <attrspat ellipsis="1">
+          <attr name="x" />
+          <attr name="y" />
+          <attr name="z" />
+        </attrspat>
+      </function>
+    </attr>
+    <attr name="f">
+      <function>
+        <attrspat>
+          <attr name="x" />
+          <attr name="y" />
+          <attr name="z" />
+        </attrspat>
+      </function>
+    </attr>
+    <attr name="id">
+      <function>
+        <varpat name="x" />
+      </function>
+    </attr>
+    <attr name="x">
+      <int value="123" />
+    </attr>
+    <attr name="y">
+      <float value="567.89" />
+    </attr>
+  </attrs>
+</expr>
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-xml.nix b/tvix/eval/src/tests/nix_tests/eval-okay-xml.nix
new file mode 100644
index 0000000000..9ee9f8a0b4
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-xml.nix
@@ -0,0 +1,21 @@
+rec {
+
+  x = 123;
+
+  y = 567.890;
+
+  a = "foo";
+
+  b = "bar";
+
+  c = "foo" + "bar";
+
+  f = {z, x, y}: if y then x else z;
+
+  id = x: x;
+
+  at = args@{x, y, z}: x;
+
+  ellipsis = {x, y, z, ...}: x;
+
+}
diff --git a/tvix/eval/src/tests/nix_tests/imported.nix b/tvix/eval/src/tests/nix_tests/imported.nix
new file mode 100644
index 0000000000..fb39ee4efa
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/imported.nix
@@ -0,0 +1,3 @@
+# The function โ€˜rangeโ€™ comes from lib.nix and was added to the lexical
+# scope by scopedImport.
+range 1 5 ++ import ./imported2.nix
diff --git a/tvix/eval/src/tests/nix_tests/imported2.nix b/tvix/eval/src/tests/nix_tests/imported2.nix
new file mode 100644
index 0000000000..6d0a2992b7
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/imported2.nix
@@ -0,0 +1 @@
+range 6 10
diff --git a/tvix/eval/src/tests/nix_tests/lib.nix b/tvix/eval/src/tests/nix_tests/lib.nix
new file mode 100644
index 0000000000..028a538314
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/lib.nix
@@ -0,0 +1,61 @@
+with builtins;
+
+rec {
+
+  fold = op: nul: list:
+    if list == []
+    then nul
+    else op (head list) (fold op nul (tail list));
+
+  concat =
+    fold (x: y: x + y) "";
+
+  and = fold (x: y: x && y) true;
+
+  flatten = x:
+    if isList x
+    then fold (x: y: (flatten x) ++ y) [] x
+    else [x];
+
+  sum = foldl' (x: y: add x y) 0;
+
+  hasSuffix = ext: fileName:
+    let lenFileName = stringLength fileName;
+        lenExt = stringLength ext;
+    in !(lessThan lenFileName lenExt) &&
+       substring (sub lenFileName lenExt) lenFileName fileName == ext;
+
+  # Split a list at the given position.
+  splitAt = pos: list:
+    if pos == 0 then {first = []; second = list;} else
+    if list == [] then {first = []; second = [];} else
+    let res = splitAt (sub pos 1) (tail list);
+    in {first = [(head list)] ++ res.first; second = res.second;};
+
+  # Stable merge sort.
+  sortBy = comp: list:
+    if lessThan 1 (length list)
+    then
+      let
+        split = splitAt (div (length list) 2) list;
+        first = sortBy comp split.first;
+        second = sortBy comp split.second;
+      in mergeLists comp first second
+    else list;
+
+  mergeLists = comp: list1: list2:
+    if list1 == [] then list2 else
+    if list2 == [] then list1 else
+    if comp (head list2) (head list1) then [(head list2)] ++ mergeLists comp list1 (tail list2) else
+    [(head list1)] ++ mergeLists comp (tail list1) list2;
+
+  id = x: x;
+
+  const = x: y: x;
+
+  range = first: last:
+    if first > last
+      then []
+      else genList (n: first + n) (last - first + 1);
+
+}
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-bad-antiquote-2.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-bad-antiquote-2.nix
new file mode 100644
index 0000000000..3745235ce9
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-bad-antiquote-2.nix
@@ -0,0 +1 @@
+"${./fnord}"
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-fromTOML-timestamps.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-fromTOML-timestamps.nix
new file mode 100644
index 0000000000..74cff9470a
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-fromTOML-timestamps.nix
@@ -0,0 +1,130 @@
+builtins.fromTOML ''
+  key = "value"
+  bare_key = "value"
+  bare-key = "value"
+  1234 = "value"
+
+  "127.0.0.1" = "value"
+  "character encoding" = "value"
+  "สŽวสž" = "value"
+  'key2' = "value"
+  'quoted "value"' = "value"
+
+  name = "Orange"
+
+  physical.color = "orange"
+  physical.shape = "round"
+  site."google.com" = true
+
+  # This is legal according to the spec, but cpptoml doesn't handle it.
+  #a.b.c = 1
+  #a.d = 2
+
+  str = "I'm a string. \"You can quote me\". Name\tJos\u00E9\nLocation\tSF."
+
+  int1 = +99
+  int2 = 42
+  int3 = 0
+  int4 = -17
+  int5 = 1_000
+  int6 = 5_349_221
+  int7 = 1_2_3_4_5
+
+  hex1 = 0xDEADBEEF
+  hex2 = 0xdeadbeef
+  hex3 = 0xdead_beef
+
+  oct1 = 0o01234567
+  oct2 = 0o755
+
+  bin1 = 0b11010110
+
+  flt1 = +1.0
+  flt2 = 3.1415
+  flt3 = -0.01
+  flt4 = 5e+22
+  flt5 = 1e6
+  flt6 = -2E-2
+  flt7 = 6.626e-34
+  flt8 = 9_224_617.445_991_228_313
+
+  bool1 = true
+  bool2 = false
+
+  odt1 = 1979-05-27T07:32:00Z
+  odt2 = 1979-05-27T00:32:00-07:00
+  odt3 = 1979-05-27T00:32:00.999999-07:00
+  odt4 = 1979-05-27 07:32:00Z
+  ldt1 = 1979-05-27T07:32:00
+  ldt2 = 1979-05-27T00:32:00.999999
+  ld1 = 1979-05-27
+  lt1 = 07:32:00
+  lt2 = 00:32:00.999999
+
+  arr1 = [ 1, 2, 3 ]
+  arr2 = [ "red", "yellow", "green" ]
+  arr3 = [ [ 1, 2 ], [3, 4, 5] ]
+  arr4 = [ "all", 'strings', """are the same""", ''''type'''']
+  arr5 = [ [ 1, 2 ], ["a", "b", "c"] ]
+
+  arr7 = [
+    1, 2, 3
+  ]
+
+  arr8 = [
+    1,
+    2, # this is ok
+  ]
+
+  [table-1]
+  key1 = "some string"
+  key2 = 123
+
+
+  [table-2]
+  key1 = "another string"
+  key2 = 456
+
+  [dog."tater.man"]
+  type.name = "pug"
+
+  [a.b.c]
+  [ d.e.f ]
+  [ g .  h  . i ]
+  [ j . "สž" . 'l' ]
+  [x.y.z.w]
+
+  name = { first = "Tom", last = "Preston-Werner" }
+  point = { x = 1, y = 2 }
+  animal = { type.name = "pug" }
+
+  [[products]]
+  name = "Hammer"
+  sku = 738594937
+
+  [[products]]
+
+  [[products]]
+  name = "Nail"
+  sku = 284758393
+  color = "gray"
+
+  [[fruit]]
+    name = "apple"
+
+    [fruit.physical]
+      color = "red"
+      shape = "round"
+
+    [[fruit.variety]]
+      name = "red delicious"
+
+    [[fruit.variety]]
+      name = "granny smith"
+
+  [[fruit]]
+    name = "banana"
+
+    [[fruit.variety]]
+      name = "plantain"
+''
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-nonexist-path.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-nonexist-path.nix
new file mode 100644
index 0000000000..f2f08107b5
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-nonexist-path.nix
@@ -0,0 +1,4 @@
+# This must fail to evaluate, since ./fnord doesn't exist.  If it did
+# exist, it would produce "/nix/store/<hash>-fnord/xyzzy" (with an
+# appropriate context).
+"${./fnord}/xyzzy"
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-scope-5.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-scope-5.nix
new file mode 100644
index 0000000000..f89a65a99b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-scope-5.nix
@@ -0,0 +1,10 @@
+let {
+
+  x = "a";
+  y = "b";
+
+  f = {x ? y, y ? x}: x + y;
+
+  body = f {};
+
+}
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-undeclared-arg.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-undeclared-arg.nix
new file mode 100644
index 0000000000..cafdf16362
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-undeclared-arg.nix
@@ -0,0 +1 @@
+({x, z}: x + z) {x = "foo"; y = "bla"; z = "bar";}
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.exp
new file mode 100644
index 0000000000..b46938032e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.exp
@@ -0,0 +1 @@
+{ __overrides = { bar = "qux"; }; bar = "qux"; foo = "bar"; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.nix
new file mode 100644
index 0000000000..2e5c85483b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.nix
@@ -0,0 +1,4 @@
+rec {
+  "${"foo"}" = "bar";
+   __overrides = { bar = "qux"; };
+}
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.exp
new file mode 100644
index 0000000000..7a8391786a
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.exp
@@ -0,0 +1 @@
+"xyzzy!xyzzy!foobar"
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.flags b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.flags
new file mode 100644
index 0000000000..217c7a5ae2
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.flags
@@ -0,0 +1 @@
+--arg lib import(nix_tests/lib.nix) --argstr xyzzy xyzzy! -A result
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.nix
new file mode 100644
index 0000000000..815f51b1d6
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-autoargs.nix
@@ -0,0 +1,15 @@
+let
+
+  foobar = "foobar";
+
+in
+
+{ xyzzy2 ? xyzzy # mutually recursive args
+, xyzzy ? "blaat" # will be overridden by --argstr
+, fb ? foobar
+, lib # will be set by --arg
+}:
+
+{
+  result = lib.concat [xyzzy xyzzy2 fb];
+}
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.exp
new file mode 100644
index 0000000000..03b400cc88
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.exp
@@ -0,0 +1 @@
+[ true true true true true true ]
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.nix
new file mode 100644
index 0000000000..50a78d946e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.nix
@@ -0,0 +1,41 @@
+let
+  drv = derivation {
+    name = "fail";
+    builder = "/bin/false";
+    system = "x86_64-linux";
+    outputs = [ "out" "foo" ];
+  };
+
+  path = "${./eval-okay-context-introspection.nix}";
+
+  desired-context = {
+    "${builtins.unsafeDiscardStringContext path}" = {
+      path = true;
+    };
+    "${builtins.unsafeDiscardStringContext drv.drvPath}" = {
+      outputs = [ "foo" "out" ];
+      allOutputs = true;
+    };
+  };
+
+  combo-path = "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+  legit-context = builtins.getContext combo-path;
+
+  reconstructed-path = builtins.appendContext
+    (builtins.unsafeDiscardStringContext combo-path)
+    desired-context;
+
+  # Eta rule for strings with context.
+  etaRule = str:
+    str == builtins.appendContext
+      (builtins.unsafeDiscardStringContext str)
+      (builtins.getContext str);
+
+in [
+  (legit-context == desired-context)
+  (reconstructed-path == combo-path)
+  (etaRule "foo")
+  (etaRule drv.drvPath)
+  (etaRule drv.foo.outPath)
+  (etaRule (builtins.unsafeDiscardOutputDependency drv.drvPath))
+]
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context.exp
new file mode 100644
index 0000000000..2f535bdbc4
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context.exp
@@ -0,0 +1 @@
+"foo eval-okay-context.nix bar"
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context.nix
new file mode 100644
index 0000000000..7b9531cfe9
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context.nix
@@ -0,0 +1,6 @@
+let s = "foo ${builtins.substring 33 100 (baseNameOf "${./eval-okay-context.nix}")} bar";
+in
+  if s != "foo eval-okay-context.nix bar"
+  then abort "context not discarded"
+  else builtins.unsafeDiscardStringContext s
+
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-curpos.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-curpos.exp
new file mode 100644
index 0000000000..65fd65b4d0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-curpos.exp
@@ -0,0 +1 @@
+[ 3 7 4 9 ]
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-curpos.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-curpos.nix
new file mode 100644
index 0000000000..b79553df0b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-curpos.nix
@@ -0,0 +1,5 @@
+# Bla
+let
+  x = __curPos;
+    y = __curPos;
+in [ x.line x.column y.line y.column ]
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-delayed-with.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-delayed-with.exp
new file mode 100644
index 0000000000..8e7c61ab8e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-delayed-with.exp
@@ -0,0 +1 @@
+"b-overridden b-overridden a"
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-delayed-with.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-delayed-with.nix
new file mode 100644
index 0000000000..3fb023e1cd
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-delayed-with.nix
@@ -0,0 +1,29 @@
+let
+
+  pkgs_ = with pkgs; {
+    a = derivation {
+      name = "a";
+      system = builtins.currentSystem;
+      builder = "/bin/sh";
+      args = [ "-c" "touch $out" ];
+      inherit b;
+    };
+
+    b = derivation {
+      name = "b";
+      system = builtins.currentSystem;
+      builder = "/bin/sh";
+      args = [ "-c" "touch $out" ];
+      inherit a;
+    };
+
+    c = b;
+  };
+
+  packageOverrides = pkgs: with pkgs; {
+    b = derivation (b.drvAttrs // { name = "${b.name}-overridden"; });
+  };
+
+  pkgs = pkgs_ // (packageOverrides pkgs_);
+
+in "${pkgs.a.b.name} ${pkgs.c.name} ${pkgs.b.a.name}"
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-eq-derivations.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-eq-derivations.exp
new file mode 100644
index 0000000000..ec04aab6ae
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-eq-derivations.exp
@@ -0,0 +1 @@
+[ true true true false ]
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-eq-derivations.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-eq-derivations.nix
new file mode 100644
index 0000000000..d526cb4a21
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-eq-derivations.nix
@@ -0,0 +1,10 @@
+let
+
+  drvA1 = derivation { name = "a"; builder = "/foo"; system = "i686-linux"; };
+  drvA2 = derivation { name = "a"; builder = "/foo"; system = "i686-linux"; };
+  drvA3 = derivation { name = "a"; builder = "/foo"; system = "i686-linux"; } // { dummy = 1; };
+  
+  drvC1 = derivation { name = "c"; builder = "/foo"; system = "i686-linux"; };
+  drvC2 = derivation { name = "c"; builder = "/bar"; system = "i686-linux"; };
+
+in [ (drvA1 == drvA1) (drvA1 == drvA2) (drvA1 == drvA3) (drvC1 == drvC2) ]
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-floor-ceil.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-floor-ceil.exp
new file mode 100644
index 0000000000..81f80420b9
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-floor-ceil.exp
@@ -0,0 +1 @@
+"23;24;23;23"
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-floor-ceil.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-floor-ceil.nix
new file mode 100644
index 0000000000..d76a0d86ea
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-floor-ceil.nix
@@ -0,0 +1,9 @@
+with import ./lib.nix;
+
+let
+  n1 = builtins.floor 23.5;
+  n2 = builtins.ceil 23.5;
+  n3 = builtins.floor 23;
+  n4 = builtins.ceil 23;
+in
+  builtins.concatStringsSep ";" (map toString [ n1 n2 n3 n4 ])
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.exp
new file mode 100644
index 0000000000..08b3c69a6c
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.exp
@@ -0,0 +1 @@
+{ "1234" = "value"; "127.0.0.1" = "value"; a = { b = { c = { }; }; }; arr1 = [ 1 2 3 ]; arr2 = [ "red" "yellow" "green" ]; arr3 = [ [ 1 2 ] [ 3 4 5 ] ]; arr4 = [ "all" "strings" "are the same" "type" ]; arr5 = [ [ 1 2 ] [ "a" "b" "c" ] ]; arr7 = [ 1 2 3 ]; arr8 = [ 1 2 ]; bare-key = "value"; bare_key = "value"; bin1 = 214; bool1 = true; bool2 = false; "character encoding" = "value"; d = { e = { f = { }; }; }; dog = { "tater.man" = { type = { name = "pug"; }; }; }; flt1 = 1; flt2 = 3.1415; flt3 = -0.01; flt4 = 5e+22; flt5 = 1e+06; flt6 = -0.02; flt7 = 6.626e-34; flt8 = 9.22462e+06; fruit = [ { name = "apple"; physical = { color = "red"; shape = "round"; }; variety = [ { name = "red delicious"; } { name = "granny smith"; } ]; } { name = "banana"; variety = [ { name = "plantain"; } ]; } ]; g = { h = { i = { }; }; }; hex1 = 3735928559; hex2 = 3735928559; hex3 = 3735928559; int1 = 99; int2 = 42; int3 = 0; int4 = -17; int5 = 1000; int6 = 5349221; int7 = 12345; j = { "สž" = { l = { }; }; }; key = "value"; key2 = "value"; ld1 = { _type = "timestamp"; value = "1979-05-27"; }; ldt1 = { _type = "timestamp"; value = "1979-05-27T07:32:00"; }; ldt2 = { _type = "timestamp"; value = "1979-05-27T00:32:00.999999"; }; lt1 = { _type = "timestamp"; value = "07:32:00"; }; lt2 = { _type = "timestamp"; value = "00:32:00.999999"; }; name = "Orange"; oct1 = 342391; oct2 = 493; odt1 = { _type = "timestamp"; value = "1979-05-27T07:32:00Z"; }; odt2 = { _type = "timestamp"; value = "1979-05-27T00:32:00-07:00"; }; odt3 = { _type = "timestamp"; value = "1979-05-27T00:32:00.999999-07:00"; }; odt4 = { _type = "timestamp"; value = "1979-05-27T07:32:00Z"; }; physical = { color = "orange"; shape = "round"; }; products = [ { name = "Hammer"; sku = 738594937; } { } { color = "gray"; name = "Nail"; sku = 284758393; } ]; "quoted \"value\"" = "value"; site = { "google.com" = true; }; str = "I'm a string. \"You can quote me\". Name\tJosรฉ\nLocation\tSF."; table-1 = { key1 = "some string"; key2 = 123; }; table-2 = { key1 = "another string"; key2 = 456; }; x = { y = { z = { w = { animal = { type = { name = "pug"; }; }; name = { first = "Tom"; last = "Preston-Werner"; }; point = { x = 1; y = 2; }; }; }; }; }; "สŽวสž" = "value"; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.flags b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.flags
new file mode 100644
index 0000000000..9ed39dc6ba
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.flags
@@ -0,0 +1 @@
+--extra-experimental-features parse-toml-timestamps
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.nix
new file mode 100644
index 0000000000..74cff9470a
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-fromTOML-timestamps.nix
@@ -0,0 +1,130 @@
+builtins.fromTOML ''
+  key = "value"
+  bare_key = "value"
+  bare-key = "value"
+  1234 = "value"
+
+  "127.0.0.1" = "value"
+  "character encoding" = "value"
+  "สŽวสž" = "value"
+  'key2' = "value"
+  'quoted "value"' = "value"
+
+  name = "Orange"
+
+  physical.color = "orange"
+  physical.shape = "round"
+  site."google.com" = true
+
+  # This is legal according to the spec, but cpptoml doesn't handle it.
+  #a.b.c = 1
+  #a.d = 2
+
+  str = "I'm a string. \"You can quote me\". Name\tJos\u00E9\nLocation\tSF."
+
+  int1 = +99
+  int2 = 42
+  int3 = 0
+  int4 = -17
+  int5 = 1_000
+  int6 = 5_349_221
+  int7 = 1_2_3_4_5
+
+  hex1 = 0xDEADBEEF
+  hex2 = 0xdeadbeef
+  hex3 = 0xdead_beef
+
+  oct1 = 0o01234567
+  oct2 = 0o755
+
+  bin1 = 0b11010110
+
+  flt1 = +1.0
+  flt2 = 3.1415
+  flt3 = -0.01
+  flt4 = 5e+22
+  flt5 = 1e6
+  flt6 = -2E-2
+  flt7 = 6.626e-34
+  flt8 = 9_224_617.445_991_228_313
+
+  bool1 = true
+  bool2 = false
+
+  odt1 = 1979-05-27T07:32:00Z
+  odt2 = 1979-05-27T00:32:00-07:00
+  odt3 = 1979-05-27T00:32:00.999999-07:00
+  odt4 = 1979-05-27 07:32:00Z
+  ldt1 = 1979-05-27T07:32:00
+  ldt2 = 1979-05-27T00:32:00.999999
+  ld1 = 1979-05-27
+  lt1 = 07:32:00
+  lt2 = 00:32:00.999999
+
+  arr1 = [ 1, 2, 3 ]
+  arr2 = [ "red", "yellow", "green" ]
+  arr3 = [ [ 1, 2 ], [3, 4, 5] ]
+  arr4 = [ "all", 'strings', """are the same""", ''''type'''']
+  arr5 = [ [ 1, 2 ], ["a", "b", "c"] ]
+
+  arr7 = [
+    1, 2, 3
+  ]
+
+  arr8 = [
+    1,
+    2, # this is ok
+  ]
+
+  [table-1]
+  key1 = "some string"
+  key2 = 123
+
+
+  [table-2]
+  key1 = "another string"
+  key2 = 456
+
+  [dog."tater.man"]
+  type.name = "pug"
+
+  [a.b.c]
+  [ d.e.f ]
+  [ g .  h  . i ]
+  [ j . "สž" . 'l' ]
+  [x.y.z.w]
+
+  name = { first = "Tom", last = "Preston-Werner" }
+  point = { x = 1, y = 2 }
+  animal = { type.name = "pug" }
+
+  [[products]]
+  name = "Hammer"
+  sku = 738594937
+
+  [[products]]
+
+  [[products]]
+  name = "Nail"
+  sku = 284758393
+  color = "gray"
+
+  [[fruit]]
+    name = "apple"
+
+    [fruit.physical]
+      color = "red"
+      shape = "round"
+
+    [[fruit.variety]]
+      name = "red delicious"
+
+    [[fruit.variety]]
+      name = "granny smith"
+
+  [[fruit]]
+    name = "banana"
+
+    [[fruit.variety]]
+      name = "plantain"
+''
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-functionargs.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-functionargs.exp
new file mode 100644
index 0000000000..7f9ac40e81
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-functionargs.exp
@@ -0,0 +1 @@
+{ column = 11; file = "eval-okay-getattrpos-functionargs.nix"; line = 2; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-functionargs.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-functionargs.nix
new file mode 100644
index 0000000000..11d6bb0e3a
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-functionargs.nix
@@ -0,0 +1,4 @@
+let
+  fun = { foo }: {};
+  pos = builtins.unsafeGetAttrPos "foo" (builtins.functionArgs fun);
+in { inherit (pos) column line; file = baseNameOf pos.file; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-undefined.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-undefined.exp
new file mode 100644
index 0000000000..19765bd501
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-undefined.exp
@@ -0,0 +1 @@
+null
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-undefined.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-undefined.nix
new file mode 100644
index 0000000000..14dd38f773
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-undefined.nix
@@ -0,0 +1 @@
+builtins.unsafeGetAttrPos "abort" builtins
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos.exp
new file mode 100644
index 0000000000..469249bbc6
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos.exp
@@ -0,0 +1 @@
+{ column = 5; file = "eval-okay-getattrpos.nix"; line = 3; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos.nix
new file mode 100644
index 0000000000..ca6b079615
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos.nix
@@ -0,0 +1,6 @@
+let
+  as = {
+    foo = "bar";
+  };
+  pos = builtins.unsafeGetAttrPos "foo" as;
+in { inherit (pos) column line; file = baseNameOf pos.file; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-import.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-import.exp
new file mode 100644
index 0000000000..c508125b55
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-import.exp
@@ -0,0 +1 @@
+[ 1 2 3 4 5 6 7 8 9 10 ]
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-import.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-import.nix
new file mode 100644
index 0000000000..76213a9541
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-import.nix
@@ -0,0 +1,11 @@
+let
+
+  overrides = {
+    import = fn: scopedImport overrides fn;
+
+    scopedImport = attrs: fn: scopedImport (overrides // attrs) fn;
+
+    builtins = builtins // overrides;
+  } // import ./../lib.nix;
+
+in scopedImport overrides ./../imported.nix
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-overrides.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-overrides.exp
new file mode 100644
index 0000000000..0cfbf08886
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-overrides.exp
@@ -0,0 +1 @@
+2
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-overrides.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-overrides.nix
new file mode 100644
index 0000000000..358742b36e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-overrides.nix
@@ -0,0 +1,9 @@
+let
+
+  overrides = { a = 2; };
+
+in (rec {
+  __overrides = overrides;
+  x = a;
+  a = 1;
+}).x
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-path-antiquotation.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-path-antiquotation.exp
new file mode 100644
index 0000000000..5b8ea02438
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-path-antiquotation.exp
@@ -0,0 +1 @@
+{ absolute = /foo; expr = /pwd/lang/foo/bar; home = /fake-home/foo; notfirst = /pwd/lang/bar/foo; simple = /pwd/lang/foo; slashes = /foo/bar; surrounded = /pwd/lang/a-foo-b; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-path-antiquotation.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-path-antiquotation.nix
new file mode 100644
index 0000000000..497d7c1c75
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-path-antiquotation.nix
@@ -0,0 +1,12 @@
+let
+  foo = "foo";
+in
+{
+  simple = ./${foo};
+  surrounded = ./a-${foo}-b;
+  absolute = /${foo};
+  expr = ./${foo + "/bar"};
+  home = ~/${foo};
+  notfirst = ./bar/${foo};
+  slashes = /${foo}/${"bar"};
+}
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-path.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-path.nix
new file mode 100644
index 0000000000..e67168cf3e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-path.nix
@@ -0,0 +1,7 @@
+builtins.path
+  { path = ./.;
+    filter = path: _: baseNameOf path == "data";
+    recursive = true;
+    sha256 = "1yhm3gwvg5a41yylymgblsclk95fs6jy72w0wv925mmidlhcq4sw";
+    name = "output";
+  }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readDir.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readDir.exp
new file mode 100644
index 0000000000..6413f6d4f9
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readDir.exp
@@ -0,0 +1 @@
+{ bar = "regular"; foo = "directory"; ldir = "symlink"; linked = "symlink"; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readDir.nix.disabled b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readDir.nix.disabled
new file mode 100644
index 0000000000..a7ec9292aa
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readDir.nix.disabled
@@ -0,0 +1 @@
+builtins.readDir ./readDir
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readFileType.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readFileType.exp
new file mode 100644
index 0000000000..6413f6d4f9
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readFileType.exp
@@ -0,0 +1 @@
+{ bar = "regular"; foo = "directory"; ldir = "symlink"; linked = "symlink"; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readFileType.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readFileType.nix
new file mode 100644
index 0000000000..174fb6c3a0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readFileType.nix
@@ -0,0 +1,6 @@
+{
+  bar    = builtins.readFileType ./readDir/bar;
+  foo    = builtins.readFileType ./readDir/foo;
+  linked = builtins.readFileType ./readDir/linked;
+  ldir   = builtins.readFileType ./readDir/ldir;
+}
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-replacestrings.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-replacestrings.exp
new file mode 100644
index 0000000000..eac67c5fed
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-replacestrings.exp
@@ -0,0 +1 @@
+[ "faabar" "fbar" "fubar" "faboor" "fubar" "XaXbXcX" "X" "a_b" "fubar" ]
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-replacestrings.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-replacestrings.nix
new file mode 100644
index 0000000000..a803e65199
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-replacestrings.nix
@@ -0,0 +1,12 @@
+with builtins;
+
+[ (replaceStrings ["o"] ["a"] "foobar")
+  (replaceStrings ["o"] [""] "foobar")
+  (replaceStrings ["oo"] ["u"] "foobar")
+  (replaceStrings ["oo" "a"] ["a" "oo"] "foobar")
+  (replaceStrings ["oo" "oo"] ["u" "i"] "foobar")
+  (replaceStrings [""] ["X"] "abc")
+  (replaceStrings [""] ["X"] "")
+  (replaceStrings ["-"] ["_"] "a-b")
+  (replaceStrings ["oo" "XX"] ["u" (throw "unreachable")] "foobar")
+]
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-search-path.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-search-path.exp
new file mode 100644
index 0000000000..4519bc406d
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-search-path.exp
@@ -0,0 +1 @@
+"abccX"
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-search-path.flags b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-search-path.flags
new file mode 100644
index 0000000000..a28e682100
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-search-path.flags
@@ -0,0 +1 @@
+-I lang/dir1 -I lang/dir2 -I dir5=lang/dir3
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-search-path.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-search-path.nix
new file mode 100644
index 0000000000..6fe33decc0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-search-path.nix
@@ -0,0 +1,10 @@
+with import ./lib.nix;
+with builtins;
+
+assert isFunction (import <nix/fetchurl.nix>);
+
+assert length __nixPath == 5;
+assert length (filter (x: baseNameOf x.path == "dir4") __nixPath) == 1;
+
+import <a.nix> + import <b.nix> + import <c.nix> + import <dir5/c.nix>
+  + (let __nixPath = [ { path = ./dir2; } { path = ./dir1; } ]; in import <a.nix>)
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-zipAttrsWith.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-zipAttrsWith.exp
new file mode 100644
index 0000000000..9c0b15d22b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-zipAttrsWith.exp
@@ -0,0 +1 @@
+{ "0" = { n = "0"; v = [ 5 23 29 ]; }; "1" = { n = "1"; v = [ 7 30 ]; }; "2" = { n = "2"; v = [ 18 ]; }; "4" = { n = "4"; v = [ 10 ]; }; "5" = { n = "5"; v = [ 15 25 26 31 ]; }; "6" = { n = "6"; v = [ 3 14 ]; }; "7" = { n = "7"; v = [ 12 ]; }; "8" = { n = "8"; v = [ 2 6 8 9 ]; }; "9" = { n = "9"; v = [ 0 16 ]; }; a = { n = "a"; v = [ 17 21 22 27 ]; }; c = { n = "c"; v = [ 11 24 ]; }; d = { n = "d"; v = [ 4 13 28 ]; }; e = { n = "e"; v = [ 20 ]; }; f = { n = "f"; v = [ 1 19 ]; }; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-zipAttrsWith.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-zipAttrsWith.nix
new file mode 100644
index 0000000000..e5d4cdccb7
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-zipAttrsWith.nix
@@ -0,0 +1,9 @@
+with import ./../lib.nix;
+
+let
+  str = builtins.hashString "sha256" "test";
+in
+builtins.zipAttrsWith
+  (n: v: { inherit n v; })
+  (map (n: { ${builtins.substring n 1 str} = n; })
+    (range 0 31))
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/bar b/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/bar
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/bar
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/foo/git-hates-directories b/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/foo/git-hates-directories
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/foo/git-hates-directories
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/ldir b/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/ldir
new file mode 120000
index 0000000000..1910281566
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/ldir
@@ -0,0 +1 @@
+foo
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/linked b/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/linked
new file mode 120000
index 0000000000..c503f86a0c
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/linked
@@ -0,0 +1 @@
+foo/git-hates-directories
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-1.nix b/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-1.nix
new file mode 100644
index 0000000000..2c02317d2a
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-1.nix
@@ -0,0 +1,4 @@
+{ x = 123;
+  y = 456;
+  x = 789;
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-2.nix b/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-2.nix
new file mode 100644
index 0000000000..864d9865e0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-2.nix
@@ -0,0 +1,13 @@
+let {
+
+  as = {
+    x = 123;
+    y = 456;
+  };
+
+  bs = {
+    x = 789;
+    inherit (as) x;
+  };
+  
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-3.nix b/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-3.nix
new file mode 100644
index 0000000000..114d19779f
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-3.nix
@@ -0,0 +1,13 @@
+let {
+
+  as = {
+    x = 123;
+    y = 456;
+  };
+
+  bs = rec {
+    x = 789;
+    inherit (as) x;
+  };
+  
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-4.nix b/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-4.nix
new file mode 100644
index 0000000000..77417432b3
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-4.nix
@@ -0,0 +1,4 @@
+{
+  services.ssh.port = 22;
+  services.ssh.port = 23;
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-7.nix b/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-7.nix
new file mode 100644
index 0000000000..bbc3eb08c0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-dup-attrs-7.nix
@@ -0,0 +1,9 @@
+rec {
+
+  x = 1;
+
+  as = {
+    inherit x;
+    inherit x;
+  };
+}
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-dup-formals.nix b/tvix/eval/src/tests/nix_tests/parse-fail-dup-formals.nix
new file mode 100644
index 0000000000..a0edd91a96
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-dup-formals.nix
@@ -0,0 +1 @@
+{x, y, x}: x
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-eof-in-string.nix b/tvix/eval/src/tests/nix_tests/parse-fail-eof-in-string.nix
new file mode 100644
index 0000000000..19775d2ec8
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-eof-in-string.nix
@@ -0,0 +1,3 @@
+# https://github.com/NixOS/nix/issues/6562
+# Note that this file must not end with a newline.
+a 1"$
\ No newline at end of file
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-mixed-nested-attrs1.nix b/tvix/eval/src/tests/nix_tests/parse-fail-mixed-nested-attrs1.nix
new file mode 100644
index 0000000000..11e40e66fd
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-mixed-nested-attrs1.nix
@@ -0,0 +1,4 @@
+{ 
+  x.z = 3; 
+  x = { y = 3; z = 3; }; 
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-mixed-nested-attrs2.nix b/tvix/eval/src/tests/nix_tests/parse-fail-mixed-nested-attrs2.nix
new file mode 100644
index 0000000000..17da82e5f0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-mixed-nested-attrs2.nix
@@ -0,0 +1,4 @@
+{ 
+  x.y.y = 3; 
+  x = { y.y= 3; z = 3; }; 
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-patterns-1.nix b/tvix/eval/src/tests/nix_tests/parse-fail-patterns-1.nix
new file mode 100644
index 0000000000..7b40616417
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-patterns-1.nix
@@ -0,0 +1 @@
+args@{args, x, y, z}: x
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-regression-20060610.nix b/tvix/eval/src/tests/nix_tests/parse-fail-regression-20060610.nix
new file mode 100644
index 0000000000..b1934f7e1e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-regression-20060610.nix
@@ -0,0 +1,11 @@
+let {
+  x =
+    {gcc}:
+    {
+      inherit gcc;
+    };
+
+  body = ({
+    inherit gcc;
+  }).gcc;
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-uft8.nix b/tvix/eval/src/tests/nix_tests/parse-fail-uft8.nix
new file mode 100644
index 0000000000..34948d48ae
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-uft8.nix
@@ -0,0 +1 @@
+123 รฉ 4
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-undef-var-2.nix b/tvix/eval/src/tests/nix_tests/parse-fail-undef-var-2.nix
new file mode 100644
index 0000000000..c10a52b1ea
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-undef-var-2.nix
@@ -0,0 +1,7 @@
+let {
+
+  f = {x, y : ["baz" "bar" z "bat"]}: x + y;
+
+  body = f {x = "foo"; y = "bar";};
+
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-fail-undef-var.nix b/tvix/eval/src/tests/nix_tests/parse-fail-undef-var.nix
new file mode 100644
index 0000000000..7b63008110
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-undef-var.nix
@@ -0,0 +1 @@
+x: y
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-1.nix b/tvix/eval/src/tests/nix_tests/parse-okay-1.nix
new file mode 100644
index 0000000000..23a58ed109
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-1.nix
@@ -0,0 +1 @@
+{x, y, z}: x + y + z
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-crlf.nix b/tvix/eval/src/tests/nix_tests/parse-okay-crlf.nix
new file mode 100644
index 0000000000..21518d4c6d
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-crlf.nix
@@ -0,0 +1,17 @@
+rec {
+
+  /* Dit is

+  een test. */
+
+  x = 
+  # Dit is een test.
y;
+  
+  y = 123;

+
+  # CR or CR/LF (but not explicit \r's) in strings should be
+  # translated to LF.
+  foo = "multi
line

+  string
+  test\r";
+
+  z = 456;
}
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-dup-attrs-5.nix b/tvix/eval/src/tests/nix_tests/parse-okay-dup-attrs-5.nix
new file mode 100644
index 0000000000..f4b9efd0c5
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-dup-attrs-5.nix
@@ -0,0 +1,4 @@
+{
+  services.ssh = { enable = true; };
+  services.ssh.port = 23;
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-dup-attrs-6.nix b/tvix/eval/src/tests/nix_tests/parse-okay-dup-attrs-6.nix
new file mode 100644
index 0000000000..ae6d7a7693
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-dup-attrs-6.nix
@@ -0,0 +1,4 @@
+{
+  services.ssh.port = 23;
+  services.ssh = { enable = true; };
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-mixed-nested-attrs-1.nix b/tvix/eval/src/tests/nix_tests/parse-okay-mixed-nested-attrs-1.nix
new file mode 100644
index 0000000000..fd1001c8ca
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-mixed-nested-attrs-1.nix
@@ -0,0 +1,4 @@
+{ 
+  x = { y = 3; z = 3; }; 
+  x.q = 3; 
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-mixed-nested-attrs-2.nix b/tvix/eval/src/tests/nix_tests/parse-okay-mixed-nested-attrs-2.nix
new file mode 100644
index 0000000000..ad066b6803
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-mixed-nested-attrs-2.nix
@@ -0,0 +1,4 @@
+{ 
+  x.q = 3; 
+  x = { y = 3; z = 3; }; 
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-mixed-nested-attrs-3.nix b/tvix/eval/src/tests/nix_tests/parse-okay-mixed-nested-attrs-3.nix
new file mode 100644
index 0000000000..45a33e4803
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-mixed-nested-attrs-3.nix
@@ -0,0 +1,7 @@
+{
+    services.ssh.enable = true;
+    services.ssh = { port = 123; };
+    services = {
+        httpd.enable = true;
+    };
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-regression-20041027.nix b/tvix/eval/src/tests/nix_tests/parse-okay-regression-20041027.nix
new file mode 100644
index 0000000000..ae2e256eea
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-regression-20041027.nix
@@ -0,0 +1,11 @@
+{stdenv, fetchurl /* pkgconfig, libX11 */ }:
+
+stdenv.mkDerivation {
+  name = "libXi-6.0.1";
+  src = fetchurl {
+    url = http://freedesktop.org/~xlibs/release/libXi-6.0.1.tar.bz2;
+    md5 = "7e935a42428d63a387b3c048be0f2756";
+  };
+/*  buildInputs = [pkgconfig];
+  propagatedBuildInputs = [libX11]; */
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-regression-751.nix b/tvix/eval/src/tests/nix_tests/parse-okay-regression-751.nix
new file mode 100644
index 0000000000..05c78b3016
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-regression-751.nix
@@ -0,0 +1,2 @@
+let const = a: "const"; in
+''${ const { x = "q"; }}''
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-subversion.nix b/tvix/eval/src/tests/nix_tests/parse-okay-subversion.nix
new file mode 100644
index 0000000000..356272815d
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-subversion.nix
@@ -0,0 +1,43 @@
+{ localServer ? false
+, httpServer ? false
+, sslSupport ? false
+, pythonBindings ? false
+, javaSwigBindings ? false
+, javahlBindings ? false
+, stdenv, fetchurl
+, openssl ? null, httpd ? null, db4 ? null, expat, swig ? null, j2sdk ? null
+}:
+
+assert expat != null;
+assert localServer -> db4 != null;
+assert httpServer -> httpd != null && httpd.expat == expat;
+assert sslSupport -> openssl != null && (httpServer -> httpd.openssl == openssl);
+assert pythonBindings -> swig != null && swig.pythonSupport;
+assert javaSwigBindings -> swig != null && swig.javaSupport;
+assert javahlBindings -> j2sdk != null;
+
+stdenv.mkDerivation {
+  name = "subversion-1.1.1";
+
+  builder = /foo/bar;
+  src = fetchurl {
+    url = http://subversion.tigris.org/tarballs/subversion-1.1.1.tar.bz2;
+    md5 = "a180c3fe91680389c210c99def54d9e0";
+  };
+
+  # This is a hopefully temporary fix for the problem that
+  # libsvnjavahl.so isn't linked against libstdc++, which causes
+  # loading the library into the JVM to fail.
+  patches = if javahlBindings then [/javahl.patch] else [];
+
+  openssl = if sslSupport then openssl else null;
+  httpd = if httpServer then httpd else null;
+  db4 = if localServer then db4 else null;
+  swig = if pythonBindings || javaSwigBindings then swig else null;
+  python = if pythonBindings then swig.python else null;
+  j2sdk = if javaSwigBindings then swig.j2sdk else
+          if javahlBindings then j2sdk else null;
+
+  inherit expat localServer httpServer sslSupport
+          pythonBindings javaSwigBindings javahlBindings;
+}
diff --git a/tvix/eval/src/tests/nix_tests/parse-okay-url.nix b/tvix/eval/src/tests/nix_tests/parse-okay-url.nix
new file mode 100644
index 0000000000..08de27d0a4
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-okay-url.nix
@@ -0,0 +1,8 @@
+[ x:x
+  https://svn.cs.uu.nl:12443/repos/trace/trunk
+  http://www2.mplayerhq.hu/MPlayer/releases/fonts/font-arial-iso-8859-1.tar.bz2
+  http://losser.st-lab.cs.uu.nl/~armijn/.nix/gcc-3.3.4-static-nix.tar.gz
+  http://fpdownload.macromedia.com/get/shockwave/flash/english/linux/7.0r25/install_flash_player_7_linux.tar.gz
+  https://ftp5.gwdg.de/pub/linux/archlinux/extra/os/x86_64/unzip-6.0-14-x86_64.pkg.tar.zst
+  ftp://ftp.gtk.org/pub/gtk/v1.2/gtk+-1.2.10.tar.gz
+]
diff --git a/tvix/eval/src/tests/one_offs.rs b/tvix/eval/src/tests/one_offs.rs
new file mode 100644
index 0000000000..565d1dd48f
--- /dev/null
+++ b/tvix/eval/src/tests/one_offs.rs
@@ -0,0 +1,36 @@
+use crate::*;
+
+#[test]
+fn test_source_builtin() {
+    // Test an evaluation with a source-only builtin. The test ensures
+    // that the artificially constructed thunking is correct.
+
+    let mut eval = Evaluation::new_impure();
+    eval.src_builtins.push(("testSourceBuiltin", "42"));
+
+    let result = eval.evaluate("builtins.testSourceBuiltin", None);
+    assert!(
+        result.errors.is_empty(),
+        "evaluation failed: {:?}",
+        result.errors
+    );
+
+    let value = result.value.unwrap();
+    assert!(
+        matches!(value, Value::Integer(42)),
+        "expected the integer 42, but got {}",
+        value,
+    );
+}
+
+#[test]
+fn skip_broken_bytecode() {
+    let result = Evaluation::new_pure().evaluate(/* code = */ "x", None);
+
+    assert_eq!(result.errors.len(), 1);
+
+    assert!(matches!(
+        result.errors[0].kind,
+        ErrorKind::UnknownStaticVariable
+    ));
+}
diff --git a/tvix/eval/src/tests/tvix_tests/README.md b/tvix/eval/src/tests/tvix_tests/README.md
new file mode 100644
index 0000000000..b493aa81f1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/README.md
@@ -0,0 +1,19 @@
+These tests are "native" to Tvix and exist in addition to the Nix test
+suite.
+
+All of these are straightforward code snippets which are expected to
+produce a certain result.
+
+# `identity-*` tests
+
+Files named `identity-*.nix` contain code that is supposed to produce
+itself exactly after evaluation.
+
+These are useful for testing literals.
+
+# `eval-okay-*` tests
+
+Files named `eval-okay-*.nix` contain code which is supposed to
+evaluate to the output in the corresponding `eval-okay-*.exp` file.
+
+This convention is taken from the original Nix test suite.
diff --git a/tvix/eval/src/tests/tvix_tests/directory/default.nix b/tvix/eval/src/tests/tvix_tests/directory/default.nix
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/directory/default.nix
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-substring-negative-start.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-substring-negative-start.nix
new file mode 100644
index 0000000000..bc7a16ded8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-substring-negative-start.nix
@@ -0,0 +1,3 @@
+# Negative start is illegal, but negative length works, see
+# eval-okay-builtins-substring-negative-length.nix
+builtins.substring (-1) 1 "Wiggly Donkers"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-thunk-error.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-thunk-error.nix
new file mode 100644
index 0000000000..6df79d13f4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-thunk-error.nix
@@ -0,0 +1 @@
+builtins.genList (_: { }.foo) 1
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-tojson-tostring-notcallable.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-tojson-tostring-notcallable.nix
new file mode 100644
index 0000000000..345b76fde0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-tojson-tostring-notcallable.nix
@@ -0,0 +1,5 @@
+# attribute sets with a non-callable `__toString` can not be
+# serialised to JSON.
+builtins.toJSON {
+  __toString = 42;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-tojson-tostring-strong.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-tojson-tostring-strong.nix
new file mode 100644
index 0000000000..d1c72dc678
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-tojson-tostring-strong.nix
@@ -0,0 +1,6 @@
+# String coercions when using builtins.toJSON on an attribute set with
+# a `__toString` attribute should be weak.
+builtins.toJSON {
+  __toString = self: self.x;
+  x = 42;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-closed-formals.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-closed-formals.nix
new file mode 100644
index 0000000000..a0cd20c470
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-closed-formals.nix
@@ -0,0 +1 @@
+({ x }: x) { x = 1; y = 2; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-deep-forced-thunk-error.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-deep-forced-thunk-error.nix
new file mode 100644
index 0000000000..b7a7583022
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-deep-forced-thunk-error.nix
@@ -0,0 +1 @@
+[ (throw "error!") ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-deepseq.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-deepseq.nix
new file mode 100644
index 0000000000..9baa49b063
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-deepseq.nix
@@ -0,0 +1 @@
+builtins.deepSeq { x = abort "foo"; } 456
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-division-by-zero-float.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-division-by-zero-float.nix
new file mode 100644
index 0000000000..82dd687321
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-division-by-zero-float.nix
@@ -0,0 +1 @@
+1.0 / 0.0
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-division-by-zero-int.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-division-by-zero-int.nix
new file mode 100644
index 0000000000..72dca4d5e4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-division-by-zero-int.nix
@@ -0,0 +1 @@
+1 / 0
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-foldlStrict-strict-op-application.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-foldlStrict-strict-op-application.nix
new file mode 100644
index 0000000000..adc029b2f2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-foldlStrict-strict-op-application.nix
@@ -0,0 +1,4 @@
+builtins.foldl'
+  (_: f: f null)
+  (throw "This doesn't explode")
+  [ (_: throw "Not the final value, but is still forced!") (_: 23) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-force-before-value-pointer-equality.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-force-before-value-pointer-equality.nix
new file mode 100644
index 0000000000..a2182a508f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-force-before-value-pointer-equality.nix
@@ -0,0 +1,5 @@
+let
+  x = throw "I have been forced";
+in
+
+x == x
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-function-formals-typecheck.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-function-formals-typecheck.nix
new file mode 100644
index 0000000000..0108f958bf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-function-formals-typecheck.nix
@@ -0,0 +1,2 @@
+# A function with formal set arguments forces its argument set and verifies its type.
+({ ... }@args: 42) [ ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-getEnv-coercion.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-getEnv-coercion.nix
new file mode 100644
index 0000000000..fe48a5690c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-getEnv-coercion.nix
@@ -0,0 +1 @@
+builtins.getEnv { var = "PATH"; __toString = self: self.var; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-infinite-recursion.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-infinite-recursion.nix
new file mode 100644
index 0000000000..5e4fd3789c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-infinite-recursion.nix
@@ -0,0 +1 @@
+let x = x; in x
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-outer-value-never-pointer-equal.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-outer-value-never-pointer-equal.nix
new file mode 100644
index 0000000000..a8c3cedf61
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-outer-value-never-pointer-equal.nix
@@ -0,0 +1,7 @@
+# For an explanation of this behavior see //tvix/docs/value-pointer-equality.md
+let
+  x = { foo = throw "foo"; };
+in
+
+# while `builtins.seq x null` would succeed, this fails!
+x == x
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-parsedrvname-coerce.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-parsedrvname-coerce.nix
new file mode 100644
index 0000000000..a1218de3fe
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-parsedrvname-coerce.nix
@@ -0,0 +1 @@
+builtins.parseDrvName { outPath = "lol"; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-remove.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-remove.nix
new file mode 100644
index 0000000000..93dd8ccd45
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-remove.nix
@@ -0,0 +1,5 @@
+let {
+attrs = { x = 123; y = 456; };
+
+body = (removeAttrs attrs [ "x" ]).x;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-seq.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-seq.nix
new file mode 100644
index 0000000000..cddbbfd326
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-seq.nix
@@ -0,0 +1 @@
+builtins.seq (abort "foo") 2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-throw-abort-cannot-be-caught.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-throw-abort-cannot-be-caught.nix
new file mode 100644
index 0000000000..10781cb4ea
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-throw-abort-cannot-be-caught.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.throw (builtins.abort "abc"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-abort-throw-can-be-caught.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-abort-throw-can-be-caught.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-abort-throw-can-be-caught.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-abort-throw-can-be-caught.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-abort-throw-can-be-caught.nix
new file mode 100644
index 0000000000..aebeca1dad
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-abort-throw-can-be-caught.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.abort (builtins.throw "abc"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-access-strange-identifier.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-access-strange-identifier.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-access-strange-identifier.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-access-strange-identifier.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-access-strange-identifier.nix
new file mode 100644
index 0000000000..433f53dc56
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-access-strange-identifier.nix
@@ -0,0 +1,9 @@
+let
+  # There is no syntax for accessing this identifier in an ordinary
+  # way.
+  "foo bar" = 42;
+in
+({
+  # but we *can* inherit it back out
+  inherit "foo bar";
+})."foo bar"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-add-paths.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-add-paths.exp
new file mode 100644
index 0000000000..94ba9a881a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-add-paths.exp
@@ -0,0 +1 @@
+[ /bin /binbar /binbar /binbar /binbar /bin/bar /bin/bin ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-add-paths.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-add-paths.nix
new file mode 100644
index 0000000000..462f670882
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-add-paths.nix
@@ -0,0 +1,9 @@
+[
+  (/bin + "/")
+  (/bin + "bar")
+  (let name = "bar"; in /bin + name)
+  (let name = "bar"; in /bin + "${name}")
+  (let name = "bar"; in /bin + "/" + "${name}")
+  (let name = "bar"; in /bin + "/${name}")
+  (/bin + /bin)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-float.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-float.exp
new file mode 100644
index 0000000000..08ef6079f8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-float.exp
@@ -0,0 +1 @@
+{ add = 37.34; div = 1.05714; mul = 105.154; sub = 14.35; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-float.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-float.nix
new file mode 100644
index 0000000000..9d12aee061
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-float.nix
@@ -0,0 +1,6 @@
+{
+  add = 12.34 + 25.0;
+  sub = 20.05 - 5.7;
+  mul = 28.42 * 3.70;
+  div = 18.5 / 17.5;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-int.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-int.exp
new file mode 100644
index 0000000000..a5711e8bfe
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-int.exp
@@ -0,0 +1 @@
+{ add = 20; div = 3; mul = 8; sub = 15; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-int.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-int.nix
new file mode 100644
index 0000000000..c53790db09
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-arithmetic-int.nix
@@ -0,0 +1,6 @@
+{
+  add = 15 + 5;
+  sub = 20 - 5;
+  mul = 4 * 2;
+  div = 9 / 3;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-assert-thunk-condition.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-assert-thunk-condition.exp
new file mode 100644
index 0000000000..aabe6ec390
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-assert-thunk-condition.exp
@@ -0,0 +1 @@
+21
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-assert-thunk-condition.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-assert-thunk-condition.nix
new file mode 100644
index 0000000000..ac65f5814d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-assert-thunk-condition.nix
@@ -0,0 +1,7 @@
+let
+  condition = x: y: x < y;
+in
+
+# The function application here will become a thunk which verifies that
+  # assert forces the condition expression correctly.
+assert condition 21 42; 21
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.nix
new file mode 100644
index 0000000000..f4ef72a88b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (throw "fred" 5)).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attr-key-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-attr-key-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attr-key-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attr-key-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-attr-key-catchable.nix
new file mode 100644
index 0000000000..b9d835bcc5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attr-key-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval { "${builtins.throw "a"}" = "b"; }).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit-literal.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit-literal.exp
new file mode 100644
index 0000000000..60d3b2f4a4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit-literal.exp
@@ -0,0 +1 @@
+15
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit-literal.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit-literal.nix
new file mode 100644
index 0000000000..9ecb4e9880
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit-literal.nix
@@ -0,0 +1,2 @@
+# the 'from' part of an `inherit` can be any expression.
+{ inherit ({ a = 15; }) a; }.a
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-simple-inherit.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-simple-inherit.exp
new file mode 100644
index 0000000000..a779fce51a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-simple-inherit.exp
@@ -0,0 +1 @@
+{ a = 1; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-simple-inherit.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-simple-inherit.nix
new file mode 100644
index 0000000000..68880bcfd8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-simple-inherit.nix
@@ -0,0 +1,4 @@
+let
+  a = 1;
+in
+{ inherit a; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-lhs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-lhs.exp
new file mode 100644
index 0000000000..fedf8f25a6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-lhs.exp
@@ -0,0 +1 @@
+{ a = "ok"; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-lhs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-lhs.nix
new file mode 100644
index 0000000000..973170cdd5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-lhs.nix
@@ -0,0 +1 @@
+{ } // { a = "ok"; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-rhs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-rhs.exp
new file mode 100644
index 0000000000..fedf8f25a6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-rhs.exp
@@ -0,0 +1 @@
+{ a = "ok"; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-rhs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-rhs.nix
new file mode 100644
index 0000000000..f51b88e93a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-rhs.nix
@@ -0,0 +1 @@
+{ a = "ok"; } // { }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-kv-lhs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-kv-lhs.exp
new file mode 100644
index 0000000000..c2234a47e2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-kv-lhs.exp
@@ -0,0 +1 @@
+{ name = "foo"; other = 42; value = "bar"; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-kv-lhs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-kv-lhs.nix
new file mode 100644
index 0000000000..6f71684902
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-kv-lhs.nix
@@ -0,0 +1 @@
+{ name = "foo"; value = "bar"; } // { other = 42; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update.exp
new file mode 100644
index 0000000000..57f4d541bd
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update.exp
@@ -0,0 +1 @@
+{ a = 15; b = "works"; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update.nix
new file mode 100644
index 0000000000..735602fe02
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update.nix
@@ -0,0 +1 @@
+{ a = 15; } // { b = "works"; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof-propagate-catchables.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof-propagate-catchables.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof-propagate-catchables.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof-propagate-catchables.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof-propagate-catchables.nix
new file mode 100644
index 0000000000..240736b12a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof-propagate-catchables.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.baseNameOf (throw "jill"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof.exp
new file mode 100644
index 0000000000..60a773f4af
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof.exp
@@ -0,0 +1 @@
+[ "bar" "foo" "" "bar" "." "" "" "" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof.nix
new file mode 100644
index 0000000000..bc59613f54
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof.nix
@@ -0,0 +1,10 @@
+[
+  (builtins.baseNameOf /foo/bar)
+  (builtins.baseNameOf "foo")
+  (builtins.baseNameOf "foo///")
+  (builtins.baseNameOf "foo/bar")
+  (builtins.baseNameOf "./.")
+  (builtins.baseNameOf "")
+  (builtins.baseNameOf /.)
+  (builtins.toString (builtins.baseNameOf /.))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-add.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-add.exp
new file mode 100644
index 0000000000..c3ac813de6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-add.exp
@@ -0,0 +1 @@
+[ 18 18.9 18.9 19.1 19 42 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-add.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-add.nix
new file mode 100644
index 0000000000..b04b1d1fa6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-add.nix
@@ -0,0 +1,8 @@
+[
+  (builtins.add 7 11)
+  (builtins.add 7.9 11)
+  (builtins.add 7 11.9)
+  (builtins.add 7.2 11.9)
+  (builtins.add 7.1 11.9)
+  (builtins.add (builtins.add 21 10) 11)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all-propagate-catchable.exp
new file mode 100644
index 0000000000..48e341f4d8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all-propagate-catchable.nix
new file mode 100644
index 0000000000..8902e27c45
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all-propagate-catchable.nix
@@ -0,0 +1 @@
+map (e: (builtins.tryEval e).success) [ (builtins.all (builtins.throw "a") [ "" ]) (builtins.all (x: true) (builtins.throw "b")) (builtins.all (_: builtins.throw "x") [ "" ]) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all.exp
new file mode 100644
index 0000000000..82ca7e6b6d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all.exp
@@ -0,0 +1 @@
+[ true true false false false false true true false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all.nix
new file mode 100644
index 0000000000..12d62632dd
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all.nix
@@ -0,0 +1,15 @@
+[
+  (builtins.all (x: x) [ ])
+  (builtins.all (x: x) [ true true true ])
+  (builtins.all (x: x) [ false false false ])
+  (builtins.all (x: x) [ true true false ])
+  (builtins.all (x: x) [ false true true ])
+
+  # evaluation should short-circuit
+  (builtins.all (x: x) [ true false (builtins.abort "should be unreachable") ])
+
+  # arbitrary functions supported
+  (builtins.all (x: x * 2 == 42) [ ])
+  (builtins.all (x: x * 2 == 42) [ 21 21 21 ])
+  (builtins.all (x: x * 2 == 42) [ 1 2 3 ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any-propagate-catchable.exp
new file mode 100644
index 0000000000..48e341f4d8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any-propagate-catchable.nix
new file mode 100644
index 0000000000..8db5c0c6dc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any-propagate-catchable.nix
@@ -0,0 +1 @@
+map (e: (builtins.tryEval e).success) [ (builtins.any (builtins.throw "a") [ "" ]) (builtins.any (x: true) (builtins.throw "b")) (builtins.any (_: builtins.throw "a") [ "" ]) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any.exp
new file mode 100644
index 0000000000..d6846ac3f7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any.exp
@@ -0,0 +1 @@
+[ false true false true true true false true false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any.nix
new file mode 100644
index 0000000000..2c659f130b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any.nix
@@ -0,0 +1,15 @@
+[
+  (builtins.any (x: x) [ ])
+  (builtins.any (x: x) [ true true true ])
+  (builtins.any (x: x) [ false false false ])
+  (builtins.any (x: x) [ true true false ])
+  (builtins.any (x: x) [ false true true ])
+
+  # evaluation should short-circuit
+  (builtins.any (x: x) [ false true (builtins.abort "should be unreachable") ])
+
+  # arbitrary functions supported
+  (builtins.any (x: x * 2 == 42) [ ])
+  (builtins.any (x: x * 2 == 42) [ 7 21 42 ])
+  (builtins.any (x: x * 2 == 42) [ 1 2 3 ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrnames.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrnames.exp
new file mode 100644
index 0000000000..6521066a8e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrnames.exp
@@ -0,0 +1 @@
+[ [ ] [ "bar" "baz" "foo" ] [ "Baz" "Foo" "bar" ] [ "Eric Idle" "Graham Chapman" "John Cleese" "Michael Palin" "Terry Gilliam" "Terry Jones" ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrnames.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrnames.nix
new file mode 100644
index 0000000000..82240a0647
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrnames.nix
@@ -0,0 +1,13 @@
+[
+  (builtins.attrNames { })
+  (builtins.attrNames { foo = 1; bar = 2; baz = 3; })
+  (builtins.attrNames { Foo = 1; bar = 2; Baz = 3; })
+  (builtins.attrNames {
+    "Graham Chapman" = true;
+    "John Cleese" = true;
+    "Terry Gilliam" = true;
+    "Eric Idle" = true;
+    "Terry Jones" = true;
+    "Michael Palin" = true;
+  })
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues-propagate-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues-propagate-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues-propagate-catchable.nix
new file mode 100644
index 0000000000..b8c15c8748
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues-propagate-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.attrValues (builtins.throw "a"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues.exp
new file mode 100644
index 0000000000..35c3697720
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues.exp
@@ -0,0 +1 @@
+[ [ ] [ 2 3 1 ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues.nix
new file mode 100644
index 0000000000..ce6c5c3816
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues.nix
@@ -0,0 +1,4 @@
+[
+  (builtins.attrValues { })
+  (builtins.attrValues { foo = 1; bar = 2; baz = 3; })
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitand.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitand.exp
new file mode 100644
index 0000000000..30b348853e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitand.exp
@@ -0,0 +1 @@
+[ 0 0 0 1 8 8 8 8 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitand.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitand.nix
new file mode 100644
index 0000000000..af40005ed9
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitand.nix
@@ -0,0 +1,10 @@
+[
+  (builtins.bitAnd 0 0)
+  (builtins.bitAnd 0 1)
+  (builtins.bitAnd 1 0)
+  (builtins.bitAnd 1 1)
+  (builtins.bitAnd 8 8)
+  (builtins.bitAnd 8 (builtins.add 4 4))
+  (builtins.bitAnd (builtins.add 4 4) 8)
+  (builtins.bitAnd (builtins.add 4 4) (builtins.add 4 4))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitor.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitor.exp
new file mode 100644
index 0000000000..2556b4183c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitor.exp
@@ -0,0 +1 @@
+[ 0 1 1 1 8 8 8 8 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitor.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitor.nix
new file mode 100644
index 0000000000..9c28f6d7ac
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitor.nix
@@ -0,0 +1,10 @@
+[
+  (builtins.bitOr 0 0)
+  (builtins.bitOr 1 0)
+  (builtins.bitOr 0 1)
+  (builtins.bitOr 1 1)
+  (builtins.bitOr 8 8)
+  (builtins.bitOr 8 (builtins.add 4 4))
+  (builtins.bitOr (builtins.add 4 4) 8)
+  (builtins.bitOr (builtins.add 4 4) (builtins.add 4 4))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitxor.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitxor.exp
new file mode 100644
index 0000000000..457157d459
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitxor.exp
@@ -0,0 +1 @@
+[ 0 1 1 0 8 8 0 0 0 0 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitxor.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitxor.nix
new file mode 100644
index 0000000000..80e363fb07
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-bitxor.nix
@@ -0,0 +1,12 @@
+[
+  (builtins.bitXor 0 0)
+  (builtins.bitXor 1 0)
+  (builtins.bitXor 0 1)
+  (builtins.bitXor 1 1)
+  (builtins.bitXor 8 0)
+  (builtins.bitXor 0 8)
+  (builtins.bitXor 8 8)
+  (builtins.bitXor 8 (builtins.add 4 4))
+  (builtins.bitXor (builtins.add 4 4) 8)
+  (builtins.bitXor (builtins.add 4 4) (builtins.add 4 4))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-builtins.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-builtins.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-builtins.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-builtins.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-builtins.nix
new file mode 100644
index 0000000000..434ccf8049
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-builtins.nix
@@ -0,0 +1 @@
+[ builtins ] == [ builtins.builtins ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catAttrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catAttrs.exp
new file mode 100644
index 0000000000..f8c0b2de5f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catAttrs.exp
@@ -0,0 +1 @@
+[ 21 "+" 21 "=" 42 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catAttrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catAttrs.nix
new file mode 100644
index 0000000000..edac76d446
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catAttrs.nix
@@ -0,0 +1,10 @@
+builtins.catAttrs "foo" [
+  { foo = 21; }
+  { bar = 23; foo = "+"; }
+  { }
+  { bar = 12; }
+  { foo = 21 + 0; }
+  { foo = "="; }
+  ({ bar = 13; } // { baz = 89; })
+  { foo = 42; bar = 33; }
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catattrs-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catattrs-propagate-catchable.exp
new file mode 100644
index 0000000000..c3bb809c9f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catattrs-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catattrs-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catattrs-propagate-catchable.nix
new file mode 100644
index 0000000000..5385591f77
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-catattrs-propagate-catchable.nix
@@ -0,0 +1 @@
+map (e: (builtins.tryEval e).success) [ (builtins.catAttrs "a" (builtins.throw "b")) (builtins.catAttrs (builtins.throw "a") { a = 1; }) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-compareVersions.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-compareVersions.exp
new file mode 100644
index 0000000000..e69498c3e1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-compareVersions.exp
@@ -0,0 +1 @@
+[ 0 -1 -1 0 0 0 1 1 -1 1 -1 1 -1 -1 -1 -1 0 1 -1 -1 1 -1 -1 0 1 1 1 1 -1 -1 -1 -1 -1 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-compareVersions.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-compareVersions.nix
new file mode 100644
index 0000000000..40a90b5070
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-compareVersions.nix
@@ -0,0 +1,46 @@
+let
+  cmp = a: b:
+    let
+      ord1 = builtins.compareVersions a b;
+      ord2 = builtins.compareVersions b a;
+    in
+    assert ord1 == -ord2; ord1;
+in
+
+[
+  (cmp "1.2.3" "1.2.3")
+  (cmp "1.2.2" "1.2.3")
+  (cmp "1.2.3" "1.2.40")
+  (cmp "1.2.3" ".1.2.3")
+  (cmp "1.2.3" "1..2.3")
+  (cmp "1.2.3" "1.2.3.")
+  (cmp "1.2.3" "1.2")
+  (cmp "1.2.3" "1.2.a")
+  (cmp "1a.b" "1a.2")
+  (cmp "1" "")
+  (cmp "1.0" "1.0.0")
+  (cmp "2.3" "2.3pre")
+  (cmp "2.3" "2.3.0pre")
+  (cmp "2.3pre" "2.3.0pre")
+  (cmp "2.3" "2.3prepre")
+  (cmp "2.3pre" "2.3prepre")
+  (cmp "2.3prepre" "2.3prepre")
+  # check that the plain word comparison (via Ord) behaves the same
+  (cmp "foo" "bar")
+  (cmp "FoO" "fOo")
+  (cmp "foo" "fooo")
+  (cmp "foopre" "foo")
+  # Subset of test cases from eval-okay-versions.nix shipped by C++ Nix
+  (cmp "1.0" "2.3")
+  (cmp "2.1" "2.3")
+  (cmp "2.3" "2.3")
+  (cmp "2.5" "2.3")
+  (cmp "3.1" "2.3")
+  (cmp "2.3.1" "2.3")
+  (cmp "2.3.1" "2.3a")
+  (cmp "2.3pre1" "2.3")
+  (cmp "2.3pre3" "2.3pre12")
+  (cmp "2.3a" "2.3c")
+  (cmp "2.3pre1" "2.3c")
+  (cmp "2.3pre1" "2.3q")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists-propagate-catchable.exp
new file mode 100644
index 0000000000..c82ddd8a80
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false true ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists-propagate-catchable.nix
new file mode 100644
index 0000000000..8071daf7fb
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists-propagate-catchable.nix
@@ -0,0 +1 @@
+map (e: (builtins.tryEval e).success) [ (builtins.concatLists (builtins.throw "a")) (builtins.concatLists [ [ ] (builtins.throw "a") ]) (builtins.concatLists [ [ ] [ ] [ builtins.throw "a" ] ]) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.exp
new file mode 100644
index 0000000000..64ae529ac2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.exp
@@ -0,0 +1 @@
+[ [ ] [ 1 2 3 4 5 6 ] [ [ 1 ] [ 2 ] [ 3 ] ] [ 1 2 3 4 5 6 ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.nix
new file mode 100644
index 0000000000..19ef5eba11
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.nix
@@ -0,0 +1,6 @@
+[
+  (builtins.concatLists [ ])
+  (builtins.concatLists [ [ 1 2 ] [ 3 4 ] [ 5 6 ] ])
+  (builtins.concatLists [ [ [ 1 ] [ 2 ] ] [ [ 3 ] ] [ ] ])
+  (builtins.concatLists [ [ 1 2 ] [ ] [ 3 4 ] [ ] [ 5 6 ] ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-map-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-map-propagate-catchable.exp
new file mode 100644
index 0000000000..48e341f4d8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-map-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-map-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-map-propagate-catchable.nix
new file mode 100644
index 0000000000..740f0d3fbc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-map-propagate-catchable.nix
@@ -0,0 +1 @@
+map (e: (builtins.tryEval e).success) [ (builtins.concatMap (builtins.throw "a") [ "" ]) (builtins.concatMap (_: builtins.throw "x") [ "" ]) (builtins.concatMap (_: [ ]) (builtins.throw "a")) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-strings-sep-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-strings-sep-propagate-catchable.exp
new file mode 100644
index 0000000000..48e341f4d8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-strings-sep-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-strings-sep-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-strings-sep-propagate-catchable.nix
new file mode 100644
index 0000000000..ce5ce4170f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-strings-sep-propagate-catchable.nix
@@ -0,0 +1 @@
+map (e: (builtins.tryEval e).success) [ (builtins.concatStringsSep (builtins.throw "a") [ "" ]) (builtins.concatStringsSep "," (builtins.throw "a")) (builtins.concatStringsSep "," [ "a" (builtins.throw "a") ]) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-div.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-div.exp
new file mode 100644
index 0000000000..44154ba6a6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-div.exp
@@ -0,0 +1 @@
+[ 3 7 0 1 0 0.5 0.5 0.5 42 1 1 -1 -1 -1 1 1 -1 -1 -1 1 1 -1 -1 -1 -74711 -74711 -74711 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-div.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-div.nix
new file mode 100644
index 0000000000..dc6ce27815
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-div.nix
@@ -0,0 +1,34 @@
+[
+  (builtins.div 9 3)
+  (builtins.div 7 1)
+  (builtins.div 3 9)
+  (builtins.div 4 4)
+  (builtins.div 1 2)
+  (builtins.div 1.0 2)
+  (builtins.div 1 2.0)
+  (builtins.div 1.0 2.0)
+  (builtins.div (builtins.div 84 4) 0.5)
+
+  # builtins.div should truncate towards 0
+  (builtins.div 3 2)
+  (builtins.div (-3) (-2))
+  (builtins.div (-3) 2)
+  (builtins.div 3 (-2))
+  (-(builtins.div 3 2))
+
+  (builtins.div 4 3)
+  (builtins.div (-4) (-3))
+  (builtins.div (-4) 3)
+  (builtins.div 4 (-3))
+  (-(builtins.div 4 3))
+
+  (builtins.div 5 3)
+  (builtins.div (-5) (-3))
+  (builtins.div (-5) 3)
+  (builtins.div 5 (-3))
+  (-(builtins.div 5 3))
+
+  (builtins.div 2147812578 (-28748))
+  (builtins.div (-2147812578) 28748)
+  (-(builtins.div 2147812578 28748))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemAt-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemAt-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemAt-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemAt-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemAt-catchable.nix
new file mode 100644
index 0000000000..97be4b013c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemAt-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.elemAt (throw "fred") 0)).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.exp
new file mode 100644
index 0000000000..3701c9d75f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.exp
@@ -0,0 +1 @@
+[ "foo" "bar" "baz" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.nix
new file mode 100644
index 0000000000..762adeebbf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.nix
@@ -0,0 +1,5 @@
+[
+  (builtins.elemAt [ "foo" "bar" "baz" ] 0)
+  (builtins.elemAt [ "foo" "bar" "baz" ] 1)
+  (builtins.elemAt [ "foo" "bar" "baz" ] 2)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-catchable.nix
new file mode 100644
index 0000000000..98d90b01bb
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.filter (_: throw "fred") [ 3 ])).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-propagate-catchable.exp
new file mode 100644
index 0000000000..48e341f4d8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-propagate-catchable.nix
new file mode 100644
index 0000000000..715a0ce34f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter-propagate-catchable.nix
@@ -0,0 +1 @@
+map (e: (builtins.tryEval e).success) [ (builtins.filter (builtins.throw "a") [ "" ]) (builtins.filter (x: true) (builtins.throw "b")) (builtins.filter (_: builtins.throw "x") [ "" ]) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter.exp
new file mode 100644
index 0000000000..fb94ebaa49
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter.exp
@@ -0,0 +1 @@
+[ [ 1 2 3 4 5 ] [ ] [ 2 2 2 ] [ [ 1 2 ] [ 3 4 ] ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter.nix
new file mode 100644
index 0000000000..b621fdb43e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter.nix
@@ -0,0 +1,13 @@
+[
+  (builtins.filter (_: true) [ 1 2 3 4 5 ])
+  (builtins.filter (_: false) [ 1 2 3 4 5 ])
+  (builtins.filter (x: x == 2) [ 1 2 1 2 1 2 ])
+
+  (builtins.filter (x: (builtins.length x) > 0) [
+    [ ]
+    [ 1 2 ]
+    [ ]
+    [ ]
+    [ 3 4 ]
+  ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-foldl-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-foldl-propagate-catchable.exp
new file mode 100644
index 0000000000..7bf6c63466
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-foldl-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false false true true ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-foldl-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-foldl-propagate-catchable.nix
new file mode 100644
index 0000000000..c11a21ce1e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-foldl-propagate-catchable.nix
@@ -0,0 +1,8 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.foldl' (builtins.throw "a") { } [{ } { } { }])
+  (builtins.foldl' (x: y: x // y) { } (builtins.throw "b"))
+  (builtins.foldl' (_: _: builtins.throw "x") { } [{ }])
+  (builtins.foldl' (x: y: x // y) (builtins.throw "x") [{ }])
+  (builtins.foldl' (x: y: x // y) { } [{ } { a = builtins.throw "z"; } { }])
+  (builtins.foldl' (x: y: x // y) { } [{ } { b = 3; a = builtins.throw "u"; } { }])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-from-json-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-from-json-propagate-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-from-json-propagate-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-from-json-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-from-json-propagate-catchable.nix
new file mode 100644
index 0000000000..aa973c1352
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-from-json-propagate-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.fromJSON (builtins.throw "a"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-function-args-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-function-args-propagate-catchable.exp
new file mode 100644
index 0000000000..bd52bff2fa
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-function-args-propagate-catchable.exp
@@ -0,0 +1 @@
+[ true false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-function-args-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-function-args-propagate-catchable.nix
new file mode 100644
index 0000000000..ca3e6772f2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-function-args-propagate-catchable.nix
@@ -0,0 +1,4 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.functionArgs (_: builtins.throw "a"))
+  (builtins.functionArgs (builtins.throw "b"))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-gen-list-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-gen-list-propagate-catchable.exp
new file mode 100644
index 0000000000..652df3d4da
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-gen-list-propagate-catchable.exp
@@ -0,0 +1 @@
+[ true false true ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-gen-list-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-gen-list-propagate-catchable.nix
new file mode 100644
index 0000000000..3d4739966e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-gen-list-propagate-catchable.nix
@@ -0,0 +1,5 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.genList (builtins.throw "a") 10)
+  (builtins.genList (i: "") (builtins.throw "b"))
+  (builtins.genList (i: builtins.throw "x") 5)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genList-function-strictness.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genList-function-strictness.exp
new file mode 100644
index 0000000000..06712ebc33
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genList-function-strictness.exp
@@ -0,0 +1 @@
+[ <LAMBDA> 0 1 2 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genList-function-strictness.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genList-function-strictness.nix
new file mode 100644
index 0000000000..e161e3b4af
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genList-function-strictness.nix
@@ -0,0 +1,8 @@
+let
+  self =
+    let
+      l = builtins.genList (builtins.head self) 3;
+    in
+    [ (x: x) ] ++ l;
+in
+self
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-pointer-equality.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-pointer-equality.exp
new file mode 100644
index 0000000000..87977137a5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-pointer-equality.exp
@@ -0,0 +1 @@
+[ { key = [ { foo = <LAMBDA>; } ]; val = null; } ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-pointer-equality.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-pointer-equality.nix
new file mode 100644
index 0000000000..f6ca340c09
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-pointer-equality.nix
@@ -0,0 +1,15 @@
+let
+  foo = x: x;
+in
+
+# key needs to be a list since it uses comparison, not equality checks:
+  # lists are comparable in Nix if all non-comparable items in them are equal (e.g.
+  # functions, attribute sets).
+builtins.genericClosure {
+  startSet = [
+    { key = [{ inherit foo; }]; val = null; }
+  ];
+  operator = { val, ... }: if val != null then [ ] else [
+    { key = [{ inherit foo; }]; val = throw "no pointer equality? ๐Ÿฅบ๐Ÿ‘‰๐Ÿ‘ˆ"; }
+  ];
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-propagate-catchable.exp
new file mode 100644
index 0000000000..6c89e78fc4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-propagate-catchable.exp
@@ -0,0 +1 @@
+{ success = false; value = false; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-propagate-catchable.nix
new file mode 100644
index 0000000000..1dfc0bb04f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-propagate-catchable.nix
@@ -0,0 +1 @@
+builtins.tryEval (builtins.genericClosure { operator = (_: [{ key = throw "lol"; }]); startSet = [{ key = "lol"; }]; })
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getAttr-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getAttr-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getAttr-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getAttr-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getAttr-catchable.nix
new file mode 100644
index 0000000000..ef4a042ffb
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getAttr-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.getAttr (throw "fred") "bob")).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getContext-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getContext-propagate-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getContext-propagate-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getContext-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getContext-propagate-catchable.nix
new file mode 100644
index 0000000000..70521665ca
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getContext-propagate-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.getContext (builtins.throw "a"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.exp
new file mode 100644
index 0000000000..89fa6c6810
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.exp
@@ -0,0 +1 @@
+[ 1 2 3 { bar = { baz = 3; }; } ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.nix
new file mode 100644
index 0000000000..87a2adbcd3
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.nix
@@ -0,0 +1,6 @@
+[
+  (builtins.getAttr "foo" { foo = 1; bar = 2; baz = 3; })
+  (builtins.getAttr "bar" { foo = 1; bar = 2; baz = 3; })
+  (builtins.getAttr "baz" { foo = 1; bar = 2; baz = 3; })
+  (builtins.getAttr "foo" { foo = { bar = { baz = 3; }; }; })
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-group-by-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-group-by-propagate-catchable.exp
new file mode 100644
index 0000000000..48e341f4d8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-group-by-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-group-by-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-group-by-propagate-catchable.nix
new file mode 100644
index 0000000000..182601abb1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-group-by-propagate-catchable.nix
@@ -0,0 +1,5 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.groupBy (builtins.throw "a") [ "" ])
+  (builtins.groupBy (x: true) (builtins.throw "b"))
+  (builtins.groupBy (_: builtins.throw "x") [ "" ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-groupby-thunk.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-groupby-thunk.exp
new file mode 100644
index 0000000000..94649819ca
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-groupby-thunk.exp
@@ -0,0 +1 @@
+{ fred = [ { x = "fred"; y = "fred"; } ]; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-groupby-thunk.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-groupby-thunk.nix
new file mode 100644
index 0000000000..eaf48045f2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-groupby-thunk.nix
@@ -0,0 +1,5 @@
+builtins.groupBy
+  (v: v.x)
+  [ (rec { y = x; x = "fred"; }) ]
+
+
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasContext-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasContext-propagate-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasContext-propagate-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasContext-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasContext-propagate-catchable.nix
new file mode 100644
index 0000000000..0c02a82730
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasContext-propagate-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.hasContext (builtins.throw "a"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasattr.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasattr.exp
new file mode 100644
index 0000000000..541fe347cb
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasattr.exp
@@ -0,0 +1 @@
+[ true true true false false true false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasattr.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasattr.nix
new file mode 100644
index 0000000000..fb786b4f09
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hasattr.nix
@@ -0,0 +1,9 @@
+[
+  (builtins.hasAttr "foo" { foo = 1; bar = 2; baz = 3; })
+  (builtins.hasAttr "bar" { foo = 1; bar = 2; baz = 3; })
+  (builtins.hasAttr "baz" { foo = 1; bar = 2; baz = 3; })
+  (builtins.hasAttr "FOO" { foo = 1; bar = 2; baz = 3; })
+  (builtins.hasAttr "foo" { })
+  (builtins.hasAttr ("f" + "o" + "o") { foo = 1; })
+  (builtins.hasAttr ("b" + "a" + "r") { foo = 1; })
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hashString.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hashString.exp
new file mode 100644
index 0000000000..e00b80e561
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hashString.exp
@@ -0,0 +1 @@
+[ "8a0614b4eaa4cffb7515ec101847e198" "8bd218cf61321d8aa05b3602b99f90d2d8cef3d6" "80ac06d74cb6c5d14af718ce8c3c1255969a1a595b76a3cf92354a95331a879a" "0edac513b6b0454705b553deda4c9b055da0939d26d2f73548862817ebeac5378cf64ff7a752ce1a0590a736735d3bbd9e8a7f04d93617cdf514313f5ab5baa4" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hashString.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hashString.nix
new file mode 100644
index 0000000000..aed723d367
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-hashString.nix
@@ -0,0 +1,6 @@
+[
+  (builtins.hashString "md5" "tvix")
+  (builtins.hashString "sha1" "tvix")
+  (builtins.hashString "sha256" "tvix")
+  (builtins.hashString "sha512" "tvix")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head-propagate-catchable.exp
new file mode 100644
index 0000000000..c3bb809c9f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head-propagate-catchable.nix
new file mode 100644
index 0000000000..0e69f2f6fc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head-propagate-catchable.nix
@@ -0,0 +1,4 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.head (builtins.throw "a"))
+  (builtins.head [ (builtins.throw "a") ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head.exp
new file mode 100644
index 0000000000..afe288459f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head.exp
@@ -0,0 +1 @@
+[ "foo" 1 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head.nix
new file mode 100644
index 0000000000..1741a7aac4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head.nix
@@ -0,0 +1,4 @@
+[
+  (builtins.head [ "foo" ])
+  (builtins.head [ 1 2 3 ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-isType-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-isType-propagate-catchable.exp
new file mode 100644
index 0000000000..cefd8652b4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-isType-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false false false false false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-isType-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-isType-propagate-catchable.nix
new file mode 100644
index 0000000000..28cf2351f6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-isType-propagate-catchable.nix
@@ -0,0 +1,14 @@
+let
+  isTypeFns = [
+    builtins.isAttrs
+    builtins.isBool
+    builtins.isFloat
+    builtins.isFunction
+    builtins.isInt
+    builtins.isList
+    builtins.isNull
+    builtins.isPath
+    builtins.isString
+  ];
+in
+map (fn: (builtins.tryEval (fn (builtins.throw "is type"))).success) isTypeFns
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length-catchable.nix
new file mode 100644
index 0000000000..037a4911ac
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.length (throw "fred"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length.exp
new file mode 100644
index 0000000000..e80eb6ef14
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length.exp
@@ -0,0 +1 @@
+[ 0 1 3 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length.nix
new file mode 100644
index 0000000000..6af6915f97
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length.nix
@@ -0,0 +1,5 @@
+[
+  (builtins.length [ ])
+  (builtins.length [ 1 ])
+  (builtins.length [ "one" "two" "three" ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-lessThan.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-lessThan.exp
new file mode 100644
index 0000000000..31f4598bb5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-lessThan.exp
@@ -0,0 +1 @@
+[ true true true true false false false false true true true true false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-lessThan.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-lessThan.nix
new file mode 100644
index 0000000000..cd2d0c209c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-lessThan.nix
@@ -0,0 +1,15 @@
+[
+  (builtins.lessThan 2 3)
+  (builtins.lessThan 2.0 3)
+  (builtins.lessThan 2 3.0)
+  (builtins.lessThan 2.0 3.0)
+  (builtins.lessThan 3 2)
+  (builtins.lessThan 3.0 2)
+  (builtins.lessThan 3 2.0)
+  (builtins.lessThan 3.0 2.0)
+  (builtins.lessThan 10 (builtins.add 9 2))
+  (builtins.lessThan (builtins.add 9 1) 11)
+  (builtins.lessThan (builtins.add 9 1) (builtins.add 9 2))
+  (builtins.lessThan "a" "b")
+  (builtins.lessThan "b" "a")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-list-to-attrs-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-list-to-attrs-propagate-catchable.exp
new file mode 100644
index 0000000000..5ee59ced83
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-list-to-attrs-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false true true false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-list-to-attrs-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-list-to-attrs-propagate-catchable.nix
new file mode 100644
index 0000000000..91b9f889bb
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-list-to-attrs-propagate-catchable.nix
@@ -0,0 +1,7 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.listToAttrs [{ name = builtins.throw "a"; value = "b"; }])
+  (builtins.listToAttrs [{ name = "a"; value = builtins.throw "b"; }])
+  (builtins.listToAttrs [{ name = "a"; value = "b"; } { name = "c"; value = builtins.throw "d"; }])
+  (builtins.listToAttrs [{ name = "a"; value = "b"; } (builtins.throw "e")])
+  (builtins.listToAttrs (builtins.throw "f"))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-function-strictness.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-function-strictness.exp
new file mode 100644
index 0000000000..050c2c4de5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-function-strictness.exp
@@ -0,0 +1 @@
+[ <LAMBDA> 2 "." 18 "https://github.com/NixOS/nix/issues/9779" "-.-" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-function-strictness.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-function-strictness.nix
new file mode 100644
index 0000000000..932d3d0eae
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-function-strictness.nix
@@ -0,0 +1,8 @@
+let
+  self =
+    let
+      l = builtins.map (builtins.head self) [ 2 "." 18 https://github.com/NixOS/nix/issues/9779 "-.-" ];
+    in
+    [ (x: x) ] ++ l;
+in
+self
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-propagate-catchable.exp
new file mode 100644
index 0000000000..652df3d4da
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-propagate-catchable.exp
@@ -0,0 +1 @@
+[ true false true ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-propagate-catchable.nix
new file mode 100644
index 0000000000..3ebb006c3f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map-propagate-catchable.nix
@@ -0,0 +1,5 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.map (builtins.throw "a") [ "" ])
+  (builtins.map (x: true) (builtins.throw "b"))
+  (builtins.map (_: builtins.throw "x") [ "" ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map.exp
new file mode 100644
index 0000000000..6cf5304032
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map.exp
@@ -0,0 +1 @@
+[ [ 1 2 3 4 5 ] [ 2 4 6 8 10 ] [ 2 4 6 8 10 ] [ 2 4 6 8 10 ] [ 1 2 3 4 5 ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map.nix
new file mode 100644
index 0000000000..71b351fd55
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map.nix
@@ -0,0 +1,19 @@
+[
+  # identity function
+  (builtins.map (x: x) [ 1 2 3 4 5 ])
+
+  # double stuff
+  (builtins.map (x: x * 2) [ 1 2 3 4 5 ])
+
+  # same but with a closure this time
+  (
+    let n = 2;
+    in builtins.map (x: x * n) [ 1 2 3 4 5 ]
+  )
+
+  # same, but with a builtin
+  (builtins.map (builtins.mul 2) [ 1 2 3 4 5 ])
+
+  # from global scope
+  (map (x: x) [ 1 2 3 4 5 ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mapAttrs-function-strictness.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mapAttrs-function-strictness.exp
new file mode 100644
index 0000000000..7e70748ffd
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mapAttrs-function-strictness.exp
@@ -0,0 +1 @@
+{ a = 1; b = 2; f = <LAMBDA>; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mapAttrs-function-strictness.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mapAttrs-function-strictness.nix
new file mode 100644
index 0000000000..2946d6de17
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mapAttrs-function-strictness.nix
@@ -0,0 +1,8 @@
+let
+  self =
+    let
+      s = builtins.mapAttrs self.f { a = 1; b = 2; };
+    in
+    { f = _: x: x; } // s;
+in
+self
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-match-propagate-catchables.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-match-propagate-catchables.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-match-propagate-catchables.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-match-propagate-catchables.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-match-propagate-catchables.nix
new file mode 100644
index 0000000000..8d00994b60
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-match-propagate-catchables.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.match (throw "foo") ".*")).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mul.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mul.exp
new file mode 100644
index 0000000000..e3e0f03a8a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mul.exp
@@ -0,0 +1 @@
+[ 36 0 0 14 42 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mul.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mul.nix
new file mode 100644
index 0000000000..2a8d6c4214
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mul.nix
@@ -0,0 +1,7 @@
+[
+  (builtins.mul 4 9)
+  (builtins.mul 0 7)
+  (builtins.mul 7 0)
+  (builtins.mul 7 2)
+  (builtins.mul (builtins.mul 4 0.5) 21)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-parse-drv-name-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-parse-drv-name-propagate-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-parse-drv-name-propagate-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-parse-drv-name-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-parse-drv-name-propagate-catchable.nix
new file mode 100644
index 0000000000..c718727e74
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-parse-drv-name-propagate-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.parseDrvName (builtins.throw "a"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition-propagate-catchable.exp
new file mode 100644
index 0000000000..48e341f4d8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition-propagate-catchable.nix
new file mode 100644
index 0000000000..1960b1790f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition-propagate-catchable.nix
@@ -0,0 +1,5 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.partition (builtins.throw "a") [ "" ])
+  (builtins.partition (x: true) (builtins.throw "b"))
+  (builtins.partition (_: builtins.throw "x") [ "" ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition.exp
new file mode 100644
index 0000000000..d2390db4f5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition.exp
@@ -0,0 +1 @@
+[ { right = [ 1 2 3 4 5 ]; wrong = [ ]; } { right = [ ]; wrong = [ 1 2 3 4 5 ]; } { right = [ 2 ]; wrong = [ 1 3 4 5 ]; } { right = [ [ 1 2 ] [ 3 4 ] ]; wrong = [ [ 1 ] [ 2 ] [ 3 ] ]; } ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition.nix
new file mode 100644
index 0000000000..44022a9e0c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition.nix
@@ -0,0 +1,13 @@
+[
+  (builtins.partition (_: true) [ 1 2 3 4 5 ])
+  (builtins.partition (_: false) [ 1 2 3 4 5 ])
+  (builtins.partition (x: x == 2) [ 1 2 3 4 5 ])
+
+  (builtins.partition (x: (builtins.length x) > 1) [
+    [ 1 ]
+    [ 1 2 ]
+    [ 2 ]
+    [ 3 ]
+    [ 3 4 ]
+  ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-remove-attrs-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-remove-attrs-propagate-catchable.exp
new file mode 100644
index 0000000000..1e950b5aa2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-remove-attrs-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false true false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-remove-attrs-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-remove-attrs-propagate-catchable.nix
new file mode 100644
index 0000000000..8ca8a414aa
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-remove-attrs-propagate-catchable.nix
@@ -0,0 +1,6 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.removeAttrs (builtins.throw "a") [ "a" ])
+  (builtins.removeAttrs { a = { }; } (builtins.throw "b"))
+  (builtins.removeAttrs { a = builtins.throw "b"; } [ "a" ])
+  (builtins.removeAttrs { "${builtins.throw "c"}" = "b"; } [ "c" ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replace-strings-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replace-strings-propagate-catchable.exp
new file mode 100644
index 0000000000..cefd8652b4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replace-strings-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false false false false false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replace-strings-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replace-strings-propagate-catchable.nix
new file mode 100644
index 0000000000..ad9734ba9a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replace-strings-propagate-catchable.nix
@@ -0,0 +1,16 @@
+map (e: (builtins.tryEval e).success) [
+  # This one may be hard to read for non-experts.
+  # Replace strings is a special built-in compared to others in the sense
+  # it might attempt to lazily evaluate things upon successful replacements,
+  # so it would not be surprising that some of the non-replacements which could throw
+  # could be ignored by laziness. It is not the case though.
+  (builtins.replaceStrings [ "a" (builtins.throw "b") ] [ "c" "d" ] "ab")
+  (builtins.replaceStrings [ "a" (builtins.throw "b") ] [ "c" "d" ] "a")
+  (builtins.replaceStrings [ "a" "b" ] [ "c" (builtins.throw "d") ] "a")
+  (builtins.replaceStrings [ "a" "b" ] [ "c" (builtins.throw "d") ] "ab")
+  (builtins.replaceStrings [ "" ] [ (builtins.throw "d") ] "ab")
+  (builtins.replaceStrings [ "a" "" ] [ "b" (builtins.throw "d") ] "ab")
+  (builtins.replaceStrings (builtins.throw "z") [ ] "ab")
+  (builtins.replaceStrings [ ] (builtins.throw "z") "ab")
+  (builtins.replaceStrings [ ] [ ] (builtins.throw "z"))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replaceStrings.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replaceStrings.exp
new file mode 100644
index 0000000000..9f20496c7a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replaceStrings.exp
@@ -0,0 +1 @@
+[ "fabir" "a" "1a1" "ABC" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replaceStrings.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replaceStrings.nix
new file mode 100644
index 0000000000..eea3f87a2f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replaceStrings.nix
@@ -0,0 +1,6 @@
+[
+  (builtins.replaceStrings [ "oo" "a" ] [ "a" "i" ] "foobar")
+  (builtins.replaceStrings [ "o" ] [ "a" ] "a")
+  (builtins.replaceStrings [ "" "" ] [ "1" "2" ] "a")
+  (builtins.replaceStrings [ "a" "b" "c" ] [ "A" "B" "C" ] "abc")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sort-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sort-propagate-catchable.exp
new file mode 100644
index 0000000000..1e950b5aa2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sort-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false true false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sort-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sort-propagate-catchable.nix
new file mode 100644
index 0000000000..66ca4b98ed
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sort-propagate-catchable.nix
@@ -0,0 +1,6 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.sort (builtins.throw "a") [ "" ])
+  (builtins.sort (x: y: true) (builtins.throw "b"))
+  (builtins.sort (_: _: builtins.throw "x") [ "" ])
+  (builtins.sort (_: _: builtins.throw "x") [ "" "" ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-split-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-split-propagate-catchable.exp
new file mode 100644
index 0000000000..c3bb809c9f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-split-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-split-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-split-propagate-catchable.nix
new file mode 100644
index 0000000000..1c86e9d6f8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-split-propagate-catchable.nix
@@ -0,0 +1,4 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.split (builtins.throw "regex") "abc")
+  (builtins.split "[^/]" (builtins.throw "string"))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitVersion.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitVersion.exp
new file mode 100644
index 0000000000..222a0093f5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitVersion.exp
@@ -0,0 +1 @@
+[ [ "1" "2" "3" ] [ "2" "3" "16" ] [ "22" "11" "pre" "408963" "823" "e" "2" "c" "9" "b" "0" "a" "0" ] [ "9" "4" "1" "rc" "1" ] [ "9" "4" "0" "20220721" ] [ "0" "1" "alpha" ] [ "unstable" "2022" "09" "20" ] [ "30" "pre" "9" ] [ "0" "pre+date=" "2021" "11" "30" ] [ "1" "2" "0" "_pre" "23" ] [ "0" "1" "0" "pre" "71" "_" "170" "f" "840" ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitVersion.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitVersion.nix
new file mode 100644
index 0000000000..4083e86714
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitVersion.nix
@@ -0,0 +1,13 @@
+[
+  (builtins.splitVersion "1.2.3")
+  (builtins.splitVersion "2.3.16")
+  (builtins.splitVersion "22.11pre408963.823e2c9b0a0")
+  (builtins.splitVersion "9.4.1-rc1")
+  (builtins.splitVersion "9.4.0.20220721")
+  (builtins.splitVersion "0.1-alpha")
+  (builtins.splitVersion "unstable-2022-09-20")
+  (builtins.splitVersion "30.pre9")
+  (builtins.splitVersion "0.pre+date=2021-11-30")
+  (builtins.splitVersion "1.2.0_pre23")
+  (builtins.splitVersion "0.1.0pre71_170f840")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitversion-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitversion-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitversion-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitversion-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitversion-catchable.nix
new file mode 100644
index 0000000000..0668ec7de2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-splitversion-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.splitVersion (throw "fred"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length-propagate-catchable.exp
new file mode 100644
index 0000000000..d181ebcff6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length-propagate-catchable.nix
new file mode 100644
index 0000000000..0904acd114
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length-propagate-catchable.nix
@@ -0,0 +1,4 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.stringLength (builtins.throw "a"))
+  # FIXME(raitobezarius): test coercions too.
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length.exp
new file mode 100644
index 0000000000..b019be4bfd
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length.exp
@@ -0,0 +1 @@
+[ 3 "hello" 9 4 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length.nix
new file mode 100644
index 0000000000..b7d51db3c5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length.nix
@@ -0,0 +1,10 @@
+[
+  (builtins.stringLength "foo")
+  (let s = "hello"; in (builtins.substring 0 (builtins.stringLength s) s))
+  (builtins.stringLength ("foo" + "${"bar" + "baz"}"))
+
+  # feel free to delete this test case at any time, it's just to show: This is a
+  # thing at the moment. We may want to break compatibility with this aspect of
+  # the C++ Nix implementation at any time.
+  (builtins.stringLength "๐Ÿ˜€")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sub.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sub.exp
new file mode 100644
index 0000000000..51842eccfa
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sub.exp
@@ -0,0 +1 @@
+[ -4 -3.1 -4.9 -4.7 -4 42 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sub.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sub.nix
new file mode 100644
index 0000000000..2929c4dddd
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sub.nix
@@ -0,0 +1,8 @@
+[
+  (builtins.sub 7 11)
+  (builtins.sub 7.9 11)
+  (builtins.sub 7 11.9)
+  (builtins.sub 7.2 11.9)
+  (builtins.sub 7.9 11.9)
+  (builtins.sub (builtins.sub 123 23) 58)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.exp
new file mode 100644
index 0000000000..192548e949
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.exp
@@ -0,0 +1 @@
+"42"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.nix
new file mode 100644
index 0000000000..626ae1d1be
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-coerce.nix
@@ -0,0 +1,5 @@
+# builtins.substring uses string coercion internally
+
+builtins.substring 0 2 {
+  __toString = _: "4200";
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-negative-length.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-negative-length.exp
new file mode 100644
index 0000000000..e614d49940
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-negative-length.exp
@@ -0,0 +1 @@
+[ "SIP dial" "Lounge" " Lounge" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-negative-length.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-negative-length.nix
new file mode 100644
index 0000000000..062e2c0581
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring-negative-length.nix
@@ -0,0 +1,5 @@
+[
+  (builtins.substring 0 (-1) "SIP dial")
+  (builtins.substring 13 (-1) "Nichtraucher Lounge")
+  (builtins.substring 12 (-2) "Nichtraucher Lounge")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring.exp
new file mode 100644
index 0000000000..1682760228
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring.exp
@@ -0,0 +1 @@
+[ "tes" "testing" "" "estin" "ting" "" "" "" "" "est" "est" "est" "est" "est" "est" "" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring.nix
new file mode 100644
index 0000000000..f4ee82e273
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring.nix
@@ -0,0 +1,18 @@
+[
+  (builtins.substring 0 3 "testing")
+  (builtins.substring 0 300 "testing")
+  (builtins.substring 3 0 "testing")
+  (builtins.substring 1 5 "testing")
+  (builtins.substring 3 5 "testing")
+  (builtins.substring 300 300 "testing")
+  (builtins.substring 301 300 "testing")
+  (builtins.substring 0 0 "")
+  (builtins.substring 0 1 "")
+  (builtins.substring (builtins.add 0 1) 3 "testing")
+  (builtins.substring 1 (builtins.add 3 0) "testing")
+  (builtins.substring (builtins.add 0 1) (builtins.add 3 0) "testing")
+  (builtins.substring (builtins.add 0 1) (builtins.add 3 0) "testing")
+  (builtins.substring (builtins.add 0 1) (builtins.add 3 0) ("test" + "ing"))
+  (builtins.substring (builtins.add 0 1) (builtins.add 3 0) ("test" + "ing"))
+  (builtins.substring 300 (-10) "testing")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail-propagate-catchable.exp
new file mode 100644
index 0000000000..d4cd584d22
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false true true true ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail-propagate-catchable.nix
new file mode 100644
index 0000000000..b4e461e12b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail-propagate-catchable.nix
@@ -0,0 +1,6 @@
+map (e: (builtins.tryEval e).success) [
+  (builtins.tail (builtins.throw "a"))
+  (builtins.tail [ (builtins.throw "a") ])
+  (builtins.tail [ (builtins.throw "a") "a" ])
+  (builtins.tail [ (builtins.throw "a") (builtins.throw "a") ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.exp
new file mode 100644
index 0000000000..b9e3aa1ef7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.exp
@@ -0,0 +1 @@
+[ [ ] [ 2 3 ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.nix
new file mode 100644
index 0000000000..2be9496a98
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.nix
@@ -0,0 +1,4 @@
+[
+  (builtins.tail [ "foo" ])
+  (builtins.tail [ 1 2 3 ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-thunked-function-calls.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-thunked-function-calls.exp
new file mode 100644
index 0000000000..3d4204d5a8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-thunked-function-calls.exp
@@ -0,0 +1 @@
+[ 2 [ "Hans" "James" "Joachim" ] 2 [ "Clawdia" "Mynheer" ] 981 3 2 2 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-thunked-function-calls.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-thunked-function-calls.nix
new file mode 100644
index 0000000000..3a1c0b9821
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-thunked-function-calls.nix
@@ -0,0 +1,31 @@
+[
+  # This is independent of builtins
+  (builtins.length [ (builtins.throw "Ferge") (builtins.throw "Wehsal") ])
+  (builtins.attrNames {
+    Hans = throw "Castorp";
+    Joachim = throw "ZiemรŸen";
+    James = "Tienappel";
+  })
+
+  (builtins.length (builtins.map builtins.throw [ "Settembrini" "Naphta" ]))
+
+  (builtins.attrNames (builtins.mapAttrs builtins.throw {
+    Clawdia = "Chauchat";
+    Mynheer = "Peeperkorn";
+  }))
+
+  (builtins.length (builtins.genList (builtins.add "Marusja") 981))
+  (builtins.length (builtins.genList builtins.throw 3))
+
+  # These are hard to get wrong since the outer layer needs to be forced anyways
+  (builtins.length (builtins.genericClosure {
+    startSet = [
+      { key = 1; initial = true; }
+    ];
+    operator = { key, initial, ... }:
+      if initial
+      then [{ key = key - 1; initial = false; value = throw "lol"; }]
+      else [ ];
+  }))
+  (builtins.length (builtins.concatMap (m: [ m (builtins.throw m) ]) [ "Marusja" ]))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-json-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-json-propagate-catchable.exp
new file mode 100644
index 0000000000..ca00e3c049
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-json-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-json-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-json-propagate-catchable.nix
new file mode 100644
index 0000000000..8ae5e48e97
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-json-propagate-catchable.nix
@@ -0,0 +1,14 @@
+map (e: (builtins.tryEval (builtins.toJSON e)).success) [
+  (builtins.throw "a")
+  {
+    a = builtins.throw "attribute a";
+  }
+  {
+    a.b.c.d.e.f.g.h.i = builtins.throw "deep i";
+  }
+  {
+    x = 32;
+    y = builtins.throw "second argument";
+  }
+  # FIXME(raitobezarius): we would like to test coercions, i.e. `toFile` and `derivation` containing throwables.
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-path-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-path-propagate-catchable.exp
new file mode 100644
index 0000000000..c3bb809c9f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-path-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-path-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-path-propagate-catchable.nix
new file mode 100644
index 0000000000..808fb8c46e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-path-propagate-catchable.nix
@@ -0,0 +1,5 @@
+map (e: (builtins.tryEval (builtins.toPath e)).success) [
+  (builtins.throw "a")
+  (./xyz + (builtins.throw "p"))
+  # FIXME: test derivations and files.
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-string-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-string-propagate-catchable.exp
new file mode 100644
index 0000000000..ca00e3c049
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-string-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-string-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-string-propagate-catchable.nix
new file mode 100644
index 0000000000..7b19ff53c3
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-string-propagate-catchable.nix
@@ -0,0 +1,7 @@
+map (e: (builtins.tryEval (builtins.toString e)).success) [
+  (builtins.throw "a")
+  [ (builtins.throw "a") ]
+  [ "abc" (builtins.throw "a") ]
+  "abc${builtins.throw "c"}"
+  # FIXME: test derivations and files.
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-xml-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-xml-propagate-catchable.exp
new file mode 100644
index 0000000000..366f7adf0d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-xml-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false false false false true false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-xml-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-xml-propagate-catchable.nix
new file mode 100644
index 0000000000..80d9e688be
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-to-xml-propagate-catchable.nix
@@ -0,0 +1,15 @@
+map (e: (builtins.tryEval (builtins.toXML e)).success) [
+  (builtins.throw "a")
+  [ (builtins.throw "a") ]
+  [ "abc" (builtins.throw "a") ]
+  "abc${builtins.throw "c"}"
+  (_: builtins.throw "d")
+  {
+    u = builtins.throw "x";
+    v = "a";
+  }
+  {
+    u.i.w.x.z = builtins.throw "n";
+  }
+  # FIXME: test derivations and files.
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-toString.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-toString.exp
new file mode 100644
index 0000000000..cd5a6c0d54
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-toString.exp
@@ -0,0 +1 @@
+[ "" " " " /deep/thought" " 2  3" " flat" "1" "4.200000" "" "" "1" "foo" "/etc" "Hello World" "Hello World" "1" "out" "2" "    /deep/thought  2  3  flat 1 4.200000   1 foo /etc Hello World Hello World 1 out 2" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-toString.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-toString.nix
new file mode 100644
index 0000000000..eb8011158f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-toString.nix
@@ -0,0 +1,28 @@
+let
+  toStringableSet = {
+    __toString = self: self.content;
+    content = "Hello World";
+  };
+
+  toStringExamples = [
+    null
+    [ null false ]
+    [ null /deep/thought ]
+    [ [ null 2 ] null 3 ]
+    [ false "flat" ]
+    1
+    4.2
+    null
+    false
+    true
+    "foo"
+    /etc
+    toStringableSet
+    { __toString = _: toStringableSet; }
+    { __toString = _: true; }
+    { outPath = "out"; }
+    { outPath = { outPath = { __toString = _: 2; }; }; }
+  ];
+in
+
+(builtins.map toString toStringExamples) ++ [ (toString toStringExamples) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-literals.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-literals.exp
new file mode 100644
index 0000000000..0a274c201f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-literals.exp
@@ -0,0 +1 @@
+"[42,\"hello\",13.37,[],[1,2,3],{},{\"name\":\"foo\",\"value\":42},{\"foo\":42}]"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-literals.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-literals.nix
new file mode 100644
index 0000000000..12e8c03b17
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-literals.nix
@@ -0,0 +1,11 @@
+# tests serialisation of literal data
+builtins.toJSON [
+  42
+  "hello"
+  13.37
+  [ ]
+  [ 1 2 3 ]
+  { }
+  { name = "foo"; value = 42; }
+  { foo = 42; }
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath-nested.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath-nested.exp
new file mode 100644
index 0000000000..69667de5a1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath-nested.exp
@@ -0,0 +1 @@
+"{\"a\":40,\"b\":2}"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath-nested.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath-nested.nix
new file mode 100644
index 0000000000..70755c8c6d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath-nested.nix
@@ -0,0 +1,8 @@
+# Attribute sets with an `outPath` can contain _any_ serialisable
+# value in that field.
+builtins.toJSON {
+  outPath = {
+    a = 40;
+    b = 2;
+  };
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath.exp
new file mode 100644
index 0000000000..82dd081798
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath.exp
@@ -0,0 +1 @@
+"\"/nix/store/jzka5ndnygkkfjfvpqwjipqp75lhz138-emacs-28.2\""
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath.nix
new file mode 100644
index 0000000000..7f9d95ac60
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath.nix
@@ -0,0 +1,5 @@
+# Attribute sets with an `outPath` have that outPath itself serialised
+# to string.
+builtins.toJSON {
+  outPath = "/nix/store/jzka5ndnygkkfjfvpqwjipqp75lhz138-emacs-28.2";
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-thunks.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-thunks.exp
new file mode 100644
index 0000000000..9ccd94224b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-thunks.exp
@@ -0,0 +1 @@
+"[42,42,\"42\"]"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-thunks.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-thunks.nix
new file mode 100644
index 0000000000..16234ab451
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-thunks.nix
@@ -0,0 +1,9 @@
+let
+  a = b * 2;
+  b = 21;
+in
+builtins.toJSON [
+  a
+  ((n: n * 2) 21)
+  (builtins.toJSON a)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-tostring.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-tostring.exp
new file mode 100644
index 0000000000..2661fd257b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-tostring.exp
@@ -0,0 +1 @@
+"\"it's 42\""
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-tostring.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-tostring.nix
new file mode 100644
index 0000000000..ec6f8d947c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-tostring.nix
@@ -0,0 +1,8 @@
+# Attribute sets with a `__toString` attribute JSON-serialise with a
+# string coercion of the function call result.
+
+builtins.toJSON {
+  __toString = self: "it's " + (builtins.toString (self.x * self.y));
+  x = 21;
+  y = 2;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of-propagate-catchable.exp
new file mode 100644
index 0000000000..41f22d3ee4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false true true false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of-propagate-catchable.nix
new file mode 100644
index 0000000000..4b70609bbf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of-propagate-catchable.nix
@@ -0,0 +1,9 @@
+map (e: (builtins.tryEval (builtins.typeOf e)).success) [
+  (builtins.throw "a")
+  {
+    a = builtins.throw "b";
+  }
+  [ (builtins.throw "c") ]
+  (./xyz + (builtins.throw "p"))
+  # FIXME: test derivations and files.
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of.exp
new file mode 100644
index 0000000000..1ea054fc2d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of.exp
@@ -0,0 +1 @@
+[ "null" "bool" "bool" "int" "int" "float" "string" "string" "set" "set" "list" "lambda" "path" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of.nix
new file mode 100644
index 0000000000..fa42c6008e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of.nix
@@ -0,0 +1,22 @@
+let
+  fix = f: let x = f x; in x;
+in
+
+fix (self:
+[
+  (builtins.typeOf null)
+  (builtins.typeOf true)
+  (builtins.typeOf (true && false))
+  (builtins.typeOf 12)
+  (builtins.typeOf (builtins.add 21 21))
+  (builtins.typeOf 1.2)
+  (builtins.typeOf "foo")
+  (builtins.typeOf "${"foo" + "bar"}baz")
+  (builtins.typeOf { })
+  # (builtins.typeOf { foo.bar = 32; }.foo) # TODO: re-enable when nested keys are done
+  (builtins.typeOf ({ name = "foo"; value = 13; } // { name = "bar"; }))
+  (builtins.typeOf self)
+  (builtins.typeOf fix)
+  (builtins.typeOf /nix/store)
+]
+)
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-predicates.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-predicates.exp
new file mode 100644
index 0000000000..724c1f9c34
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-predicates.exp
@@ -0,0 +1 @@
+[ true true false true true false true true false true true false true true false true true false true true false true true false true true true false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-predicates.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-predicates.nix
new file mode 100644
index 0000000000..e67b219159
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-predicates.nix
@@ -0,0 +1,34 @@
+let
+  # apply is thunked, so we can create a thunked value using the identity function
+  thunk = x: x;
+in
+[
+  (builtins.isAttrs { bar = throw "baz"; })
+  (builtins.isAttrs (thunk { foo = 13; }))
+  (builtins.isAttrs (thunk 123))
+  (builtins.isBool true)
+  (builtins.isBool (thunk false))
+  (builtins.isBool (thunk "lol"))
+  (builtins.isFloat 1.2)
+  (builtins.isFloat (thunk (1 * 1.0)))
+  (builtins.isFloat 1)
+  (builtins.isFunction thunk)
+  (builtins.isFunction (thunk thunk))
+  (builtins.isFunction { })
+  (builtins.isInt 1)
+  (builtins.isInt (thunk 42))
+  (builtins.isInt 1.0)
+  (builtins.isList [ (throw "oh no") (abort "it's over") ])
+  (builtins.isList (thunk [ 21 21 ]))
+  (builtins.isList (thunk { }))
+  (builtins.isNull null)
+  (builtins.isNull (thunk null))
+  (builtins.isNull 42)
+  (builtins.isPath ./relative)
+  (builtins.isPath (thunk /absolute))
+  (builtins.isPath "/not/a/path")
+  (builtins.isString "simple")
+  (builtins.isString "${{ outPath = "coerced"; }}")
+  (builtins.isString "hello ${"interpolation"}")
+  (builtins.isString true)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-unsafe-discard-string-context-propagate-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-unsafe-discard-string-context-propagate-catchable.exp
new file mode 100644
index 0000000000..d181ebcff6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-unsafe-discard-string-context-propagate-catchable.exp
@@ -0,0 +1 @@
+[ false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-unsafe-discard-string-context-propagate-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-unsafe-discard-string-context-propagate-catchable.nix
new file mode 100644
index 0000000000..8ef5f35a17
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-unsafe-discard-string-context-propagate-catchable.nix
@@ -0,0 +1,4 @@
+map (e: (builtins.tryEval (builtins.unsafeDiscardStringContext e)).success) [
+  (builtins.throw "a")
+  # FIXME: test derivations with throwables.
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-double-throw.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-double-throw.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-double-throw.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-double-throw.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-double-throw.nix
new file mode 100644
index 0000000000..7c2132b502
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-double-throw.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.throw (builtins.throw "a"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-attrNames.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-attrNames.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-attrNames.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-attrNames.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-attrNames.nix
new file mode 100644
index 0000000000..75531d56a3
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-attrNames.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.attrNames (throw "fred"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-inequality.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-inequality.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-inequality.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-inequality.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-inequality.nix
new file mode 100644
index 0000000000..93836bd8fe
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-inequality.nix
@@ -0,0 +1 @@
+(builtins.tryEval (throw "bob" != 3)).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-intersectattrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-intersectattrs.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-intersectattrs.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-intersectattrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-intersectattrs.nix
new file mode 100644
index 0000000000..a06a383342
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-intersectattrs.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.intersectAttrs (throw "fred") { })).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-string-interpolation.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-string-interpolation.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-string-interpolation.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-string-interpolation.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-string-interpolation.nix
new file mode 100644
index 0000000000..7a0cf16709
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-string-interpolation.nix
@@ -0,0 +1 @@
+(builtins.tryEval ("${toString 3}  ${throw "bob"}")).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-update-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-update-attrs.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-update-attrs.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-update-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-update-attrs.nix
new file mode 100644
index 0000000000..38a9169034
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-in-update-attrs.nix
@@ -0,0 +1 @@
+(builtins.tryEval (throw "bob" // { })).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-passed-to-function-with-formals.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-passed-to-function-with-formals.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-passed-to-function-with-formals.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-passed-to-function-with-formals.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-passed-to-function-with-formals.nix
new file mode 100644
index 0000000000..df6726db76
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-catchable-passed-to-function-with-formals.nix
@@ -0,0 +1 @@
+(builtins.tryEval (({ fred }: "bob") (throw "3"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-ceil.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-ceil.exp
new file mode 100644
index 0000000000..dffbbe59f0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-ceil.exp
@@ -0,0 +1 @@
+[ 4 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-ceil.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-ceil.nix
new file mode 100644
index 0000000000..5835bf829b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-ceil.nix
@@ -0,0 +1 @@
+[ (builtins.ceil 3.4) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-closure-pointer-compare.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-pointer-compare.exp
new file mode 100644
index 0000000000..c3bb809c9f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-pointer-compare.exp
@@ -0,0 +1 @@
+[ false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-closure-pointer-compare.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-pointer-compare.nix
new file mode 100644
index 0000000000..639191be5d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-pointer-compare.nix
@@ -0,0 +1,14 @@
+# For an explanation of this behavior see //tvix/docs/value-pointer-equality.md
+let
+  g = x:
+    owo: "th" + x;
+in
+[
+  (
+    { q = g "ia"; } == { q = g ("i" + "a"); }
+  )
+
+  (
+    [ (g "ia") ] == [ (g ("i" + "a")) ]
+  )
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-closure-self.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-self.exp
new file mode 100644
index 0000000000..be54b4b4e3
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-self.exp
@@ -0,0 +1 @@
+"done"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-closure-self.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-self.nix
new file mode 100644
index 0000000000..7be6660009
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-self.nix
@@ -0,0 +1,5 @@
+let
+  # self-recursive function should be able to close over itself
+  f = n: if n <= 0 then "done" else f (n - 1);
+in
+f 10
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-closure-with-shadowing.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-with-shadowing.exp
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-with-shadowing.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-closure-with-shadowing.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-with-shadowing.nix
new file mode 100644
index 0000000000..2c4de65e76
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-with-shadowing.nix
@@ -0,0 +1,14 @@
+# If a closure closes over a variable that is statically known *and*
+# available dynamically through `with`, the statically known one must
+# have precedence.
+
+let
+  # introduce statically known `a` (this should be the result)
+  a = 1;
+in
+
+# introduce some closure depth to force both kinds of upvalue
+  # resolution, and introduce a dynamically known `a` within the
+  # closures
+let f = b: with { a = 2; }; c: a + b + c;
+in f 0 0
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-false.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-false.exp
new file mode 100644
index 0000000000..95a0e7378b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-false.exp
@@ -0,0 +1 @@
+{ eq = false; ge = false; gt = false; le = false; lt = false; ne = false; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-false.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-false.nix
new file mode 100644
index 0000000000..2b511f56ee
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-false.nix
@@ -0,0 +1,8 @@
+{
+  eq = 6.9 == 4.2;
+  ne = 4.2 != 4.2;
+  lt = 2.5 < 1.5;
+  le = 2.5 <= 1.5;
+  gt = 1.5 > 2.5;
+  ge = 1.5 >= 2.5;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-true.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-true.exp
new file mode 100644
index 0000000000..9160829dde
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-true.exp
@@ -0,0 +1 @@
+{ eq = true; ge = true; gt = true; le = true; lt = true; ne = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-true.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-true.nix
new file mode 100644
index 0000000000..c505a85b1f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-float-true.nix
@@ -0,0 +1,8 @@
+{
+  eq = 4.2 == 4.2;
+  ne = 6.9 != 4.2;
+  lt = 1.5 < 2.5;
+  le = 2.5 <= 2.5;
+  gt = 2.3 > 1.2;
+  ge = 2.3 >= 2.3;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-false.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-false.exp
new file mode 100644
index 0000000000..95a0e7378b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-false.exp
@@ -0,0 +1 @@
+{ eq = false; ge = false; gt = false; le = false; lt = false; ne = false; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-false.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-false.nix
new file mode 100644
index 0000000000..7d6b30419f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-false.nix
@@ -0,0 +1,8 @@
+{
+  eq = 69 == 42;
+  ne = 42 != 42;
+  lt = 2 < 1;
+  le = 2 <= 1;
+  gt = 1 > 2;
+  ge = 1 >= 2;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-true.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-true.exp
new file mode 100644
index 0000000000..9160829dde
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-true.exp
@@ -0,0 +1 @@
+{ eq = true; ge = true; gt = true; le = true; lt = true; ne = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-true.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-true.nix
new file mode 100644
index 0000000000..0bf474e53f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-int-true.nix
@@ -0,0 +1,8 @@
+{
+  eq = 42 == 42;
+  ne = 69 != 42;
+  lt = 1 < 2;
+  le = 2 <= 2;
+  gt = 2 > 1;
+  ge = 2 >= 2;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-false.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-false.exp
new file mode 100644
index 0000000000..95a0e7378b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-false.exp
@@ -0,0 +1 @@
+{ eq = false; ge = false; gt = false; le = false; lt = false; ne = false; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-false.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-false.nix
new file mode 100644
index 0000000000..61b206c033
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-false.nix
@@ -0,0 +1,8 @@
+{
+  eq = 6.9 == 4;
+  ne = 4.0 != 4;
+  lt = 2.5 < 1;
+  le = 2 <= 1.5;
+  gt = 1 > 1.1;
+  ge = 1.5 >= 2;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-true.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-true.exp
new file mode 100644
index 0000000000..9160829dde
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-true.exp
@@ -0,0 +1 @@
+{ eq = true; ge = true; gt = true; le = true; lt = true; ne = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-true.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-true.nix
new file mode 100644
index 0000000000..ad77074710
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-num-true.nix
@@ -0,0 +1,8 @@
+{
+  eq = 42.0 == 42;
+  ne = 6.9 != 4;
+  lt = 1.5 < 2;
+  le = 2.0 <= 2.0;
+  gt = 1.1 > 1;
+  ge = 2.3 >= 2.3;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-false.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-false.exp
new file mode 100644
index 0000000000..95a0e7378b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-false.exp
@@ -0,0 +1 @@
+{ eq = false; ge = false; gt = false; le = false; lt = false; ne = false; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-false.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-false.nix
new file mode 100644
index 0000000000..b5773a21d3
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-false.nix
@@ -0,0 +1,8 @@
+{
+  eq = "test" == "not test";
+  ne = "test" != "test";
+  lt = "bcd" < "abc";
+  le = "bcd" <= "abc";
+  gt = "abc" > "bcd";
+  ge = "abc" >= "bcd";
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-true.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-true.exp
new file mode 100644
index 0000000000..9160829dde
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-true.exp
@@ -0,0 +1 @@
+{ eq = true; ge = true; gt = true; le = true; lt = true; ne = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-true.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-true.nix
new file mode 100644
index 0000000000..172d2237e9
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-cmp-str-true.nix
@@ -0,0 +1,8 @@
+{
+  eq = "test" == "test";
+  ne = "test" != "not test";
+  lt = "abc" < "bcd";
+  le = "bcd" <= "bcd";
+  gt = "bcd" > "abc";
+  ge = "bcd" >= "bcd";
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-coerce-opadd.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-coerce-opadd.exp
new file mode 100644
index 0000000000..d874518a37
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-coerce-opadd.exp
@@ -0,0 +1 @@
+[ "lordnikon" "zerocool" /tmp/31337h4x0r "fooblah" "blahfoo" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-coerce-opadd.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-coerce-opadd.nix
new file mode 100644
index 0000000000..e79e521f8a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-coerce-opadd.nix
@@ -0,0 +1,7 @@
+[
+  ({ __toString = _: "lord"; } + "nikon")
+  ("zero" + { __toString = _: "cool"; })
+  (/tmp/31337 + "h4x0r")
+  ("foo" + { outPath = "blah"; })
+  ({ outPath = "blah"; } + "foo")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-compare-lists.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-compare-lists.exp
new file mode 100644
index 0000000000..3b7fd39819
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-compare-lists.exp
@@ -0,0 +1 @@
+[ false true true true false true false false false true false false false true true ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-compare-lists.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-compare-lists.nix
new file mode 100644
index 0000000000..1837f4d820
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-compare-lists.nix
@@ -0,0 +1,17 @@
+[
+  ([ 1 2 ] < [ 1 ])
+  ([ 1 2 ] < [ 2 3 ])
+  ([ 1 2 ] < [ 2 ])
+  ([ 1 2 ] < [ 1 2 3 ])
+  ([ 3 4 ] < [ 1 ])
+  ([ 1 2 ] > [ 1 ])
+  ([ 1 2 ] > [ 2 3 ])
+  ([ 1 2 ] > [ 2 ])
+  ([ 1 2 ] > [ 1 2 3 ])
+  ([ 3 4 ] > [ 1 ])
+  ([ 1 2 ] <= [ 1 ])
+  ([ 1 2 ] >= [ 2 3 ])
+  ([ 1 2 ] >= [ 2 ])
+  ([ 1 2 ] <= [ 1 2 3 ])
+  ([ 3 4 ] >= [ 1 ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-compare-ordering-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-compare-ordering-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-compare-ordering-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-compare-ordering-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-compare-ordering-catchable.nix
new file mode 100644
index 0000000000..9000160e57
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-compare-ordering-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval ((throw "x") < 3)).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-concat-lists.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-concat-lists.exp
new file mode 100644
index 0000000000..3bed31f76e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-concat-lists.exp
@@ -0,0 +1 @@
+[ 1 2 3 4 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-concat-lists.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-concat-lists.nix
new file mode 100644
index 0000000000..de332cd29f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-concat-lists.nix
@@ -0,0 +1 @@
+[ 1 2 ] ++ [ 3 4 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-concat-strings.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-concat-strings.exp
new file mode 100644
index 0000000000..cd4bc1ab64
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-concat-strings.exp
@@ -0,0 +1 @@
+"hello world"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-concat-strings.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-concat-strings.nix
new file mode 100644
index 0000000000..1fc7089299
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-concat-strings.nix
@@ -0,0 +1 @@
+"hello " + "world"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-concatmap.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-concatmap.exp
new file mode 100644
index 0000000000..14d804aa22
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-concatmap.exp
@@ -0,0 +1 @@
+[ "a" "z" "b" "z" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-concatmap.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-concatmap.nix
new file mode 100644
index 0000000000..cff39b05e6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-concatmap.nix
@@ -0,0 +1 @@
+(builtins.concatMap (x: [ x ] ++ [ "z" ]) [ "a" "b" ])
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-concatstringssep.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-concatstringssep.exp
new file mode 100644
index 0000000000..93987647ff
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-concatstringssep.exp
@@ -0,0 +1 @@
+[ "" "foobarxyzzy" "foo, bar, xyzzy" "foo" "" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-concatstringssep.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-concatstringssep.nix
new file mode 100644
index 0000000000..cd94ca99b4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-concatstringssep.nix
@@ -0,0 +1,9 @@
+with builtins;
+
+[
+  (concatStringsSep "" [ ])
+  (concatStringsSep "" [ "foo" "bar" "xyzzy" ])
+  (concatStringsSep ", " [ "foo" "bar" "xyzzy" ])
+  (concatStringsSep ", " [ "foo" ])
+  (concatStringsSep ", " [ ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-contains-nested-non-set.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-contains-nested-non-set.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-contains-nested-non-set.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-contains-nested-non-set.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-contains-nested-non-set.nix
new file mode 100644
index 0000000000..361ba91445
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-contains-nested-non-set.nix
@@ -0,0 +1,3 @@
+# ? operator should work even if encountering a non-set value on the
+# walk
+{ a.b = 42; } ? a.b.c
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-contains-non-set.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-contains-non-set.exp
new file mode 100644
index 0000000000..ca00e3c049
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-contains-non-set.exp
@@ -0,0 +1 @@
+[ false false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-contains-non-set.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-contains-non-set.nix
new file mode 100644
index 0000000000..c086759f45
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-contains-non-set.nix
@@ -0,0 +1,3 @@
+# Nix allows using the ? operator on non-set types, in which case it
+# should always return false.
+[ (123 ? key) ("foo" ? key) (null ? key) ([ "key" ] ? key) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-attrs.exp
new file mode 100644
index 0000000000..7cf54d9596
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-attrs.exp
@@ -0,0 +1 @@
+{ a = { b = { c = { d = { e = { f = { g = "deep!"; }; }; }; }; }; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-attrs.nix
new file mode 100644
index 0000000000..91649d0c6d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-attrs.nix
@@ -0,0 +1 @@
+{ a.b.c.d.e.f.g = "deep!"; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.exp
new file mode 100644
index 0000000000..3bed31f76e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.exp
@@ -0,0 +1 @@
+[ 1 2 3 4 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.nix
new file mode 100644
index 0000000000..f65e8ee537
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.nix
@@ -0,0 +1,21 @@
+# This convoluted test constructs a situation in which dynamically
+# resolved upvalues refer `with` blocks introduced at different lambda
+# context boundaries, i.e. the access to a, b in the innermost closure
+# must be threaded through upvalues in several levels.
+
+(_:
+with { a = 1; b = 1; };
+
+_:
+with { b = 2; c = 2; };
+
+_:
+with { c = 3; d = 3; };
+
+_:
+with { d = 4; };
+
+[ a b c d ]) null
+  null
+  null
+  null
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with.exp
new file mode 100644
index 0000000000..3bed31f76e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with.exp
@@ -0,0 +1 @@
+[ 1 2 3 4 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with.nix
new file mode 100644
index 0000000000..7f1128b670
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with.nix
@@ -0,0 +1,6 @@
+with { a = 1; b = 1; };
+with { b = 2; c = 2; };
+with { c = 3; d = 3; };
+with { d = 4; };
+
+[ a b c d ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deepseq.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-deepseq.exp
new file mode 100644
index 0000000000..8d38505c16
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deepseq.exp
@@ -0,0 +1 @@
+456
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deepseq.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-deepseq.nix
new file mode 100644
index 0000000000..53aa4b1dc2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deepseq.nix
@@ -0,0 +1 @@
+builtins.deepSeq (let as = { x = 123; y = as; }; in as) 456
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.exp
new file mode 100644
index 0000000000..5993db7ccc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.exp
@@ -0,0 +1 @@
+[ false -2 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.nix
new file mode 100644
index 0000000000..1fbb3e853a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-unary-formals.nix
@@ -0,0 +1,6 @@
+# Application of unary operators on deferred formals arguments (via
+# defaulting), see also b/255.
+[
+  (({ b ? !a, a }: b) { a = true; })
+  (({ b ? -a, a }: b) { a = 2; })
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-with.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-with.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-with.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-with.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-with.nix
new file mode 100644
index 0000000000..ccafdf74ce
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-with.nix
@@ -0,0 +1,9 @@
+# Tests using `with` on a set that does not yet exist on the stack.
+
+let
+  result = with set; value;
+  set = {
+    value = 42;
+  };
+in
+result
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-dirof.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-dirof.exp
new file mode 100644
index 0000000000..ff464e4c30
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-dirof.exp
@@ -0,0 +1 @@
+[ /foo "." "foo//" "foo" "." "." / "/" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-dirof.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-dirof.nix
new file mode 100644
index 0000000000..13cf473205
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-dirof.nix
@@ -0,0 +1,10 @@
+[
+  (builtins.dirOf /foo/bar)
+  (builtins.dirOf "foo")
+  (builtins.dirOf "foo///")
+  (builtins.dirOf "foo/bar")
+  (builtins.dirOf "./.")
+  (builtins.dirOf "")
+  (builtins.dirOf /.)
+  (builtins.toString (builtins.dirOf /.))
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-elem.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-elem.exp
new file mode 100644
index 0000000000..3cf6c0e962
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-elem.exp
@@ -0,0 +1 @@
+[ true false 30 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-elem.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-elem.nix
new file mode 100644
index 0000000000..71ea7a4ed0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-elem.nix
@@ -0,0 +1,6 @@
+with import ./lib.nix;
+
+let xs = range 10 40; in
+
+[ (builtins.elem 23 xs) (builtins.elem 42 xs) (builtins.elemAt xs 20) ]
+
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-empty-rec-inherit.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-empty-rec-inherit.exp
new file mode 100644
index 0000000000..ffcd4415b0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-empty-rec-inherit.exp
@@ -0,0 +1 @@
+{ }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-empty-rec-inherit.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-empty-rec-inherit.nix
new file mode 100644
index 0000000000..a1181431de
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-empty-rec-inherit.nix
@@ -0,0 +1 @@
+rec { inherit; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-eq-float.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-float.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-float.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-eq-float.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-float.nix
new file mode 100644
index 0000000000..398f4a9dfc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-float.nix
@@ -0,0 +1 @@
+4.2 == 4.2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-eq-int.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-int.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-int.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-eq-int.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-int.nix
new file mode 100644
index 0000000000..dc52ba112a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-int.nix
@@ -0,0 +1 @@
+42 == 42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-eq-nested-list.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-nested-list.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-nested-list.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-eq-nested-list.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-nested-list.nix
new file mode 100644
index 0000000000..cc39e2415f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-eq-nested-list.nix
@@ -0,0 +1 @@
+[ [ "f" "" ] ] == [ [ "f" "" ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-equality-tolerate-catchable-in-type-field.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-equality-tolerate-catchable-in-type-field.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-equality-tolerate-catchable-in-type-field.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-equality-tolerate-catchable-in-type-field.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-equality-tolerate-catchable-in-type-field.nix
new file mode 100644
index 0000000000..6bd018b68d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-equality-tolerate-catchable-in-type-field.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.elem { type = rec { x = throw "fred"; }.x; } [{ type = 3; }])).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-escape-string-correct-char-boundaries.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-escape-string-correct-char-boundaries.exp
new file mode 100644
index 0000000000..d889063f9a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-escape-string-correct-char-boundaries.exp
@@ -0,0 +1 @@
+"๐Ÿ’ญ(\":thonking:\")"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-escape-string-correct-char-boundaries.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-escape-string-correct-char-boundaries.nix
new file mode 100644
index 0000000000..49f4b62731
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-escape-string-correct-char-boundaries.nix
@@ -0,0 +1,6 @@
+# Regression test for a bug where tvix would crash in nix_escape_string
+# because it counted the string position by unicode code point count,
+# but then used it as a byte index for slicing. Consequently, it would
+# try slicing ๐Ÿ’ญ in half, thinking the first element to be escaped was
+# at byte index 2 (i.e. the quote).
+"๐Ÿ’ญ(\":thonking:\")"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-escapify-integer-keys.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-escapify-integer-keys.exp
new file mode 100644
index 0000000000..aa98a082a8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-escapify-integer-keys.exp
@@ -0,0 +1 @@
+{ "3" = 3; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-escapify-integer-keys.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-escapify-integer-keys.nix
new file mode 100644
index 0000000000..aa98a082a8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-escapify-integer-keys.nix
@@ -0,0 +1 @@
+{ "3" = 3; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-fib.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-fib.exp
new file mode 100644
index 0000000000..8643cf6deb
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fib.exp
@@ -0,0 +1 @@
+89
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-fib.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-fib.nix
new file mode 100644
index 0000000000..04cb52e033
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fib.nix
@@ -0,0 +1,9 @@
+let
+  fib' = i: n: m:
+    if i == 0
+    then n
+    else fib' (i - 1) m (n + m);
+
+  fib = n: fib' n 1 1;
+in
+fib 10
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-fix.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-fix.exp
new file mode 100644
index 0000000000..c158154351
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fix.exp
@@ -0,0 +1 @@
+{ a = 1; b = 21; c = 42; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-fix.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-fix.nix
new file mode 100644
index 0000000000..6069950194
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fix.nix
@@ -0,0 +1,8 @@
+let
+  fix = f: let x = f x; in x;
+in
+fix (self: {
+  a = 1;
+  b = self.a + 20;
+  c = self.b * 2;
+})
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-float-repr.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-float-repr.exp
new file mode 100644
index 0000000000..c55d2be717
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-float-repr.exp
@@ -0,0 +1 @@
+1.23457
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-float-repr.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-float-repr.nix
new file mode 100644
index 0000000000..447bd5af7f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-float-repr.nix
@@ -0,0 +1,2 @@
+# Floats are displayed with a maximum of 5 digits
+1.23456789
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-floor.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-floor.exp
new file mode 100644
index 0000000000..6f98a7f48f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-floor.exp
@@ -0,0 +1 @@
+[ 3 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-floor.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-floor.nix
new file mode 100644
index 0000000000..c6b79c91a1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-floor.nix
@@ -0,0 +1 @@
+[ (builtins.floor 3.4) ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-elements.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-elements.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-elements.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-elements.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-elements.nix
new file mode 100644
index 0000000000..fc4129a254
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-elements.nix
@@ -0,0 +1,8 @@
+let
+  lst = builtins.foldl'
+    (acc: x: acc ++ [ x ])
+    [ ]
+    [ 42 (throw "this shouldn't be evaluated") ];
+in
+
+builtins.head lst
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.nix
new file mode 100644
index 0000000000..59fd29b552
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict-lazy-initial-accumulator.nix
@@ -0,0 +1,4 @@
+builtins.foldl'
+  (_: x: x)
+  (throw "This is never forced")
+  [ "but the results of applying op are" 42 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict.exp
new file mode 100644
index 0000000000..8d683a20fa
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict.exp
@@ -0,0 +1 @@
+[ 6 [ 0 1 2 3 ] 2 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict.nix
new file mode 100644
index 0000000000..aadf5e1121
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-foldlStrict.nix
@@ -0,0 +1,5 @@
+[
+  (builtins.foldl' builtins.add 0 [ 1 2 3 ])
+  (builtins.foldl' (l1: l2: l1 ++ l2) [ 0 ] [ [ 1 ] [ 2 3 ] ])
+  (builtins.foldl' (x: y: if x == 0 then y else x * y) 0 [ 1 2 ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.exp
new file mode 100644
index 0000000000..721a052bcc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.exp
@@ -0,0 +1 @@
+[ true null ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.nix
new file mode 100644
index 0000000000..772fa6f386
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-formals-miscompilation-b-261-regression.nix
@@ -0,0 +1,20 @@
+# This is a regression test for https://b.tvl.fyi/261.
+#
+# The bug occurred when Tvix would unconditionally finalise the stack slot of
+# `finalise` (as its default expression needs a finaliser): Finalising an
+# manually provided, already forced thunk would cause the VM to crash.
+let
+  thunk = x: x;
+  bomb = thunk true;
+  f =
+    { finalise ? later == null
+    , later ? null
+    }:
+    [ finalise later ];
+in
+
+# Note that the crash did not occur if the offending expression was the rhs
+  # argument to `builtins.seq`, hence we need to put the assert in between.
+assert builtins.seq bomb true;
+
+f { finalise = bomb; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson-escapes.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson-escapes.exp
new file mode 100644
index 0000000000..add5505a82
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson-escapes.exp
@@ -0,0 +1 @@
+"quote \" reverse solidus \\ solidus / backspace  formfeed  newline \n carriage return \r horizontal tab \t 1 char unicode encoded backspace  1 char unicode encoded e with accent รฉ 2 char unicode encoded s with caron ลก 3 char unicode encoded rightwards arrow โ†’"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson-escapes.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson-escapes.nix
new file mode 100644
index 0000000000..f007135077
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson-escapes.nix
@@ -0,0 +1,3 @@
+# This string contains all supported escapes in a JSON string, per json.org
+# \b and \f are not supported by Nix
+builtins.fromJSON ''"quote \" reverse solidus \\ solidus \/ backspace \b formfeed \f newline \n carriage return \r horizontal tab \t 1 char unicode encoded backspace \u0008 1 char unicode encoded e with accent \u00e9 2 char unicode encoded s with caron \u0161 3 char unicode encoded rightwards arrow \u2192"''
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson.exp
new file mode 100644
index 0000000000..24aa21d78f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson.exp
@@ -0,0 +1 @@
+[ { Image = { Animated = false; Height = 600; IDs = [ 116 943 234 38793 true false null -100 ]; Latitude = 37.7668; Longitude = -122.396; Thumbnail = { Height = 125; Url = "http://www.example.com/image/481989943"; Width = 100; }; Title = "View from 15th Floor"; Width = 800; }; } { name = "a"; value = "b"; } [ 1 2 3 4 ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson.nix
new file mode 100644
index 0000000000..1083919af8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson.nix
@@ -0,0 +1,24 @@
+[
+  # RFC 7159, section 13.
+  (builtins.fromJSON
+    ''
+      {
+        "Image": {
+            "Width":  800,
+            "Height": 600,
+            "Title":  "View from 15th Floor",
+            "Thumbnail": {
+                "Url":    "http://www.example.com/image/481989943",
+                "Height": 125,
+                "Width":  100
+            },
+            "Animated" : false,
+            "IDs": [116, 943, 234, 38793, true  ,false,null, -100],
+            "Latitude":  37.7668,
+            "Longitude": -122.396
+        }
+      }
+    '')
+  (builtins.fromJSON ''{"name": "a", "value": "b"}'')
+  (builtins.fromJSON "[ 1, 2, 3, 4 ]")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-functionargs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-functionargs.exp
new file mode 100644
index 0000000000..c1c9f8ffaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-functionargs.exp
@@ -0,0 +1 @@
+[ "stdenv" "fetchurl" "aterm-stdenv" "aterm-stdenv2" "libX11" "libXv" "mplayer-stdenv2.libXv-libX11" "mplayer-stdenv2.libXv-libX11_2" "nix-stdenv-aterm-stdenv" "nix-stdenv2-aterm2-stdenv2" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-functionargs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-functionargs.nix
new file mode 100644
index 0000000000..6db04c562c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-functionargs.nix
@@ -0,0 +1,83 @@
+let
+
+  stdenvFun = {}: { name = "stdenv"; };
+  stdenv2Fun = {}: { name = "stdenv2"; };
+  fetchurlFun = { stdenv }: assert stdenv.name == "stdenv"; { name = "fetchurl"; };
+  atermFun = { stdenv, fetchurl }: { name = "aterm-${stdenv.name}"; };
+  aterm2Fun = { stdenv, fetchurl }: { name = "aterm2-${stdenv.name}"; };
+  nixFun = { stdenv, fetchurl, aterm }: { name = "nix-${stdenv.name}-${aterm.name}"; };
+
+  mplayerFun =
+    { stdenv, fetchurl, enableX11 ? false, xorg ? null, enableFoo ? true, foo ? null }:
+      assert stdenv.name == "stdenv2";
+      assert enableX11 -> xorg.libXv.name == "libXv";
+      assert enableFoo -> foo != null;
+      { name = "mplayer-${stdenv.name}.${xorg.libXv.name}-${xorg.libX11.name}"; };
+
+  makeOverridable = f: origArgs: f origArgs //
+    {
+      override = newArgs:
+        makeOverridable f (origArgs // (if builtins.isFunction newArgs then newArgs origArgs else newArgs));
+    };
+
+  callPackage_ = pkgs: f: args:
+    makeOverridable f ((builtins.intersectAttrs (builtins.functionArgs f) pkgs) // args);
+
+  allPackages =
+    { overrides ? (pkgs: pkgsPrev: { }) }:
+    let
+      callPackage = callPackage_ pkgs;
+      pkgs = pkgsStd // (overrides pkgs pkgsStd);
+      pkgsStd = {
+        inherit pkgs;
+        stdenv = callPackage stdenvFun { };
+        stdenv2 = callPackage stdenv2Fun { };
+        fetchurl = callPackage fetchurlFun { };
+        aterm = callPackage atermFun { };
+        xorg = callPackage xorgFun { };
+        mplayer = callPackage mplayerFun { stdenv = pkgs.stdenv2; enableFoo = false; };
+        nix = callPackage nixFun { };
+      };
+    in
+    pkgs;
+
+  libX11Fun = { stdenv, fetchurl }: { name = "libX11"; };
+  libX11_2Fun = { stdenv, fetchurl }: { name = "libX11_2"; };
+  libXvFun = { stdenv, fetchurl, libX11 }: { name = "libXv"; };
+
+  xorgFun =
+    { pkgs }:
+    let callPackage = callPackage_ (pkgs // pkgs.xorg); in
+    {
+      libX11 = callPackage libX11Fun { };
+      libXv = callPackage libXvFun { };
+    };
+
+in
+
+let
+
+  pkgs = allPackages { };
+
+  pkgs2 = allPackages {
+    overrides = pkgs: pkgsPrev: {
+      stdenv = pkgs.stdenv2;
+      nix = pkgsPrev.nix.override { aterm = aterm2Fun { inherit (pkgs) stdenv fetchurl; }; };
+      xorg = pkgsPrev.xorg // { libX11 = libX11_2Fun { inherit (pkgs) stdenv fetchurl; }; };
+    };
+  };
+
+in
+
+[
+  pkgs.stdenv.name
+  pkgs.fetchurl.name
+  pkgs.aterm.name
+  pkgs2.aterm.name
+  pkgs.xorg.libX11.name
+  pkgs.xorg.libXv.name
+  pkgs.mplayer.name
+  pkgs2.mplayer.name
+  pkgs.nix.name
+  pkgs2.nix.name
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-functor-call.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-functor-call.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-functor-call.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-functor-call.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-functor-call.nix
new file mode 100644
index 0000000000..80ae345d83
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-functor-call.nix
@@ -0,0 +1 @@
+{ x = 21; __functor = self: y: self.x * y; } 2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-genlist.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-genlist.exp
new file mode 100644
index 0000000000..cd4ca34f14
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-genlist.exp
@@ -0,0 +1 @@
+[ 0 1 4 9 16 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-genlist.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-genlist.nix
new file mode 100644
index 0000000000..2c4dfba203
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-genlist.nix
@@ -0,0 +1 @@
+builtins.genList (x: x * x) 5
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-hasattr-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-hasattr-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-hasattr-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-hasattr-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-hasattr-catchable.nix
new file mode 100644
index 0000000000..ba85d6b776
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-hasattr-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval ((throw "fred") ? bob)).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.exp
new file mode 100644
index 0000000000..9800c675fc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.exp
@@ -0,0 +1 @@
+{ "'quoted'" = false; "-20ยฐ" = false; "2normal" = false; "45 44 43-'3 2 1" = false; "9front" = false; Very2Normal = true; VeryNormal = true; _'12 = true; "_'12.5" = false; __internal = true; _internal = true; abort = true; "assert" = false; "attr.path" = false; "else" = false; false = true; foldl' = true; "if" = false; "in" = false; "inherit" = false; "let" = false; normal = true; normal2 = true; null = true; or = true; "rec" = false; "then" = false; throw = true; true = true; "with" = false; x = true; x' = true; x'' = true; "๐Ÿ˜€" = false; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.nix
new file mode 100644
index 0000000000..58af3d6d16
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.nix
@@ -0,0 +1,42 @@
+# Note: the attribute values in this set aren't just dummies!  They
+# are booleans which indicate whether or not the corresponding
+# attrname is valid without quotification.
+{
+  __internal = true;
+  _internal = true;
+  normal = true;
+  VeryNormal = true;
+  normal2 = true;
+  Very2Normal = true;
+  _'12 = true;
+  foldl' = true;
+  x = true;
+  x' = true;
+  x'' = true;
+
+  true = true;
+  false = true;
+  null = true;
+  or = true;
+  "assert" = false;
+  throw = true;
+  abort = true;
+
+  "9front" = false;
+  "2normal" = false;
+  "-20ยฐ" = false;
+  "45 44 43-'3 2 1" = false;
+  "attr.path" = false;
+  "'quoted'" = false;
+  "_'12.5" = false;
+  "๐Ÿ˜€" = false;
+
+  "if" = false;
+  "then" = false;
+  "else" = false;
+  "with" = false;
+  "let" = false;
+  "in" = false;
+  "rec" = false;
+  "inherit" = false;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-import-display.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-import-display.exp
new file mode 100644
index 0000000000..15d838950e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-import-display.exp
@@ -0,0 +1 @@
+<PRIMOP>
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-import-display.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-import-display.nix
new file mode 100644
index 0000000000..411f3cd6ef
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-import-display.nix
@@ -0,0 +1,2 @@
+# In C++ Nix 2.3 this used to be <PRIMOP-APP>
+import
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-import.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-import.exp
new file mode 100644
index 0000000000..5ba7f64d78
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-import.exp
@@ -0,0 +1 @@
+[ 42 42 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-import.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-import.nix
new file mode 100644
index 0000000000..49cd244f06
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-import.nix
@@ -0,0 +1,4 @@
+[
+  (import ./directory)
+  (import ./directory/default.nix)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-inherit-string-ident.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-inherit-string-ident.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-inherit-string-ident.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-inherit-string-ident.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-inherit-string-ident.nix
new file mode 100644
index 0000000000..75794d3337
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-inherit-string-ident.nix
@@ -0,0 +1,8 @@
+# identifiers in inherits can be string-like expressions
+
+let
+  set = {
+    inherit ({ value = 42; }) "value";
+  };
+in
+set.value
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.nix
new file mode 100644
index 0000000000..5c6702120f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.nix
@@ -0,0 +1,3 @@
+# Tests formals which have internal default values that must be deferred.
+
+({ optional ? defaultValue, defaultValue }: optional) { defaultValue = 42; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.nix
new file mode 100644
index 0000000000..c6dd5e9d54
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.nix
@@ -0,0 +1,3 @@
+# Tests formals which have internal default values.
+
+({ defaultValue, optional ? defaultValue }: optional) { defaultValue = 42; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-intersectattrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-intersectattrs.exp
new file mode 100644
index 0000000000..25001b211f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-intersectattrs.exp
@@ -0,0 +1 @@
+{ a = 100; b = 200; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-intersectattrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-intersectattrs.nix
new file mode 100644
index 0000000000..f02d963226
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-intersectattrs.nix
@@ -0,0 +1,3 @@
+builtins.intersectAttrs
+{ a = 1; b = 2; c = 3; }
+{ a = 100; b = 200; d = 5; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-lambda-identity.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-lambda-identity.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lambda-identity.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-lambda-identity.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-lambda-identity.nix
new file mode 100644
index 0000000000..f2ee49df80
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lambda-identity.nix
@@ -0,0 +1,2 @@
+# Identity function is the simplest possible function.
+(x: x) 42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding-closure.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding-closure.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding-closure.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding-closure.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding-closure.nix
new file mode 100644
index 0000000000..4312ec9a52
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding-closure.nix
@@ -0,0 +1,5 @@
+let
+  f = n: n + a;
+  a = 2;
+in
+f 40
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding.nix
new file mode 100644
index 0000000000..6b6875cf1a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding.nix
@@ -0,0 +1,5 @@
+let
+  a = b;
+  b = 42;
+in
+a
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-assert.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-assert.exp
new file mode 100644
index 0000000000..48082f72f0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-assert.exp
@@ -0,0 +1 @@
+12
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-assert.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-assert.nix
new file mode 100644
index 0000000000..5a36964976
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-assert.nix
@@ -0,0 +1,8 @@
+assert true;
+
+let
+  x = assert false; 13;
+  y = 12;
+in
+
+{ inherit x y; }.y
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-equality.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-equality.exp
new file mode 100644
index 0000000000..1c70d1bcf1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-equality.exp
@@ -0,0 +1 @@
+[ true true false true true ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-equality.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-equality.nix
new file mode 100644
index 0000000000..92363245f8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-equality.nix
@@ -0,0 +1,16 @@
+let
+  attrs1 = { x = 1 + 2; };
+  attrs2 = { x = 2 + 1; };
+  list1 = [ (1 + 2) ];
+  list2 = [ (2 + 1) ];
+  list3 = [ (2 + 2) ];
+  list4 = [ (2 + 2) ];
+  list5 = [ (2 + 2) ];
+in
+[
+  (attrs1 == attrs2)
+  (list1 == list2)
+  (list3 == list2)
+  (list4 == [ 4 ])
+  ([ 4 ] == list5)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with-nested.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with-nested.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with-nested.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with-nested.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with-nested.nix
new file mode 100644
index 0000000000..22ac14b3f1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with-nested.nix
@@ -0,0 +1,5 @@
+# The 'namespace' of a with should only be evaluated if an identifier
+# from it is actually accessed.
+
+with (abort "should not be evaluated");
+let a = dynamic; in 42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with.nix
new file mode 100644
index 0000000000..8b1a0191dc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-with.nix
@@ -0,0 +1,6 @@
+# The 'namespace' of a with should only be evaluated if an identifier
+# from it is actually accessed.
+
+with (abort "should not be evaluated");
+
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-fix.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-fix.exp
new file mode 100644
index 0000000000..5d2955ffd5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-fix.exp
@@ -0,0 +1 @@
+{ one = 42; two = 42; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-fix.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-fix.nix
new file mode 100644
index 0000000000..a411b1c4a4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-fix.nix
@@ -0,0 +1,9 @@
+let {
+a = 21;
+b = body.one;
+
+body = {
+  one = a * 2;
+  two = b;
+};
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-in-with.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-in-with.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-in-with.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-in-with.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-in-with.nix
new file mode 100644
index 0000000000..7d95efa5c3
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let-in-with.nix
@@ -0,0 +1 @@
+with { }; let { body = 42; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let.nix
new file mode 100644
index 0000000000..faabe25457
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-legacy-let.nix
@@ -0,0 +1,4 @@
+let {
+a = 21;
+body = a * 2;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-identifiers.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-let-identifiers.exp
new file mode 100644
index 0000000000..5776134d0e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-identifiers.exp
@@ -0,0 +1 @@
+[ 1 2 3 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-identifiers.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-let-identifiers.nix
new file mode 100644
index 0000000000..ce588be069
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-identifiers.nix
@@ -0,0 +1,6 @@
+let
+  a = 1;
+  "b" = 2;
+  ${"c"} = 3;
+in
+[ a b c ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit-from-later-bound.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit-from-later-bound.exp
new file mode 100644
index 0000000000..409940768f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit-from-later-bound.exp
@@ -0,0 +1 @@
+23
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit-from-later-bound.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit-from-later-bound.nix
new file mode 100644
index 0000000000..21196f48bc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit-from-later-bound.nix
@@ -0,0 +1,13 @@
+let
+  inherit (c) d;
+  inherit (a) b c;
+
+  a = {
+    b = 20;
+    c = {
+      d = 3;
+    };
+  };
+in
+
+b + d
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit.exp
new file mode 100644
index 0000000000..0cfbf08886
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit.exp
@@ -0,0 +1 @@
+2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit.nix
new file mode 100644
index 0000000000..3aa7c0f8d2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit.nix
@@ -0,0 +1,13 @@
+let
+  set = {
+    a = 1;
+  };
+in
+let
+  set2 = {
+    b = 1;
+  };
+  inherit (set) a;
+  inherit (set2) b;
+in
+a + b
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-sibling-access.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-let-sibling-access.exp
new file mode 100644
index 0000000000..00750edc07
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-sibling-access.exp
@@ -0,0 +1 @@
+3
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-sibling-access.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-let-sibling-access.nix
new file mode 100644
index 0000000000..faad81a213
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-sibling-access.nix
@@ -0,0 +1,6 @@
+let
+  a = 1;
+  b = 2;
+  c = a + b;
+in
+c
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit-mixed.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit-mixed.exp
new file mode 100644
index 0000000000..3bed31f76e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit-mixed.exp
@@ -0,0 +1 @@
+[ 1 2 3 4 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit-mixed.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit-mixed.nix
new file mode 100644
index 0000000000..30981099cb
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit-mixed.nix
@@ -0,0 +1,20 @@
+# This test mixes different ways of creating bindings in a let โ€ฆ in expression
+# to make sure that the compiler initialises the locals in the same order as
+# they are declared.
+
+let
+  d = 4;
+in
+
+# Trick to allow useless inherits in the following let
+with { _unused = null; };
+
+let
+  set = { b = 2; };
+  a = 1;
+  inherit (set) b;
+  c = 3;
+  inherit d;
+in
+
+[ a b c d ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit.exp
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit.nix
new file mode 100644
index 0000000000..3d1c46b10b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-useful-plain-inherit.nix
@@ -0,0 +1,9 @@
+with { a = 1; };
+
+let
+  inherit a;
+in
+
+with { a = 2; };
+
+a
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-list-comparison.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-list-comparison.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-list-comparison.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-list-comparison.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-list-comparison.nix
new file mode 100644
index 0000000000..7796fe4cbb
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-list-comparison.nix
@@ -0,0 +1 @@
+[ 1 2 ] > [ ((rec{ x = 1; }).x) 2 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-listtoattrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-listtoattrs.exp
new file mode 100644
index 0000000000..74abef7bc6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-listtoattrs.exp
@@ -0,0 +1 @@
+"AAbar"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-listtoattrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-listtoattrs.nix
new file mode 100644
index 0000000000..551db72cb0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-listtoattrs.nix
@@ -0,0 +1,16 @@
+with builtins;
+let
+  fold = op: nul: list:
+    if list == [ ]
+    then nul
+    else op (head list) (fold op nul (tail list));
+  concat =
+    fold (x: y: x + y) "";
+  asi = name: value: { inherit name value; };
+  list = [ (asi "a" "A") (asi "b" "B") ];
+  a = builtins.listToAttrs list;
+  b = builtins.listToAttrs (list ++ list);
+  r = builtins.listToAttrs [ (asi "result" [ a b ]) (asi "throw" (throw "this should not be thrown")) ];
+  x = builtins.listToAttrs [ (asi "foo" "bar") (asi "foo" "bla") ];
+in
+concat (map (x: x.a) r.result) + x.foo
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-logical-and-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-logical-and-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-logical-and-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-logical-and-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-logical-and-catchable.nix
new file mode 100644
index 0000000000..dd2a9baa75
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-logical-and-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval ((throw "fred") && (throw "jill"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-logical-or-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-logical-or-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-logical-or-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-logical-or-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-logical-or-catchable.nix
new file mode 100644
index 0000000000..3adccfa441
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-logical-or-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval ((throw "fred") || (throw "jill"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-manual-rec.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-manual-rec.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-manual-rec.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-manual-rec.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-manual-rec.nix
new file mode 100644
index 0000000000..23459e384a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-manual-rec.nix
@@ -0,0 +1,10 @@
+# Manual desugaring of something similar to `rec`, to test lower level
+# recursion primitives.
+
+let
+  set = with set; {
+    a = 21;
+    b = a * 2;
+  };
+in
+set.b
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-attrs.exp
new file mode 100644
index 0000000000..911ab51de5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-attrs.exp
@@ -0,0 +1 @@
+{ set = { a = 1; b = 2; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-attrs.nix
new file mode 100644
index 0000000000..78b28909a2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-attrs.nix
@@ -0,0 +1,9 @@
+{
+  set = {
+    a = 1;
+  };
+
+  set = {
+    b = 2;
+  };
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-rec-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-rec-attrs.exp
new file mode 100644
index 0000000000..768eaae61c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-rec-attrs.exp
@@ -0,0 +1 @@
+{ set = { a = 21; b = 42; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-rec-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-rec-attrs.nix
new file mode 100644
index 0000000000..cea4cb1b4f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-merge-nested-rec-attrs.nix
@@ -0,0 +1,12 @@
+{
+  set = rec {
+    a = 21;
+  };
+
+  set = {
+    # Fun fact: This might be the only case in Nix where a lexical
+    # resolution of an identifier can only be resolved by looking at
+    # *siblings* in the AST.
+    b = 2 * a;
+  };
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-multiline-string.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-multiline-string.exp
new file mode 100644
index 0000000000..9839e480b7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-multiline-string.exp
@@ -0,0 +1 @@
+"hello\nworld"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-multiline-string.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-multiline-string.nix
new file mode 100644
index 0000000000..84beb22ed5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-multiline-string.nix
@@ -0,0 +1,2 @@
+''hello
+world''
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-multiple-nested-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-multiple-nested-attrs.exp
new file mode 100644
index 0000000000..b5c707cf46
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-multiple-nested-attrs.exp
@@ -0,0 +1 @@
+{ a = { b = 15; }; b = { c = "test"; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-multiple-nested-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-multiple-nested-attrs.nix
new file mode 100644
index 0000000000..5d611930ca
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-multiple-nested-attrs.nix
@@ -0,0 +1 @@
+{ a.b = 15; b.c = "test"; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-mutually-recursive-let-binding.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-mutually-recursive-let-binding.exp
new file mode 100644
index 0000000000..edca9baca9
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-mutually-recursive-let-binding.exp
@@ -0,0 +1 @@
+{ a = 1; b = 2; c = 3; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-mutually-recursive-let-binding.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-mutually-recursive-let-binding.nix
new file mode 100644
index 0000000000..1b3feda432
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-mutually-recursive-let-binding.nix
@@ -0,0 +1,14 @@
+let
+  a = {
+    a = 3;
+    b = b.b;
+  };
+
+  b = {
+    a = a.a - 2;
+    b = 2;
+    c = a.c or 3;
+  };
+in
+
+a // b
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-ne-int.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-ne-int.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-ne-int.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-ne-int.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-ne-int.nix
new file mode 100644
index 0000000000..e06b571a28
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-ne-int.nix
@@ -0,0 +1 @@
+42 != 69
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-ne-string.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-ne-string.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-ne-string.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-ne-string.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-ne-string.nix
new file mode 100644
index 0000000000..a83471e500
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-ne-string.nix
@@ -0,0 +1 @@
+"this" != "that"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-assertions.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-assertions.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-assertions.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-assertions.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-assertions.nix
new file mode 100644
index 0000000000..b0397e268e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-assertions.nix
@@ -0,0 +1 @@
+(builtins.tryEval (assert (assert false; true); true)).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-closure.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-closure.exp
new file mode 100644
index 0000000000..b6a7d89c68
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-closure.exp
@@ -0,0 +1 @@
+16
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-closure.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-closure.nix
new file mode 100644
index 0000000000..97bff7f077
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-closure.nix
@@ -0,0 +1 @@
+(a: b: c: d: a + b + c + d) 1 3 5 7
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-deferred-upvalue.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-deferred-upvalue.exp
new file mode 100644
index 0000000000..209e3ef4b6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-deferred-upvalue.exp
@@ -0,0 +1 @@
+20
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-deferred-upvalue.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-deferred-upvalue.nix
new file mode 100644
index 0000000000..3fa1d3ed05
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-deferred-upvalue.nix
@@ -0,0 +1,10 @@
+let
+  doubler = n: outer n;
+  outer =
+    let
+      inner = n: a * n;
+      a = 2;
+    in
+    inner;
+in
+doubler 10
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-has-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-has-attrs.exp
new file mode 100644
index 0000000000..d2c1c04da3
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-has-attrs.exp
@@ -0,0 +1 @@
+[ true true true true true true true false false false false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-has-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-has-attrs.nix
new file mode 100644
index 0000000000..47dcec7a95
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-has-attrs.nix
@@ -0,0 +1,26 @@
+let
+  set = {
+    a.b.c = 123;
+    foo = {
+      bar = 23;
+    };
+    baz = 1;
+  };
+
+  tes = "random value";
+in
+
+[
+  (set ? a)
+  (set ? a.b)
+  (set ? a.b.c)
+  (set ? foo)
+  (set ? foo.bar)
+  (set.foo ? bar)
+  (set ? baz)
+  (set ? x)
+  (set ? x.y.z)
+  (tes ? bar)
+  (tes ? x.y.z)
+  (null ? null)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-let.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-let.exp
new file mode 100644
index 0000000000..6db47b033e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-let.exp
@@ -0,0 +1 @@
+{ a = { b = 42; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-let.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-let.nix
new file mode 100644
index 0000000000..c99ac748e6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-let.nix
@@ -0,0 +1,5 @@
+let
+  inner = 21;
+  set.a.b = inner * 2;
+in
+set
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-rec.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-rec.exp
new file mode 100644
index 0000000000..77eb325dde
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-rec.exp
@@ -0,0 +1 @@
+{ a = { b = { c = 42; }; }; outer = 21; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-rec.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-rec.nix
new file mode 100644
index 0000000000..797d11108f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-rec.nix
@@ -0,0 +1,4 @@
+rec {
+  outer = 21;
+  a.b.c = outer * 2;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let-slots.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let-slots.exp
new file mode 100644
index 0000000000..e45ef1da2f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let-slots.exp
@@ -0,0 +1 @@
+[ 1 2 3 4 5 6 7 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let-slots.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let-slots.nix
new file mode 100644
index 0000000000..eec5940875
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let-slots.nix
@@ -0,0 +1,22 @@
+# This test deals with a tricky edge-case around scopes, where the
+# stack slot accounting must correctly account for the position at
+# which the body of a let expression is being initialised when
+# resolving upvalues.
+
+let
+  a = 1;
+  b = 2;
+  outer =
+    let
+      c = 3;
+      d = 4;
+      inner =
+        let
+          e = 5;
+          f = 6;
+        in
+        g: [ a b c d e f g ];
+    in
+    inner;
+in
+outer 7
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let.exp
new file mode 100644
index 0000000000..7f8f011eb7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let.exp
@@ -0,0 +1 @@
+7
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let.nix
new file mode 100644
index 0000000000..f40c04b139
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let.nix
@@ -0,0 +1,10 @@
+let
+  a =
+    let
+      b = 1;
+      c = 2;
+    in
+    b + c;
+  b = 4;
+in
+a + b
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-poisoning.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-poisoning.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-poisoning.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-poisoning.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-poisoning.nix
new file mode 100644
index 0000000000..0fd22a671c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-poisoning.nix
@@ -0,0 +1,5 @@
+let
+  null = 1;
+  f = n: n + null;
+in
+f 41
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-set-thunks.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-set-thunks.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-set-thunks.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-set-thunks.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-set-thunks.nix
new file mode 100644
index 0000000000..f3ad829354
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-set-thunks.nix
@@ -0,0 +1,5 @@
+({
+  x = {
+    y = 42;
+  };
+}).x.y
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-siblings.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-siblings.exp
new file mode 100644
index 0000000000..d757cae1f5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-siblings.exp
@@ -0,0 +1 @@
+{ outer = 42; sibling = 42; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-siblings.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-siblings.nix
new file mode 100644
index 0000000000..31111d8081
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-siblings.nix
@@ -0,0 +1,7 @@
+rec {
+  outer =
+    let inner = sibling;
+    in inner;
+
+  sibling = 42;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-thunks.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-thunks.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-thunks.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-thunks.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-thunks.nix
new file mode 100644
index 0000000000..2519221e97
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-thunks.nix
@@ -0,0 +1,8 @@
+# If a thunk yields another thunk, OpForce should keep forcing until
+# there is a value.
+let
+  a = b;
+  b = c;
+  c = 42;
+in
+a
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-with.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-with.exp
new file mode 100644
index 0000000000..0cfbf08886
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-with.exp
@@ -0,0 +1 @@
+2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nested-with.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-with.nix
new file mode 100644
index 0000000000..fa832b2099
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-with.nix
@@ -0,0 +1,5 @@
+let
+  set1 = { a = 1; };
+  set2 = { a = 2; };
+in
+with set1; with set2; a
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nix-version-cmp.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nix-version-cmp.exp
new file mode 100644
index 0000000000..3a2e3f4984
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nix-version-cmp.exp
@@ -0,0 +1 @@
+-1
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-nix-version-cmp.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-nix-version-cmp.nix
new file mode 100644
index 0000000000..6f35305612
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nix-version-cmp.nix
@@ -0,0 +1,5 @@
+# nixpkgs checks against the `builtins.nixVersion` and fails if it
+# doesn't like what it sees. To work around this we have a "user-agent
+# style" version (see cl/6858) that ensures compatibility.
+
+builtins.compareVersions "2.3" builtins.nixVersion
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-observable-eval-cache.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-observable-eval-cache.exp
new file mode 100644
index 0000000000..aaa53b6025
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-observable-eval-cache.exp
@@ -0,0 +1 @@
+[ true true false false true ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-observable-eval-cache.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-observable-eval-cache.nix
new file mode 100644
index 0000000000..24003d0637
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-observable-eval-cache.nix
@@ -0,0 +1,7 @@
+[
+  (import ./observable-eval-cache1.nix == import ./observable-eval-cache1.nix)
+  (import ./observable-eval-cache1.nix == import ./observable-eval-cache2.nix)
+  (import ./observable-eval-cache1.nix == import ./observable-eval-cache3.nix)
+  (import ./observable-eval-cache2.nix == import ./observable-eval-cache3.nix)
+  (import ./observable-eval-cache3.nix == import ./observable-eval-cache3.nix)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-observe-infinite-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-observe-infinite-attrs.exp
new file mode 100644
index 0000000000..bbb332a5ee
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-observe-infinite-attrs.exp
@@ -0,0 +1 @@
+[ "x" "y" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-observe-infinite-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-observe-infinite-attrs.nix
new file mode 100644
index 0000000000..684c88f800
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-observe-infinite-attrs.nix
@@ -0,0 +1,4 @@
+# The below attribute set is infinitely large, but we should be able
+# to observe it as long as we don't access its entire value.
+
+let as = { x = 123; y = as; }; in builtins.attrNames as.y.y
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-optimised-bools.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-optimised-bools.exp
new file mode 100644
index 0000000000..9d9185fcd1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-optimised-bools.exp
@@ -0,0 +1 @@
+[ true true false false true true false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-optimised-bools.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-optimised-bools.nix
new file mode 100644
index 0000000000..650d7f028d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-optimised-bools.nix
@@ -0,0 +1,21 @@
+let
+  makeTrue = _: true;
+  makeFalse = _: false;
+in
+[
+  # useless `false`
+  (false || makeTrue null) # true
+  (makeTrue null || false) # true
+
+  # useless `true`
+  (true && makeFalse null) # false
+  (makeFalse null && true) # false
+
+  # useless `||`
+  (true || makeFalse null) # true
+  (makeFalse null || true) # true
+
+  # useless `&&`
+  (false && makeTrue null) # false
+  (makeTrue null && false) # false
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-default.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-default.exp
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-default.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-default.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-default.nix
new file mode 100644
index 0000000000..444f270af6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-default.nix
@@ -0,0 +1 @@
+{ b = 1; }.b or 2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested-default.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested-default.exp
new file mode 100644
index 0000000000..0cfbf08886
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested-default.exp
@@ -0,0 +1 @@
+2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested-default.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested-default.nix
new file mode 100644
index 0000000000..ceffd0697b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested-default.nix
@@ -0,0 +1 @@
+{ a.b = 1; }.a.c or 2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested.exp
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested.nix
new file mode 100644
index 0000000000..1a76594546
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-nested.nix
@@ -0,0 +1 @@
+{ a.b = 1; }.a.b or 2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-non-set.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-non-set.exp
new file mode 100644
index 0000000000..a833e32892
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-non-set.exp
@@ -0,0 +1 @@
+"works fine"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-non-set.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-non-set.nix
new file mode 100644
index 0000000000..fd09bfee64
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator-non-set.nix
@@ -0,0 +1,2 @@
+# `or` operator should keep working if it encounters a non-set type.
+{ a.b = 42; }.a.b.c or "works fine"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator.exp
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator.nix
new file mode 100644
index 0000000000..ce1e6e67c2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-or-operator.nix
@@ -0,0 +1 @@
+{ a = 1; }.a or 2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-overlapping-nested-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-overlapping-nested-attrs.exp
new file mode 100644
index 0000000000..2483a27183
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-overlapping-nested-attrs.exp
@@ -0,0 +1 @@
+{ a = { b = 15; c = "test"; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-overlapping-nested-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-overlapping-nested-attrs.nix
new file mode 100644
index 0000000000..4154ff9da2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-overlapping-nested-attrs.nix
@@ -0,0 +1,4 @@
+{
+  a.b = 15;
+  a.c = "test";
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-parsedrvname.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-parsedrvname.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-parsedrvname.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-parsedrvname.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-parsedrvname.nix
new file mode 100644
index 0000000000..5997c99b47
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-parsedrvname.nix
@@ -0,0 +1,11 @@
+# the first dash followed by a non-alphabetic character separates
+# the "name" from the "version"
+
+assert builtins.parseDrvName "ripgrep-1.2" == { name = "ripgrep"; version = "1.2"; };
+assert builtins.parseDrvName "rip-grep-1.2" == { name = "rip-grep"; version = "1.2"; };
+assert builtins.parseDrvName "7zip_archiver-0.2" == { name = "7zip_archiver"; version = "0.2"; };
+assert builtins.parseDrvName "gcc-1-2" == { name = "gcc"; version = "1-2"; };
+assert builtins.parseDrvName "bash--1-2" == { name = "bash"; version = "-1-2"; };
+assert builtins.parseDrvName "xvidtune-?1-2" == { name = "xvidtune"; version = "?1-2"; };
+
+true
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-pathexists.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-pathexists.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-pathexists.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-pathexists.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-pathexists.nix
new file mode 100644
index 0000000000..c9eedb44ff
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-pathexists.nix
@@ -0,0 +1,2 @@
+builtins.pathExists ./lib.nix
+  && !builtins.pathExists ./bla.nix
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-poisoned-scopes.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-poisoned-scopes.exp
new file mode 100644
index 0000000000..5776134d0e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-poisoned-scopes.exp
@@ -0,0 +1 @@
+[ 1 2 3 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-poisoned-scopes.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-poisoned-scopes.nix
new file mode 100644
index 0000000000..81f03d9e2b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-poisoned-scopes.nix
@@ -0,0 +1,6 @@
+let
+  true = 1;
+  false = 2;
+  null = 3;
+in
+[ true false null ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-readDir.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-readDir.exp
new file mode 100644
index 0000000000..bf8d2c14ea
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-readDir.exp
@@ -0,0 +1 @@
+{ bar = "regular"; foo = "directory"; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-readDir.nix.disabled b/tvix/eval/src/tests/tvix_tests/eval-okay-readDir.nix.disabled
new file mode 100644
index 0000000000..a7ec9292aa
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-readDir.nix.disabled
@@ -0,0 +1 @@
+builtins.readDir ./readDir
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.exp
new file mode 100644
index 0000000000..a2c87d0c43
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.exp
@@ -0,0 +1 @@
+"builtins.readFile ./eval-okay-readfile.nix\n"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.nix
new file mode 100644
index 0000000000..82f7cb1743
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-readfile.nix
@@ -0,0 +1 @@
+builtins.readFile ./eval-okay-readfile.nix
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-rec-dynamic-keys.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-rec-dynamic-keys.exp
new file mode 100644
index 0000000000..ac8d062a69
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-rec-dynamic-keys.exp
@@ -0,0 +1 @@
+{ barbaz = 42; foobar = 42; val = 21; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-rec-dynamic-keys.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-rec-dynamic-keys.nix
new file mode 100644
index 0000000000..8d7a8cef8e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-rec-dynamic-keys.nix
@@ -0,0 +1,5 @@
+rec {
+  val = 21;
+  ${"foo" + "bar"} = 42;
+  ${"bar" + "baz"} = val * 2;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-rec-nested-access.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-rec-nested-access.exp
new file mode 100644
index 0000000000..a1dca9bb68
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-rec-nested-access.exp
@@ -0,0 +1 @@
+{ a = { b = 1; c = 2; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-rec-nested-access.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-rec-nested-access.nix
new file mode 100644
index 0000000000..7d037c6b37
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-rec-nested-access.nix
@@ -0,0 +1,4 @@
+rec {
+  a.b = 1;
+  a.c = a.b * 2;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-recursive-attrs-all-features.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-recursive-attrs-all-features.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-recursive-attrs-all-features.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-recursive-attrs-all-features.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-recursive-attrs-all-features.nix
new file mode 100644
index 0000000000..a234705b5e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-recursive-attrs-all-features.nix
@@ -0,0 +1,13 @@
+let a = 1;
+in
+(rec {
+  inherit a;
+
+  b = {
+    c = a + 20;
+  };
+
+  inherit (b) c;
+
+  d = c * 2;
+}).d
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-regex-match.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-regex-match.exp
new file mode 100644
index 0000000000..9501035391
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-regex-match.exp
@@ -0,0 +1 @@
+[ true true false true true true true false false true false [ "foobar" ] [ "FOO" ] [ "/path/to/" "/path/to" "foobar" "nix" ] [ null null "foobar" "cc" ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-regex-match.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-regex-match.nix
new file mode 100644
index 0000000000..f774e00a21
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-regex-match.nix
@@ -0,0 +1,29 @@
+with builtins;
+
+let
+
+  matches = pat: s: match pat s != null;
+
+  splitFN = match "((.*)/)?([^/]*)\\.(nix|cc)";
+
+in
+
+[
+  (matches "foobar" "foobar")
+  (matches "fo*" "f")
+  (matches "fo+" "f")
+  (matches "fo*" "fo")
+  (matches "fo*" "foo")
+  (matches "fo+" "foo")
+  (matches "fo{1,2}" "foo")
+  (matches "fo{1,2}" "fooo")
+  (matches "fo*" "foobar")
+  (matches "[[:space:]]+([^[:space:]]+)[[:space:]]+" "  foo   ")
+  (matches "[[:space:]]+([[:upper:]]+)[[:space:]]+" "  foo   ")
+
+  (match "(.*)\\.nix" "foobar.nix")
+  (match "[[:space:]]+([[:upper:]]+)[[:space:]]+" "  FOO   ")
+
+  (splitFN "/path/to/foobar.nix")
+  (splitFN "foobar.cc")
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-remove.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-remove.exp
new file mode 100644
index 0000000000..8d38505c16
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-remove.exp
@@ -0,0 +1 @@
+456
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-remove.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-remove.nix
new file mode 100644
index 0000000000..62c5aa1fd4
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-remove.nix
@@ -0,0 +1,5 @@
+let {
+attrs = { x = 123; y = 456; };
+
+body = (removeAttrs attrs [ "x" ]).y;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-repeated-list-to-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-repeated-list-to-attrs.exp
new file mode 100644
index 0000000000..b4a1e66d6b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-repeated-list-to-attrs.exp
@@ -0,0 +1 @@
+[ 1 2 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-repeated-list-to-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-repeated-list-to-attrs.nix
new file mode 100644
index 0000000000..ed819d76c7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-repeated-list-to-attrs.nix
@@ -0,0 +1,14 @@
+# Ensure that builtins.listToAttrs returns the first instance of a key.
+
+let
+  inherit (builtins) foldl' listToAttrs;
+
+  input = [{ name = "result"; value = 1; } { name = "result"; value = 2; }];
+
+  # foldl-based version of listToAttrs with the _opposite_ behaviour.
+  listToAttrs' = list: foldl' (acc: elem: acc // { ${elem.name} = elem.value; }) { } list;
+in
+[
+  (listToAttrs input).result
+  (listToAttrs' input).result
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-seq.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-seq.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-seq.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-seq.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-seq.nix
new file mode 100644
index 0000000000..fd0806c199
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-seq.nix
@@ -0,0 +1 @@
+(builtins.seq 1 2) + (builtins.seq [ (throw "list") ] 20) + (builtins.seq { boing = throw "set"; } 20)
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-closure.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-closure.exp
new file mode 100644
index 0000000000..7f8f011eb7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-closure.exp
@@ -0,0 +1 @@
+7
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-closure.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-closure.nix
new file mode 100644
index 0000000000..56445454fe
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-closure.nix
@@ -0,0 +1 @@
+(a: b: a + b) 2 5
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-interpol.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-interpol.exp
new file mode 100644
index 0000000000..cd4bc1ab64
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-interpol.exp
@@ -0,0 +1 @@
+"hello world"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-interpol.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-interpol.nix
new file mode 100644
index 0000000000..125b0859ac
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-interpol.nix
@@ -0,0 +1 @@
+"hello ${"world"}"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-let.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-let.exp
new file mode 100644
index 0000000000..00750edc07
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-let.exp
@@ -0,0 +1 @@
+3
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-let.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-let.nix
new file mode 100644
index 0000000000..b4da0f824a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-let.nix
@@ -0,0 +1,5 @@
+let
+  a = 1;
+  b = 2;
+in
+a + b
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-nested-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-nested-attrs.exp
new file mode 100644
index 0000000000..6db47b033e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-nested-attrs.exp
@@ -0,0 +1 @@
+{ a = { b = 42; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-nested-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-nested-attrs.nix
new file mode 100644
index 0000000000..a97394d165
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-nested-attrs.nix
@@ -0,0 +1 @@
+{ a.b = 42; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-recursive-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-recursive-attrs.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-recursive-attrs.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-recursive-attrs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-recursive-attrs.nix
new file mode 100644
index 0000000000..c86ff80383
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-recursive-attrs.nix
@@ -0,0 +1,4 @@
+(rec {
+  a = 21;
+  b = a * 2;
+}).b
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-with.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-with.exp
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-with.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-simple-with.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-with.nix
new file mode 100644
index 0000000000..3d375be4f9
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-with.nix
@@ -0,0 +1,6 @@
+let
+  set = {
+    a = 1;
+  };
+in
+with set; a
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-stable-sort.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-stable-sort.exp
new file mode 100644
index 0000000000..9d78376214
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-stable-sort.exp
@@ -0,0 +1 @@
+[ { index = 7; key = 0; } { index = 0; key = 1; } { index = 13; key = 1; } { index = 1; key = 2; } { index = 3; key = 2; } { index = 4; key = 2; } { index = 5; key = 2; } { index = 12; key = 2; } { index = 14; key = 2; } { index = 2; key = 3; } { index = 11; key = 3; } { index = 15; key = 3; } { index = 10; key = 4; } { index = 6; key = 5; } { index = 8; key = 5; } { index = 9; key = 5; } { index = 16; key = 22; } ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-stable-sort.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-stable-sort.nix
new file mode 100644
index 0000000000..9969e0a294
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-stable-sort.nix
@@ -0,0 +1,7 @@
+let
+  keys = [ 1 2 3 2 2 2 5 0 5 5 4 3 2 1 2 3 22 ];
+in
+
+builtins.sort
+  (a: b: a.key < b.key)
+  (builtins.genList (index: { inherit index; key = builtins.elemAt keys index; }) (builtins.length keys))
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-substring-propagate-catchables.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-substring-propagate-catchables.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-substring-propagate-catchables.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-substring-propagate-catchables.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-substring-propagate-catchables.nix
new file mode 100644
index 0000000000..78d5dda38e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-substring-propagate-catchables.nix
@@ -0,0 +1 @@
+(builtins.tryEval (builtins.substring 0 4 (throw "jill"))).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-default-args.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-default-args.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-default-args.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-default-args.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-default-args.nix
new file mode 100644
index 0000000000..0523cf864c
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-default-args.nix
@@ -0,0 +1 @@
+(builtins.tryEval (({ foo ? throw "up" }: if foo then 1 else 2) { })).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-implications.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-implications.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-implications.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-implications.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-implications.nix
new file mode 100644
index 0000000000..126738d883
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-test-catchables-in-implications.nix
@@ -0,0 +1 @@
+(builtins.tryEval (({ foo ? throw "up" }: foo -> true) { })).success
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-functor.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-functor.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-functor.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-functor.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-functor.nix
new file mode 100644
index 0000000000..568a5c5413
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-functor.nix
@@ -0,0 +1,8 @@
+let
+  __functor = f;
+  f = self: x: self.out * x;
+in
+{
+  inherit __functor;
+  out = 21;
+} 2
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-if.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-if.exp
new file mode 100644
index 0000000000..ffcd4415b0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-if.exp
@@ -0,0 +1 @@
+{ }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-if.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-if.nix
new file mode 100644
index 0000000000..3810ebe784
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-if.nix
@@ -0,0 +1,8 @@
+let
+  a = { };
+in
+let
+  c = if builtins.isFunction a then a b else a;
+  b = { };
+in
+c
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-string-interpolation.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-string-interpolation.exp
new file mode 100644
index 0000000000..fc2f21e930
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-string-interpolation.exp
@@ -0,0 +1 @@
+"strict literal"
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-string-interpolation.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-string-interpolation.nix
new file mode 100644
index 0000000000..bd3555bb24
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-string-interpolation.nix
@@ -0,0 +1,7 @@
+let
+  final = { text = "strict literal"; inherit x y; };
+  x = "lazy ${throw "interpolation"}";
+  y = "${throw "also lazy!"}";
+in
+
+final.text
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-with.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-with.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-with.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-with.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-with.nix
new file mode 100644
index 0000000000..799408b2e6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-with.nix
@@ -0,0 +1,8 @@
+# Creates a `with` across multiple thunk boundaries.
+
+let
+  set = {
+    a = with { b = 42; }; b;
+  };
+in
+set.a
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-toplevel-finaliser.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-toplevel-finaliser.exp
new file mode 100644
index 0000000000..edca9baca9
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-toplevel-finaliser.exp
@@ -0,0 +1 @@
+{ a = 1; b = 2; c = 3; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-toplevel-finaliser.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-toplevel-finaliser.nix
new file mode 100644
index 0000000000..5f25f80671
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-toplevel-finaliser.nix
@@ -0,0 +1,11 @@
+# A simple expression with upvalue resolution beyond the target stack
+# index of the root expression.
+
+let
+  a = 1;
+  b = 2;
+  c = 3;
+in
+{
+  inherit a b c;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval-thunk-twice.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval-thunk-twice.exp
new file mode 100644
index 0000000000..b5ba0757c1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval-thunk-twice.exp
@@ -0,0 +1 @@
+[ { success = false; value = false; } { success = false; value = false; } ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval-thunk-twice.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval-thunk-twice.nix
new file mode 100644
index 0000000000..1749643f82
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval-thunk-twice.nix
@@ -0,0 +1 @@
+let x = throw "lol"; in builtins.map (f: f x) [ builtins.tryEval builtins.tryEval ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.exp
new file mode 100644
index 0000000000..8b6ed7dbac
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.exp
@@ -0,0 +1 @@
+{ v = false; w = { success = false; value = false; }; x = { success = true; value = "x"; }; y = { success = false; value = false; }; z = { success = false; value = false; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.nix
new file mode 100644
index 0000000000..e2357c7987
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.nix
@@ -0,0 +1,7 @@
+{
+  v = (builtins.tryEval (toString <oink>)).value;
+  w = builtins.tryEval <nope>;
+  x = builtins.tryEval "x";
+  y = builtins.tryEval (assert false; "y");
+  z = builtins.tryEval (throw "bla");
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-unpoison-scope.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-unpoison-scope.exp
new file mode 100644
index 0000000000..5462431496
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-unpoison-scope.exp
@@ -0,0 +1 @@
+[ true false null 1 2 3 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-unpoison-scope.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-unpoison-scope.nix
new file mode 100644
index 0000000000..539735a8ef
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-unpoison-scope.nix
@@ -0,0 +1,10 @@
+let
+  poisoned =
+    let
+      true = 1;
+      false = 2;
+      null = 3;
+    in
+    [ true false null ];
+in
+[ true false null ] ++ poisoned
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.exp
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.nix
new file mode 100644
index 0000000000..dd768c1aca
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.nix
@@ -0,0 +1,16 @@
+# Normally using an `inherit` without a source attribute set within a
+# `let` is a no-op, *unless* there is a with in-scope that might
+# provide the value.
+
+# Provide a dynamic `x` identifier in the scope.
+with ({ x = 1; });
+
+# inherit this `x` as a static identifier
+let inherit x;
+
+  # Provide another dynamic `x` identifier
+in
+with ({ x = 3; });
+
+# Inherited static identifier should have precedence
+x
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-value-display.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-value-display.exp
new file mode 100644
index 0000000000..c7e3fc6503
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-value-display.exp
@@ -0,0 +1 @@
+[ null true false 42 42 "foo\t\nbar" /home/arthur [ 1 2 3 ] <LAMBDA> <PRIMOP> <PRIMOP-APP> { hello = "world"; } ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-value-display.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-value-display.nix
new file mode 100644
index 0000000000..d34ed1697e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-value-display.nix
@@ -0,0 +1,16 @@
+# Sanity check of how values are rendered by tvix vs. nix-instantiate(1).
+# Ensures that we can use this test suite to compare against C++ Nix.
+[
+  null
+  true
+  false
+  42
+  42.0
+  "foo\t\nbar"
+  /home/arthur
+  [ 1 2 3 ]
+  (x: x)
+  builtins.add
+  (builtins.substring 0 1)
+  { hello = "world"; }
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-compare.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-compare.exp
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-compare.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-compare.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-compare.nix
new file mode 100644
index 0000000000..c2ca913af2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-compare.nix
@@ -0,0 +1,6 @@
+# For an explanation of this behavior see //tvix/docs/value-pointer-equality.md
+let
+  f = owo: "thia";
+in
+
+[ f 42 ] > [ f 21 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-equality.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-equality.exp
new file mode 100644
index 0000000000..aec30f297a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-equality.exp
@@ -0,0 +1 @@
+[ true true true true false false false true true true true true true true true true false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-equality.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-equality.nix
new file mode 100644
index 0000000000..b5cfbeb12e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-equality.nix
@@ -0,0 +1,46 @@
+# For an explanation of this behavior see //tvix/docs/value-pointer-equality.md
+let
+  # Some incomparable values
+  f = MC: "Boing";
+  t = [ (throw "is a little blue man") ];
+  a = { "with" = abort "headphones and a big smile."; };
+
+  # Aliases
+  f' = f;
+  t' = t;
+  a' = a;
+
+  peq1 = a: b: [ a ] == [ b ];
+  peq2 = a: b: { x = a; } == { x = b; };
+in
+
+[
+  # pointer equality of functions
+  (peq1 f f)
+  (peq2 f f)
+  (peq1 f f')
+  (peq2 f f')
+
+  # encapsulation is necessary for pointer equality
+  (f == f)
+  (f == f')
+  # works with !=
+  ([ f ] != [ f' ])
+
+  # thunks that fail to evaluated wrapped in sets/lists
+  (peq1 t t)
+  (peq2 t t)
+  (peq1 a a)
+  (peq2 a a)
+  (peq1 t t')
+  (peq2 t t')
+  (peq1 a' a)
+  (peq2 a' a)
+
+  # function equality with builtins.elem
+  (builtins.elem f [ 21 f 42 ])
+
+  # pointer inequality
+  (peq1 f (x: x))
+  (peq2 (x: x) f)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-with-closure.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-with-closure.exp
new file mode 100644
index 0000000000..fa8f08cb6f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-with-closure.exp
@@ -0,0 +1 @@
+150
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-with-closure.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-with-closure.nix
new file mode 100644
index 0000000000..7e2f7c073b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-with-closure.nix
@@ -0,0 +1,5 @@
+# Upvalues from `with` require special runtime handling. Do they work?
+let
+  f = with { a = 15; }; n: n * a;
+in
+f 10
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-dynamic-key.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-dynamic-key.exp
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-dynamic-key.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-dynamic-key.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-dynamic-key.nix
new file mode 100644
index 0000000000..bf221746c0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-dynamic-key.nix
@@ -0,0 +1,13 @@
+# Tests correct tracking of stack indices within construction of an
+# attribute set. Dynamic keys can be any expression, so something that
+# is extremely sensitive to stack offsets (like `with`) can be tricky.
+
+let
+  set1 = { key = "b"; };
+  set2 = {
+    a = 20;
+    ${with set1; key} = 20;
+    ${with { key = "c"; }; key} = 2;
+  };
+in
+set2.a + set2.b + set2.c
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-list.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-list.exp
new file mode 100644
index 0000000000..5776134d0e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-list.exp
@@ -0,0 +1 @@
+[ 1 2 3 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-list.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-list.nix
new file mode 100644
index 0000000000..3e85cbee45
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-list.nix
@@ -0,0 +1,14 @@
+# This code causes a situation where a list element causes an
+# additional phantom value to temporarily be placed on the locals
+# stack, which must be correctly accounted for by the compiler.
+
+let
+  set = {
+    value = 2;
+  };
+in
+[
+  1
+  (with set; value)
+  3
+]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-xml.exp.xml b/tvix/eval/src/tests/tvix_tests/eval-okay-xml.exp.xml
new file mode 100644
index 0000000000..1521bcc97a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-xml.exp.xml
@@ -0,0 +1,41 @@
+<?xml version='1.0' encoding='utf-8'?>
+<expr>
+  <attrs>
+    <attr name="attrspat">
+      <function>
+        <attrspat name="args">
+          <attr name="x" />
+          <attr name="y" />
+          <attr name="z" />
+        </attrspat>
+      </function>
+    </attr>
+    <attr name="attrspat-ellipsis">
+      <function>
+        <attrspat ellipsis="1" name="args">
+          <attr name="x" />
+          <attr name="y" />
+          <attr name="z" />
+        </attrspat>
+      </function>
+    </attr>
+    <attr name="noattrspat">
+      <function>
+        <attrspat>
+          <attr name="x" />
+          <attr name="y" />
+          <attr name="z" />
+        </attrspat>
+      </function>
+    </attr>
+    <attr name="noattrspat-ellipsis">
+      <function>
+        <attrspat ellipsis="1">
+          <attr name="x" />
+          <attr name="y" />
+          <attr name="z" />
+        </attrspat>
+      </function>
+    </attr>
+  </attrs>
+</expr>
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-xml.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-xml.nix
new file mode 100644
index 0000000000..3cc5acf430
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-xml.nix
@@ -0,0 +1,7 @@
+{
+  attrspat = args@{ x, y, z }: x;
+  attrspat-ellipsis = args@{ x, y, z, ... }: x;
+
+  noattrspat = { x, y, z }: x;
+  noattrspat-ellipsis = { x, y, z, ... }: x;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/identity-bool-false.nix b/tvix/eval/src/tests/tvix_tests/identity-bool-false.nix
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-bool-false.nix
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/identity-bool-true.nix b/tvix/eval/src/tests/tvix_tests/identity-bool-true.nix
new file mode 100644
index 0000000000..27ba77ddaf
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-bool-true.nix
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/tvix_tests/identity-dollar-escape.nix b/tvix/eval/src/tests/tvix_tests/identity-dollar-escape.nix
new file mode 100644
index 0000000000..08951d7637
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-dollar-escape.nix
@@ -0,0 +1 @@
+"\${foobar}"
diff --git a/tvix/eval/src/tests/tvix_tests/identity-empty-attrs.nix b/tvix/eval/src/tests/tvix_tests/identity-empty-attrs.nix
new file mode 100644
index 0000000000..ffcd4415b0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-empty-attrs.nix
@@ -0,0 +1 @@
+{ }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-empty-list.nix b/tvix/eval/src/tests/tvix_tests/identity-empty-list.nix
new file mode 100644
index 0000000000..1e3ec7217a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-empty-list.nix
@@ -0,0 +1 @@
+[ ]
diff --git a/tvix/eval/src/tests/tvix_tests/identity-flat-attrs.nix b/tvix/eval/src/tests/tvix_tests/identity-flat-attrs.nix
new file mode 100644
index 0000000000..e7c2ae18a6
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-flat-attrs.nix
@@ -0,0 +1 @@
+{ a = 15; b = "string"; c = null; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-float.nix b/tvix/eval/src/tests/tvix_tests/identity-float.nix
new file mode 100644
index 0000000000..bf77d54968
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-float.nix
@@ -0,0 +1 @@
+4.2
diff --git a/tvix/eval/src/tests/tvix_tests/identity-heterogeneous-list.nix b/tvix/eval/src/tests/tvix_tests/identity-heterogeneous-list.nix
new file mode 100644
index 0000000000..87f7fb0d06
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-heterogeneous-list.nix
@@ -0,0 +1 @@
+[ 1 2.1 "three" null ]
diff --git a/tvix/eval/src/tests/tvix_tests/identity-homogeneous-float-list.nix b/tvix/eval/src/tests/tvix_tests/identity-homogeneous-float-list.nix
new file mode 100644
index 0000000000..48e6655fb1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-homogeneous-float-list.nix
@@ -0,0 +1 @@
+[ 4.2 6.9 13.37 ]
diff --git a/tvix/eval/src/tests/tvix_tests/identity-homogeneous-int-list.nix b/tvix/eval/src/tests/tvix_tests/identity-homogeneous-int-list.nix
new file mode 100644
index 0000000000..d23a5c3814
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-homogeneous-int-list.nix
@@ -0,0 +1 @@
+[ 0 1 2 3 4 5 7 8 9 ]
diff --git a/tvix/eval/src/tests/tvix_tests/identity-homogeneous-string-list.nix b/tvix/eval/src/tests/tvix_tests/identity-homogeneous-string-list.nix
new file mode 100644
index 0000000000..d78a54e5b0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-homogeneous-string-list.nix
@@ -0,0 +1 @@
+[ "string" "list" ]
diff --git a/tvix/eval/src/tests/tvix_tests/identity-int.nix b/tvix/eval/src/tests/tvix_tests/identity-int.nix
new file mode 100644
index 0000000000..d81cc0710e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-int.nix
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/identity-kv-attrs.nix b/tvix/eval/src/tests/tvix_tests/identity-kv-attrs.nix
new file mode 100644
index 0000000000..f1398b8d05
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-kv-attrs.nix
@@ -0,0 +1 @@
+{ name = "foo"; value = 12; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-nested-attrs.nix b/tvix/eval/src/tests/tvix_tests/identity-nested-attrs.nix
new file mode 100644
index 0000000000..6a139452ef
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-nested-attrs.nix
@@ -0,0 +1 @@
+{ a = { b = null; }; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-null.nix b/tvix/eval/src/tests/tvix_tests/identity-null.nix
new file mode 100644
index 0000000000..19765bd501
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-null.nix
@@ -0,0 +1 @@
+null
diff --git a/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-assert.nix b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-assert.nix
new file mode 100644
index 0000000000..575b1af588
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-assert.nix
@@ -0,0 +1 @@
+{ "assert" = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-else.nix b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-else.nix
new file mode 100644
index 0000000000..7601f14b32
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-else.nix
@@ -0,0 +1 @@
+{ "else" = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-if.nix b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-if.nix
new file mode 100644
index 0000000000..1c391fc9a3
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-if.nix
@@ -0,0 +1 @@
+{ "if" = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-in.nix b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-in.nix
new file mode 100644
index 0000000000..b4f238651d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-in.nix
@@ -0,0 +1 @@
+{ "in" = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-inherit.nix b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-inherit.nix
new file mode 100644
index 0000000000..e62ed32b04
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-inherit.nix
@@ -0,0 +1 @@
+{ "inherit" = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-let.nix b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-let.nix
new file mode 100644
index 0000000000..196ec7cc88
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-let.nix
@@ -0,0 +1 @@
+{ "let" = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-rec.nix b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-rec.nix
new file mode 100644
index 0000000000..d2c4f93a2a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-rec.nix
@@ -0,0 +1 @@
+{ "rec" = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-then.nix b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-then.nix
new file mode 100644
index 0000000000..f2af8d6970
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-then.nix
@@ -0,0 +1 @@
+{ "then" = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-with.nix b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-with.nix
new file mode 100644
index 0000000000..cbcfa970c2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-quoted-attrname-with.nix
@@ -0,0 +1 @@
+{ "with" = true; }
diff --git a/tvix/eval/src/tests/tvix_tests/identity-signed-float.nix b/tvix/eval/src/tests/tvix_tests/identity-signed-float.nix
new file mode 100644
index 0000000000..50c9d06aa5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-signed-float.nix
@@ -0,0 +1 @@
+-4.2
diff --git a/tvix/eval/src/tests/tvix_tests/identity-signed-int.nix b/tvix/eval/src/tests/tvix_tests/identity-signed-int.nix
new file mode 100644
index 0000000000..6a0e60d48b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-signed-int.nix
@@ -0,0 +1 @@
+-42
diff --git a/tvix/eval/src/tests/tvix_tests/identity-string.nix b/tvix/eval/src/tests/tvix_tests/identity-string.nix
new file mode 100644
index 0000000000..d71ddbcf82
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/identity-string.nix
@@ -0,0 +1 @@
+"test string"
diff --git a/tvix/eval/src/tests/tvix_tests/lib.nix b/tvix/eval/src/tests/tvix_tests/lib.nix
new file mode 100644
index 0000000000..ab509bc85f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/lib.nix
@@ -0,0 +1,64 @@
+with builtins;
+
+rec {
+
+  fold = op: nul: list:
+    if list == [ ]
+    then nul
+    else op (head list) (fold op nul (tail list));
+
+  concat =
+    fold (x: y: x + y) "";
+
+  and = fold (x: y: x && y) true;
+
+  flatten = x:
+    if isList x
+    then fold (x: y: (flatten x) ++ y) [ ] x
+    else [ x ];
+
+  sum = foldl' (x: y: add x y) 0;
+
+  hasSuffix = ext: fileName:
+    let
+      lenFileName = stringLength fileName;
+      lenExt = stringLength ext;
+    in
+    !(lessThan lenFileName lenExt) &&
+    substring (sub lenFileName lenExt) lenFileName fileName == ext;
+
+  # Split a list at the given position.
+  splitAt = pos: list:
+    if pos == 0 then { first = [ ]; second = list; } else
+    if list == [ ] then { first = [ ]; second = [ ]; } else
+    let res = splitAt (sub pos 1) (tail list);
+    in { first = [ (head list) ] ++ res.first; second = res.second; };
+
+  # Stable merge sort.
+  sortBy = comp: list:
+    if lessThan 1 (length list)
+    then
+      let
+        split = splitAt (div (length list) 2) list;
+        first = sortBy comp split.first;
+        second = sortBy comp split.second;
+      in
+      mergeLists comp first second
+    else list;
+
+  mergeLists = comp: list1: list2:
+    if list1 == [ ] then list2 else
+    if list2 == [ ] then list1 else
+    if comp (head list2) (head list1) then [ (head list2) ] ++ mergeLists comp list1 (tail list2) else
+    [ (head list1) ] ++ mergeLists comp (tail list1) list2;
+
+  id = x: x;
+
+  const = x: y: x;
+
+  range = first: last:
+    if first > last
+    then [ ]
+    else genList (n: first + n) (last - first + 1);
+
+}
diff --git a/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-fail-builtins-genericClosure-uncomparable-keys.nix b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-fail-builtins-genericClosure-uncomparable-keys.nix
new file mode 100644
index 0000000000..d4e93e1f28
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-fail-builtins-genericClosure-uncomparable-keys.nix
@@ -0,0 +1,9 @@
+# Attribute sets can't be compared, only checked for equality
+builtins.genericClosure {
+  startSet = [
+    { key = { foo = 21; }; }
+  ];
+  operator = _: [
+    { key = { bar = 21; }; }
+  ];
+}
diff --git a/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-fail-builtins-genericClosure-uncomparable-keys2.nix b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-fail-builtins-genericClosure-uncomparable-keys2.nix
new file mode 100644
index 0000000000..0589a3ab59
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-fail-builtins-genericClosure-uncomparable-keys2.nix
@@ -0,0 +1,12 @@
+let
+  id = x: x;
+in
+
+builtins.genericClosure {
+  startSet = [{ key = id; first = true; }];
+  operator =
+    { first, ... }:
+    if first then [
+      { key = id; first = false; }
+    ] else [ ];
+}
diff --git a/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-builtins-set-pointer-equality.exp b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-builtins-set-pointer-equality.exp
new file mode 100644
index 0000000000..097eb2033a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-builtins-set-pointer-equality.exp
@@ -0,0 +1 @@
+[ true true true true true true true true true true ]
diff --git a/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-builtins-set-pointer-equality.nix b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-builtins-set-pointer-equality.nix
new file mode 100644
index 0000000000..aa2a0a1e19
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-builtins-set-pointer-equality.nix
@@ -0,0 +1,25 @@
+let
+  alias = builtins;
+in
+
+[
+  (builtins == builtins)
+  (alias == builtins)
+  (builtins == builtins.builtins)
+  (builtins.builtins == builtins.builtins)
+  (builtins.builtins == builtins.builtins.builtins)
+  (alias == alias)
+  (alias == builtins.builtins)
+  ([ builtins ] == [ builtins ])
+
+  # Surprisingly the following expressions don't work. They are
+  # here for documentation purposes and covered only
+  # by eval-okay-select-pointer-inequality.nix. Reasoning is that
+  # we may not want / be able to replicate this behavior at all.
+  #   ([ builtins.add ] == [ builtins.add ])
+  #   ({ inherit (builtins) import; } == { inherit (builtins) import; })
+
+  # These expressions work as expected, however:
+  (let x = { inherit (builtins) add; }; in x == x)
+  (let inherit (builtins) add; in [ add ] == [ add ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-cycle-display-cpp-nix-2.13.exp b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-cycle-display-cpp-nix-2.13.exp
new file mode 100644
index 0000000000..9c44023f02
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-cycle-display-cpp-nix-2.13.exp
@@ -0,0 +1 @@
+[ { car = 42; cdr = ยซrepeatedยป; } [ ยซrepeatedยป ยซrepeatedยป ยซrepeatedยป ] { val = 42; wal = ยซrepeatedยป; xal = ยซrepeatedยป; } { tail1 = ยซrepeatedยป; tail2 = ยซrepeatedยป; val = 42; } { tail1 = ยซrepeatedยป; tail2 = ยซrepeatedยป; val = 21; } ]
diff --git a/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-cycle-display-cpp-nix-2.13.nix b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-cycle-display-cpp-nix-2.13.nix
new file mode 100644
index 0000000000..ac849a58fe
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-cycle-display-cpp-nix-2.13.nix
@@ -0,0 +1,34 @@
+let
+  linkedList = {
+    car = 42;
+    cdr = linkedList;
+  };
+
+  list = [
+    linkedList
+    linkedList
+    linkedList
+  ];
+
+  set = {
+    val = 42;
+    wal = set;
+    xal = set;
+  };
+
+  multiTail = {
+    val = 42;
+    tail1 = multiTail;
+    tail2 = multiTail;
+  };
+in
+
+[
+  linkedList
+  list
+  set
+
+  # In C++ Nix 2.3 these would be displayed differently
+  multiTail
+  (let multiTail = { val = 21; tail1 = multiTail; tail2 = multiTail; }; in multiTail)
+]
diff --git a/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-minimal-2.3-builtins.exp b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-minimal-2.3-builtins.exp
new file mode 100644
index 0000000000..967fc858bc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-minimal-2.3-builtins.exp
@@ -0,0 +1 @@
+[ "abort" "add" "addErrorContext" "all" "any" "appendContext" "attrNames" "attrValues" "baseNameOf" "bitAnd" "bitOr" "bitXor" "builtins" "catAttrs" "compareVersions" "concatLists" "concatMap" "concatStringsSep" "currentSystem" "currentTime" "deepSeq" "derivation" "derivationStrict" "dirOf" "div" "elem" "elemAt" "false" "fetchGit" "fetchMercurial" "fetchTarball" "fetchurl" "filter" "filterSource" "findFile" "foldl'" "fromJSON" "fromTOML" "functionArgs" "genList" "genericClosure" "getAttr" "getContext" "getEnv" "hasAttr" "hasContext" "hashFile" "hashString" "head" "import" "intersectAttrs" "isAttrs" "isBool" "isFloat" "isFunction" "isInt" "isList" "isNull" "isPath" "isString" "langVersion" "length" "lessThan" "listToAttrs" "map" "mapAttrs" "match" "mul" "nixPath" "nixVersion" "null" "parseDrvName" "partition" "path" "pathExists" "placeholder" "readDir" "readFile" "removeAttrs" "replaceStrings" "scopedImport" "seq" "sort" "split" "splitVersion" "storeDir" "storePath" "stringLength" "sub" "substring" "tail" "throw" "toFile" "toJSON" "toPath" "toString" "toXML" "trace" "true" "tryEval" "typeOf" "unsafeDiscardOutputDependency" "unsafeDiscardStringContext" "unsafeGetAttrPos" ]
diff --git a/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-minimal-2.3-builtins.nix b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-minimal-2.3-builtins.nix
new file mode 100644
index 0000000000..4480daecd9
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-minimal-2.3-builtins.nix
@@ -0,0 +1,122 @@
+# This tests verifies that the Nix implementation evaluating this has at least
+# all the builtins given in `minimalBuiltins`. We don't test a precise list of
+# builtins since we accept that there will always be difference between the
+# builtins sets of Tvix, C++ Nix 2.3 and newer C++ Nix versions, as new builtins
+# are added.
+#
+# Tvix also may choose never to implement some builtins if they are only useful
+# for flakes or perform well enough via the shims nixpkgs usually provides.
+
+let
+  # C++ Nix 2.3 builtins except valueSize which is removed in later versions
+  minimalBuiltins = [
+    "abort"
+    "add"
+    "addErrorContext"
+    "all"
+    "any"
+    "appendContext"
+    "attrNames"
+    "attrValues"
+    "baseNameOf"
+    "bitAnd"
+    "bitOr"
+    "bitXor"
+    "builtins"
+    "catAttrs"
+    "compareVersions"
+    "concatLists"
+    "concatMap"
+    "concatStringsSep"
+    "currentSystem"
+    "currentTime"
+    "deepSeq"
+    "derivation"
+    "derivationStrict"
+    "dirOf"
+    "div"
+    "elem"
+    "elemAt"
+    "false"
+    "fetchGit"
+    "fetchMercurial"
+    "fetchTarball"
+    "fetchurl"
+    "filter"
+    "filterSource"
+    "findFile"
+    "foldl'"
+    "fromJSON"
+    "fromTOML"
+    "functionArgs"
+    "genList"
+    "genericClosure"
+    "getAttr"
+    "getContext"
+    "getEnv"
+    "hasAttr"
+    "hasContext"
+    "hashFile"
+    "hashString"
+    "head"
+    "import"
+    "intersectAttrs"
+    "isAttrs"
+    "isBool"
+    "isFloat"
+    "isFunction"
+    "isInt"
+    "isList"
+    "isNull"
+    "isPath"
+    "isString"
+    "langVersion"
+    "length"
+    "lessThan"
+    "listToAttrs"
+    "map"
+    "mapAttrs"
+    "match"
+    "mul"
+    "nixPath"
+    "nixVersion"
+    "null"
+    "parseDrvName"
+    "partition"
+    "path"
+    "pathExists"
+    "placeholder"
+    "readDir"
+    "readFile"
+    "removeAttrs"
+    "replaceStrings"
+    "scopedImport"
+    "seq"
+    "sort"
+    "split"
+    "splitVersion"
+    "storeDir"
+    "storePath"
+    "stringLength"
+    "sub"
+    "substring"
+    "tail"
+    "throw"
+    "toFile"
+    "toJSON"
+    "toPath"
+    "toString"
+    "toXML"
+    "trace"
+    "true"
+    "tryEval"
+    "typeOf"
+    "unsafeDiscardOutputDependency"
+    "unsafeDiscardStringContext"
+    "unsafeGetAttrPos"
+  ];
+
+  intersectLists = as: bs: builtins.filter (a: builtins.elem a bs) as;
+in
+
+intersectLists minimalBuiltins (builtins.attrNames builtins)
diff --git a/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-non-identifier-pointer-inequality.exp b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-non-identifier-pointer-inequality.exp
new file mode 100644
index 0000000000..69fd1d0847
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-non-identifier-pointer-inequality.exp
@@ -0,0 +1 @@
+[ false false false false false true false false ]
diff --git a/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-non-identifier-pointer-inequality.nix b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-non-identifier-pointer-inequality.nix
new file mode 100644
index 0000000000..821aa47a0d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-non-identifier-pointer-inequality.nix
@@ -0,0 +1,28 @@
+# C++ Nix frequently creates copies of Value structs when evaluating
+# a variety of expressions. As a result, pointer equality doesn't
+# work for many (all?) expressions that go beyond simple identifier
+# access from the scope: Even if the inner representation of the
+# value still has the same memory location, C++ Nix has created
+# a copy of the struct that holds the pointer to this memory.
+# Since pointer equality is established via the location of
+# the latter, not the former, the values are no longer equal
+# by pointer.
+let
+  foo = { bar = x: x; };
+
+  id = x: x;
+in
+
+[
+  ({ inherit (foo) bar; } == { inherit (foo) bar; })
+  ([ foo.bar ] == [ foo.bar ])
+
+  ([ builtins.add ] == [ builtins.add ])
+  ({ inherit (builtins) import; } == { inherit (builtins) import; })
+
+  ([ (id id) ] == [ (id id) ])
+  ([ id ] == [ id ])
+
+  (with foo; [ bar ] == [ bar ])
+  (with builtins; [ add ] == [ add ])
+]
diff --git a/tvix/eval/src/tests/tvix_tests/observable-eval-cache1.nix b/tvix/eval/src/tests/tvix_tests/observable-eval-cache1.nix
new file mode 100644
index 0000000000..b5f3f59a79
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/observable-eval-cache1.nix
@@ -0,0 +1 @@
+let id = x: x; in { inherit id; }
diff --git a/tvix/eval/src/tests/tvix_tests/observable-eval-cache2.nix b/tvix/eval/src/tests/tvix_tests/observable-eval-cache2.nix
new file mode 120000
index 0000000000..7f69c0eb47
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/observable-eval-cache2.nix
@@ -0,0 +1 @@
+observable-eval-cache1.nix
\ No newline at end of file
diff --git a/tvix/eval/src/tests/tvix_tests/observable-eval-cache3.nix b/tvix/eval/src/tests/tvix_tests/observable-eval-cache3.nix
new file mode 100644
index 0000000000..b5f3f59a79
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/observable-eval-cache3.nix
@@ -0,0 +1 @@
+let id = x: x; in { inherit id; }
diff --git a/tvix/eval/src/tests/tvix_tests/readDir/bar b/tvix/eval/src/tests/tvix_tests/readDir/bar
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/readDir/bar
diff --git a/tvix/eval/src/tests/tvix_tests/readDir/foo/.keep b/tvix/eval/src/tests/tvix_tests/readDir/foo/.keep
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/readDir/foo/.keep
diff --git a/tvix/eval/src/upvalues.rs b/tvix/eval/src/upvalues.rs
new file mode 100644
index 0000000000..687d6850cc
--- /dev/null
+++ b/tvix/eval/src/upvalues.rs
@@ -0,0 +1,86 @@
+//! This module encapsulates some logic for upvalue handling, which is
+//! relevant to both thunks (delayed computations for lazy-evaluation)
+//! as well as closures (lambdas that capture variables from the
+//! surrounding scope).
+//!
+//! The upvalues of a scope are whatever data are needed at runtime
+//! in order to resolve each free variable in the scope to a value.
+//! "Upvalue" is a term taken from Lua.
+
+use std::ops::Index;
+
+use crate::{opcode::UpvalueIdx, Value};
+
+/// Structure for carrying upvalues of an UpvalueCarrier.  The
+/// implementation of this struct encapsulates the logic for
+/// capturing and accessing upvalues.
+///
+/// Nix's `with` cannot be used to shadow an enclosing binding --
+/// like Rust's `use xyz::*` construct, but unlike Javascript's
+/// `with (xyz)`.  This means that Nix has two kinds of identifiers,
+/// which can be distinguished at compile time:
+///
+/// - Static identifiers, which are bound in some enclosing scope by
+///   `let`, `name:` or `{name}:`
+/// - Dynamic identifiers, which are not bound in any enclosing
+///   scope
+#[derive(Clone, Debug)]
+pub struct Upvalues {
+    /// The upvalues of static identifiers.  Each static identifier
+    /// is assigned an integer identifier at compile time, which is
+    /// an index into this Vec.
+    static_upvalues: Vec<Value>,
+
+    /// The upvalues of dynamic identifiers, if any exist.  This
+    /// consists of the value passed to each enclosing `with val;`,
+    /// from outermost to innermost.
+    with_stack: Option<Vec<Value>>,
+}
+
+impl Upvalues {
+    pub fn with_capacity(count: usize) -> Self {
+        Upvalues {
+            static_upvalues: Vec::with_capacity(count),
+            with_stack: None,
+        }
+    }
+
+    /// Push an upvalue at the end of the upvalue list.
+    pub fn push(&mut self, value: Value) {
+        self.static_upvalues.push(value);
+    }
+
+    /// Set the captured with stack.
+    pub fn set_with_stack(&mut self, with_stack: Vec<Value>) {
+        self.with_stack = Some(with_stack);
+    }
+
+    pub fn with_stack(&self) -> Option<&Vec<Value>> {
+        self.with_stack.as_ref()
+    }
+
+    pub fn with_stack_len(&self) -> usize {
+        match &self.with_stack {
+            None => 0,
+            Some(stack) => stack.len(),
+        }
+    }
+
+    /// Resolve deferred upvalues from the provided stack slice,
+    /// mutating them in the internal upvalue slots.
+    pub fn resolve_deferred_upvalues(&mut self, stack: &[Value]) {
+        for upvalue in self.static_upvalues.iter_mut() {
+            if let Value::DeferredUpvalue(update_from_idx) = upvalue {
+                *upvalue = stack[update_from_idx.0].clone();
+            }
+        }
+    }
+}
+
+impl Index<UpvalueIdx> for Upvalues {
+    type Output = Value;
+
+    fn index(&self, index: UpvalueIdx) -> &Self::Output {
+        &self.static_upvalues[index.0]
+    }
+}
diff --git a/tvix/eval/src/value/arbitrary.rs b/tvix/eval/src/value/arbitrary.rs
new file mode 100644
index 0000000000..bf53f4fcb2
--- /dev/null
+++ b/tvix/eval/src/value/arbitrary.rs
@@ -0,0 +1,106 @@
+//! Support for configurable generation of arbitrary nix values
+
+use imbl::proptest::{ord_map, vector};
+use proptest::{prelude::*, strategy::BoxedStrategy};
+use std::ffi::OsString;
+
+use super::{attrs::AttrsRep, NixAttrs, NixList, NixString, Value};
+
+#[derive(Clone)]
+pub enum Parameters {
+    Strategy(BoxedStrategy<Value>),
+    Parameters {
+        generate_internal_values: bool,
+        generate_functions: bool,
+        generate_nested: bool,
+    },
+}
+
+impl Default for Parameters {
+    fn default() -> Self {
+        Self::Parameters {
+            generate_internal_values: false,
+            generate_functions: false,
+            generate_nested: true,
+        }
+    }
+}
+
+impl Arbitrary for NixAttrs {
+    type Parameters = Parameters;
+    type Strategy = BoxedStrategy<Self>;
+
+    fn arbitrary_with(args: Self::Parameters) -> Self::Strategy {
+        prop_oneof![
+            // Empty attrs representation
+            Just(Self(AttrsRep::Empty)),
+            // KV representation (name/value pairs)
+            (
+                any_with::<Value>(args.clone()),
+                any_with::<Value>(args.clone())
+            )
+                .prop_map(|(name, value)| Self(AttrsRep::KV { name, value })),
+            // Map representation
+            ord_map(NixString::arbitrary(), Value::arbitrary_with(args), 0..100)
+                .prop_map(|map| Self(AttrsRep::Im(map)))
+        ]
+        .boxed()
+    }
+}
+
+impl Arbitrary for NixList {
+    type Parameters = <Value as Arbitrary>::Parameters;
+    type Strategy = BoxedStrategy<Self>;
+
+    fn arbitrary_with(args: Self::Parameters) -> Self::Strategy {
+        vector(<Value as Arbitrary>::arbitrary_with(args), 0..100)
+            .prop_map(NixList::from)
+            .boxed()
+    }
+}
+
+impl Arbitrary for Value {
+    type Parameters = Parameters;
+    type Strategy = BoxedStrategy<Self>;
+
+    fn arbitrary_with(args: Self::Parameters) -> Self::Strategy {
+        match args {
+            Parameters::Strategy(s) => s,
+            Parameters::Parameters {
+                generate_internal_values,
+                generate_functions,
+                generate_nested,
+            } => {
+                if generate_internal_values || generate_functions {
+                    todo!("Generating internal values and functions not implemented yet")
+                } else if generate_nested {
+                    non_internal_value().boxed()
+                } else {
+                    leaf_value().boxed()
+                }
+            }
+        }
+    }
+}
+
+fn leaf_value() -> impl Strategy<Value = Value> {
+    use Value::*;
+
+    prop_oneof![
+        Just(Null),
+        any::<bool>().prop_map(Bool),
+        any::<i64>().prop_map(Integer),
+        any::<f64>().prop_map(Float),
+        any::<NixString>().prop_map(String),
+        any::<OsString>().prop_map(|s| Path(Box::new(s.into()))),
+    ]
+}
+
+fn non_internal_value() -> impl Strategy<Value = Value> {
+    leaf_value().prop_recursive(3, 5, 5, |inner| {
+        prop_oneof![
+            NixAttrs::arbitrary_with(Parameters::Strategy(inner.clone())).prop_map(Value::attrs),
+            any_with::<NixList>(Parameters::Strategy(inner)).prop_map(Value::List)
+        ]
+    })
+}
diff --git a/tvix/eval/src/value/attrs.rs b/tvix/eval/src/value/attrs.rs
index e2ebc7cb34..33259c8058 100644
--- a/tvix/eval/src/value/attrs.rs
+++ b/tvix/eval/src/value/attrs.rs
@@ -1,43 +1,621 @@
-/// This module implements Nix attribute sets. They have flexible
-/// backing implementations, as they are used in very versatile
-/// use-cases that are all exposed the same way in the language
-/// surface.
-use std::collections::BTreeMap;
-use std::fmt::Display;
+//! This module implements Nix attribute sets. They have flexible
+//! backing implementations, as they are used in very versatile
+//! use-cases that are all exposed the same way in the language
+//! surface.
+//!
+//! Due to this, construction and management of attribute sets has
+//! some peculiarities that are encapsulated within this module.
+use std::borrow::Borrow;
+use std::iter::FromIterator;
+
+use bstr::BStr;
+use imbl::{ordmap, OrdMap};
+use lazy_static::lazy_static;
+use serde::de::{Deserializer, Error, Visitor};
+use serde::Deserialize;
 
 use super::string::NixString;
+use super::thunk::ThunkSet;
+use super::TotalDisplay;
 use super::Value;
+use crate::errors::ErrorKind;
+use crate::CatchableErrorKind;
 
-#[derive(Debug)]
-pub enum NixAttrs {
-    Map(BTreeMap<NixString, Value>),
-    KV { name: Value, value: Value },
+lazy_static! {
+    static ref NAME_S: NixString = "name".into();
+    static ref NAME_REF: &'static NixString = &NAME_S;
+    static ref VALUE_S: NixString = "value".into();
+    static ref VALUE_REF: &'static NixString = &VALUE_S;
 }
 
-impl Display for NixAttrs {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        f.write_str("{ ")?;
+#[cfg(test)]
+mod tests;
+
+#[derive(Clone, Debug, Deserialize, Default)]
+pub(super) enum AttrsRep {
+    #[default]
+    Empty,
+
+    Im(OrdMap<NixString, Value>),
+
+    /// Warning: this represents a **two**-attribute attrset, with
+    /// attribute names "name" and "value", like `{name="foo";
+    /// value="bar";}`, *not* `{foo="bar";}`!
+    KV {
+        name: Value,
+        value: Value,
+    },
+}
 
+impl AttrsRep {
+    /// Retrieve reference to a mutable map inside of an attrs,
+    /// optionally changing the representation if required.
+    fn map_mut(&mut self) -> &mut OrdMap<NixString, Value> {
         match self {
-            NixAttrs::KV { name, value } => {
-                f.write_fmt(format_args!("name = \"{}\"; ", name))?;
-                f.write_fmt(format_args!("value = {}; ", value))?;
-                f.write_str("/* optimised pair! */")?;
+            AttrsRep::Im(m) => m,
+
+            AttrsRep::Empty => {
+                *self = AttrsRep::Im(OrdMap::new());
+                self.map_mut()
+            }
+
+            AttrsRep::KV { name, value } => {
+                *self = AttrsRep::Im(ordmap! {
+                    NAME_S.clone() => name.clone(),
+                    VALUE_S.clone() => value.clone()
+                });
+
+                self.map_mut()
+            }
+        }
+    }
+
+    fn select(&self, key: &BStr) -> Option<&Value> {
+        match self {
+            AttrsRep::Empty => None,
+
+            AttrsRep::KV { name, value } => match &**key {
+                b"name" => Some(name),
+                b"value" => Some(value),
+                _ => None,
+            },
+
+            AttrsRep::Im(map) => map.get(key),
+        }
+    }
+
+    fn contains(&self, key: &BStr) -> bool {
+        match self {
+            AttrsRep::Empty => false,
+            AttrsRep::KV { .. } => key == "name" || key == "value",
+            AttrsRep::Im(map) => map.contains_key(key),
+        }
+    }
+}
+
+#[repr(transparent)]
+#[derive(Clone, Debug, Default)]
+pub struct NixAttrs(pub(super) AttrsRep);
+
+impl From<OrdMap<NixString, Value>> for NixAttrs {
+    fn from(map: OrdMap<NixString, Value>) -> Self {
+        NixAttrs(AttrsRep::Im(map))
+    }
+}
+
+impl<K, V> FromIterator<(K, V)> for NixAttrs
+where
+    NixString: From<K>,
+    Value: From<V>,
+{
+    fn from_iter<T>(iter: T) -> NixAttrs
+    where
+        T: IntoIterator<Item = (K, V)>,
+    {
+        NixAttrs(AttrsRep::Im(iter.into_iter().collect()))
+    }
+}
+
+impl TotalDisplay for NixAttrs {
+    fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result {
+        f.write_str("{ ")?;
+
+        match &self.0 {
+            AttrsRep::KV { name, value } => {
+                f.write_str("name = ")?;
+                name.total_fmt(f, set)?;
+                f.write_str("; ")?;
+
+                f.write_str("value = ")?;
+                value.total_fmt(f, set)?;
+                f.write_str("; ")?;
             }
 
-            NixAttrs::Map(map) => {
+            AttrsRep::Im(map) => {
                 for (name, value) in map {
-                    f.write_fmt(format_args!("{} = {}; ", name, value))?;
+                    write!(f, "{} = ", name.ident_str())?;
+                    value.total_fmt(f, set)?;
+                    f.write_str("; ")?;
                 }
             }
+
+            AttrsRep::Empty => { /* no values to print! */ }
         }
 
         f.write_str("}")
     }
 }
 
-impl PartialEq for NixAttrs {
-    fn eq(&self, _other: &Self) -> bool {
-        todo!("attrset equality")
+impl<'de> Deserialize<'de> for NixAttrs {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct MapVisitor;
+
+        impl<'de> Visitor<'de> for MapVisitor {
+            type Value = NixAttrs;
+
+            fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
+                formatter.write_str("a valid Nix attribute set")
+            }
+
+            fn visit_map<A>(self, mut map: A) -> Result<Self::Value, A::Error>
+            where
+                A: serde::de::MapAccess<'de>,
+            {
+                let mut stack_array = Vec::with_capacity(map.size_hint().unwrap_or(0) * 2);
+
+                while let Some((key, value)) = map.next_entry()? {
+                    stack_array.push(key);
+                    stack_array.push(value);
+                }
+
+                Ok(NixAttrs::construct(stack_array.len() / 2, stack_array)
+                    .map_err(A::Error::custom)?
+                    .expect("Catchable values are unreachable here"))
+            }
+        }
+
+        deserializer.deserialize_map(MapVisitor)
+    }
+}
+
+impl NixAttrs {
+    pub fn empty() -> Self {
+        Self(AttrsRep::Empty)
+    }
+
+    /// Compare two attribute sets by pointer equality. Only makes
+    /// sense for some attribute set reprsentations, i.e. returning
+    /// `false` does not mean that the two attribute sets do not have
+    /// equal *content*.
+    pub fn ptr_eq(&self, other: &Self) -> bool {
+        match (&self.0, &other.0) {
+            (AttrsRep::Im(lhs), AttrsRep::Im(rhs)) => lhs.ptr_eq(rhs),
+            _ => false,
+        }
+    }
+
+    /// Return an attribute set containing the merge of the two
+    /// provided sets. Keys from the `other` set have precedence.
+    pub fn update(self, other: Self) -> Self {
+        // Short-circuit on some optimal cases:
+        match (&self.0, &other.0) {
+            (AttrsRep::Empty, AttrsRep::Empty) => return self,
+            (AttrsRep::Empty, _) => return other,
+            (_, AttrsRep::Empty) => return self,
+            (AttrsRep::KV { .. }, AttrsRep::KV { .. }) => return other,
+
+            // Explicitly handle all branches instead of falling
+            // through, to ensure that we get at least some compiler
+            // errors if variants are modified.
+            (AttrsRep::Im(_), AttrsRep::Im(_))
+            | (AttrsRep::Im(_), AttrsRep::KV { .. })
+            | (AttrsRep::KV { .. }, AttrsRep::Im(_)) => {}
+        };
+
+        // Slightly more advanced, but still optimised updates
+        match (self.0, other.0) {
+            (AttrsRep::Im(mut m), AttrsRep::KV { name, value }) => {
+                m.insert(NAME_S.clone(), name);
+                m.insert(VALUE_S.clone(), value);
+                NixAttrs(AttrsRep::Im(m))
+            }
+
+            (AttrsRep::KV { name, value }, AttrsRep::Im(mut m)) => {
+                match m.entry(NAME_S.clone()) {
+                    imbl::ordmap::Entry::Vacant(e) => {
+                        e.insert(name);
+                    }
+
+                    imbl::ordmap::Entry::Occupied(_) => { /* name from `m` has precedence */ }
+                };
+
+                match m.entry(VALUE_S.clone()) {
+                    imbl::ordmap::Entry::Vacant(e) => {
+                        e.insert(value);
+                    }
+
+                    imbl::ordmap::Entry::Occupied(_) => { /* value from `m` has precedence */ }
+                };
+
+                NixAttrs(AttrsRep::Im(m))
+            }
+
+            // Plain merge of maps.
+            (AttrsRep::Im(m1), AttrsRep::Im(m2)) => NixAttrs(AttrsRep::Im(m2.union(m1))),
+
+            // Cases handled above by the borrowing match:
+            _ => unreachable!(),
+        }
+    }
+
+    /// Return the number of key-value entries in an attrset.
+    pub fn len(&self) -> usize {
+        match &self.0 {
+            AttrsRep::Im(map) => map.len(),
+            AttrsRep::Empty => 0,
+            AttrsRep::KV { .. } => 2,
+        }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        match &self.0 {
+            AttrsRep::Im(map) => map.is_empty(),
+            AttrsRep::Empty => true,
+            AttrsRep::KV { .. } => false,
+        }
+    }
+
+    /// Select a value from an attribute set by key.
+    pub fn select<K>(&self, key: &K) -> Option<&Value>
+    where
+        K: Borrow<BStr> + ?Sized,
+    {
+        self.0.select(key.borrow())
+    }
+
+    /// Select a required value from an attribute set by key, return
+    /// an `AttributeNotFound` error if it is missing.
+    pub fn select_required<K>(&self, key: &K) -> Result<&Value, ErrorKind>
+    where
+        K: Borrow<BStr> + ?Sized,
+    {
+        self.select(key)
+            .ok_or_else(|| ErrorKind::AttributeNotFound {
+                name: key.borrow().to_string(),
+            })
+    }
+
+    pub fn contains<'a, K: 'a>(&self, key: K) -> bool
+    where
+        &'a BStr: From<K>,
+    {
+        self.0.contains(key.into())
+    }
+
+    /// Construct an iterator over all the key-value pairs in the attribute set.
+    #[allow(clippy::needless_lifetimes)]
+    pub fn iter<'a>(&'a self) -> Iter<KeyValue<'a>> {
+        Iter(match &self.0 {
+            AttrsRep::Im(map) => KeyValue::Im(map.iter()),
+            AttrsRep::Empty => KeyValue::Empty,
+
+            AttrsRep::KV {
+                ref name,
+                ref value,
+            } => KeyValue::KV {
+                name,
+                value,
+                at: IterKV::default(),
+            },
+        })
+    }
+
+    /// Same as iter(), but marks call sites which rely on the
+    /// iteration being lexicographic.
+    pub fn iter_sorted(&self) -> Iter<KeyValue<'_>> {
+        self.iter()
+    }
+
+    /// Same as [IntoIterator::into_iter], but marks call sites which rely on the
+    /// iteration being lexicographic.
+    pub fn into_iter_sorted(self) -> OwnedAttrsIterator {
+        self.into_iter()
+    }
+
+    /// Construct an iterator over all the keys of the attribute set
+    pub fn keys(&self) -> Keys {
+        Keys(match &self.0 {
+            AttrsRep::Empty => KeysInner::Empty,
+            AttrsRep::Im(m) => KeysInner::Im(m.keys()),
+            AttrsRep::KV { .. } => KeysInner::KV(IterKV::default()),
+        })
+    }
+
+    /// Implement construction logic of an attribute set, to encapsulate
+    /// logic about attribute set optimisations inside of this module.
+    pub fn construct(
+        count: usize,
+        mut stack_slice: Vec<Value>,
+    ) -> Result<Result<Self, CatchableErrorKind>, ErrorKind> {
+        debug_assert!(
+            stack_slice.len() == count * 2,
+            "construct_attrs called with count == {}, but slice.len() == {}",
+            count,
+            stack_slice.len(),
+        );
+
+        // Optimisation: Empty attribute set
+        if count == 0 {
+            return Ok(Ok(NixAttrs(AttrsRep::Empty)));
+        }
+
+        // Optimisation: KV pattern
+        if count == 2 {
+            if let Some(kv) = attempt_optimise_kv(&mut stack_slice) {
+                return Ok(Ok(kv));
+            }
+        }
+
+        let mut attrs = NixAttrs(AttrsRep::Im(OrdMap::new()));
+
+        for _ in 0..count {
+            let value = stack_slice.pop().unwrap();
+            let key = stack_slice.pop().unwrap();
+
+            match key {
+                Value::String(ks) => set_attr(&mut attrs, ks, value)?,
+
+                Value::Null => {
+                    // This is in fact valid, but leads to the value
+                    // being ignored and nothing being set, i.e. `{
+                    // ${null} = 1; } => { }`.
+                    continue;
+                }
+
+                Value::Catchable(err) => return Ok(Err(*err)),
+
+                other => return Err(ErrorKind::InvalidAttributeName(other)),
+            }
+        }
+
+        Ok(Ok(attrs))
+    }
+
+    /// Construct an optimized "KV"-style attribute set given the value for the
+    /// `"name"` key, and the value for the `"value"` key
+    pub(crate) fn from_kv(name: Value, value: Value) -> Self {
+        NixAttrs(AttrsRep::KV { name, value })
+    }
+}
+
+impl IntoIterator for NixAttrs {
+    type Item = (NixString, Value);
+    type IntoIter = OwnedAttrsIterator;
+
+    fn into_iter(self) -> Self::IntoIter {
+        match self.0 {
+            AttrsRep::Empty => OwnedAttrsIterator(IntoIterRepr::Empty),
+            AttrsRep::KV { name, value } => OwnedAttrsIterator(IntoIterRepr::Finite(
+                vec![(NAME_REF.clone(), name), (VALUE_REF.clone(), value)].into_iter(),
+            )),
+            AttrsRep::Im(map) => OwnedAttrsIterator(IntoIterRepr::Im(map.into_iter())),
+        }
+    }
+}
+
+/// In Nix, name/value attribute pairs are frequently constructed from
+/// literals. This particular case should avoid allocation of a map,
+/// additional heap values etc. and use the optimised `KV` variant
+/// instead.
+///
+/// ```norust
+/// `slice` is the top of the stack from which the attrset is being
+/// constructed, e.g.
+///
+///   slice: [ "value" 5 "name" "foo" ]
+///   index:   0       1 2      3
+///   stack:   3       2 1      0
+/// ```
+fn attempt_optimise_kv(slice: &mut [Value]) -> Option<NixAttrs> {
+    let (name_idx, value_idx) = {
+        match (&slice[2], &slice[0]) {
+            (Value::String(s1), Value::String(s2)) if (*s1 == *NAME_S && *s2 == *VALUE_S) => (3, 1),
+            (Value::String(s1), Value::String(s2)) if (*s1 == *VALUE_S && *s2 == *NAME_S) => (1, 3),
+
+            // Technically this branch lets type errors pass,
+            // but they will be caught during normal attribute
+            // set construction instead.
+            _ => return None,
+        }
+    };
+
+    Some(NixAttrs::from_kv(
+        slice[name_idx].clone(),
+        slice[value_idx].clone(),
+    ))
+}
+
+/// Set an attribute on an in-construction attribute set, while
+/// checking against duplicate keys.
+fn set_attr(attrs: &mut NixAttrs, key: NixString, value: Value) -> Result<(), ErrorKind> {
+    match attrs.0.map_mut().entry(key) {
+        imbl::ordmap::Entry::Occupied(entry) => Err(ErrorKind::DuplicateAttrsKey {
+            key: entry.key().to_string(),
+        }),
+
+        imbl::ordmap::Entry::Vacant(entry) => {
+            entry.insert(value);
+            Ok(())
+        }
+    }
+}
+
+/// Internal helper type to track the iteration status of an iterator
+/// over the name/value representation.
+#[derive(Debug, Default)]
+pub enum IterKV {
+    #[default]
+    Name,
+    Value,
+    Done,
+}
+
+impl IterKV {
+    fn next(&mut self) {
+        match *self {
+            Self::Name => *self = Self::Value,
+            Self::Value => *self = Self::Done,
+            Self::Done => {}
+        }
+    }
+}
+
+/// Iterator representation over the keys *and* values of an attribute
+/// set.
+pub enum KeyValue<'a> {
+    Empty,
+
+    KV {
+        name: &'a Value,
+        value: &'a Value,
+        at: IterKV,
+    },
+
+    Im(imbl::ordmap::Iter<'a, NixString, Value>),
+}
+
+/// Iterator over a Nix attribute set.
+// This wrapper type exists to make the inner "raw" iterator
+// inaccessible.
+#[repr(transparent)]
+pub struct Iter<T>(T);
+
+impl<'a> Iterator for Iter<KeyValue<'a>> {
+    type Item = (&'a NixString, &'a Value);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match &mut self.0 {
+            KeyValue::Im(inner) => inner.next(),
+            KeyValue::Empty => None,
+
+            KeyValue::KV { name, value, at } => match at {
+                IterKV::Name => {
+                    at.next();
+                    Some((&NAME_REF, name))
+                }
+
+                IterKV::Value => {
+                    at.next();
+                    Some((&VALUE_REF, value))
+                }
+
+                IterKV::Done => None,
+            },
+        }
+    }
+}
+
+impl<'a> ExactSizeIterator for Iter<KeyValue<'a>> {
+    fn len(&self) -> usize {
+        match &self.0 {
+            KeyValue::Empty => 0,
+            KeyValue::KV { .. } => 2,
+            KeyValue::Im(inner) => inner.len(),
+        }
+    }
+}
+
+enum KeysInner<'a> {
+    Empty,
+    KV(IterKV),
+    Im(imbl::ordmap::Keys<'a, NixString, Value>),
+}
+
+pub struct Keys<'a>(KeysInner<'a>);
+
+impl<'a> Iterator for Keys<'a> {
+    type Item = &'a NixString;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match &mut self.0 {
+            KeysInner::Empty => None,
+            KeysInner::KV(at @ IterKV::Name) => {
+                at.next();
+                Some(&NAME_REF)
+            }
+            KeysInner::KV(at @ IterKV::Value) => {
+                at.next();
+                Some(&VALUE_REF)
+            }
+            KeysInner::KV(IterKV::Done) => None,
+            KeysInner::Im(m) => m.next(),
+        }
+    }
+}
+
+impl<'a> IntoIterator for &'a NixAttrs {
+    type Item = (&'a NixString, &'a Value);
+
+    type IntoIter = Iter<KeyValue<'a>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+impl<'a> ExactSizeIterator for Keys<'a> {
+    fn len(&self) -> usize {
+        match &self.0 {
+            KeysInner::Empty => 0,
+            KeysInner::KV(_) => 2,
+            KeysInner::Im(m) => m.len(),
+        }
+    }
+}
+
+/// Internal representation of an owning attrset iterator
+pub enum IntoIterRepr {
+    Empty,
+    Finite(std::vec::IntoIter<(NixString, Value)>),
+    Im(imbl::ordmap::ConsumingIter<(NixString, Value)>),
+}
+
+/// Wrapper type which hides the internal implementation details from
+/// users.
+#[repr(transparent)]
+pub struct OwnedAttrsIterator(IntoIterRepr);
+
+impl Iterator for OwnedAttrsIterator {
+    type Item = (NixString, Value);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match &mut self.0 {
+            IntoIterRepr::Empty => None,
+            IntoIterRepr::Finite(inner) => inner.next(),
+            IntoIterRepr::Im(inner) => inner.next(),
+        }
+    }
+}
+
+impl ExactSizeIterator for OwnedAttrsIterator {
+    fn len(&self) -> usize {
+        match &self.0 {
+            IntoIterRepr::Empty => 0,
+            IntoIterRepr::Finite(inner) => inner.len(),
+            IntoIterRepr::Im(inner) => inner.len(),
+        }
+    }
+}
+
+impl DoubleEndedIterator for OwnedAttrsIterator {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        match &mut self.0 {
+            IntoIterRepr::Empty => None,
+            IntoIterRepr::Finite(inner) => inner.next_back(),
+            IntoIterRepr::Im(inner) => inner.next_back(),
+        }
     }
 }
diff --git a/tvix/eval/src/value/attrs/tests.rs b/tvix/eval/src/value/attrs/tests.rs
new file mode 100644
index 0000000000..534b78a00d
--- /dev/null
+++ b/tvix/eval/src/value/attrs/tests.rs
@@ -0,0 +1,106 @@
+use bstr::B;
+
+use super::*;
+
+#[test]
+fn test_empty_attrs() {
+    let attrs = NixAttrs::construct(0, vec![])
+        .expect("empty attr construction should succeed")
+        .unwrap();
+
+    assert!(
+        matches!(attrs, NixAttrs(AttrsRep::Empty)),
+        "empty attribute set should use optimised representation"
+    );
+}
+
+#[test]
+fn test_simple_attrs() {
+    let attrs = NixAttrs::construct(1, vec![Value::from("key"), Value::from("value")])
+        .expect("simple attr construction should succeed")
+        .unwrap();
+
+    assert!(
+        matches!(attrs, NixAttrs(AttrsRep::Im(_))),
+        "simple attribute set should use map representation",
+    )
+}
+
+#[test]
+fn test_kv_attrs() {
+    let name_val = Value::from("name");
+    let value_val = Value::from("value");
+    let meaning_val = Value::from("meaning");
+    let forty_two_val = Value::Integer(42);
+
+    let kv_attrs = NixAttrs::construct(
+        2,
+        vec![
+            value_val,
+            forty_two_val.clone(),
+            name_val,
+            meaning_val.clone(),
+        ],
+    )
+    .expect("constructing K/V pair attrs should succeed")
+    .unwrap();
+
+    match kv_attrs {
+        NixAttrs(AttrsRep::KV { name, value })
+            if name.to_str().unwrap() == meaning_val.to_str().unwrap()
+                || value.to_str().unwrap() == forty_two_val.to_str().unwrap() => {}
+
+        _ => panic!(
+            "K/V attribute set should use optimised representation, but got {:?}",
+            kv_attrs
+        ),
+    }
+}
+
+#[test]
+fn test_empty_attrs_iter() {
+    let attrs = NixAttrs::construct(0, vec![]).unwrap().unwrap();
+    assert!(attrs.iter().next().is_none());
+}
+
+#[test]
+fn test_kv_attrs_iter() {
+    let name_val = Value::from("name");
+    let value_val = Value::from("value");
+    let meaning_val = Value::from("meaning");
+    let forty_two_val = Value::Integer(42);
+
+    let kv_attrs = NixAttrs::construct(
+        2,
+        vec![
+            value_val,
+            forty_two_val.clone(),
+            name_val,
+            meaning_val.clone(),
+        ],
+    )
+    .expect("constructing K/V pair attrs should succeed")
+    .unwrap();
+
+    let mut iter = kv_attrs.iter().collect::<Vec<_>>().into_iter();
+    let (k, v) = iter.next().unwrap();
+    assert!(k == *NAME_REF);
+    assert!(v.to_str().unwrap() == meaning_val.to_str().unwrap());
+    let (k, v) = iter.next().unwrap();
+    assert!(k == *VALUE_REF);
+    assert!(v.as_int().unwrap() == forty_two_val.as_int().unwrap());
+    assert!(iter.next().is_none());
+}
+
+#[test]
+fn test_map_attrs_iter() {
+    let attrs = NixAttrs::construct(1, vec![Value::from("key"), Value::from("value")])
+        .expect("simple attr construction should succeed")
+        .unwrap();
+
+    let mut iter = attrs.iter().collect::<Vec<_>>().into_iter();
+    let (k, v) = iter.next().unwrap();
+    assert!(k == &NixString::from("key"));
+    assert_eq!(v.to_str().unwrap(), B("value"));
+    assert!(iter.next().is_none());
+}
diff --git a/tvix/eval/src/value/builtin.rs b/tvix/eval/src/value/builtin.rs
new file mode 100644
index 0000000000..346f06cb77
--- /dev/null
+++ b/tvix/eval/src/value/builtin.rs
@@ -0,0 +1,137 @@
+//! This module implements the runtime representation of a Nix
+//! builtin.
+//!
+//! Builtins are directly backed by Rust code operating on Nix values.
+
+use crate::vm::generators::Generator;
+
+use super::Value;
+
+use std::{
+    fmt::{Debug, Display},
+    rc::Rc,
+};
+
+/// Trait for closure types of builtins.
+///
+/// Builtins are expected to yield a generator which can be run by the VM to
+/// produce the final value.
+///
+/// Implementors should use the builtins-macros to create these functions
+/// instead of handling the argument-passing logic manually.
+pub trait BuiltinGen: Fn(Vec<Value>) -> Generator {}
+impl<F: Fn(Vec<Value>) -> Generator> BuiltinGen for F {}
+
+#[derive(Clone)]
+pub struct BuiltinRepr {
+    name: &'static str,
+    /// Optional documentation for the builtin.
+    documentation: Option<&'static str>,
+    arg_count: usize,
+
+    func: Rc<dyn BuiltinGen>,
+
+    /// Partially applied function arguments.
+    partials: Vec<Value>,
+}
+
+pub enum BuiltinResult {
+    /// Builtin was not ready to be called (arguments missing) and remains
+    /// partially applied.
+    Partial(Builtin),
+
+    /// Builtin was called and constructed a generator that the VM must run.
+    Called(&'static str, Generator),
+}
+
+/// Represents a single built-in function which directly executes Rust
+/// code that operates on a Nix value.
+///
+/// Builtins are the only functions in Nix that have varying arities
+/// (for example, `hasAttr` has an arity of 2, but `isAttrs` an arity
+/// of 1). To facilitate this generically, builtins expect to be
+/// called with a vector of Nix values corresponding to their
+/// arguments in order.
+///
+/// Partially applied builtins act similar to closures in that they
+/// "capture" the partially applied arguments, and are treated
+/// specially when printing their representation etc.
+#[derive(Clone)]
+pub struct Builtin(Box<BuiltinRepr>);
+
+impl From<BuiltinRepr> for Builtin {
+    fn from(value: BuiltinRepr) -> Self {
+        Builtin(Box::new(value))
+    }
+}
+
+impl Builtin {
+    pub fn new<F: BuiltinGen + 'static>(
+        name: &'static str,
+        documentation: Option<&'static str>,
+        arg_count: usize,
+        func: F,
+    ) -> Self {
+        BuiltinRepr {
+            name,
+            documentation,
+            arg_count,
+            func: Rc::new(func),
+            partials: vec![],
+        }
+        .into()
+    }
+
+    pub fn name(&self) -> &'static str {
+        self.0.name
+    }
+
+    pub fn documentation(&self) -> Option<&'static str> {
+        self.0.documentation
+    }
+
+    /// Apply an additional argument to the builtin.
+    /// After this, [`Builtin::call`] *must* be called, otherwise it may leave
+    /// the builtin in an incorrect state.
+    pub fn apply_arg(&mut self, arg: Value) {
+        self.0.partials.push(arg);
+
+        debug_assert!(
+            self.0.partials.len() <= self.0.arg_count,
+            "Tvix bug: pushed too many arguments to builtin"
+        );
+    }
+
+    /// Attempt to call a builtin, which will produce a generator if it is fully
+    /// applied or return the builtin if it is partially applied.
+    pub fn call(self) -> BuiltinResult {
+        if self.0.partials.len() == self.0.arg_count {
+            BuiltinResult::Called(self.0.name, (self.0.func)(self.0.partials))
+        } else {
+            BuiltinResult::Partial(self)
+        }
+    }
+}
+
+impl Debug for Builtin {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "builtin[{}]", self.0.name)
+    }
+}
+
+impl Display for Builtin {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        if !self.0.partials.is_empty() {
+            f.write_str("<PRIMOP-APP>")
+        } else {
+            f.write_str("<PRIMOP>")
+        }
+    }
+}
+
+/// Builtins are uniquely identified by their name
+impl PartialEq for Builtin {
+    fn eq(&self, other: &Self) -> bool {
+        self.0.name == other.0.name
+    }
+}
diff --git a/tvix/eval/src/value/function.rs b/tvix/eval/src/value/function.rs
new file mode 100644
index 0000000000..7592e3d641
--- /dev/null
+++ b/tvix/eval/src/value/function.rs
@@ -0,0 +1,112 @@
+//! This module implements the runtime representation of functions.
+use std::{collections::BTreeMap, hash::Hash, rc::Rc};
+
+use codemap::Span;
+use smol_str::SmolStr;
+
+use crate::{chunk::Chunk, upvalues::Upvalues};
+
+use super::NixString;
+
+#[derive(Clone, Debug, PartialEq)]
+pub(crate) struct Formals {
+    /// Map from argument name, to whether that argument is required
+    pub(crate) arguments: BTreeMap<NixString, bool>,
+
+    /// Do the formals of this function accept extra arguments
+    pub(crate) ellipsis: bool,
+
+    /// The span of the formals themselves, to use to emit errors
+    pub(crate) span: Span,
+
+    /// Optionally tracks a name for all function arguments (args@ style).
+    /// Used by toXML.
+    pub(crate) name: Option<String>,
+}
+
+impl Formals {
+    /// Returns true if the given arg is a valid argument to these formals.
+    ///
+    /// This is true if it is either listed in the list of arguments, or the formals have an
+    /// ellipsis
+    pub(crate) fn contains<Q>(&self, arg: &Q) -> bool
+    where
+        Q: ?Sized + Hash + Ord + Eq,
+        NixString: std::borrow::Borrow<Q>,
+    {
+        self.ellipsis || self.arguments.contains_key(arg)
+    }
+}
+
+/// The opcodes for a thunk or closure, plus the number of
+/// non-executable opcodes which are allowed after an OpThunkClosure or
+/// OpThunkSuspended referencing it.  At runtime `Lambda` is usually wrapped
+/// in `Rc` to avoid copying the `Chunk` it holds (which can be
+/// quite large).
+///
+/// In order to correctly reproduce cppnix's "pointer equality"
+/// semantics it is important that we never clone a Lambda --
+/// use `Rc<Lambda>::clone()` instead.  This struct deliberately
+/// does not `derive(Clone)` in order to prevent this from being
+/// done accidentally.
+///
+#[derive(/* do not add Clone here */ Debug, Default)]
+pub struct Lambda {
+    pub(crate) chunk: Chunk,
+
+    /// Name of the function (equivalent to the name of the
+    /// identifier (e.g. a value in a let-expression or an attribute
+    /// set entry) it is located in).
+    pub(crate) name: Option<SmolStr>,
+
+    /// Number of upvalues which the code in this Lambda closes
+    /// over, and which need to be initialised at
+    /// runtime.  Information about the variables is emitted using
+    /// data-carrying opcodes (see [`crate::opcode::OpCode::DataStackIdx`]).
+    pub(crate) upvalue_count: usize,
+    pub(crate) formals: Option<Formals>,
+}
+
+impl Lambda {
+    pub fn chunk(&mut self) -> &mut Chunk {
+        &mut self.chunk
+    }
+}
+
+///
+/// In order to correctly reproduce cppnix's "pointer equality"
+/// semantics it is important that we never clone a Lambda --
+/// use `Rc<Lambda>::clone()` instead.  This struct deliberately
+/// does not `derive(Clone)` in order to prevent this from being
+/// done accidentally.
+///
+#[derive(/* do not add Clone here */ Debug)]
+pub struct Closure {
+    pub lambda: Rc<Lambda>,
+    pub upvalues: Rc<Upvalues>,
+}
+
+impl Closure {
+    pub fn new(lambda: Rc<Lambda>) -> Self {
+        Self::new_with_upvalues(
+            Rc::new(Upvalues::with_capacity(lambda.upvalue_count)),
+            lambda,
+        )
+    }
+
+    pub fn new_with_upvalues(upvalues: Rc<Upvalues>, lambda: Rc<Lambda>) -> Self {
+        Closure { upvalues, lambda }
+    }
+
+    pub fn chunk(&self) -> &Chunk {
+        &self.lambda.chunk
+    }
+
+    pub fn lambda(&self) -> Rc<Lambda> {
+        self.lambda.clone()
+    }
+
+    pub fn upvalues(&self) -> Rc<Upvalues> {
+        self.upvalues.clone()
+    }
+}
diff --git a/tvix/eval/src/value/json.rs b/tvix/eval/src/value/json.rs
new file mode 100644
index 0000000000..c48e9c1f4e
--- /dev/null
+++ b/tvix/eval/src/value/json.rs
@@ -0,0 +1,154 @@
+/// Implementation of Value serialisation *to* JSON.
+///
+/// This can not be implemented through standard serde-derive methods,
+/// as there is internal Nix logic that must happen within the
+/// serialisation methods.
+use super::{CoercionKind, Value};
+use crate::errors::{CatchableErrorKind, ErrorKind};
+use crate::generators::{self, GenCo};
+use crate::NixContext;
+
+use bstr::ByteSlice;
+use serde_json::value::to_value;
+use serde_json::Value as Json; // name clash with *our* `Value`
+use serde_json::{Map, Number};
+
+impl Value {
+    /// Transforms the structure into a JSON
+    /// and accumulate all encountered context in the second's element
+    /// of the return type.
+    pub async fn into_contextful_json(
+        self,
+        co: &GenCo,
+    ) -> Result<Result<(Json, NixContext), CatchableErrorKind>, ErrorKind> {
+        let self_forced = generators::request_force(co, self).await;
+        let mut context = NixContext::new();
+
+        let value = match self_forced {
+            Value::Null => Json::Null,
+            Value::Bool(b) => Json::Bool(b),
+            Value::Integer(i) => Json::Number(Number::from(i)),
+            Value::Float(f) => to_value(f)?,
+            Value::String(s) => {
+                context.mimic(&s);
+
+                Json::String(s.to_str()?.to_owned())
+            }
+
+            Value::Path(p) => {
+                let imported = generators::request_path_import(co, *p).await;
+                let path = imported.to_string_lossy().to_string();
+                context = context.append(crate::NixContextElement::Plain(path.clone()));
+                Json::String(path)
+            }
+
+            Value::List(l) => {
+                let mut out = vec![];
+
+                for val in l.into_iter() {
+                    match generators::request_to_json(co, val).await {
+                        Ok((v, mut ctx)) => {
+                            context = context.join(&mut ctx);
+                            out.push(v)
+                        }
+                        Err(cek) => return Ok(Err(cek)),
+                    }
+                }
+
+                Json::Array(out)
+            }
+
+            Value::Attrs(attrs) => {
+                // Attribute sets with a callable `__toString` attribute
+                // serialise to the string-coerced version of the result of
+                // calling that.
+                if attrs.select("__toString").is_some() {
+                    let span = generators::request_span(co).await;
+                    match Value::Attrs(attrs)
+                        .coerce_to_string_(
+                            co,
+                            CoercionKind {
+                                strong: false,
+                                import_paths: false,
+                            },
+                            span,
+                        )
+                        .await?
+                    {
+                        Value::Catchable(cek) => return Ok(Err(*cek)),
+                        Value::String(s) => {
+                            // We need a fresh context here because `__toString` will discard
+                            // everything.
+                            let mut fresh = NixContext::new();
+                            fresh.mimic(&s);
+
+                            return Ok(Ok((Json::String(s.to_str()?.to_owned()), fresh)));
+                        }
+                        _ => panic!("Value::coerce_to_string_() returned a non-string!"),
+                    }
+                }
+
+                // Attribute sets with an `outPath` attribute
+                // serialise to a JSON serialisation of that inner
+                // value (regardless of what it is!).
+                if let Some(out_path) = attrs.select("outPath") {
+                    return Ok(generators::request_to_json(co, out_path.clone()).await);
+                }
+
+                let mut out = Map::with_capacity(attrs.len());
+                for (name, value) in attrs.into_iter_sorted() {
+                    out.insert(
+                        name.to_str()?.to_owned(),
+                        match generators::request_to_json(co, value).await {
+                            Ok((v, mut ctx)) => {
+                                context = context.join(&mut ctx);
+                                v
+                            }
+                            Err(cek) => return Ok(Err(cek)),
+                        },
+                    );
+                }
+
+                Json::Object(out)
+            }
+
+            Value::Catchable(c) => return Ok(Err(*c)),
+
+            val @ Value::Closure(_)
+            | val @ Value::Thunk(_)
+            | val @ Value::Builtin(_)
+            | val @ Value::AttrNotFound
+            | val @ Value::Blueprint(_)
+            | val @ Value::DeferredUpvalue(_)
+            | val @ Value::UnresolvedPath(_)
+            | val @ Value::Json(..)
+            | val @ Value::FinaliseRequest(_) => {
+                return Err(ErrorKind::NotSerialisableToJson(val.type_of()))
+            }
+        };
+
+        Ok(Ok((value, context)))
+    }
+
+    /// Generator version of the above, which wraps responses in
+    /// [`Value::Json`].
+    pub(crate) async fn into_contextful_json_generator(
+        self,
+        co: GenCo,
+    ) -> Result<Value, ErrorKind> {
+        match self.into_contextful_json(&co).await? {
+            Err(cek) => Ok(Value::from(cek)),
+            Ok((json, ctx)) => Ok(Value::Json(Box::new((json, ctx)))),
+        }
+    }
+
+    /// Transforms the structure into a JSON
+    /// All the accumulated context is ignored, use [`into_contextful_json`]
+    /// to obtain the resulting context of the JSON object.
+    pub async fn into_json(
+        self,
+        co: &GenCo,
+    ) -> Result<Result<Json, CatchableErrorKind>, ErrorKind> {
+        Ok(self.into_contextful_json(co).await?.map(|(json, _)| json))
+    }
+}
diff --git a/tvix/eval/src/value/list.rs b/tvix/eval/src/value/list.rs
index d5f7c8b2ba..2b8b3de28d 100644
--- a/tvix/eval/src/value/list.rs
+++ b/tvix/eval/src/value/list.rs
@@ -1,20 +1,102 @@
-/// This module implements Nix lists.
-use std::fmt::Display;
+//! This module implements Nix lists.
+use std::ops::Index;
+use std::rc::Rc;
 
+use imbl::{vector, Vector};
+
+use serde::Deserialize;
+
+use super::thunk::ThunkSet;
+use super::TotalDisplay;
 use super::Value;
 
-#[derive(Clone, Debug, PartialEq)]
-pub struct NixList(pub Vec<Value>);
+#[repr(transparent)]
+#[derive(Clone, Debug, Deserialize)]
+pub struct NixList(Rc<Vector<Value>>);
 
-impl Display for NixList {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl TotalDisplay for NixList {
+    fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result {
         f.write_str("[ ")?;
 
-        for v in &self.0 {
-            v.fmt(f)?;
+        for v in self {
+            v.total_fmt(f, set)?;
             f.write_str(" ")?;
         }
 
         f.write_str("]")
     }
 }
+
+impl From<Vector<Value>> for NixList {
+    fn from(vs: Vector<Value>) -> Self {
+        Self(Rc::new(vs))
+    }
+}
+
+impl NixList {
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    pub fn get(&self, i: usize) -> Option<&Value> {
+        self.0.get(i)
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    pub fn construct(count: usize, stack_slice: Vec<Value>) -> Self {
+        debug_assert!(
+            count == stack_slice.len(),
+            "NixList::construct called with count == {}, but slice.len() == {}",
+            count,
+            stack_slice.len(),
+        );
+
+        NixList(Rc::new(Vector::from_iter(stack_slice)))
+    }
+
+    pub fn iter(&self) -> vector::Iter<Value> {
+        self.0.iter()
+    }
+
+    pub fn ptr_eq(&self, other: &Self) -> bool {
+        Rc::ptr_eq(&self.0, &other.0)
+    }
+
+    pub fn into_inner(self) -> Vector<Value> {
+        Rc::try_unwrap(self.0).unwrap_or_else(|rc| (*rc).clone())
+    }
+
+    #[deprecated(note = "callers should avoid constructing from Vec")]
+    pub fn from_vec(vs: Vec<Value>) -> Self {
+        Self(Rc::new(Vector::from_iter(vs)))
+    }
+}
+
+impl IntoIterator for NixList {
+    type Item = Value;
+    type IntoIter = imbl::vector::ConsumingIter<Value>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.into_inner().into_iter()
+    }
+}
+
+impl<'a> IntoIterator for &'a NixList {
+    type Item = &'a Value;
+    type IntoIter = imbl::vector::Iter<'a, Value>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.iter()
+    }
+}
+
+impl Index<usize> for NixList {
+    type Output = Value;
+
+    fn index(&self, index: usize) -> &Self::Output {
+        &self.0[index]
+    }
+}
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index 55d44048b6..c171c9a04e 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -1,38 +1,663 @@
 //! This module implements the backing representation of runtime
 //! values in the Nix language.
+use std::cmp::Ordering;
 use std::fmt::Display;
+use std::num::{NonZeroI32, NonZeroUsize};
+use std::path::PathBuf;
 use std::rc::Rc;
 
+use bstr::{BString, ByteVec};
+use lexical_core::format::CXX_LITERAL;
+use serde::Deserialize;
+
+#[cfg(feature = "arbitrary")]
+mod arbitrary;
 mod attrs;
+mod builtin;
+mod function;
+mod json;
 mod list;
+mod path;
 mod string;
+mod thunk;
 
-use crate::errors::{Error, EvalResult};
+use crate::errors::{CatchableErrorKind, ErrorKind};
+use crate::opcode::StackIdx;
+use crate::spans::LightSpan;
+use crate::vm::generators::{self, GenCo};
+use crate::AddContext;
 pub use attrs::NixAttrs;
+pub use builtin::{Builtin, BuiltinResult};
+pub(crate) use function::Formals;
+pub use function::{Closure, Lambda};
 pub use list::NixList;
-pub use string::NixString;
+pub use path::canon_path;
+pub use string::{NixContext, NixContextElement, NixString};
+pub use thunk::Thunk;
+
+pub use self::thunk::ThunkSet;
+
+use lazy_static::lazy_static;
 
-#[derive(Clone, Debug)]
+#[warn(variant_size_differences)]
+#[derive(Clone, Debug, Deserialize)]
+#[serde(untagged)]
 pub enum Value {
     Null,
     Bool(bool),
     Integer(i64),
     Float(f64),
     String(NixString),
-    Attrs(Rc<NixAttrs>),
+
+    #[serde(skip)]
+    Path(Box<PathBuf>),
+    Attrs(Box<NixAttrs>),
     List(NixList),
 
+    #[serde(skip)]
+    Closure(Rc<Closure>), // must use Rc<Closure> here in order to get proper pointer equality
+
+    #[serde(skip)]
+    Builtin(Builtin),
+
     // Internal values that, while they technically exist at runtime,
     // are never returned to or created directly by users.
-    AttrPath(Vec<NixString>),
+    #[serde(skip_deserializing)]
+    Thunk(Thunk),
+
+    // See [`compiler::compile_select_or()`] for explanation
+    #[serde(skip)]
+    AttrNotFound,
+
+    // this can only occur in Chunk::Constants and nowhere else
+    #[serde(skip)]
+    Blueprint(Rc<Lambda>),
+
+    #[serde(skip)]
+    DeferredUpvalue(StackIdx),
+    #[serde(skip)]
+    UnresolvedPath(Box<PathBuf>),
+    #[serde(skip)]
+    Json(Box<(serde_json::Value, NixContext)>),
+
+    #[serde(skip)]
+    FinaliseRequest(bool),
+
+    #[serde(skip)]
+    // TODO(tazjin): why is this in a Box?
+    Catchable(Box<CatchableErrorKind>),
+}
+
+impl From<CatchableErrorKind> for Value {
+    #[inline]
+    fn from(c: CatchableErrorKind) -> Value {
+        Value::Catchable(Box::new(c))
+    }
+}
+
+impl<V> From<Result<V, CatchableErrorKind>> for Value
+where
+    Value: From<V>,
+{
+    #[inline]
+    fn from(v: Result<V, CatchableErrorKind>) -> Value {
+        match v {
+            Ok(v) => v.into(),
+            Err(e) => Value::Catchable(Box::new(e)),
+        }
+    }
+}
+
+lazy_static! {
+    static ref WRITE_FLOAT_OPTIONS: lexical_core::WriteFloatOptions =
+        lexical_core::WriteFloatOptionsBuilder::new()
+            .trim_floats(true)
+            .round_mode(lexical_core::write_float_options::RoundMode::Round)
+            .positive_exponent_break(Some(NonZeroI32::new(5).unwrap()))
+            .max_significant_digits(Some(NonZeroUsize::new(6).unwrap()))
+            .build()
+            .unwrap();
+}
+
+// Helper macros to generate the to_*/as_* macros while accounting for
+// thunks.
+
+/// Generate an `as_*` method returning a reference to the expected
+/// type, or a type error. This only works for types that implement
+/// `Copy`, as returning a reference to an inner thunk value is not
+/// possible.
+
+/// Generate an `as_*/to_*` accessor method that returns either the
+/// expected type, or a type error.
+macro_rules! gen_cast {
+    ( $name:ident, $type:ty, $expected:expr, $variant:pat, $result:expr ) => {
+        pub fn $name(&self) -> Result<$type, ErrorKind> {
+            match self {
+                $variant => Ok($result),
+                Value::Thunk(thunk) => Self::$name(&thunk.value()),
+                other => Err(type_error($expected, &other)),
+            }
+        }
+    };
 }
 
+/// Generate an `as_*_mut/to_*_mut` accessor method that returns either the
+/// expected type, or a type error.
+macro_rules! gen_cast_mut {
+    ( $name:ident, $type:ty, $expected:expr, $variant:ident) => {
+        pub fn $name(&mut self) -> Result<&mut $type, ErrorKind> {
+            match self {
+                Value::$variant(x) => Ok(x),
+                other => Err(type_error($expected, &other)),
+            }
+        }
+    };
+}
+
+/// Generate an `is_*` type-checking method.
+macro_rules! gen_is {
+    ( $name:ident, $variant:pat ) => {
+        pub fn $name(&self) -> bool {
+            match self {
+                $variant => true,
+                Value::Thunk(thunk) => Self::$name(&thunk.value()),
+                _ => false,
+            }
+        }
+    };
+}
+
+/// Describes what input types are allowed when coercing a `Value` to a string
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub struct CoercionKind {
+    /// If false only coerce already "stringly" types like strings and paths, but
+    /// also coerce sets that have a `__toString` attribute. In Tvix, this is
+    /// usually called a weak coercion. Equivalent to passing `false` as the
+    /// `coerceMore` argument of `EvalState::coerceToString` in C++ Nix.
+    ///
+    /// If true coerce all value types included by a weak coercion, but also
+    /// coerce `null`, booleans, integers, floats and lists of coercible types.
+    /// Consequently, we call this a strong coercion. Equivalent to passing
+    /// `true` as `coerceMore` in C++ Nix.
+    pub strong: bool,
+
+    /// If `import_paths` is `true`, paths are imported into the store and their
+    /// store path is the result of the coercion (equivalent to the
+    /// `copyToStore` argument of `EvalState::coerceToString` in C++ Nix).
+    pub import_paths: bool,
+}
+
+impl<T> From<T> for Value
+where
+    T: Into<NixString>,
+{
+    fn from(t: T) -> Self {
+        Self::String(t.into())
+    }
+}
+
+/// Constructors
 impl Value {
-    pub fn is_number(&self) -> bool {
-        match self {
-            Value::Integer(_) => true,
-            Value::Float(_) => true,
-            _ => false,
+    /// Construct a [`Value::Attrs`] from a [`NixAttrs`].
+    pub fn attrs(attrs: NixAttrs) -> Self {
+        Self::Attrs(Box::new(attrs))
+    }
+}
+
+/// Controls what kind of by-pointer equality comparison is allowed.
+///
+/// See `//tvix/docs/value-pointer-equality.md` for details.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub enum PointerEquality {
+    /// Pointer equality not allowed at all.
+    ForbidAll,
+
+    /// Pointer equality comparisons only allowed for nested values.
+    AllowNested,
+
+    /// Pointer equality comparisons are allowed in all contexts.
+    AllowAll,
+}
+
+impl Value {
+    /// Deeply forces a value, traversing e.g. lists and attribute sets and forcing
+    /// their contents, too.
+    ///
+    /// This is a generator function.
+    pub(super) async fn deep_force(self, co: GenCo, span: LightSpan) -> Result<Value, ErrorKind> {
+        if let Some(v) = Self::deep_force_(self.clone(), co, span).await? {
+            Ok(v)
+        } else {
+            Ok(self)
+        }
+    }
+
+    /// Returns Some(v) or None to indicate the returned value is myself
+    async fn deep_force_(
+        myself: Value,
+        co: GenCo,
+        span: LightSpan,
+    ) -> Result<Option<Value>, ErrorKind> {
+        // This is a stack of values which still remain to be forced.
+        let mut vals = vec![myself];
+
+        let mut thunk_set: ThunkSet = Default::default();
+
+        loop {
+            let v = if let Some(v) = vals.pop() {
+                v
+            } else {
+                return Ok(None);
+            };
+
+            // Get rid of any top-level thunks, and bail out of self-recursive
+            // thunks.
+            let value = if let Value::Thunk(t) = &v {
+                if !thunk_set.insert(t) {
+                    continue;
+                }
+                Thunk::force_(t.clone(), &co, span.clone()).await?
+            } else {
+                v
+            };
+
+            match value {
+                // Short-circuit on already evaluated values, or fail on internal values.
+                Value::Null
+                | Value::Bool(_)
+                | Value::Integer(_)
+                | Value::Float(_)
+                | Value::String(_)
+                | Value::Path(_)
+                | Value::Closure(_)
+                | Value::Builtin(_) => continue,
+
+                Value::List(list) => {
+                    for val in list.into_iter().rev() {
+                        vals.push(val);
+                    }
+                    continue;
+                }
+
+                Value::Attrs(attrs) => {
+                    for (_, val) in attrs.into_iter().rev() {
+                        vals.push(val);
+                    }
+                    continue;
+                }
+
+                Value::Thunk(_) => panic!("Tvix bug: force_value() returned a thunk"),
+
+                Value::Catchable(_) => return Ok(Some(value)),
+
+                Value::AttrNotFound
+                | Value::Blueprint(_)
+                | Value::DeferredUpvalue(_)
+                | Value::UnresolvedPath(_)
+                | Value::Json(..)
+                | Value::FinaliseRequest(_) => panic!(
+                    "Tvix bug: internal value left on stack: {}",
+                    value.type_of()
+                ),
+            }
+        }
+    }
+
+    pub async fn coerce_to_string(
+        self,
+        co: GenCo,
+        kind: CoercionKind,
+        span: LightSpan,
+    ) -> Result<Value, ErrorKind> {
+        self.coerce_to_string_(&co, kind, span).await
+    }
+
+    /// Coerce a `Value` to a string. See `CoercionKind` for a rundown of what
+    /// input types are accepted under what circumstances.
+    pub async fn coerce_to_string_(
+        self,
+        co: &GenCo,
+        kind: CoercionKind,
+        span: LightSpan,
+    ) -> Result<Value, ErrorKind> {
+        let mut result = BString::default();
+        let mut vals = vec![self];
+        // Track if we are coercing the first value of a list to correctly emit
+        // separating white spaces.
+        let mut is_list_head = None;
+        // FIXME(raitobezarius): as per https://b.tvl.fyi/issues/364
+        // we might be interested into more powerful context-related coercion kinds.
+        let mut context: NixContext = NixContext::new();
+
+        loop {
+            let value = if let Some(v) = vals.pop() {
+                v.force(co, span.clone()).await?
+            } else {
+                return Ok(Value::String(NixString::new_context_from(context, result)));
+            };
+            let coerced: Result<BString, _> = match (value, kind) {
+                // coercions that are always done
+                (Value::String(mut s), _) => {
+                    if let Some(ctx) = s.context_mut() {
+                        context = context.join(ctx);
+                    }
+                    Ok((*s).into())
+                }
+
+                // TODO(sterni): Think about proper encoding handling here. This needs
+                // general consideration anyways, since one current discrepancy between
+                // C++ Nix and Tvix is that the former's strings are arbitrary byte
+                // sequences without NUL bytes, whereas Tvix only allows valid
+                // Unicode. See also b/189.
+                (
+                    Value::Path(p),
+                    CoercionKind {
+                        import_paths: true, ..
+                    },
+                ) => {
+                    let imported = generators::request_path_import(co, *p).await;
+                    // When we import a path from the evaluator, we must attach
+                    // its original path as its context.
+                    context = context.append(NixContextElement::Plain(
+                        imported.to_string_lossy().to_string(),
+                    ));
+                    Ok(imported.into_os_string().into_encoded_bytes().into())
+                }
+                (
+                    Value::Path(p),
+                    CoercionKind {
+                        import_paths: false,
+                        ..
+                    },
+                ) => Ok(p.into_os_string().into_encoded_bytes().into()),
+
+                // Attribute sets can be converted to strings if they either have an
+                // `__toString` attribute which holds a function that receives the
+                // set itself or an `outPath` attribute which should be a string.
+                // `__toString` is preferred.
+                (Value::Attrs(attrs), kind) => {
+                    if let Some(to_string) = attrs.select("__toString") {
+                        let callable = to_string.clone().force(co, span.clone()).await?;
+
+                        // Leave the attribute set on the stack as an argument
+                        // to the function call.
+                        generators::request_stack_push(co, Value::Attrs(attrs.clone())).await;
+
+                        // Call the callable ...
+                        let result = generators::request_call(co, callable).await;
+
+                        // Recurse on the result, as attribute set coercion
+                        // actually works recursively, e.g. you can even return
+                        // /another/ set with a __toString attr.
+                        vals.push(result);
+                        continue;
+                    } else if let Some(out_path) = attrs.select("outPath") {
+                        vals.push(out_path.clone());
+                        continue;
+                    } else {
+                        return Err(ErrorKind::NotCoercibleToString { from: "set", kind });
+                    }
+                }
+
+                // strong coercions
+                (Value::Null, CoercionKind { strong: true, .. })
+                | (Value::Bool(false), CoercionKind { strong: true, .. }) => Ok("".into()),
+                (Value::Bool(true), CoercionKind { strong: true, .. }) => Ok("1".into()),
+
+                (Value::Integer(i), CoercionKind { strong: true, .. }) => Ok(format!("{i}").into()),
+                (Value::Float(f), CoercionKind { strong: true, .. }) => {
+                    // contrary to normal Display, coercing a float to a string will
+                    // result in unconditional 6 decimal places
+                    Ok(format!("{:.6}", f).into())
+                }
+
+                // Lists are coerced by coercing their elements and interspersing spaces
+                (Value::List(list), CoercionKind { strong: true, .. }) => {
+                    for elem in list.into_iter().rev() {
+                        vals.push(elem);
+                    }
+                    // In case we are coercing a list within a list we don't want
+                    // to touch this. Since the algorithm is nonrecursive, the
+                    // space would not have been created yet (due to continue).
+                    if is_list_head.is_none() {
+                        is_list_head = Some(true);
+                    }
+                    continue;
+                }
+
+                (Value::Thunk(_), _) => panic!("Tvix bug: force returned unforced thunk"),
+
+                val @ (Value::Closure(_), _)
+                | val @ (Value::Builtin(_), _)
+                | val @ (Value::Null, _)
+                | val @ (Value::Bool(_), _)
+                | val @ (Value::Integer(_), _)
+                | val @ (Value::Float(_), _)
+                | val @ (Value::List(_), _) => Err(ErrorKind::NotCoercibleToString {
+                    from: val.0.type_of(),
+                    kind,
+                }),
+
+                (c @ Value::Catchable(_), _) => return Ok(c),
+
+                (Value::AttrNotFound, _)
+                | (Value::Blueprint(_), _)
+                | (Value::DeferredUpvalue(_), _)
+                | (Value::UnresolvedPath(_), _)
+                | (Value::Json(..), _)
+                | (Value::FinaliseRequest(_), _) => {
+                    panic!("tvix bug: .coerce_to_string() called on internal value")
+                }
+            };
+
+            if let Some(head) = is_list_head {
+                if !head {
+                    result.push(b' ');
+                } else {
+                    is_list_head = Some(false);
+                }
+            }
+
+            result.push_str(&coerced?);
+        }
+    }
+
+    pub(crate) async fn nix_eq_owned_genco(
+        self,
+        other: Value,
+        co: GenCo,
+        ptr_eq: PointerEquality,
+        span: LightSpan,
+    ) -> Result<Value, ErrorKind> {
+        self.nix_eq(other, &co, ptr_eq, span).await
+    }
+
+    /// Compare two Nix values for equality, forcing nested parts of the structure
+    /// as needed.
+    ///
+    /// This comparison needs to be invoked for nested values (e.g. in lists and
+    /// attribute sets) as well, which is done by suspending and asking the VM to
+    /// perform the nested comparison.
+    ///
+    /// The `top_level` parameter controls whether this invocation is the top-level
+    /// comparison, or a nested value comparison. See
+    /// `//tvix/docs/value-pointer-equality.md`
+    pub(crate) async fn nix_eq(
+        self,
+        other: Value,
+        co: &GenCo,
+        ptr_eq: PointerEquality,
+        span: LightSpan,
+    ) -> Result<Value, ErrorKind> {
+        // this is a stack of ((v1,v2),peq) triples to be compared;
+        // after each triple is popped off of the stack, v1 is
+        // compared to v2 using peq-mode PointerEquality
+        let mut vals = vec![((self, other), ptr_eq)];
+
+        loop {
+            let ((a, b), ptr_eq) = if let Some(abp) = vals.pop() {
+                abp
+            } else {
+                // stack is empty, so comparison has succeeded
+                return Ok(Value::Bool(true));
+            };
+            let a = match a {
+                Value::Thunk(thunk) => {
+                    // If both values are thunks, and thunk comparisons are allowed by
+                    // pointer, do that and move on.
+                    if ptr_eq == PointerEquality::AllowAll {
+                        if let Value::Thunk(t1) = &b {
+                            if t1.ptr_eq(&thunk) {
+                                continue;
+                            }
+                        }
+                    };
+
+                    Thunk::force_(thunk, co, span.clone()).await?
+                }
+
+                _ => a,
+            };
+
+            let b = b.force(co, span.clone()).await?;
+
+            debug_assert!(!matches!(a, Value::Thunk(_)));
+            debug_assert!(!matches!(b, Value::Thunk(_)));
+
+            let result = match (a, b) {
+                // Trivial comparisons
+                (c @ Value::Catchable(_), _) => return Ok(c),
+                (_, c @ Value::Catchable(_)) => return Ok(c),
+                (Value::Null, Value::Null) => true,
+                (Value::Bool(b1), Value::Bool(b2)) => b1 == b2,
+                (Value::String(s1), Value::String(s2)) => s1 == s2,
+                (Value::Path(p1), Value::Path(p2)) => p1 == p2,
+
+                // Numerical comparisons (they work between float & int)
+                (Value::Integer(i1), Value::Integer(i2)) => i1 == i2,
+                (Value::Integer(i), Value::Float(f)) => i as f64 == f,
+                (Value::Float(f1), Value::Float(f2)) => f1 == f2,
+                (Value::Float(f), Value::Integer(i)) => i as f64 == f,
+
+                // List comparisons
+                (Value::List(l1), Value::List(l2)) => {
+                    if ptr_eq >= PointerEquality::AllowNested && l1.ptr_eq(&l2) {
+                        continue;
+                    }
+
+                    if l1.len() != l2.len() {
+                        return Ok(Value::Bool(false));
+                    }
+
+                    vals.extend(l1.into_iter().rev().zip(l2.into_iter().rev()).zip(
+                        std::iter::repeat(std::cmp::max(ptr_eq, PointerEquality::AllowNested)),
+                    ));
+                    continue;
+                }
+
+                (_, Value::List(_)) | (Value::List(_), _) => return Ok(Value::Bool(false)),
+
+                // Attribute set comparisons
+                (Value::Attrs(a1), Value::Attrs(a2)) => {
+                    if ptr_eq >= PointerEquality::AllowNested && a1.ptr_eq(&a2) {
+                        continue;
+                    }
+
+                    // Special-case for derivation comparisons: If both attribute sets
+                    // have `type = derivation`, compare them by `outPath`.
+                    #[allow(clippy::single_match)] // might need more match arms later
+                    match (a1.select("type"), a2.select("type")) {
+                        (Some(v1), Some(v2)) => {
+                            let s1 = v1.clone().force(co, span.clone()).await?;
+                            if s1.is_catchable() {
+                                return Ok(s1);
+                            }
+                            let s2 = v2.clone().force(co, span.clone()).await?;
+                            if s2.is_catchable() {
+                                return Ok(s2);
+                            }
+                            let s1 = s1.to_str();
+                            let s2 = s2.to_str();
+
+                            if let (Ok(s1), Ok(s2)) = (s1, s2) {
+                                if s1 == "derivation" && s2 == "derivation" {
+                                    // TODO(tazjin): are the outPaths really required,
+                                    // or should it fall through?
+                                    let out1 = a1
+                                        .select_required("outPath")
+                                        .context("comparing derivations")?
+                                        .clone();
+
+                                    let out2 = a2
+                                        .select_required("outPath")
+                                        .context("comparing derivations")?
+                                        .clone();
+
+                                    let out1 = out1.clone().force(co, span.clone()).await?;
+                                    let out2 = out2.clone().force(co, span.clone()).await?;
+
+                                    if out1.is_catchable() {
+                                        return Ok(out1);
+                                    }
+
+                                    if out2.is_catchable() {
+                                        return Ok(out2);
+                                    }
+
+                                    let result =
+                                        out1.to_contextful_str()? == out2.to_contextful_str()?;
+                                    if !result {
+                                        return Ok(Value::Bool(false));
+                                    } else {
+                                        continue;
+                                    }
+                                }
+                            }
+                        }
+                        _ => {}
+                    };
+
+                    if a1.len() != a2.len() {
+                        return Ok(Value::Bool(false));
+                    }
+
+                    // note that it is important to be careful here with the
+                    // order we push the keys and values in order to properly
+                    // compare attrsets containing `throw` elements.
+                    let iter1 = a1.into_iter_sorted().rev();
+                    let iter2 = a2.into_iter_sorted().rev();
+                    for ((k1, v1), (k2, v2)) in iter1.zip(iter2) {
+                        vals.push((
+                            (v1, v2),
+                            std::cmp::max(ptr_eq, PointerEquality::AllowNested),
+                        ));
+                        vals.push((
+                            (k1.into(), k2.into()),
+                            std::cmp::max(ptr_eq, PointerEquality::AllowNested),
+                        ));
+                    }
+                    continue;
+                }
+
+                (Value::Attrs(_), _) | (_, Value::Attrs(_)) => return Ok(Value::Bool(false)),
+
+                (Value::Closure(c1), Value::Closure(c2))
+                    if ptr_eq >= PointerEquality::AllowNested =>
+                {
+                    if Rc::ptr_eq(&c1, &c2) {
+                        continue;
+                    } else {
+                        return Ok(Value::Bool(false));
+                    }
+                }
+
+                // Everything else is either incomparable (e.g. internal types) or
+                // false.
+                _ => return Ok(Value::Bool(false)),
+            };
+            if !result {
+                return Ok(Value::Bool(false));
+            }
         }
     }
 
@@ -43,74 +668,406 @@ impl Value {
             Value::Integer(_) => "int",
             Value::Float(_) => "float",
             Value::String(_) => "string",
+            Value::Path(_) => "path",
             Value::Attrs(_) => "set",
             Value::List(_) => "list",
+            Value::Closure(_) | Value::Builtin(_) => "lambda",
 
-            // Internal types
-            Value::AttrPath(_) => "internal",
+            // Internal types. Note: These are only elaborated here
+            // because it makes debugging easier. If a user ever sees
+            // any of these strings, it's a bug.
+            Value::Thunk(_) => "internal[thunk]",
+            Value::AttrNotFound => "internal[attr_not_found]",
+            Value::Blueprint(_) => "internal[blueprint]",
+            Value::DeferredUpvalue(_) => "internal[deferred_upvalue]",
+            Value::UnresolvedPath(_) => "internal[unresolved_path]",
+            Value::Json(..) => "internal[json]",
+            Value::FinaliseRequest(_) => "internal[finaliser_sentinel]",
+            Value::Catchable(_) => "internal[catchable]",
         }
     }
 
-    pub fn as_bool(self) -> EvalResult<bool> {
+    gen_cast!(as_bool, bool, "bool", Value::Bool(b), *b);
+    gen_cast!(as_int, i64, "int", Value::Integer(x), *x);
+    gen_cast!(as_float, f64, "float", Value::Float(x), *x);
+
+    /// Cast the current value into a **context-less** string.
+    /// If you wanted to cast it into a potentially contextful string,
+    /// you have to explicitly use `to_contextful_str`.
+    /// Contextful strings are special, they should not be obtained
+    /// everytime you want a string.
+    pub fn to_str(&self) -> Result<NixString, ErrorKind> {
         match self {
-            Value::Bool(b) => Ok(b),
-            other => Err(Error::TypeError {
-                expected: "bool",
-                actual: other.type_of(),
-            }),
+            Value::String(s) if !s.has_context() => Ok((*s).clone()),
+            Value::Thunk(thunk) => Self::to_str(&thunk.value()),
+            other => Err(type_error("contextless strings", other)),
         }
     }
 
-    pub fn as_string(self) -> EvalResult<NixString> {
+    gen_cast!(
+        to_contextful_str,
+        NixString,
+        "contextful string",
+        Value::String(s),
+        (*s).clone()
+    );
+    gen_cast!(to_path, Box<PathBuf>, "path", Value::Path(p), p.clone());
+    gen_cast!(to_attrs, Box<NixAttrs>, "set", Value::Attrs(a), a.clone());
+    gen_cast!(to_list, NixList, "list", Value::List(l), l.clone());
+    gen_cast!(
+        as_closure,
+        Rc<Closure>,
+        "lambda",
+        Value::Closure(c),
+        c.clone()
+    );
+
+    gen_cast_mut!(as_list_mut, NixList, "list", List);
+
+    gen_is!(is_path, Value::Path(_));
+    gen_is!(is_number, Value::Integer(_) | Value::Float(_));
+    gen_is!(is_bool, Value::Bool(_));
+    gen_is!(is_attrs, Value::Attrs(_));
+    gen_is!(is_catchable, Value::Catchable(_));
+
+    /// Returns `true` if the value is a [`Thunk`].
+    ///
+    /// [`Thunk`]: Value::Thunk
+    pub fn is_thunk(&self) -> bool {
+        matches!(self, Self::Thunk(..))
+    }
+
+    /// Compare `self` against other using (fallible) Nix ordering semantics.
+    ///
+    /// The function is intended to be used from within other generator
+    /// functions or `gen!` blocks.
+    pub async fn nix_cmp_ordering(
+        self,
+        other: Self,
+        co: GenCo,
+        span: LightSpan,
+    ) -> Result<Result<Ordering, CatchableErrorKind>, ErrorKind> {
+        Self::nix_cmp_ordering_(self, other, co, span).await
+    }
+
+    async fn nix_cmp_ordering_(
+        myself: Self,
+        other: Self,
+        co: GenCo,
+        span: LightSpan,
+    ) -> Result<Result<Ordering, CatchableErrorKind>, ErrorKind> {
+        // this is a stack of ((v1,v2),peq) triples to be compared;
+        // after each triple is popped off of the stack, v1 is
+        // compared to v2 using peq-mode PointerEquality
+        let mut vals = vec![((myself, other), PointerEquality::ForbidAll)];
+
+        loop {
+            let ((mut a, mut b), ptr_eq) = if let Some(abp) = vals.pop() {
+                abp
+            } else {
+                // stack is empty, so they are equal
+                return Ok(Ok(Ordering::Equal));
+            };
+            if ptr_eq == PointerEquality::AllowAll {
+                if a.clone()
+                    .nix_eq(b.clone(), &co, PointerEquality::AllowAll, span.clone())
+                    .await?
+                    .as_bool()?
+                {
+                    continue;
+                }
+                a = a.force(&co, span.clone()).await?;
+                b = b.force(&co, span.clone()).await?;
+            }
+            let result = match (a, b) {
+                (Value::Catchable(c), _) => return Ok(Err(*c)),
+                (_, Value::Catchable(c)) => return Ok(Err(*c)),
+                // same types
+                (Value::Integer(i1), Value::Integer(i2)) => i1.cmp(&i2),
+                (Value::Float(f1), Value::Float(f2)) => f1.total_cmp(&f2),
+                (Value::String(s1), Value::String(s2)) => s1.cmp(&s2),
+                (Value::List(l1), Value::List(l2)) => {
+                    let max = l1.len().max(l2.len());
+                    for j in 0..max {
+                        let i = max - 1 - j;
+                        if i >= l2.len() {
+                            vals.push(((1.into(), 0.into()), PointerEquality::ForbidAll));
+                        } else if i >= l1.len() {
+                            vals.push(((0.into(), 1.into()), PointerEquality::ForbidAll));
+                        } else {
+                            vals.push(((l1[i].clone(), l2[i].clone()), PointerEquality::AllowAll));
+                        }
+                    }
+                    continue;
+                }
+
+                // different types
+                (Value::Integer(i1), Value::Float(f2)) => (i1 as f64).total_cmp(&f2),
+                (Value::Float(f1), Value::Integer(i2)) => f1.total_cmp(&(i2 as f64)),
+
+                // unsupported types
+                (lhs, rhs) => {
+                    return Err(ErrorKind::Incomparable {
+                        lhs: lhs.type_of(),
+                        rhs: rhs.type_of(),
+                    })
+                }
+            };
+            if result != Ordering::Equal {
+                return Ok(Ok(result));
+            }
+        }
+    }
+
+    // TODO(amjoseph): de-asyncify this (when called directly by the VM)
+    pub async fn force(self, co: &GenCo, span: LightSpan) -> Result<Value, ErrorKind> {
+        if let Value::Thunk(thunk) = self {
+            // TODO(amjoseph): use #[tailcall::mutual]
+            return Thunk::force_(thunk, co, span).await;
+        }
+        Ok(self)
+    }
+
+    // need two flavors, because async
+    pub async fn force_owned_genco(self, co: GenCo, span: LightSpan) -> Result<Value, ErrorKind> {
+        if let Value::Thunk(thunk) = self {
+            // TODO(amjoseph): use #[tailcall::mutual]
+            return Thunk::force_(thunk, &co, span).await;
+        }
+        Ok(self)
+    }
+
+    /// Explain a value in a human-readable way, e.g. by presenting
+    /// the docstrings of functions if present.
+    pub fn explain(&self) -> String {
         match self {
-            Value::String(s) => Ok(s),
-            other => Err(Error::TypeError {
-                expected: "string",
-                actual: other.type_of(),
-            }),
+            Value::Null => "the 'null' value".into(),
+            Value::Bool(b) => format!("the boolean value '{}'", b),
+            Value::Integer(i) => format!("the integer '{}'", i),
+            Value::Float(f) => format!("the float '{}'", f),
+            Value::String(s) if s.has_context() => format!("the contextful string '{}'", s),
+            Value::String(s) => format!("the contextless string '{}'", s),
+            Value::Path(p) => format!("the path '{}'", p.to_string_lossy()),
+            Value::Attrs(attrs) => format!("a {}-item attribute set", attrs.len()),
+            Value::List(list) => format!("a {}-item list", list.len()),
+
+            Value::Closure(f) => {
+                if let Some(name) = &f.lambda.name {
+                    format!("the user-defined Nix function '{}'", name)
+                } else {
+                    "a user-defined Nix function".to_string()
+                }
+            }
+
+            Value::Builtin(b) => {
+                let mut out = format!("the builtin function '{}'", b.name());
+                if let Some(docs) = b.documentation() {
+                    out.push_str("\n\n");
+                    out.push_str(docs);
+                }
+                out
+            }
+
+            // TODO: handle suspended thunks with a different explanation instead of panicking
+            Value::Thunk(t) => t.value().explain(),
+
+            Value::Catchable(_) => "a catchable failure".into(),
+
+            Value::AttrNotFound
+            | Value::Blueprint(_)
+            | Value::DeferredUpvalue(_)
+            | Value::UnresolvedPath(_)
+            | Value::Json(..)
+            | Value::FinaliseRequest(_) => "an internal Tvix evaluator value".into(),
         }
     }
 }
 
+trait TotalDisplay {
+    fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result;
+}
+
 impl Display for Value {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.total_fmt(f, &mut Default::default())
+    }
+}
+
+/// Emulates the C++-Nix style formatting of floats, which diverges
+/// significantly from Rust's native float formatting.
+fn total_fmt_float<F: std::fmt::Write>(num: f64, mut f: F) -> std::fmt::Result {
+    let mut buf = [b'0'; lexical_core::BUFFER_SIZE];
+    let mut s = lexical_core::write_with_options::<f64, { CXX_LITERAL }>(
+        num,
+        &mut buf,
+        &WRITE_FLOAT_OPTIONS,
+    );
+
+    // apply some postprocessing on the buffer. If scientific
+    // notation is used (we see an `e`), and the next character is
+    // a digit, add the missing `+` sign.)
+    let mut new_s = Vec::with_capacity(s.len());
+
+    if s.contains(&b'e') {
+        for (i, c) in s.iter().enumerate() {
+            // encountered `e`
+            if c == &b'e' {
+                // next character is a digit (so no negative exponent)
+                if s.len() > i && s[i + 1].is_ascii_digit() {
+                    // copy everything from the start up to (including) the e
+                    new_s.extend_from_slice(&s[0..=i]);
+                    // add the missing '+'
+                    new_s.push(b'+');
+                    // check for the remaining characters.
+                    // If it's only one, we need to prepend a trailing zero
+                    if s.len() == i + 2 {
+                        new_s.push(b'0');
+                    }
+                    new_s.extend_from_slice(&s[i + 1..]);
+                    break;
+                }
+            }
+        }
+
+        // if we modified the scientific notation, flip the reference
+        if !new_s.is_empty() {
+            s = &mut new_s
+        }
+    } else if s.contains(&b'.') {
+        // else, if this is not scientific notation, and there's a
+        // decimal point, make sure we really drop trailing zeroes.
+        // In some cases, lexical_core doesn't.
+        for (i, c) in s.iter().enumerate() {
+            // at `.``
+            if c == &b'.' {
+                // trim zeroes from the right side.
+                let frac = String::from_utf8_lossy(&s[i + 1..]);
+                let frac_no_trailing_zeroes = frac.trim_end_matches('0');
+
+                if frac.len() != frac_no_trailing_zeroes.len() {
+                    // we managed to strip something, construct new_s
+                    if frac_no_trailing_zeroes.is_empty() {
+                        // if frac_no_trailing_zeroes is empty, the fractional part was all zeroes, so we can drop the decimal point as well
+                        new_s.extend_from_slice(&s[0..=i - 1]);
+                    } else {
+                        // else, assemble the rest of the string
+                        new_s.extend_from_slice(&s[0..=i]);
+                        new_s.extend_from_slice(frac_no_trailing_zeroes.as_bytes());
+                    }
+
+                    // flip the reference
+                    s = &mut new_s;
+                    break;
+                }
+            }
+        }
+    }
+
+    write!(f, "{}", String::from_utf8_lossy(s))
+}
+
+impl TotalDisplay for Value {
+    fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result {
         match self {
             Value::Null => f.write_str("null"),
             Value::Bool(true) => f.write_str("true"),
             Value::Bool(false) => f.write_str("false"),
-            Value::Integer(num) => f.write_fmt(format_args!("{}", num)),
-            Value::Float(num) => f.write_fmt(format_args!("{}", num)),
+            Value::Integer(num) => write!(f, "{}", num),
             Value::String(s) => s.fmt(f),
-            Value::Attrs(attrs) => attrs.fmt(f),
-            Value::List(list) => list.fmt(f),
+            Value::Path(p) => p.display().fmt(f),
+            Value::Attrs(attrs) => attrs.total_fmt(f, set),
+            Value::List(list) => list.total_fmt(f, set),
+            // TODO: fancy REPL display with position
+            Value::Closure(_) => f.write_str("<LAMBDA>"),
+            Value::Builtin(builtin) => builtin.fmt(f),
+
+            // Nix prints floats with a maximum precision of 5 digits
+            // only. Except when it decides to use scientific notation
+            // (with a + after the `e`, and zero-padded to 0 digits)
+            Value::Float(num) => total_fmt_float(*num, f),
 
             // internal types
-            Value::AttrPath(_) => f.write_str("internal"),
+            Value::AttrNotFound => f.write_str("internal[not found]"),
+            Value::Blueprint(_) => f.write_str("internal[blueprint]"),
+            Value::DeferredUpvalue(_) => f.write_str("internal[deferred_upvalue]"),
+            Value::UnresolvedPath(_) => f.write_str("internal[unresolved_path]"),
+            Value::Json(..) => f.write_str("internal[json]"),
+            Value::FinaliseRequest(_) => f.write_str("internal[finaliser_sentinel]"),
+
+            // Delegate thunk display to the type, as it must handle
+            // the case of already evaluated or cyclic thunks.
+            Value::Thunk(t) => t.total_fmt(f, set),
+            Value::Catchable(_) => panic!("total_fmt() called on a CatchableErrorKind"),
         }
     }
 }
 
-impl PartialEq for Value {
-    fn eq(&self, other: &Self) -> bool {
-        match (self, other) {
-            // Trivial comparisons
-            (Value::Null, Value::Null) => true,
-            (Value::Bool(b1), Value::Bool(b2)) => b1 == b2,
-            (Value::List(l1), Value::List(l2)) => l1 == l2,
-            (Value::String(s1), Value::String(s2)) => s1 == s2,
+impl From<bool> for Value {
+    fn from(b: bool) -> Self {
+        Value::Bool(b)
+    }
+}
+
+impl From<i64> for Value {
+    fn from(i: i64) -> Self {
+        Self::Integer(i)
+    }
+}
+
+impl From<f64> for Value {
+    fn from(i: f64) -> Self {
+        Self::Float(i)
+    }
+}
+
+impl From<PathBuf> for Value {
+    fn from(path: PathBuf) -> Self {
+        Self::Path(Box::new(path))
+    }
+}
 
-            // Numerical comparisons (they work between float & int)
-            (Value::Integer(i1), Value::Integer(i2)) => i1 == i2,
-            (Value::Integer(i), Value::Float(f)) => *i as f64 == *f,
-            (Value::Float(f1), Value::Float(f2)) => f1 == f2,
-            (Value::Float(f), Value::Integer(i)) => *i as f64 == *f,
+fn type_error(expected: &'static str, actual: &Value) -> ErrorKind {
+    ErrorKind::TypeError {
+        expected,
+        actual: actual.type_of(),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::mem::size_of;
+
+    #[test]
+    fn size() {
+        assert_eq!(size_of::<Value>(), 16);
+    }
 
-            // Optimised attribute set comparison
-            (Value::Attrs(a1), Value::Attrs(a2)) => Rc::ptr_eq(a1, a2) || { a1 == a2 },
+    mod floats {
+        use crate::value::total_fmt_float;
 
-            // Everything else is either incomparable (e.g. internal
-            // types) or false.
-            _ => false,
+        #[test]
+        fn format_float() {
+            let ff = [
+                (0f64, "0"),
+                (1.0f64, "1"),
+                (-0.01, "-0.01"),
+                (5e+22, "5e+22"),
+                (1e6, "1e+06"),
+                (-2E-2, "-0.02"),
+                (6.626e-34, "6.626e-34"),
+                (9_224_617.445_991_227, "9.22462e+06"),
+            ];
+            for (n, expected) in ff.iter() {
+                let mut buf = String::new();
+                let res = total_fmt_float(*n, &mut buf);
+                assert!(res.is_ok());
+                assert_eq!(
+                    expected, &buf,
+                    "{} should be formatted as {}, but got {}",
+                    n, expected, &buf
+                );
+            }
         }
     }
 }
diff --git a/tvix/eval/src/value/path.rs b/tvix/eval/src/value/path.rs
new file mode 100644
index 0000000000..ad526a8746
--- /dev/null
+++ b/tvix/eval/src/value/path.rs
@@ -0,0 +1,14 @@
+use path_clean::PathClean;
+use std::path::PathBuf;
+
+/// This function should match the behavior of canonPath() in
+/// src/libutil/util.cc of cppnix.  Currently it does not match that
+/// behavior; it uses the `path_clean` library which is based on the
+/// Go standard library
+///
+/// TODO: make this match the behavior of cppnix
+/// TODO: write tests for this
+
+pub fn canon_path(path: PathBuf) -> PathBuf {
+    path.clean()
+}
diff --git a/tvix/eval/src/value/string.rs b/tvix/eval/src/value/string.rs
index 531bcf547b..ceb43f1ea5 100644
--- a/tvix/eval/src/value/string.rs
+++ b/tvix/eval/src/value/string.rs
@@ -1,13 +1,873 @@
-use std::fmt::Display;
+//! This module implements Nix language strings.
+//!
+//! Nix language strings never need to be modified on the language
+//! level, allowing us to shave off some memory overhead and only
+//! paying the cost when creating new strings.
+use bstr::{BStr, BString, ByteSlice, Chars};
+use rnix::ast;
+use std::alloc::{alloc, dealloc, handle_alloc_error, Layout};
+use std::borrow::{Borrow, Cow};
+use std::collections::HashSet;
+use std::ffi::c_void;
+use std::fmt::{self, Debug, Display};
+use std::hash::Hash;
+use std::ops::Deref;
+use std::ptr::{self, NonNull};
+use std::slice;
 
-/// This module implements Nix language strings and their different
-/// backing implementations.
+use serde::de::{Deserializer, Visitor};
+use serde::{Deserialize, Serialize};
 
-#[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
-pub struct NixString(pub String);
+#[derive(Clone, Debug, Serialize, Hash, PartialEq, Eq)]
+pub enum NixContextElement {
+    /// A plain store path (e.g. source files copied to the store)
+    Plain(String),
+
+    /// Single output of a derivation, represented by its name and its derivation path.
+    Single { name: String, derivation: String },
+
+    /// A reference to a complete derivation
+    /// including its source and its binary closure.
+    /// It is used for the `drvPath` attribute context.
+    /// The referred string is the store path to
+    /// the derivation path.
+    Derivation(String),
+}
+
+/// Nix context strings representation in Tvix. This tracks a set of different kinds of string
+/// dependencies that we can come across during manipulation of our language primitives, mostly
+/// strings. There's some simple algebra of context strings and how they propagate w.r.t. primitive
+/// operations, e.g. concatenation, interpolation and other string operations.
+#[repr(transparent)]
+#[derive(Clone, Debug, Serialize, Default)]
+pub struct NixContext(HashSet<NixContextElement>);
+
+impl From<NixContextElement> for NixContext {
+    fn from(value: NixContextElement) -> Self {
+        Self([value].into())
+    }
+}
+
+impl From<HashSet<NixContextElement>> for NixContext {
+    fn from(value: HashSet<NixContextElement>) -> Self {
+        Self(value)
+    }
+}
+
+impl NixContext {
+    /// Creates an empty context that can be populated
+    /// and passed to form a contextful [NixString], albeit
+    /// if the context is concretly empty, the resulting [NixString]
+    /// will be contextless.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// For internal consumers, we let people observe
+    /// if the [NixContext] is actually empty or not
+    /// to decide whether they want to skip the allocation
+    /// of a full blown [HashSet].
+    pub(crate) fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    /// Consumes a new [NixContextElement] and add it if not already
+    /// present in this context.
+    pub fn append(mut self, other: NixContextElement) -> Self {
+        self.0.insert(other);
+        self
+    }
+
+    /// Consumes both ends of the join into a new NixContent
+    /// containing the union of elements of both ends.
+    pub fn join(mut self, other: &mut NixContext) -> Self {
+        let other_set = std::mem::take(&mut other.0);
+        let mut set: HashSet<NixContextElement> = std::mem::take(&mut self.0);
+        set.extend(other_set);
+        Self(set)
+    }
+
+    /// Copies from another [NixString] its context strings
+    /// in this context.
+    pub fn mimic(&mut self, other: &NixString) {
+        if let Some(context) = other.context() {
+            self.0.extend(context.iter().cloned());
+        }
+    }
+
+    /// Iterates over "plain" context elements, e.g. sources imported
+    /// in the store without more information, i.e. `toFile` or coerced imported paths.
+    /// It yields paths to the store.
+    pub fn iter_plain(&self) -> impl Iterator<Item = &str> {
+        self.iter().filter_map(|elt| {
+            if let NixContextElement::Plain(s) = elt {
+                Some(s.as_str())
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Iterates over "full derivations" context elements, e.g. something
+    /// referring to their `drvPath`, i.e. their full sources and binary closure.
+    /// It yields derivation paths.
+    pub fn iter_derivation(&self) -> impl Iterator<Item = &str> {
+        self.iter().filter_map(|elt| {
+            if let NixContextElement::Derivation(s) = elt {
+                Some(s.as_str())
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Iterates over "single" context elements, e.g. single derived paths,
+    /// or also known as the single output of a given derivation.
+    /// The first element of the tuple is the output name
+    /// and the second element is the derivation path.
+    pub fn iter_single_outputs(&self) -> impl Iterator<Item = (&str, &str)> {
+        self.iter().filter_map(|elt| {
+            if let NixContextElement::Single { name, derivation } = elt {
+                Some((name.as_str(), derivation.as_str()))
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Iterates over any element of the context.
+    pub fn iter(&self) -> impl Iterator<Item = &NixContextElement> {
+        self.0.iter()
+    }
+
+    /// Produces a list of owned references to this current context,
+    /// no matter its type.
+    pub fn to_owned_references(self) -> Vec<String> {
+        self.0
+            .into_iter()
+            .map(|ctx| match ctx {
+                NixContextElement::Derivation(drv_path) => drv_path,
+                NixContextElement::Plain(store_path) => store_path,
+                NixContextElement::Single { derivation, .. } => derivation,
+            })
+            .collect()
+    }
+}
+
+/// This type is never instantiated, but serves to document the memory layout of the actual heap
+/// allocation for Nix strings.
+#[allow(dead_code)]
+struct NixStringInner {
+    /// The string context, if any.  Note that this is boxed to take advantage of the null pointer
+    /// niche, otherwise this field ends up being very large:
+    ///
+    /// ```notrust
+    /// >> std::mem::size_of::<Option<HashSet<String>>>()
+    /// 48
+    ///
+    /// >> std::mem::size_of::<Option<Box<HashSet<String>>>>()
+    /// 8
+    /// ```
+    context: Option<Box<NixContext>>,
+    /// The length of the data, stored *inline in the allocation*
+    length: usize,
+    /// The actual data for the string itself. Will always be `length` bytes long
+    data: [u8],
+}
+
+#[allow(clippy::zst_offset)]
+impl NixStringInner {
+    /// Construct a [`Layout`] for a nix string allocation with the given length.
+    ///
+    /// Returns a tuple of:
+    /// 1. The layout itself.
+    /// 2. The offset of [`Self::length`] within the allocation, assuming the allocation starts at 0
+    /// 3. The offset of the data array within the allocation, assuming the allocation starts at 0
+    fn layout(len: usize) -> (Layout, usize, usize) {
+        let layout = Layout::new::<Option<Box<NixContext>>>();
+        let (layout, len_offset) = layout.extend(Layout::new::<usize>()).unwrap();
+        let (layout, data_offset) = layout.extend(Layout::array::<u8>(len).unwrap()).unwrap();
+        (layout, len_offset, data_offset)
+    }
+
+    /// Returns the [`Layout`] for an *already-allocated* nix string, loading the length from the
+    /// pointer.
+    ///
+    /// Returns a tuple of:
+    /// 1. The layout itself.
+    /// 2. The offset of [`Self::length`] within the allocation, assuming the allocation starts at 0
+    /// 3. The offset of the data array within the allocation, assuming the allocation starts at 0
+    ///
+    /// # Safety
+    ///
+    /// This function must only be called on a pointer that has been properly initialized with
+    /// [`Self::alloc`]. The data buffer may not necessarily be initialized
+    unsafe fn layout_of(this: NonNull<c_void>) -> (Layout, usize, usize) {
+        let layout = Layout::new::<Option<Box<NixContext>>>();
+        let (_, len_offset) = layout.extend(Layout::new::<usize>()).unwrap();
+        // SAFETY: Layouts are linear, so even though we haven't involved data at all yet, we know
+        // the len_offset is a valid offset into the second field of the allocation
+        let len = *(this.as_ptr().add(len_offset) as *const usize);
+        Self::layout(len)
+    }
+
+    /// Allocate an *uninitialized* nix string with the given length. Writes the length to the
+    /// length value in the pointer, but leaves both context and data uninitialized
+    ///
+    /// This function is safe to call (as constructing pointers of any sort of validity is always
+    /// safe in Rust) but it is unsafe to use the resulting pointer to do anything other than
+    ///
+    /// 1. Read the length
+    /// 2. Write the context
+    /// 3. Write the data
+    ///
+    /// until the string is fully initialized
+    fn alloc(len: usize) -> NonNull<c_void> {
+        let (layout, len_offset, _data_offset) = Self::layout(len);
+        debug_assert_ne!(layout.size(), 0);
+        unsafe {
+            // SAFETY: Layout has non-zero size, since the layout of the context and the
+            // layout of the len both have non-zero size
+            let ptr = alloc(layout);
+
+            if let Some(this) = NonNull::new(ptr as *mut _) {
+                // SAFETY: We've allocated with a layout that causes the len_offset to be in-bounds
+                // and writeable, and if the allocation succeeded it won't wrap
+                ((this.as_ptr() as *mut u8).add(len_offset) as *mut usize).write(len);
+                debug_assert_eq!(Self::len(this), len);
+                this
+            } else {
+                handle_alloc_error(layout);
+            }
+        }
+    }
+
+    /// Deallocate the Nix string at the given pointer
+    ///
+    /// # Safety
+    ///
+    /// This function must only be called with a pointer that has been properly initialized with
+    /// [`Self::alloc`]
+    unsafe fn dealloc(this: NonNull<c_void>) {
+        let (layout, _, _) = Self::layout_of(this);
+        // SAFETY: okay because of the safety guarantees of this method
+        dealloc(this.as_ptr() as *mut u8, layout)
+    }
+
+    /// Return the length of the Nix string at the given pointer
+    ///
+    /// # Safety
+    ///
+    /// This function must only be called with a pointer that has been properly initialized with
+    /// [`Self::alloc`]
+    unsafe fn len(this: NonNull<c_void>) -> usize {
+        let (_, len_offset, _) = Self::layout_of(this);
+        // SAFETY: As long as the safety guarantees of this method are upheld, we've allocated with
+        // a layout that causes the len_offset to be in-bounds and writeable, and if the allocation
+        // succeeded it won't wrap
+        *(this.as_ptr().add(len_offset) as *const usize)
+    }
+
+    /// Return a pointer to the context value within the given Nix string pointer
+    ///
+    /// # Safety
+    ///
+    /// This function must only be called with a pointer that has been properly initialized with
+    /// [`Self::alloc`]
+    unsafe fn context_ptr(this: NonNull<c_void>) -> *mut Option<Box<NixContext>> {
+        // SAFETY: The context is the first field in the layout of the allocation
+        this.as_ptr() as *mut Option<Box<NixContext>>
+    }
+
+    /// Construct a shared reference to the context value within the given Nix string pointer
+    ///
+    /// # Safety
+    ///
+    /// This function must only be called with a pointer that has been properly initialized with
+    /// [`Self::alloc`], and where the context has been properly initialized (by writing to the
+    /// pointer returned from [`Self::context_ptr`]).
+    ///
+    /// Also, all the normal Rust rules about pointer-to-reference conversion apply. See
+    /// [`NonNull::as_ref`] for more.
+    unsafe fn context_ref<'a>(this: NonNull<c_void>) -> &'a Option<Box<NixContext>> {
+        Self::context_ptr(this).as_ref().unwrap()
+    }
+
+    /// Construct a mutable reference to the context value within the given Nix string pointer
+    ///
+    /// # Safety
+    ///
+    /// This function must only be called with a pointer that has been properly initialized with
+    /// [`Self::alloc`], and where the context has been properly initialized (by writing to the
+    /// pointer returned from [`Self::context_ptr`]).
+    ///
+    /// Also, all the normal Rust rules about pointer-to-reference conversion apply. See
+    /// [`NonNull::as_mut`] for more.
+    unsafe fn context_mut<'a>(this: NonNull<c_void>) -> &'a mut Option<Box<NixContext>> {
+        Self::context_ptr(this).as_mut().unwrap()
+    }
+
+    /// Return a pointer to the data array within the given Nix string pointer
+    ///
+    /// # Safety
+    ///
+    /// This function must only be called with a pointer that has been properly initialized with
+    /// [`Self::alloc`]
+    unsafe fn data_ptr(this: NonNull<c_void>) -> *mut u8 {
+        let (_, _, data_offset) = Self::layout_of(this);
+        // SAFETY: data is the third field in the layout of the allocation
+        this.as_ptr().add(data_offset) as *mut u8
+    }
+
+    /// Construct a shared reference to the data slice within the given Nix string pointer
+    ///
+    /// # Safety
+    ///
+    /// This function must only be called with a pointer that has been properly initialized with
+    /// [`Self::alloc`], and where the data array has been properly initialized (by writing to the
+    /// pointer returned from [`Self::data_ptr`]).
+    ///
+    /// Also, all the normal Rust rules about pointer-to-reference conversion apply. See
+    /// [`slice::from_raw_parts`] for more.
+    unsafe fn data_slice<'a>(this: NonNull<c_void>) -> &'a [u8] {
+        let len = Self::len(this);
+        let data = Self::data_ptr(this);
+        slice::from_raw_parts(data, len)
+    }
+
+    /// Construct a mutable reference to the data slice within the given Nix string pointer
+    ///
+    /// # Safety
+    ///
+    /// This function must only be called with a pointer that has been properly initialized with
+    /// [`Self::alloc`], and where the data array has been properly initialized (by writing to the
+    /// pointer returned from [`Self::data_ptr`]).
+    ///
+    /// Also, all the normal Rust rules about pointer-to-reference conversion apply. See
+    /// [`slice::from_raw_parts_mut`] for more.
+    #[allow(dead_code)]
+    unsafe fn data_slice_mut<'a>(this: NonNull<c_void>) -> &'a mut [u8] {
+        let len = Self::len(this);
+        let data = Self::data_ptr(this);
+        slice::from_raw_parts_mut(data, len)
+    }
+
+    /// Clone the Nix string pointed to by this pointer, and return a pointer to a new Nix string
+    /// containing the same data and context.
+    ///
+    /// # Safety
+    ///
+    /// This function must only be called with a pointer that has been properly initialized with
+    /// [`Self::alloc`], and where the context has been properly initialized (by writing to the
+    /// pointer returned from [`Self::context_ptr`]), and the data array has been properly
+    /// initialized (by writing to the pointer returned from [`Self::data_ptr`]).
+    unsafe fn clone(this: NonNull<c_void>) -> NonNull<c_void> {
+        let (layout, _, _) = Self::layout_of(this);
+        let ptr = alloc(layout);
+        if let Some(new) = NonNull::new(ptr as *mut _) {
+            ptr::copy_nonoverlapping(this.as_ptr(), new.as_ptr(), layout.size());
+            Self::context_ptr(new).write(Self::context_ref(this).clone());
+            new
+        } else {
+            handle_alloc_error(layout);
+        }
+    }
+}
+
+/// Nix string values
+///
+/// # Internals
+///
+/// For performance reasons (to keep allocations small, and to avoid indirections), [`NixString`] is
+/// represented as a single *thin* pointer to a packed data structure containing the
+/// [context][NixContext] and the string data itself (which is a raw byte array, to match the Nix
+/// string semantics that allow any array of bytes to be represented by a string).
+
+/// This memory representation is documented in [`NixStringInner`], but since Rust prefers to deal
+/// with slices via *fat pointers* (pointers that include the length in the *pointer*, not in the
+/// heap allocation), we have to do mostly manual layout management and allocation for this
+/// representation. See the documentation for the methods of [`NixStringInner`] for more information
+pub struct NixString(NonNull<c_void>);
+
+unsafe impl Send for NixString {}
+unsafe impl Sync for NixString {}
+
+impl Drop for NixString {
+    fn drop(&mut self) {
+        // SAFETY: There's no way to construct a NixString that doesn't leave the allocation correct
+        // according to the rules of dealloc
+        unsafe {
+            NixStringInner::dealloc(self.0);
+        }
+    }
+}
+
+impl Clone for NixString {
+    fn clone(&self) -> Self {
+        // SAFETY: There's no way to construct a NixString that doesn't leave the allocation correct
+        // according to the rules of clone
+        unsafe { Self(NixStringInner::clone(self.0)) }
+    }
+}
+
+impl Debug for NixString {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Some(ctx) = self.context() {
+            f.debug_struct("NixString")
+                .field("context", ctx)
+                .field("data", &self.as_bstr())
+                .finish()
+        } else {
+            write!(f, "{:?}", self.as_bstr())
+        }
+    }
+}
+
+impl PartialEq for NixString {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_bstr() == other.as_bstr()
+    }
+}
+
+impl Eq for NixString {}
+
+impl PartialEq<&[u8]> for NixString {
+    fn eq(&self, other: &&[u8]) -> bool {
+        **self == **other
+    }
+}
+
+impl PartialEq<&str> for NixString {
+    fn eq(&self, other: &&str) -> bool {
+        **self == other.as_bytes()
+    }
+}
+
+impl PartialOrd for NixString {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for NixString {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.as_bstr().cmp(other.as_bstr())
+    }
+}
+
+impl From<Box<BStr>> for NixString {
+    fn from(value: Box<BStr>) -> Self {
+        Self::new(&value, None)
+    }
+}
+
+impl From<BString> for NixString {
+    fn from(value: BString) -> Self {
+        Self::new(&value, None)
+    }
+}
+
+impl From<&BStr> for NixString {
+    fn from(value: &BStr) -> Self {
+        value.to_owned().into()
+    }
+}
+
+impl From<&[u8]> for NixString {
+    fn from(value: &[u8]) -> Self {
+        Self::from(value.to_owned())
+    }
+}
+
+impl From<Vec<u8>> for NixString {
+    fn from(value: Vec<u8>) -> Self {
+        value.into_boxed_slice().into()
+    }
+}
+
+impl From<Box<[u8]>> for NixString {
+    fn from(value: Box<[u8]>) -> Self {
+        Self::new(&value, None)
+    }
+}
+
+impl From<&str> for NixString {
+    fn from(s: &str) -> Self {
+        s.as_bytes().into()
+    }
+}
+
+impl From<String> for NixString {
+    fn from(s: String) -> Self {
+        s.into_bytes().into()
+    }
+}
+
+impl<T> From<(T, Option<Box<NixContext>>)> for NixString
+where
+    NixString: From<T>,
+{
+    fn from((s, ctx): (T, Option<Box<NixContext>>)) -> Self {
+        Self::new(NixString::from(s).as_ref(), ctx)
+    }
+}
+
+impl From<Box<str>> for NixString {
+    fn from(s: Box<str>) -> Self {
+        s.into_boxed_bytes().into()
+    }
+}
+
+impl From<ast::Ident> for NixString {
+    fn from(ident: ast::Ident) -> Self {
+        ident.ident_token().unwrap().text().into()
+    }
+}
+
+impl<'a> From<&'a NixString> for &'a BStr {
+    fn from(s: &'a NixString) -> Self {
+        s.as_bstr()
+    }
+}
+
+// No impl From<NixString> for String, that one quotes.
+
+impl From<NixString> for BString {
+    fn from(s: NixString) -> Self {
+        s.as_bstr().to_owned()
+    }
+}
+
+impl AsRef<[u8]> for NixString {
+    fn as_ref(&self) -> &[u8] {
+        self.as_bytes()
+    }
+}
+
+impl Borrow<BStr> for NixString {
+    fn borrow(&self) -> &BStr {
+        self.as_bstr()
+    }
+}
+
+impl Hash for NixString {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.as_bstr().hash(state)
+    }
+}
+
+impl<'de> Deserialize<'de> for NixString {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct StringVisitor;
+
+        impl<'de> Visitor<'de> for StringVisitor {
+            type Value = NixString;
+
+            fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
+                formatter.write_str("a valid Nix string")
+            }
+
+            fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+            where
+                E: serde::de::Error,
+            {
+                Ok(v.into())
+            }
+
+            fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+            where
+                E: serde::de::Error,
+            {
+                Ok(v.into())
+            }
+        }
+
+        deserializer.deserialize_string(StringVisitor)
+    }
+}
+
+impl Deref for NixString {
+    type Target = BStr;
+
+    fn deref(&self) -> &Self::Target {
+        self.as_bstr()
+    }
+}
+
+#[cfg(feature = "arbitrary")]
+mod arbitrary {
+    use super::*;
+    use proptest::prelude::{any_with, Arbitrary};
+    use proptest::strategy::{BoxedStrategy, Strategy};
+
+    impl Arbitrary for NixString {
+        type Parameters = <String as Arbitrary>::Parameters;
+
+        type Strategy = BoxedStrategy<Self>;
+
+        fn arbitrary_with(args: Self::Parameters) -> Self::Strategy {
+            any_with::<String>(args).prop_map(Self::from).boxed()
+        }
+    }
+}
+
+impl NixString {
+    fn new(contents: &[u8], context: Option<Box<NixContext>>) -> Self {
+        // SAFETY: We're always fully initializing a NixString here:
+        //
+        // 1. NixStringInner::alloc sets up the len for us
+        // 2. We set the context, using ptr::write to make sure that the uninitialized memory isn't
+        //    read or dropped
+        // 3. We set the data, using copy_from_nonoverlapping to make sure that the uninitialized
+        //    memory isn't read or dropped
+        //
+        // Only *then* can we construct a NixString
+        unsafe {
+            let inner = NixStringInner::alloc(contents.len());
+            NixStringInner::context_ptr(inner).write(context);
+            NixStringInner::data_ptr(inner)
+                .copy_from_nonoverlapping(contents.as_ptr(), contents.len());
+            Self(inner)
+        }
+    }
+
+    pub fn new_inherit_context_from<T>(other: &NixString, new_contents: T) -> Self
+    where
+        NixString: From<T>,
+    {
+        Self::new(
+            Self::from(new_contents).as_ref(),
+            other.context().map(|c| Box::new(c.clone())),
+        )
+    }
+
+    pub fn new_context_from<T>(context: NixContext, contents: T) -> Self
+    where
+        NixString: From<T>,
+    {
+        Self::new(
+            Self::from(contents).as_ref(),
+            if context.is_empty() {
+                None
+            } else {
+                Some(Box::new(context))
+            },
+        )
+    }
+
+    pub fn as_bstr(&self) -> &BStr {
+        BStr::new(self.as_bytes())
+    }
+
+    pub fn as_bytes(&self) -> &[u8] {
+        // SAFETY: There's no way to construct an uninitialized NixString (see the SAFETY comment in
+        // `new`)
+        unsafe { NixStringInner::data_slice(self.0) }
+    }
+
+    pub fn into_bstring(self) -> BString {
+        self.as_bstr().to_owned()
+    }
+
+    /// Return a displayable representation of the string as an
+    /// identifier.
+    ///
+    /// This is used when printing out strings used as e.g. attribute
+    /// set keys, as those are only escaped in the presence of special
+    /// characters.
+    pub fn ident_str(&self) -> Cow<str> {
+        let escaped = match self.to_str_lossy() {
+            Cow::Borrowed(s) => nix_escape_string(s),
+            Cow::Owned(s) => nix_escape_string(&s).into_owned().into(),
+        };
+        match escaped {
+            // A borrowed string is unchanged and can be returned as
+            // is.
+            Cow::Borrowed(_) => {
+                if is_valid_nix_identifier(&escaped) && !is_keyword(&escaped) {
+                    escaped
+                } else {
+                    Cow::Owned(format!("\"{}\"", escaped))
+                }
+            }
+
+            // An owned string has escapes, and needs the outer quotes
+            // for display.
+            Cow::Owned(s) => Cow::Owned(format!("\"{}\"", s)),
+        }
+    }
+
+    pub fn concat(&self, other: &Self) -> Self {
+        let mut s = self.to_vec();
+        s.extend(&(***other));
+
+        let context = [self.context(), other.context()]
+            .into_iter()
+            .flatten()
+            .fold(NixContext::new(), |acc_ctx, new_ctx| {
+                acc_ctx.join(&mut new_ctx.clone())
+            });
+        Self::new_context_from(context, s)
+    }
+
+    pub(crate) fn context(&self) -> Option<&NixContext> {
+        // SAFETY: There's no way to construct an uninitialized or invalid NixString (see the SAFETY
+        // comment in `new`).
+        //
+        // Also, we're using the same lifetime and mutability as self, to fit the
+        // pointer-to-reference conversion rules
+        unsafe { NixStringInner::context_ref(self.0).as_deref() }
+    }
+
+    pub(crate) fn context_mut(&mut self) -> Option<&mut NixContext> {
+        // SAFETY: There's no way to construct an uninitialized or invalid NixString (see the SAFETY
+        // comment in `new`).
+        //
+        // Also, we're using the same lifetime and mutability as self, to fit the
+        // pointer-to-reference conversion rules
+        unsafe { NixStringInner::context_mut(self.0).as_deref_mut() }
+    }
+
+    pub fn iter_context(&self) -> impl Iterator<Item = &NixContext> {
+        self.context().into_iter()
+    }
+
+    pub fn iter_plain(&self) -> impl Iterator<Item = &str> {
+        self.iter_context().flat_map(|context| context.iter_plain())
+    }
+
+    pub fn iter_derivation(&self) -> impl Iterator<Item = &str> {
+        return self
+            .iter_context()
+            .flat_map(|context| context.iter_derivation());
+    }
+
+    pub fn iter_single_outputs(&self) -> impl Iterator<Item = (&str, &str)> {
+        return self
+            .iter_context()
+            .flat_map(|context| context.iter_single_outputs());
+    }
+
+    /// Returns whether this Nix string possess a context or not.
+    pub fn has_context(&self) -> bool {
+        self.context().is_some()
+    }
+
+    /// This clears the context of that string, losing
+    /// all dependency tracking information.
+    pub fn clear_context(&mut self) {
+        // SAFETY: There's no way to construct an uninitialized or invalid NixString (see the SAFETY
+        // comment in `new`).
+        *unsafe { NixStringInner::context_mut(self.0) } = None;
+    }
+
+    pub fn chars(&self) -> Chars<'_> {
+        self.as_bstr().chars()
+    }
+}
+
+fn nix_escape_char(ch: char, next: Option<&char>) -> Option<&'static str> {
+    match (ch, next) {
+        ('\\', _) => Some("\\\\"),
+        ('"', _) => Some("\\\""),
+        ('\n', _) => Some("\\n"),
+        ('\t', _) => Some("\\t"),
+        ('\r', _) => Some("\\r"),
+        ('$', Some('{')) => Some("\\$"),
+        _ => None,
+    }
+}
+
+/// Return true if this string is a keyword -- character strings
+/// which lexically match the "identifier" production but are not
+/// parsed as identifiers.  See also cppnix commit
+/// b72bc4a972fe568744d98b89d63adcd504cb586c.
+fn is_keyword(s: &str) -> bool {
+    matches!(
+        s,
+        "if" | "then" | "else" | "assert" | "with" | "let" | "in" | "rec" | "inherit"
+    )
+}
+
+/// Return true if this string can be used as an identifier in Nix.
+fn is_valid_nix_identifier(s: &str) -> bool {
+    // adapted from rnix-parser's tokenizer.rs
+    let mut chars = s.chars();
+    match chars.next() {
+        Some('a'..='z' | 'A'..='Z' | '_') => (),
+        _ => return false,
+    }
+    for c in chars {
+        match c {
+            'a'..='z' | 'A'..='Z' | '0'..='9' | '_' | '-' | '\'' => (),
+            _ => return false,
+        }
+    }
+    true
+}
+
+/// Escape a Nix string for display, as most user-visible representation
+/// are escaped strings.
+///
+/// Note that this does not add the outer pair of surrounding quotes.
+fn nix_escape_string(input: &str) -> Cow<str> {
+    let mut iter = input.char_indices().peekable();
+
+    while let Some((i, c)) = iter.next() {
+        if let Some(esc) = nix_escape_char(c, iter.peek().map(|(_, c)| c)) {
+            let mut escaped = String::with_capacity(input.len());
+            escaped.push_str(&input[..i]);
+            escaped.push_str(esc);
+
+            // In theory we calculate how many bytes it takes to represent `esc`
+            // in UTF-8 and use that for the offset. It is, however, safe to
+            // assume that to be 1, as all characters that can be escaped in a
+            // Nix string are ASCII.
+            let mut inner_iter = input[i + 1..].chars().peekable();
+            while let Some(c) = inner_iter.next() {
+                match nix_escape_char(c, inner_iter.peek()) {
+                    Some(esc) => escaped.push_str(esc),
+                    None => escaped.push(c),
+                }
+            }
+
+            return Cow::Owned(escaped);
+        }
+    }
+
+    Cow::Borrowed(input)
+}
 
 impl Display for NixString {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        f.write_str(self.0.as_str())
+        f.write_str("\"")?;
+        f.write_str(&nix_escape_string(&self.to_str_lossy()))?;
+        f.write_str("\"")
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use test_strategy::proptest;
+
+    use super::*;
+
+    use crate::properties::{eq_laws, hash_laws, ord_laws};
+
+    #[test]
+    fn size() {
+        assert_eq!(std::mem::size_of::<NixString>(), 8);
+    }
+
+    #[proptest]
+    fn clone_strings(s: NixString) {
+        drop(s.clone())
+    }
+
+    eq_laws!(NixString);
+    hash_laws!(NixString);
+    ord_laws!(NixString);
+}
diff --git a/tvix/eval/src/value/thunk.rs b/tvix/eval/src/value/thunk.rs
new file mode 100644
index 0000000000..a67537f945
--- /dev/null
+++ b/tvix/eval/src/value/thunk.rs
@@ -0,0 +1,434 @@
+//! This module implements the runtime representation of Thunks.
+//!
+//! Thunks are a special kind of Nix value, similar to a 0-argument
+//! closure that yields some value. Thunks are used to implement the
+//! lazy evaluation behaviour of Nix:
+//!
+//! Whenever the compiler determines that an expression should be
+//! evaluated lazily, it creates a thunk instead of compiling the
+//! expression value directly. At any point in the runtime where the
+//! actual value of a thunk is required, it is "forced", meaning that
+//! the encompassing computation takes place and the thunk takes on
+//! its new value.
+//!
+//! Thunks have interior mutability to be able to memoise their
+//! computation. Once a thunk is evaluated, its internal
+//! representation becomes the result of the expression. It is legal
+//! for the runtime to replace a thunk object directly with its value
+//! object, but when forcing a thunk, the runtime *must* mutate the
+//! memoisable slot.
+
+use std::{
+    cell::{Ref, RefCell, RefMut},
+    collections::HashSet,
+    fmt::Debug,
+    rc::Rc,
+};
+
+use crate::{
+    errors::ErrorKind,
+    opcode::OpCode,
+    spans::LightSpan,
+    upvalues::Upvalues,
+    value::Closure,
+    vm::generators::{self, GenCo},
+    Value,
+};
+
+use super::{Lambda, TotalDisplay};
+use codemap::Span;
+
+/// Internal representation of a suspended native thunk.
+struct SuspendedNative(Box<dyn Fn() -> Result<Value, ErrorKind>>);
+
+impl Debug for SuspendedNative {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "SuspendedNative({:p})", self.0)
+    }
+}
+
+/// Internal representation of the different states of a thunk.
+///
+/// Upvalues must be finalised before leaving the initial state
+/// (Suspended or RecursiveClosure).  The [`value()`] function may
+/// not be called until the thunk is in the final state (Evaluated).
+#[derive(Debug)]
+enum ThunkRepr {
+    /// Thunk is closed over some values, suspended and awaiting
+    /// execution.
+    Suspended {
+        lambda: Rc<Lambda>,
+        upvalues: Rc<Upvalues>,
+        light_span: LightSpan,
+    },
+
+    /// Thunk is a suspended native computation.
+    Native(SuspendedNative),
+
+    /// Thunk currently under-evaluation; encountering a blackhole
+    /// value means that infinite recursion has occured.
+    Blackhole {
+        /// Span at which the thunk was first forced.
+        forced_at: LightSpan,
+
+        /// Span at which the thunk was originally suspended.
+        suspended_at: Option<LightSpan>,
+
+        /// Span of the first instruction of the actual code inside
+        /// the thunk.
+        content_span: Option<Span>,
+    },
+
+    // TODO(amjoseph): consider changing `Value` to `Rc<Value>` to avoid
+    // expensive clone()s in Thunk::force().
+    /// Fully evaluated thunk.
+    Evaluated(Value),
+}
+
+impl ThunkRepr {
+    fn debug_repr(&self) -> String {
+        match self {
+            ThunkRepr::Evaluated(v) => format!("thunk(val|{})", v),
+            ThunkRepr::Blackhole { .. } => "thunk(blackhole)".to_string(),
+            ThunkRepr::Native(_) => "thunk(native)".to_string(),
+            ThunkRepr::Suspended { lambda, .. } => format!("thunk({:p})", *lambda),
+        }
+    }
+
+    /// Return the Value within a fully-evaluated ThunkRepr; panics
+    /// if the thunk is not fully-evaluated.
+    fn expect(self) -> Value {
+        match self {
+            ThunkRepr::Evaluated(value) => value,
+            ThunkRepr::Blackhole { .. } => panic!("Thunk::expect() called on a black-holed thunk"),
+            ThunkRepr::Suspended { .. } | ThunkRepr::Native(_) => {
+                panic!("Thunk::expect() called on a suspended thunk")
+            }
+        }
+    }
+
+    fn expect_ref(&self) -> &Value {
+        match self {
+            ThunkRepr::Evaluated(value) => value,
+            ThunkRepr::Blackhole { .. } => panic!("Thunk::expect() called on a black-holed thunk"),
+            ThunkRepr::Suspended { .. } | ThunkRepr::Native(_) => {
+                panic!("Thunk::expect() called on a suspended thunk")
+            }
+        }
+    }
+
+    pub fn is_forced(&self) -> bool {
+        match self {
+            ThunkRepr::Evaluated(Value::Thunk(_)) => false,
+            ThunkRepr::Evaluated(_) => true,
+            _ => false,
+        }
+    }
+}
+
+/// A thunk is created for any value which requires non-strict
+/// evaluation due to self-reference or lazy semantics (or both).
+/// Every reference cycle involving `Value`s will contain at least
+/// one `Thunk`.
+#[derive(Clone, Debug)]
+pub struct Thunk(Rc<RefCell<ThunkRepr>>);
+
+impl Thunk {
+    pub fn new_closure(lambda: Rc<Lambda>) -> Self {
+        Thunk(Rc::new(RefCell::new(ThunkRepr::Evaluated(Value::Closure(
+            Rc::new(Closure {
+                upvalues: Rc::new(Upvalues::with_capacity(lambda.upvalue_count)),
+                lambda: lambda.clone(),
+            }),
+        )))))
+    }
+
+    pub fn new_suspended(lambda: Rc<Lambda>, light_span: LightSpan) -> Self {
+        Thunk(Rc::new(RefCell::new(ThunkRepr::Suspended {
+            upvalues: Rc::new(Upvalues::with_capacity(lambda.upvalue_count)),
+            lambda: lambda.clone(),
+            light_span,
+        })))
+    }
+
+    pub fn new_suspended_native(native: Box<dyn Fn() -> Result<Value, ErrorKind>>) -> Self {
+        Thunk(Rc::new(RefCell::new(ThunkRepr::Native(SuspendedNative(
+            native,
+        )))))
+    }
+
+    /// Helper function to create a [`Thunk`] that calls a function given as the
+    /// [`Value`] `callee` with the argument `arg` when it is forced. This is
+    /// particularly useful in builtin implementations if the result of calling
+    /// a function does not need to be forced immediately, because e.g. it is
+    /// stored in an attribute set.
+    pub fn new_suspended_call(callee: Value, arg: Value, light_span: LightSpan) -> Self {
+        let mut lambda = Lambda::default();
+        let span = light_span.span();
+
+        let arg_idx = lambda.chunk().push_constant(arg);
+        let f_idx = lambda.chunk().push_constant(callee);
+
+        // This is basically a recreation of compile_apply():
+        // We need to push the argument onto the stack and then the function.
+        // The function (not the argument) needs to be forced before calling.
+        lambda.chunk.push_op(OpCode::OpConstant(arg_idx), span);
+        lambda.chunk().push_op(OpCode::OpConstant(f_idx), span);
+        lambda.chunk.push_op(OpCode::OpForce, span);
+        lambda.chunk.push_op(OpCode::OpCall, span);
+
+        // Inform the VM that the chunk has ended
+        lambda.chunk.push_op(OpCode::OpReturn, span);
+
+        Thunk(Rc::new(RefCell::new(ThunkRepr::Suspended {
+            upvalues: Rc::new(Upvalues::with_capacity(0)),
+            lambda: Rc::new(lambda),
+            light_span,
+        })))
+    }
+
+    fn prepare_blackhole(&self, forced_at: LightSpan) -> ThunkRepr {
+        match &*self.0.borrow() {
+            ThunkRepr::Suspended {
+                light_span, lambda, ..
+            } => ThunkRepr::Blackhole {
+                forced_at,
+                suspended_at: Some(light_span.clone()),
+                content_span: Some(lambda.chunk.first_span()),
+            },
+
+            _ => ThunkRepr::Blackhole {
+                forced_at,
+                suspended_at: None,
+                content_span: None,
+            },
+        }
+    }
+
+    pub async fn force(myself: Thunk, co: GenCo, span: LightSpan) -> Result<Value, ErrorKind> {
+        Self::force_(myself, &co, span).await
+    }
+    pub async fn force_(
+        mut myself: Thunk,
+        co: &GenCo,
+        span: LightSpan,
+    ) -> Result<Value, ErrorKind> {
+        // This vector of "thunks which point to the thunk-being-forced", to
+        // be updated along with it, is necessary in order to write this
+        // function in iterative (and later, mutual-tail-call) form.
+        let mut also_update: Vec<Rc<RefCell<ThunkRepr>>> = vec![];
+
+        loop {
+            // If the current thunk is already fully evaluated, return its evaluated
+            // value. The VM will continue running the code that landed us here.
+            if myself.is_forced() {
+                let val = myself.unwrap_or_clone();
+                for other_thunk in also_update.into_iter() {
+                    other_thunk.replace(ThunkRepr::Evaluated(val.clone()));
+                }
+                return Ok(val);
+            }
+
+            // Begin evaluation of this thunk by marking it as a blackhole, meaning
+            // that any other forcing frame encountering this thunk before its
+            // evaluation is completed detected an evaluation cycle.
+            let inner = myself.0.replace(myself.prepare_blackhole(span.clone()));
+
+            match inner {
+                // If there was already a blackhole in the thunk, this is an
+                // evaluation cycle.
+                ThunkRepr::Blackhole {
+                    forced_at,
+                    suspended_at,
+                    content_span,
+                } => {
+                    return Err(ErrorKind::InfiniteRecursion {
+                        first_force: forced_at.span(),
+                        suspended_at: suspended_at.map(|s| s.span()),
+                        content_span,
+                    })
+                }
+
+                // If there is a native function stored in the thunk, evaluate it
+                // and replace this thunk's representation with the result.
+                ThunkRepr::Native(native) => {
+                    let value = native.0()?;
+                    myself.0.replace(ThunkRepr::Evaluated(value));
+                    continue;
+                }
+
+                // When encountering a suspended thunk, request that the VM enters
+                // it and produces the result.
+                ThunkRepr::Suspended {
+                    lambda,
+                    upvalues,
+                    light_span,
+                } => {
+                    // TODO(amjoseph): use #[tailcall::mutual] here.  This can
+                    // be turned into a tailcall to vm::execute_bytecode() by
+                    // passing `also_update` to it.
+                    let value =
+                        generators::request_enter_lambda(co, lambda, upvalues, light_span).await;
+                    myself.0.replace(ThunkRepr::Evaluated(value));
+                    continue;
+                }
+
+                // nested thunks -- try to flatten before forcing
+                ThunkRepr::Evaluated(Value::Thunk(inner_thunk)) => {
+                    match Rc::try_unwrap(inner_thunk.0) {
+                        Ok(refcell) => {
+                            // we are the only reference to the inner thunk,
+                            // so steal it
+                            myself.0.replace(refcell.into_inner());
+                            continue;
+                        }
+                        Err(rc) => {
+                            let inner_thunk = Thunk(rc);
+                            if inner_thunk.is_forced() {
+                                // tail call to force the inner thunk; note that
+                                // this means the outer thunk remains unforced
+                                // even after calling force() on it; however the
+                                // next time it is forced we will be one
+                                // thunk-forcing closer to it being
+                                // fully-evaluated.
+                                myself
+                                    .0
+                                    .replace(ThunkRepr::Evaluated(inner_thunk.value().clone()));
+                                continue;
+                            }
+                            also_update.push(myself.0.clone());
+                            myself = inner_thunk;
+                            continue;
+                        }
+                    }
+                }
+
+                ThunkRepr::Evaluated(val) => {
+                    return Ok(val);
+                }
+            }
+        }
+    }
+
+    pub fn finalise(&self, stack: &[Value]) {
+        self.upvalues_mut().resolve_deferred_upvalues(stack);
+    }
+
+    pub fn is_evaluated(&self) -> bool {
+        matches!(*self.0.borrow(), ThunkRepr::Evaluated(_))
+    }
+
+    pub fn is_suspended(&self) -> bool {
+        matches!(
+            *self.0.borrow(),
+            ThunkRepr::Suspended { .. } | ThunkRepr::Native(_)
+        )
+    }
+
+    /// Returns true if forcing this thunk will not change it.
+    pub fn is_forced(&self) -> bool {
+        self.0.borrow().is_forced()
+    }
+
+    /// Returns a reference to the inner evaluated value of a thunk.
+    /// It is an error to call this on a thunk that has not been
+    /// forced, or is not otherwise known to be fully evaluated.
+    // Note: Due to the interior mutability of thunks this is
+    // difficult to represent in the type system without impacting the
+    // API too much.
+    pub fn value(&self) -> Ref<Value> {
+        Ref::map(self.0.borrow(), |thunk| match thunk {
+            ThunkRepr::Evaluated(value) => value,
+            ThunkRepr::Blackhole { .. } => panic!("Thunk::value called on a black-holed thunk"),
+            ThunkRepr::Suspended { .. } | ThunkRepr::Native(_) => {
+                panic!("Thunk::value called on a suspended thunk")
+            }
+        })
+    }
+
+    /// Returns the inner evaluated value of a thunk, cloning it if
+    /// the Rc has more than one strong reference.  It is an error
+    /// to call this on a thunk that has not been forced, or is not
+    /// otherwise known to be fully evaluated.
+    fn unwrap_or_clone(self) -> Value {
+        match Rc::try_unwrap(self.0) {
+            Ok(refcell) => refcell.into_inner().expect(),
+            Err(rc) => Ref::map(rc.borrow(), |thunkrepr| thunkrepr.expect_ref()).clone(),
+        }
+    }
+
+    pub fn upvalues(&self) -> Ref<'_, Upvalues> {
+        Ref::map(self.0.borrow(), |thunk| match thunk {
+            ThunkRepr::Suspended { upvalues, .. } => upvalues.as_ref(),
+            ThunkRepr::Evaluated(Value::Closure(c)) => &c.upvalues,
+            _ => panic!("upvalues() on non-suspended thunk"),
+        })
+    }
+
+    pub fn upvalues_mut(&self) -> RefMut<'_, Upvalues> {
+        RefMut::map(self.0.borrow_mut(), |thunk| match thunk {
+            ThunkRepr::Suspended { upvalues, .. } => Rc::get_mut(upvalues).unwrap(),
+            ThunkRepr::Evaluated(Value::Closure(c)) => Rc::get_mut(
+                &mut Rc::get_mut(c).unwrap().upvalues,
+            )
+            .expect(
+                "upvalues_mut() was called on a thunk which already had multiple references to it",
+            ),
+            thunk => panic!("upvalues() on non-suspended thunk: {thunk:?}"),
+        })
+    }
+
+    /// Do not use this without first reading and understanding
+    /// `tvix/docs/value-pointer-equality.md`.
+    pub(crate) fn ptr_eq(&self, other: &Self) -> bool {
+        if Rc::ptr_eq(&self.0, &other.0) {
+            return true;
+        }
+        match &*self.0.borrow() {
+            ThunkRepr::Evaluated(Value::Closure(c1)) => match &*other.0.borrow() {
+                ThunkRepr::Evaluated(Value::Closure(c2)) => Rc::ptr_eq(c1, c2),
+                _ => false,
+            },
+            _ => false,
+        }
+    }
+
+    /// Helper function to format thunks in observer output.
+    pub(crate) fn debug_repr(&self) -> String {
+        self.0.borrow().debug_repr()
+    }
+}
+
+impl TotalDisplay for Thunk {
+    fn total_fmt(&self, f: &mut std::fmt::Formatter<'_>, set: &mut ThunkSet) -> std::fmt::Result {
+        if !set.insert(self) {
+            return f.write_str("<CYCLE>");
+        }
+
+        match &*self.0.borrow() {
+            ThunkRepr::Evaluated(v) => v.total_fmt(f, set),
+            ThunkRepr::Suspended { .. } | ThunkRepr::Native(_) => f.write_str("<CODE>"),
+            other => write!(f, "internal[{}]", other.debug_repr()),
+        }
+    }
+}
+
+/// A wrapper type for tracking which thunks have already been seen
+/// in a context. This is necessary for printing and deeply forcing
+/// cyclic non-diverging data structures like `rec { f = [ f ]; }`.
+/// This is separate from the ThunkRepr::Blackhole mechanism, which
+/// detects diverging data structures like `(rec { f = f; }).f`.
+///
+/// The inner `HashSet` is not available on the outside, as it would be
+/// potentially unsafe to interact with the pointers in the set.
+#[derive(Default)]
+pub struct ThunkSet(HashSet<*const ThunkRepr>);
+
+impl ThunkSet {
+    /// Check whether the given thunk has already been seen. Will mark the thunk
+    /// as seen otherwise.
+    pub fn insert(&mut self, thunk: &Thunk) -> bool {
+        let ptr: *const ThunkRepr = thunk.0.as_ptr();
+        self.0.insert(ptr)
+    }
+}
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
deleted file mode 100644
index 90f5f45f61..0000000000
--- a/tvix/eval/src/vm.rs
+++ /dev/null
@@ -1,385 +0,0 @@
-//! This module implements the virtual (or abstract) machine that runs
-//! Tvix bytecode.
-
-use std::{collections::BTreeMap, rc::Rc};
-
-use crate::{
-    chunk::Chunk,
-    errors::{Error, EvalResult},
-    opcode::OpCode,
-    value::{NixAttrs, NixList, NixString, Value},
-};
-
-pub struct VM {
-    ip: usize,
-    chunk: Chunk,
-    stack: Vec<Value>,
-}
-
-impl VM {
-    fn inc_ip(&mut self) -> OpCode {
-        let op = self.chunk.code[self.ip];
-        self.ip += 1;
-        op
-    }
-
-    fn peek(&self, at: usize) -> &Value {
-        &self.stack[self.stack.len() - 1 - at]
-    }
-
-    fn pop(&mut self) -> Value {
-        self.stack.pop().expect("TODO")
-    }
-
-    fn pop_number_pair(&mut self) -> EvalResult<NumberPair> {
-        let v2 = self.pop();
-        let v1 = self.pop();
-
-        match (v1, v2) {
-            (Value::Integer(i1), Value::Integer(i2)) => Ok(NumberPair::Integer(i1, i2)),
-
-            (Value::Float(f1), Value::Float(f2)) => Ok(NumberPair::Floats(f1, f2)),
-
-            (Value::Integer(i1), Value::Float(f2)) => Ok(NumberPair::Floats(i1 as f64, f2)),
-
-            (Value::Float(f1), Value::Integer(i2)) => Ok(NumberPair::Floats(f1, i2 as f64)),
-
-            (v1, v2) => Err(Error::TypeError {
-                expected: "number (either int or float)",
-                actual: if v1.is_number() {
-                    v2.type_of()
-                } else {
-                    v1.type_of()
-                },
-            }),
-        }
-    }
-
-    fn push(&mut self, value: Value) {
-        self.stack.push(value)
-    }
-
-    fn run(&mut self) -> EvalResult<Value> {
-        loop {
-            match self.inc_ip() {
-                OpCode::OpConstant(idx) => {
-                    let c = self.chunk.constant(idx).clone();
-                    self.push(c);
-                }
-
-                OpCode::OpAdd => match self.pop_number_pair()? {
-                    NumberPair::Floats(f1, f2) => self.push(Value::Float(f1 + f2)),
-                    NumberPair::Integer(i1, i2) => self.push(Value::Integer(i1 + i2)),
-                },
-
-                OpCode::OpSub => match self.pop_number_pair()? {
-                    NumberPair::Floats(f1, f2) => self.push(Value::Float(f1 - f2)),
-                    NumberPair::Integer(i1, i2) => self.push(Value::Integer(i1 - i2)),
-                },
-
-                OpCode::OpMul => match self.pop_number_pair()? {
-                    NumberPair::Floats(f1, f2) => self.push(Value::Float(f1 * f2)),
-                    NumberPair::Integer(i1, i2) => self.push(Value::Integer(i1 * i2)),
-                },
-
-                OpCode::OpDiv => match self.pop_number_pair()? {
-                    NumberPair::Floats(f1, f2) => self.push(Value::Float(f1 / f2)),
-                    NumberPair::Integer(i1, i2) => self.push(Value::Integer(i1 / i2)),
-                },
-
-                OpCode::OpInvert => {
-                    let v = self.pop().as_bool()?;
-                    self.push(Value::Bool(!v));
-                }
-
-                OpCode::OpNegate => match self.pop() {
-                    Value::Integer(i) => self.push(Value::Integer(-i)),
-                    Value::Float(f) => self.push(Value::Float(-f)),
-                    v => {
-                        return Err(Error::TypeError {
-                            expected: "number (either int or float)",
-                            actual: v.type_of(),
-                        })
-                    }
-                },
-
-                OpCode::OpEqual => {
-                    let v2 = self.pop();
-                    let v1 = self.pop();
-
-                    let eq = match (v1, v2) {
-                        (Value::Float(f), Value::Integer(i))
-                        | (Value::Integer(i), Value::Float(f)) => f == (i as f64),
-
-                        (v1, v2) => v1 == v2,
-                    };
-
-                    self.push(Value::Bool(eq))
-                }
-
-                OpCode::OpNull => self.push(Value::Null),
-                OpCode::OpTrue => self.push(Value::Bool(true)),
-                OpCode::OpFalse => self.push(Value::Bool(false)),
-                OpCode::OpAttrs(count) => self.run_attrset(count)?,
-                OpCode::OpAttrPath(count) => self.run_attr_path(count)?,
-                OpCode::OpList(count) => self.run_list(count)?,
-                OpCode::OpInterpolate(count) => self.run_interpolate(count)?,
-            }
-
-            if self.ip == self.chunk.code.len() {
-                return Ok(self.pop());
-            }
-        }
-    }
-
-    // Construct runtime representation of an attr path (essentially
-    // just a list of strings).
-    //
-    // The difference to the list construction operation is that this
-    // forces all elements into strings, as attribute set keys are
-    // required to be strict in Nix.
-    fn run_attr_path(&mut self, count: usize) -> EvalResult<()> {
-        debug_assert!(count > 1, "AttrPath needs at least two fragments");
-        let mut path = Vec::with_capacity(count);
-
-        for _ in 0..count {
-            path.push(self.pop().as_string()?);
-        }
-
-        self.push(Value::AttrPath(path));
-        Ok(())
-    }
-
-    fn run_attrset(&mut self, count: usize) -> EvalResult<()> {
-        // If the attribute count happens to be 2, we might be able to
-        // create the optimised name/value struct instead.
-        if count == 2 {
-            // When determining whether we are dealing with a
-            // name/value pair, we return the stack locations of name
-            // and value, using `0` as a sentinel value (i.e. if
-            // either is 0, we are dealing with some other attrset).
-            let is_pair = {
-                // The keys are located 1 & 3 values back in the
-                // stack.
-                let k1 = self.peek(1);
-                let k2 = self.peek(3);
-
-                match (k1, k2) {
-                    (Value::String(NixString(s1)), Value::String(NixString(s2)))
-                        if (s1 == "name" && s2 == "value") =>
-                    {
-                        (1, 2)
-                    }
-
-                    (Value::String(NixString(s1)), Value::String(NixString(s2)))
-                        if (s1 == "value" && s2 == "name") =>
-                    {
-                        (2, 1)
-                    }
-
-                    // Technically this branch lets type errors pass,
-                    // but they will be caught during normal attribute
-                    // set construction instead.
-                    _ => (0, 0),
-                }
-            };
-
-            match is_pair {
-                (1, 2) => {
-                    // The value of 'name' is at stack slot 0, the
-                    // value of 'value' is at stack slot 2.
-                    let pair = Value::Attrs(Rc::new(NixAttrs::KV {
-                        name: self.pop(),
-                        value: {
-                            self.pop(); // ignore the key
-                            self.pop()
-                        },
-                    }));
-
-                    // Clean up the last key fragment.
-                    self.pop();
-
-                    self.push(pair);
-                    return Ok(());
-                }
-
-                (2, 1) => {
-                    // The value of 'name' is at stack slot 2, the
-                    // value of 'value' is at stack slot 0.
-                    let pair = Value::Attrs(Rc::new(NixAttrs::KV {
-                        value: self.pop(),
-                        name: {
-                            self.pop(); // ignore the key
-                            self.pop()
-                        },
-                    }));
-
-                    // Clean up the last key fragment.
-                    self.pop();
-
-                    self.push(pair);
-                    return Ok(());
-                }
-                _ => {}
-            }
-        }
-
-        let mut attrs: BTreeMap<NixString, Value> = BTreeMap::new();
-
-        for _ in 0..count {
-            let value = self.pop();
-
-            // It is at this point that nested attribute sets need to
-            // be constructed (if they exist).
-            //
-            let key = self.pop();
-            match key {
-                Value::String(ks) => set_attr(&mut attrs, ks, value)?,
-
-                Value::AttrPath(mut path) => {
-                    set_nested_attr(
-                        &mut attrs,
-                        path.pop().expect("AttrPath is never empty"),
-                        path,
-                        value,
-                    )?;
-                }
-
-                other => {
-                    return Err(Error::InvalidKeyType {
-                        given: other.type_of(),
-                    })
-                }
-            }
-        }
-
-        // TODO(tazjin): extend_reserve(count) (rust#72631)
-        self.push(Value::Attrs(Rc::new(NixAttrs::Map(attrs))));
-        Ok(())
-    }
-
-    // Interpolate string fragments by popping the specified number of
-    // fragments of the stack, evaluating them to strings, and pushing
-    // the concatenated result string back on the stack.
-    fn run_interpolate(&mut self, count: usize) -> EvalResult<()> {
-        let mut out = String::new();
-
-        for _ in 0..count {
-            out.push_str(&self.pop().as_string()?.0);
-        }
-
-        self.push(Value::String(NixString(out)));
-        Ok(())
-    }
-
-    // Construct runtime representation of a list. Because the list
-    // items are on the stack in reverse order, the vector is created
-    // initialised and elements are directly assigned to their
-    // respective indices.
-    fn run_list(&mut self, count: usize) -> EvalResult<()> {
-        let mut list = vec![Value::Null; count];
-
-        for idx in 0..count {
-            list[count - idx - 1] = self.pop();
-        }
-
-        self.push(Value::List(NixList(list)));
-        Ok(())
-    }
-}
-
-#[derive(Clone, Copy, Debug, PartialEq)]
-pub enum NumberPair {
-    Floats(f64, f64),
-    Integer(i64, i64),
-}
-
-// Set an attribute on an in-construction attribute set, while
-// checking against duplicate key.s
-fn set_attr(
-    attrs: &mut BTreeMap<NixString, Value>,
-    key: NixString,
-    value: Value,
-) -> EvalResult<()> {
-    let entry = attrs.entry(key);
-
-    match entry {
-        std::collections::btree_map::Entry::Occupied(entry) => {
-            return Err(Error::DuplicateAttrsKey {
-                key: entry.key().0.clone(),
-            })
-        }
-
-        std::collections::btree_map::Entry::Vacant(entry) => {
-            entry.insert(value);
-            return Ok(());
-        }
-    };
-}
-
-// Set a nested attribute inside of an attribute set, throwing a
-// duplicate key error if a non-hashmap entry already exists on the
-// path.
-//
-// There is some optimisation potential for this simple implementation
-// if it becomes a problem.
-fn set_nested_attr(
-    attrs: &mut BTreeMap<NixString, Value>,
-    key: NixString,
-    mut path: Vec<NixString>,
-    value: Value,
-) -> EvalResult<()> {
-    // If there is no next key we are at the point where we
-    // should insert the value itself.
-    if path.is_empty() {
-        return set_attr(attrs, key, value);
-    }
-
-    let entry = attrs.entry(key);
-
-    // If there is not we go one step further down, in which case we
-    // need to ensure that there either is no entry, or the existing
-    // entry is a hashmap into which to insert the next value.
-    //
-    // If a value of a different type exists, the user specified a
-    // duplicate key.
-    match entry {
-        // Vacant entry -> new attribute set is needed.
-        std::collections::btree_map::Entry::Vacant(entry) => {
-            let mut map = BTreeMap::new();
-
-            // TODO(tazjin): technically recursing further is not
-            // required, we can create the whole hierarchy here, but
-            // it's noisy.
-            set_nested_attr(&mut map, path.pop().expect("next key exists"), path, value)?;
-
-            entry.insert(Value::Attrs(Rc::new(NixAttrs::Map(map))));
-        }
-
-        // Occupied entry: Either error out if there is something
-        // other than attrs, or insert the next value.
-        std::collections::btree_map::Entry::Occupied(mut entry) => match entry.get_mut() {
-            Value::Attrs(_attrs) => {
-                todo!("implement mutable attrsets")
-            }
-
-            _ => {
-                return Err(Error::DuplicateAttrsKey {
-                    key: entry.key().0.clone(),
-                })
-            }
-        },
-    }
-
-    Ok(())
-}
-
-pub fn run_chunk(chunk: Chunk) -> EvalResult<Value> {
-    let mut vm = VM {
-        chunk,
-        ip: 0,
-        stack: vec![],
-    };
-
-    vm.run()
-}
diff --git a/tvix/eval/src/vm/generators.rs b/tvix/eval/src/vm/generators.rs
new file mode 100644
index 0000000000..79de688692
--- /dev/null
+++ b/tvix/eval/src/vm/generators.rs
@@ -0,0 +1,809 @@
+//! This module implements generator logic for the VM. Generators are functions
+//! used during evaluation which can suspend their execution during their
+//! control flow, and request that the VM do something.
+//!
+//! This is used to keep the VM's stack size constant even when evaluating
+//! deeply nested recursive data structures.
+//!
+//! We implement generators using the [`genawaiter`] crate.
+
+use core::pin::Pin;
+use genawaiter::rc::Co;
+pub use genawaiter::rc::Gen;
+use std::fmt::Display;
+use std::future::Future;
+
+use crate::value::PointerEquality;
+use crate::warnings::{EvalWarning, WarningKind};
+use crate::FileType;
+use crate::NixString;
+
+use super::*;
+
+// -- Implementation of generic generator logic.
+
+/// States that a generator can be in while being driven by the VM.
+pub(crate) enum GeneratorState {
+    /// Normal execution of the generator.
+    Running,
+
+    /// Generator is awaiting the result of a forced value.
+    AwaitingValue,
+}
+
+/// Messages that can be sent from generators *to* the VM. In most
+/// cases, the VM will suspend the generator when receiving a message
+/// and enter some other frame to process the request.
+///
+/// Responses are returned to generators via the [`GeneratorResponse`] type.
+pub enum VMRequest {
+    /// Request that the VM forces this value. This message is first sent to the
+    /// VM with the unforced value, then returned to the generator with the
+    /// forced result.
+    ForceValue(Value),
+
+    /// Request that the VM deep-forces the value.
+    DeepForceValue(Value),
+
+    /// Request the value at the given index from the VM's with-stack, in forced
+    /// state.
+    ///
+    /// The value is returned in the `ForceValue` message.
+    WithValue(usize),
+
+    /// Request the value at the given index from the *captured* with-stack, in
+    /// forced state.
+    CapturedWithValue(usize),
+
+    /// Request that the two values be compared for Nix equality. The result is
+    /// returned in the `ForceValue` message.
+    NixEquality(Box<(Value, Value)>, PointerEquality),
+
+    /// Push the given value to the VM's stack. This is used to prepare the
+    /// stack for requesting a function call from the VM.
+    ///
+    /// The VM does not respond to this request, so the next message received is
+    /// `Empty`.
+    StackPush(Value),
+
+    /// Pop a value from the stack and return it to the generator.
+    StackPop,
+
+    /// Request that the VM coerces this value to a string.
+    StringCoerce(Value, CoercionKind),
+
+    /// Request that the VM calls the given value, with arguments already
+    /// prepared on the stack. Value must already be forced.
+    Call(Value),
+
+    /// Request a call frame entering the given lambda immediately. This can be
+    /// used to force thunks.
+    EnterLambda {
+        lambda: Rc<Lambda>,
+        upvalues: Rc<Upvalues>,
+        light_span: LightSpan,
+    },
+
+    /// Emit a runtime warning (already containing a span) through the VM.
+    EmitWarning(EvalWarning),
+
+    /// Emit a runtime warning through the VM. The span of the current generator
+    /// is used for the final warning.
+    EmitWarningKind(WarningKind),
+
+    /// Request a lookup in the VM's import cache, which tracks the
+    /// thunks yielded by previously imported files.
+    ImportCacheLookup(PathBuf),
+
+    /// Provide the VM with an imported value for a given path, which
+    /// it can populate its input cache with.
+    ImportCachePut(PathBuf, Value),
+
+    /// Request that the VM imports the given path through its I/O interface.
+    PathImport(PathBuf),
+
+    /// Request that the VM opens the specified file and provides a reader.
+    OpenFile(PathBuf),
+
+    /// Request that the VM checks whether the given path exists.
+    PathExists(PathBuf),
+
+    /// Request that the VM reads the given path.
+    ReadDir(PathBuf),
+
+    /// Request a reasonable span from the VM.
+    Span,
+
+    /// Request evaluation of `builtins.tryEval` from the VM. See
+    /// [`VM::catch_result`] for an explanation of how this works.
+    TryForce(Value),
+
+    /// Request serialisation of a value to JSON, according to the
+    /// slightly odd Nix evaluation rules.
+    ToJson(Value),
+}
+
+/// Human-readable representation of a generator message, used by observers.
+impl Display for VMRequest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            VMRequest::ForceValue(v) => write!(f, "force_value({})", v.type_of()),
+            VMRequest::DeepForceValue(v) => {
+                write!(f, "deep_force_value({})", v.type_of())
+            }
+            VMRequest::WithValue(_) => write!(f, "with_value"),
+            VMRequest::CapturedWithValue(_) => write!(f, "captured_with_value"),
+            VMRequest::NixEquality(values, ptr_eq) => {
+                write!(
+                    f,
+                    "nix_eq({}, {}, PointerEquality::{:?})",
+                    values.0.type_of(),
+                    values.1.type_of(),
+                    ptr_eq
+                )
+            }
+            VMRequest::StackPush(v) => write!(f, "stack_push({})", v.type_of()),
+            VMRequest::StackPop => write!(f, "stack_pop"),
+            VMRequest::StringCoerce(
+                v,
+                CoercionKind {
+                    strong,
+                    import_paths,
+                },
+            ) => write!(
+                f,
+                "{}_{}importing_string_coerce({})",
+                if *strong { "strong" } else { "weak" },
+                if *import_paths { "" } else { "non_" },
+                v.type_of()
+            ),
+            VMRequest::Call(v) => write!(f, "call({})", v),
+            VMRequest::EnterLambda { lambda, .. } => {
+                write!(f, "enter_lambda({:p})", *lambda)
+            }
+            VMRequest::EmitWarning(_) => write!(f, "emit_warning"),
+            VMRequest::EmitWarningKind(_) => write!(f, "emit_warning_kind"),
+            VMRequest::ImportCacheLookup(p) => {
+                write!(f, "import_cache_lookup({})", p.to_string_lossy())
+            }
+            VMRequest::ImportCachePut(p, _) => {
+                write!(f, "import_cache_put({})", p.to_string_lossy())
+            }
+            VMRequest::PathImport(p) => write!(f, "path_import({})", p.to_string_lossy()),
+            VMRequest::OpenFile(p) => {
+                write!(f, "open_file({})", p.to_string_lossy())
+            }
+            VMRequest::PathExists(p) => write!(f, "path_exists({})", p.to_string_lossy()),
+            VMRequest::ReadDir(p) => write!(f, "read_dir({})", p.to_string_lossy()),
+            VMRequest::Span => write!(f, "span"),
+            VMRequest::TryForce(v) => write!(f, "try_force({})", v.type_of()),
+            VMRequest::ToJson(v) => write!(f, "to_json({})", v.type_of()),
+        }
+    }
+}
+
+/// Responses returned to generators *from* the VM.
+pub enum VMResponse {
+    /// Empty message. Passed to the generator as the first message,
+    /// or when return values were optional.
+    Empty,
+
+    /// Value produced by the VM and returned to the generator.
+    Value(Value),
+
+    /// Path produced by the VM in response to some IO operation.
+    Path(PathBuf),
+
+    /// VM response with the contents of a directory.
+    Directory(Vec<(bytes::Bytes, FileType)>),
+
+    /// VM response with a span to use at the current point.
+    Span(LightSpan),
+
+    /// [std::io::Reader] produced by the VM in response to some IO operation.
+    Reader(Box<dyn std::io::Read>),
+}
+
+impl Display for VMResponse {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            VMResponse::Empty => write!(f, "empty"),
+            VMResponse::Value(v) => write!(f, "value({})", v),
+            VMResponse::Path(p) => write!(f, "path({})", p.to_string_lossy()),
+            VMResponse::Directory(d) => write!(f, "dir(len = {})", d.len()),
+            VMResponse::Span(_) => write!(f, "span"),
+            VMResponse::Reader(_) => write!(f, "reader"),
+        }
+    }
+}
+
+pub(crate) type Generator =
+    Gen<VMRequest, VMResponse, Pin<Box<dyn Future<Output = Result<Value, ErrorKind>>>>>;
+
+/// Helper function to provide type annotations which are otherwise difficult to
+/// infer.
+pub fn pin_generator(
+    f: impl Future<Output = Result<Value, ErrorKind>> + 'static,
+) -> Pin<Box<dyn Future<Output = Result<Value, ErrorKind>>>> {
+    Box::pin(f)
+}
+
+impl<'o, IO> VM<'o, IO>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
+    /// Helper function to re-enqueue the current generator while it
+    /// is awaiting a value.
+    fn reenqueue_generator(&mut self, name: &'static str, span: LightSpan, generator: Generator) {
+        self.frames.push(Frame::Generator {
+            name,
+            generator,
+            span,
+            state: GeneratorState::AwaitingValue,
+        });
+    }
+
+    /// Helper function to enqueue a new generator.
+    pub(super) fn enqueue_generator<F, G>(&mut self, name: &'static str, span: LightSpan, gen: G)
+    where
+        F: Future<Output = Result<Value, ErrorKind>> + 'static,
+        G: FnOnce(GenCo) -> F,
+    {
+        self.frames.push(Frame::Generator {
+            name,
+            span,
+            state: GeneratorState::Running,
+            generator: Gen::new(|co| pin_generator(gen(co))),
+        });
+    }
+
+    /// Run a generator frame until it yields to the outer control loop, or runs
+    /// to completion.
+    ///
+    /// The return value indicates whether the generator has completed (true),
+    /// or was suspended (false).
+    pub(crate) fn run_generator(
+        &mut self,
+        name: &'static str,
+        span: LightSpan,
+        frame_id: usize,
+        state: GeneratorState,
+        mut generator: Generator,
+        initial_message: Option<VMResponse>,
+    ) -> EvalResult<bool> {
+        // Determine what to send to the generator based on its state.
+        let mut message = match (initial_message, state) {
+            (Some(msg), _) => msg,
+            (_, GeneratorState::Running) => VMResponse::Empty,
+
+            // If control returned here, and the generator is
+            // awaiting a value, send it the top of the stack.
+            (_, GeneratorState::AwaitingValue) => VMResponse::Value(self.stack_pop()),
+        };
+
+        loop {
+            match generator.resume_with(message) {
+                // If the generator yields, it contains an instruction
+                // for what the VM should do.
+                genawaiter::GeneratorState::Yielded(request) => {
+                    self.observer.observe_generator_request(name, &request);
+
+                    match request {
+                        VMRequest::StackPush(value) => {
+                            self.stack.push(value);
+                            message = VMResponse::Empty;
+                        }
+
+                        VMRequest::StackPop => {
+                            message = VMResponse::Value(self.stack_pop());
+                        }
+
+                        // Generator has requested a force, which means that
+                        // this function prepares the frame stack and yields
+                        // back to the outer VM loop.
+                        VMRequest::ForceValue(value) => {
+                            self.reenqueue_generator(name, span.clone(), generator);
+                            self.enqueue_generator("force", span.clone(), |co| {
+                                value.force_owned_genco(co, span)
+                            });
+                            return Ok(false);
+                        }
+
+                        // Generator has requested a deep-force.
+                        VMRequest::DeepForceValue(value) => {
+                            self.reenqueue_generator(name, span.clone(), generator);
+                            self.enqueue_generator("deep_force", span.clone(), |co| {
+                                value.deep_force(co, span)
+                            });
+                            return Ok(false);
+                        }
+
+                        // Generator has requested a value from the with-stack.
+                        // Logic is similar to `ForceValue`, except with the
+                        // value being taken from that stack.
+                        VMRequest::WithValue(idx) => {
+                            self.reenqueue_generator(name, span.clone(), generator);
+
+                            let value = self.stack[self.with_stack[idx]].clone();
+                            self.enqueue_generator("force", span.clone(), |co| {
+                                value.force_owned_genco(co, span)
+                            });
+
+                            return Ok(false);
+                        }
+
+                        // Generator has requested a value from the *captured*
+                        // with-stack. Logic is same as above, except for the
+                        // value being from that stack.
+                        VMRequest::CapturedWithValue(idx) => {
+                            self.reenqueue_generator(name, span.clone(), generator);
+
+                            let call_frame = self.last_call_frame()
+                                .expect("Tvix bug: generator requested captured with-value, but there is no call frame");
+
+                            let value = call_frame.upvalues.with_stack().unwrap()[idx].clone();
+                            self.enqueue_generator("force", span.clone(), |co| {
+                                value.force_owned_genco(co, span)
+                            });
+
+                            return Ok(false);
+                        }
+
+                        VMRequest::NixEquality(values, ptr_eq) => {
+                            let values = *values;
+                            self.reenqueue_generator(name, span.clone(), generator);
+                            self.enqueue_generator("nix_eq", span.clone(), |co| {
+                                values.0.nix_eq_owned_genco(values.1, co, ptr_eq, span)
+                            });
+                            return Ok(false);
+                        }
+
+                        VMRequest::StringCoerce(val, kind) => {
+                            self.reenqueue_generator(name, span.clone(), generator);
+                            self.enqueue_generator("coerce_to_string", span.clone(), |co| {
+                                val.coerce_to_string(co, kind, span)
+                            });
+                            return Ok(false);
+                        }
+
+                        VMRequest::Call(callable) => {
+                            self.reenqueue_generator(name, span.clone(), generator);
+                            self.call_value(span, None, callable)?;
+                            return Ok(false);
+                        }
+
+                        VMRequest::EnterLambda {
+                            lambda,
+                            upvalues,
+                            light_span,
+                        } => {
+                            self.reenqueue_generator(name, span, generator);
+
+                            self.frames.push(Frame::CallFrame {
+                                span: light_span,
+                                call_frame: CallFrame {
+                                    lambda,
+                                    upvalues,
+                                    ip: CodeIdx(0),
+                                    stack_offset: self.stack.len(),
+                                },
+                            });
+
+                            return Ok(false);
+                        }
+
+                        VMRequest::EmitWarning(warning) => {
+                            self.push_warning(warning);
+                            message = VMResponse::Empty;
+                        }
+
+                        VMRequest::EmitWarningKind(kind) => {
+                            self.emit_warning(kind);
+                            message = VMResponse::Empty;
+                        }
+
+                        VMRequest::ImportCacheLookup(path) => {
+                            if let Some(cached) = self.import_cache.get(path) {
+                                message = VMResponse::Value(cached.clone());
+                            } else {
+                                message = VMResponse::Empty;
+                            }
+                        }
+
+                        VMRequest::ImportCachePut(path, value) => {
+                            self.import_cache.insert(path, value);
+                            message = VMResponse::Empty;
+                        }
+
+                        VMRequest::PathImport(path) => {
+                            let imported = self
+                                .io_handle
+                                .as_ref()
+                                .import_path(&path)
+                                .map_err(|e| ErrorKind::IO {
+                                    path: Some(path),
+                                    error: e.into(),
+                                })
+                                .with_span(&span, self)?;
+
+                            message = VMResponse::Path(imported);
+                        }
+
+                        VMRequest::OpenFile(path) => {
+                            let reader = self
+                                .io_handle
+                                .as_ref()
+                                .open(&path)
+                                .map_err(|e| ErrorKind::IO {
+                                    path: Some(path),
+                                    error: e.into(),
+                                })
+                                .with_span(&span, self)?;
+
+                            message = VMResponse::Reader(reader)
+                        }
+
+                        VMRequest::PathExists(path) => {
+                            let exists = self
+                                .io_handle
+                                .as_ref()
+                                .path_exists(&path)
+                                .map_err(|e| ErrorKind::IO {
+                                    path: Some(path),
+                                    error: e.into(),
+                                })
+                                .map(Value::Bool)
+                                .with_span(&span, self)?;
+
+                            message = VMResponse::Value(exists);
+                        }
+
+                        VMRequest::ReadDir(path) => {
+                            let dir = self
+                                .io_handle
+                                .as_ref()
+                                .read_dir(&path)
+                                .map_err(|e| ErrorKind::IO {
+                                    path: Some(path),
+                                    error: e.into(),
+                                })
+                                .with_span(&span, self)?;
+                            message = VMResponse::Directory(dir);
+                        }
+
+                        VMRequest::Span => {
+                            message = VMResponse::Span(self.reasonable_light_span());
+                        }
+
+                        VMRequest::TryForce(value) => {
+                            self.try_eval_frames.push(frame_id);
+                            self.reenqueue_generator(name, span.clone(), generator);
+
+                            debug_assert!(
+                                self.frames.len() == frame_id + 1,
+                                "generator should be reenqueued with the same frame ID"
+                            );
+
+                            self.enqueue_generator("force", span.clone(), |co| {
+                                value.force_owned_genco(co, span)
+                            });
+                            return Ok(false);
+                        }
+
+                        VMRequest::ToJson(value) => {
+                            self.reenqueue_generator(name, span.clone(), generator);
+                            self.enqueue_generator("to_json", span, |co| {
+                                value.into_contextful_json_generator(co)
+                            });
+                            return Ok(false);
+                        }
+                    }
+                }
+
+                // Generator has completed, and its result value should
+                // be left on the stack.
+                genawaiter::GeneratorState::Complete(result) => {
+                    let value = result.with_span(&span, self)?;
+                    self.stack.push(value);
+                    return Ok(true);
+                }
+            }
+        }
+    }
+}
+
+pub type GenCo = Co<VMRequest, VMResponse>;
+
+// -- Implementation of concrete generator use-cases.
+
+/// Request that the VM place the given value on its stack.
+pub async fn request_stack_push(co: &GenCo, val: Value) {
+    match co.yield_(VMRequest::StackPush(val)).await {
+        VMResponse::Empty => {}
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM pop a value from the stack and return it to the
+/// generator.
+pub async fn request_stack_pop(co: &GenCo) -> Value {
+    match co.yield_(VMRequest::StackPop).await {
+        VMResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Force any value and return the evaluated result from the VM.
+pub async fn request_force(co: &GenCo, val: Value) -> Value {
+    if let Value::Thunk(_) = val {
+        match co.yield_(VMRequest::ForceValue(val)).await {
+            VMResponse::Value(value) => value,
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        }
+    } else {
+        val
+    }
+}
+
+/// Force a value
+pub(crate) async fn request_try_force(co: &GenCo, val: Value) -> Value {
+    if let Value::Thunk(_) = val {
+        match co.yield_(VMRequest::TryForce(val)).await {
+            VMResponse::Value(value) => value,
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        }
+    } else {
+        val
+    }
+}
+
+/// Call the given value as a callable. The argument(s) must already be prepared
+/// on the stack.
+pub async fn request_call(co: &GenCo, val: Value) -> Value {
+    let val = request_force(co, val).await;
+    match co.yield_(VMRequest::Call(val)).await {
+        VMResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Helper function to call the given value with the provided list of arguments.
+/// This uses the StackPush and Call messages under the hood.
+pub async fn request_call_with<I>(co: &GenCo, mut callable: Value, args: I) -> Value
+where
+    I: IntoIterator<Item = Value>,
+    I::IntoIter: DoubleEndedIterator,
+{
+    let mut num_args = 0_usize;
+    for arg in args.into_iter().rev() {
+        num_args += 1;
+        request_stack_push(co, arg).await;
+    }
+
+    debug_assert!(num_args > 0, "call_with called with an empty list of args");
+
+    while num_args > 0 {
+        callable = request_call(co, callable).await;
+        num_args -= 1;
+    }
+
+    callable
+}
+
+pub async fn request_string_coerce(
+    co: &GenCo,
+    val: Value,
+    kind: CoercionKind,
+) -> Result<NixString, CatchableErrorKind> {
+    match val {
+        Value::String(s) => Ok(s),
+        _ => match co.yield_(VMRequest::StringCoerce(val, kind)).await {
+            VMResponse::Value(Value::Catchable(c)) => Err(*c),
+            VMResponse::Value(value) => Ok(value
+                .to_contextful_str()
+                .expect("coerce_to_string always returns a string")),
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        },
+    }
+}
+
+/// Deep-force any value and return the evaluated result from the VM.
+pub async fn request_deep_force(co: &GenCo, val: Value) -> Value {
+    match co.yield_(VMRequest::DeepForceValue(val)).await {
+        VMResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Ask the VM to compare two values for equality.
+pub(crate) async fn check_equality(
+    co: &GenCo,
+    a: Value,
+    b: Value,
+    ptr_eq: PointerEquality,
+) -> Result<Result<bool, CatchableErrorKind>, ErrorKind> {
+    match co
+        .yield_(VMRequest::NixEquality(Box::new((a, b)), ptr_eq))
+        .await
+    {
+        VMResponse::Value(Value::Bool(b)) => Ok(Ok(b)),
+        VMResponse::Value(Value::Catchable(cek)) => Ok(Err(*cek)),
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Emit a fully constructed runtime warning.
+pub(crate) async fn emit_warning(co: &GenCo, warning: EvalWarning) {
+    match co.yield_(VMRequest::EmitWarning(warning)).await {
+        VMResponse::Empty => {}
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Emit a runtime warning with the span of the current generator.
+pub async fn emit_warning_kind(co: &GenCo, kind: WarningKind) {
+    match co.yield_(VMRequest::EmitWarningKind(kind)).await {
+        VMResponse::Empty => {}
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM enter the given lambda.
+pub(crate) async fn request_enter_lambda(
+    co: &GenCo,
+    lambda: Rc<Lambda>,
+    upvalues: Rc<Upvalues>,
+    light_span: LightSpan,
+) -> Value {
+    let msg = VMRequest::EnterLambda {
+        lambda,
+        upvalues,
+        light_span,
+    };
+
+    match co.yield_(msg).await {
+        VMResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request a lookup in the VM's import cache.
+pub(crate) async fn request_import_cache_lookup(co: &GenCo, path: PathBuf) -> Option<Value> {
+    match co.yield_(VMRequest::ImportCacheLookup(path)).await {
+        VMResponse::Value(value) => Some(value),
+        VMResponse::Empty => None,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM populate its input cache for the given path.
+pub(crate) async fn request_import_cache_put(co: &GenCo, path: PathBuf, value: Value) {
+    match co.yield_(VMRequest::ImportCachePut(path, value)).await {
+        VMResponse::Empty => {}
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM import the given path.
+pub(crate) async fn request_path_import(co: &GenCo, path: PathBuf) -> PathBuf {
+    match co.yield_(VMRequest::PathImport(path)).await {
+        VMResponse::Path(path) => path,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM open a [std::io::Read] for the specified file.
+pub async fn request_open_file(co: &GenCo, path: PathBuf) -> Box<dyn std::io::Read> {
+    match co.yield_(VMRequest::OpenFile(path)).await {
+        VMResponse::Reader(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_path_exists(co: &GenCo, path: PathBuf) -> Value {
+    match co.yield_(VMRequest::PathExists(path)).await {
+        VMResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_read_dir(co: &GenCo, path: PathBuf) -> Vec<(bytes::Bytes, FileType)> {
+    match co.yield_(VMRequest::ReadDir(path)).await {
+        VMResponse::Directory(dir) => dir,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_span(co: &GenCo) -> LightSpan {
+    match co.yield_(VMRequest::Span).await {
+        VMResponse::Span(span) => span,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_to_json(
+    co: &GenCo,
+    value: Value,
+) -> Result<(serde_json::Value, NixContext), CatchableErrorKind> {
+    match co.yield_(VMRequest::ToJson(value)).await {
+        VMResponse::Value(Value::Json(json_with_ctx)) => Ok(*json_with_ctx),
+        VMResponse::Value(Value::Catchable(cek)) => Err(*cek),
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Call the given value as if it was an attribute set containing a functor. The
+/// arguments must already be prepared on the stack when a generator frame from
+/// this function is invoked.
+///
+pub(crate) async fn call_functor(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+    let attrs = value.to_attrs()?;
+
+    match attrs.select("__functor") {
+        None => Err(ErrorKind::NotCallable("set without `__functor_` attribute")),
+        Some(functor) => {
+            // The functor receives the set itself as its first argument and
+            // needs to be called with it.
+            let functor = request_force(&co, functor.clone()).await;
+            let primed = request_call_with(&co, functor, [value]).await;
+            Ok(request_call(&co, primed).await)
+        }
+    }
+}
diff --git a/tvix/eval/src/vm/macros.rs b/tvix/eval/src/vm/macros.rs
new file mode 100644
index 0000000000..d8a09706ab
--- /dev/null
+++ b/tvix/eval/src/vm/macros.rs
@@ -0,0 +1,93 @@
+/// This module provides macros which are used in the implementation
+/// of the VM for the implementation of repetitive operations.
+
+/// This macro simplifies the implementation of arithmetic operations,
+/// correctly handling the behaviour on different pairings of number
+/// types.
+#[macro_export]
+macro_rules! arithmetic_op {
+    ( $self:ident, $op:tt ) => {{ // TODO: remove
+        let b = $self.pop();
+        let a = $self.pop();
+        let result = fallible!($self, arithmetic_op!(&a, &b, $op));
+        $self.push(result);
+    }};
+
+    ( $a:expr, $b:expr, $op:tt ) => {{
+        match ($a, $b) {
+            (Value::Integer(i1), Value::Integer(i2)) => Ok(Value::Integer(i1 $op i2)),
+            (Value::Float(f1), Value::Float(f2)) => Ok(Value::Float(f1 $op f2)),
+            (Value::Integer(i1), Value::Float(f2)) => Ok(Value::Float(*i1 as f64 $op f2)),
+            (Value::Float(f1), Value::Integer(i2)) => Ok(Value::Float(f1 $op *i2 as f64)),
+
+            (v1, v2) => Err(ErrorKind::TypeError {
+                expected: "number (either int or float)",
+                actual: if v1.is_number() {
+                    v2.type_of()
+                } else {
+                    v1.type_of()
+                },
+            }),
+        }
+    }};
+}
+
+/// This macro simplifies the implementation of comparison operations.
+#[macro_export]
+macro_rules! cmp_op {
+    ( $vm:ident, $frame:ident, $span:ident, $op:tt ) => {{
+        lifted_pop! {
+            $vm(b, a) => {
+                async fn compare(a: Value, b: Value, co: GenCo) -> Result<Value, ErrorKind> {
+                    let a = generators::request_force(&co, a).await;
+                    let b = generators::request_force(&co, b).await;
+                    let span = generators::request_span(&co).await;
+                    let ordering = a.nix_cmp_ordering(b, co, span).await?;
+                    match ordering {
+                        Err(cek) => Ok(Value::from(cek)),
+                        Ok(ordering) => Ok(Value::Bool(cmp_op!(@order $op ordering))),
+                    }
+                }
+
+                let gen_span = $frame.current_light_span();
+                $vm.push_call_frame($span, $frame);
+                $vm.enqueue_generator("compare", gen_span, |co| compare(a, b, co));
+                return Ok(false);
+            }
+        }
+    }};
+
+    (@order < $ordering:expr) => {
+        $ordering == Ordering::Less
+    };
+
+    (@order > $ordering:expr) => {
+        $ordering == Ordering::Greater
+    };
+
+    (@order <= $ordering:expr) => {
+        matches!($ordering, Ordering::Equal | Ordering::Less)
+    };
+
+    (@order >= $ordering:expr) => {
+        matches!($ordering, Ordering::Equal | Ordering::Greater)
+    };
+}
+
+#[macro_export]
+macro_rules! lifted_pop {
+    ($vm:ident ($($bind:ident),+) => $body:expr) => {
+        {
+            $(
+                let $bind = $vm.stack_pop();
+            )+
+            $(
+                if $bind.is_catchable() {
+                    $vm.stack.push($bind);
+                    continue;
+                }
+            )+
+            $body
+        }
+    }
+}
diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
new file mode 100644
index 0000000000..5c244cc3ca
--- /dev/null
+++ b/tvix/eval/src/vm/mod.rs
@@ -0,0 +1,1368 @@
+//! This module implements the abstract/virtual machine that runs Tvix
+//! bytecode.
+//!
+//! The operation of the VM is facilitated by the [`Frame`] type,
+//! which controls the current execution state of the VM and is
+//! processed within the VM's operating loop.
+//!
+//! A [`VM`] is used by instantiating it with an initial [`Frame`],
+//! then triggering its execution and waiting for the VM to return or
+//! yield an error.
+
+pub mod generators;
+mod macros;
+
+use bstr::{BString, ByteSlice, ByteVec};
+use codemap::Span;
+use serde_json::json;
+use std::{cmp::Ordering, collections::HashMap, ops::DerefMut, path::PathBuf, rc::Rc};
+
+use crate::{
+    arithmetic_op,
+    chunk::Chunk,
+    cmp_op,
+    compiler::GlobalsMap,
+    errors::{CatchableErrorKind, Error, ErrorKind, EvalResult},
+    io::EvalIO,
+    lifted_pop,
+    nix_search_path::NixSearchPath,
+    observer::RuntimeObserver,
+    opcode::{CodeIdx, Count, JumpOffset, OpCode, StackIdx, UpvalueIdx},
+    spans::LightSpan,
+    upvalues::Upvalues,
+    value::{
+        Builtin, BuiltinResult, Closure, CoercionKind, Lambda, NixAttrs, NixContext, NixList,
+        PointerEquality, Thunk, Value,
+    },
+    vm::generators::GenCo,
+    warnings::{EvalWarning, WarningKind},
+    NixString, SourceCode,
+};
+
+use generators::{call_functor, Generator, GeneratorState};
+
+use self::generators::{VMRequest, VMResponse};
+
+/// Internal helper trait for taking a span from a variety of types, to make use
+/// of `WithSpan` (defined below) more ergonomic at call sites.
+trait GetSpan {
+    fn get_span(self) -> Span;
+}
+
+impl<'o, IO> GetSpan for &VM<'o, IO> {
+    fn get_span(self) -> Span {
+        self.reasonable_span.span()
+    }
+}
+
+impl GetSpan for &CallFrame {
+    fn get_span(self) -> Span {
+        self.current_span()
+    }
+}
+
+impl GetSpan for &LightSpan {
+    fn get_span(self) -> Span {
+        self.span()
+    }
+}
+
+impl GetSpan for Span {
+    fn get_span(self) -> Span {
+        self
+    }
+}
+
+/// Internal helper trait for ergonomically converting from a `Result<T,
+/// ErrorKind>` to a `Result<T, Error>` using the current span of a call frame,
+/// and chaining the VM's frame stack around it for printing a cause chain.
+trait WithSpan<T, S: GetSpan, IO> {
+    fn with_span(self, top_span: S, vm: &VM<IO>) -> Result<T, Error>;
+}
+
+impl<T, S: GetSpan, IO> WithSpan<T, S, IO> for Result<T, ErrorKind> {
+    fn with_span(self, top_span: S, vm: &VM<IO>) -> Result<T, Error> {
+        match self {
+            Ok(something) => Ok(something),
+            Err(kind) => {
+                let mut error = Error::new(kind, top_span.get_span(), vm.source.clone());
+
+                // Wrap the top-level error in chaining errors for each element
+                // of the frame stack.
+                for frame in vm.frames.iter().rev() {
+                    match frame {
+                        Frame::CallFrame { span, .. } => {
+                            error = Error::new(
+                                ErrorKind::BytecodeError(Box::new(error)),
+                                span.span(),
+                                vm.source.clone(),
+                            );
+                        }
+                        Frame::Generator { name, span, .. } => {
+                            error = Error::new(
+                                ErrorKind::NativeError {
+                                    err: Box::new(error),
+                                    gen_type: name,
+                                },
+                                span.span(),
+                                vm.source.clone(),
+                            );
+                        }
+                    }
+                }
+
+                Err(error)
+            }
+        }
+    }
+}
+
+struct CallFrame {
+    /// The lambda currently being executed.
+    lambda: Rc<Lambda>,
+
+    /// Optional captured upvalues of this frame (if a thunk or
+    /// closure if being evaluated).
+    upvalues: Rc<Upvalues>,
+
+    /// Instruction pointer to the instruction currently being
+    /// executed.
+    ip: CodeIdx,
+
+    /// Stack offset, i.e. the frames "view" into the VM's full stack.
+    stack_offset: usize,
+}
+
+impl CallFrame {
+    /// Retrieve an upvalue from this frame at the given index.
+    fn upvalue(&self, idx: UpvalueIdx) -> &Value {
+        &self.upvalues[idx]
+    }
+
+    /// Borrow the chunk of this frame's lambda.
+    fn chunk(&self) -> &Chunk {
+        &self.lambda.chunk
+    }
+
+    /// Increment this frame's instruction pointer and return the operation that
+    /// the pointer moved past.
+    fn inc_ip(&mut self) -> OpCode {
+        let op = self.chunk()[self.ip];
+        self.ip += 1;
+        op
+    }
+
+    /// Construct an error result from the given ErrorKind and the source span
+    /// of the current instruction.
+    pub fn error<T, IO>(&self, vm: &VM<IO>, kind: ErrorKind) -> Result<T, Error> {
+        Err(kind).with_span(self, vm)
+    }
+
+    /// Returns the current span. This is potentially expensive and should only
+    /// be used when actually constructing an error or warning.
+    pub fn current_span(&self) -> Span {
+        self.chunk().get_span(self.ip - 1)
+    }
+
+    /// Returns the information needed to calculate the current span,
+    /// but without performing that calculation.
+    // TODO: why pub?
+    pub(crate) fn current_light_span(&self) -> LightSpan {
+        LightSpan::new_actual(self.current_span())
+    }
+}
+
+/// A frame represents an execution state of the VM. The VM has a stack of
+/// frames representing the nesting of execution inside of the VM, and operates
+/// on the frame at the top.
+///
+/// When a frame has been fully executed, it is removed from the VM's frame
+/// stack and expected to leave a result [`Value`] on the top of the stack.
+enum Frame {
+    /// CallFrame represents the execution of Tvix bytecode within a thunk,
+    /// function or closure.
+    CallFrame {
+        /// The call frame itself, separated out into another type to pass it
+        /// around easily.
+        call_frame: CallFrame,
+
+        /// Span from which the call frame was launched.
+        span: LightSpan,
+    },
+
+    /// Generator represents a frame that can yield further
+    /// instructions to the VM while its execution is being driven.
+    ///
+    /// A generator is essentially an asynchronous function that can
+    /// be suspended while waiting for the VM to do something (e.g.
+    /// thunk forcing), and resume at the same point.
+    Generator {
+        /// human-readable description of the generator,
+        name: &'static str,
+
+        /// Span from which the generator was launched.
+        span: LightSpan,
+
+        state: GeneratorState,
+
+        /// Generator itself, which can be resumed with `.resume()`.
+        generator: Generator,
+    },
+}
+
+impl Frame {
+    pub fn span(&self) -> LightSpan {
+        match self {
+            Frame::CallFrame { span, .. } | Frame::Generator { span, .. } => span.clone(),
+        }
+    }
+}
+
+#[derive(Default)]
+struct ImportCache(HashMap<PathBuf, Value>);
+
+/// The `ImportCache` holds the `Value` resulting from `import`ing a certain
+/// file, so that the same file doesn't need to be re-evaluated multiple times.
+/// Currently the real path of the imported file (determined using
+/// [`std::fs::canonicalize()`], not to be confused with our
+/// [`crate::value::canon_path()`]) is used to identify the file,
+/// just like C++ Nix does.
+///
+/// Errors while determining the real path are currently just ignored, since we
+/// pass around some fake paths like `/__corepkgs__/fetchurl.nix`.
+///
+/// In the future, we could use something more sophisticated, like file hashes.
+/// However, a consideration is that the eval cache is observable via impurities
+/// like pointer equality and `builtins.trace`.
+impl ImportCache {
+    fn get(&self, path: PathBuf) -> Option<&Value> {
+        let path = match std::fs::canonicalize(path.as_path()).map_err(ErrorKind::from) {
+            Ok(path) => path,
+            Err(_) => path,
+        };
+        self.0.get(&path)
+    }
+
+    fn insert(&mut self, path: PathBuf, value: Value) -> Option<Value> {
+        self.0.insert(
+            match std::fs::canonicalize(path.as_path()).map_err(ErrorKind::from) {
+                Ok(path) => path,
+                Err(_) => path,
+            },
+            value,
+        )
+    }
+}
+
+struct VM<'o, IO> {
+    /// VM's frame stack, representing the execution contexts the VM is working
+    /// through. Elements are usually pushed when functions are called, or
+    /// thunks are being forced.
+    frames: Vec<Frame>,
+
+    /// The VM's top-level value stack. Within this stack, each code-executing
+    /// frame holds a "view" of the stack representing the slice of the
+    /// top-level stack that is relevant to its operation. This is done to avoid
+    /// allocating a new `Vec` for each frame's stack.
+    pub(crate) stack: Vec<Value>,
+
+    /// Stack indices (absolute indexes into `stack`) of attribute
+    /// sets from which variables should be dynamically resolved
+    /// (`with`).
+    with_stack: Vec<usize>,
+
+    /// Runtime warnings collected during evaluation.
+    warnings: Vec<EvalWarning>,
+
+    /// Import cache, mapping absolute file paths to the value that
+    /// they compile to. Note that this reuses thunks, too!
+    // TODO: should probably be based on a file hash
+    pub import_cache: ImportCache,
+
+    /// Data structure holding all source code evaluated in this VM,
+    /// used for pretty error reporting.
+    source: SourceCode,
+
+    /// Parsed Nix search path, which is used to resolve `<...>`
+    /// references.
+    nix_search_path: NixSearchPath,
+
+    /// Implementation of I/O operations used for impure builtins and
+    /// features like `import`.
+    io_handle: IO,
+
+    /// Runtime observer which can print traces of runtime operations.
+    observer: &'o mut dyn RuntimeObserver,
+
+    /// Strong reference to the globals, guaranteeing that they are
+    /// kept alive for the duration of evaluation.
+    ///
+    /// This is important because recursive builtins (specifically
+    /// `import`) hold a weak reference to the builtins, while the
+    /// original strong reference is held by the compiler which does
+    /// not exist anymore at runtime.
+    #[allow(dead_code)]
+    globals: Rc<GlobalsMap>,
+
+    /// A reasonably applicable span that can be used for errors in each
+    /// execution situation.
+    ///
+    /// The VM should update this whenever control flow changes take place (i.e.
+    /// entering or exiting a frame to yield control somewhere).
+    reasonable_span: LightSpan,
+
+    /// This field is responsible for handling `builtins.tryEval`. When that
+    /// builtin is encountered, it sends a special message to the VM which
+    /// pushes the frame index that requested to be informed of catchable
+    /// errors in this field.
+    ///
+    /// The frame stack is then laid out like this:
+    ///
+    /// ```notrust
+    /// โ”Œโ”€โ”€โ”ฌโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”
+    /// โ”‚ 0โ”‚ `Result`-producing frame โ”‚
+    /// โ”œโ”€โ”€โ”ผโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”ค
+    /// โ”‚-1โ”‚ `builtins.tryEval` frame โ”‚
+    /// โ”œโ”€โ”€โ”ผโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”ค
+    /// โ”‚..โ”‚ ... other frames ...     โ”‚
+    /// โ””โ”€โ”€โ”ดโ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”€โ”˜
+    /// ```
+    ///
+    /// Control is yielded to the outer VM loop, which evaluates the next frame
+    /// and returns the result itself to the `builtins.tryEval` frame.
+    try_eval_frames: Vec<usize>,
+}
+
+impl<'o, IO> VM<'o, IO>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
+    pub fn new(
+        nix_search_path: NixSearchPath,
+        io_handle: IO,
+        observer: &'o mut dyn RuntimeObserver,
+        source: SourceCode,
+        globals: Rc<GlobalsMap>,
+        reasonable_span: LightSpan,
+    ) -> Self {
+        Self {
+            nix_search_path,
+            io_handle,
+            observer,
+            globals,
+            reasonable_span,
+            source,
+            frames: vec![],
+            stack: vec![],
+            with_stack: vec![],
+            warnings: vec![],
+            import_cache: Default::default(),
+            try_eval_frames: vec![],
+        }
+    }
+
+    /// Push a call frame onto the frame stack.
+    fn push_call_frame(&mut self, span: LightSpan, call_frame: CallFrame) {
+        self.frames.push(Frame::CallFrame { span, call_frame })
+    }
+
+    /// Run the VM's primary (outer) execution loop, continuing execution based
+    /// on the current frame at the top of the frame stack.
+    fn execute(mut self) -> EvalResult<RuntimeResult> {
+        while let Some(frame) = self.frames.pop() {
+            self.reasonable_span = frame.span();
+            let frame_id = self.frames.len();
+
+            match frame {
+                Frame::CallFrame { call_frame, span } => {
+                    self.observer
+                        .observe_enter_call_frame(0, &call_frame.lambda, frame_id);
+
+                    match self.execute_bytecode(span, call_frame) {
+                        Ok(true) => self.observer.observe_exit_call_frame(frame_id, &self.stack),
+                        Ok(false) => self
+                            .observer
+                            .observe_suspend_call_frame(frame_id, &self.stack),
+
+                        Err(err) => return Err(err),
+                    };
+                }
+
+                // Handle generator frames, which can request thunk forcing
+                // during their execution.
+                Frame::Generator {
+                    name,
+                    span,
+                    state,
+                    generator,
+                } => {
+                    self.observer
+                        .observe_enter_generator(frame_id, name, &self.stack);
+
+                    match self.run_generator(name, span, frame_id, state, generator, None) {
+                        Ok(true) => {
+                            self.observer
+                                .observe_exit_generator(frame_id, name, &self.stack)
+                        }
+                        Ok(false) => {
+                            self.observer
+                                .observe_suspend_generator(frame_id, name, &self.stack)
+                        }
+
+                        Err(err) => return Err(err),
+                    };
+                }
+            }
+        }
+
+        // Once no more frames are present, return the stack's top value as the
+        // result.
+        let value = self
+            .stack
+            .pop()
+            .expect("tvix bug: runtime stack empty after execution");
+        Ok(RuntimeResult {
+            value,
+            warnings: self.warnings,
+        })
+    }
+
+    /// Run the VM's inner execution loop, processing Tvix bytecode from a
+    /// chunk. This function returns if:
+    ///
+    /// 1. The code has run to the end, and has left a value on the top of the
+    ///    stack. In this case, the frame is not returned to the frame stack.
+    ///
+    /// 2. The code encounters a generator, in which case the frame in its
+    /// current state is pushed back on the stack, and the generator is left on
+    /// top of it for the outer loop to execute.
+    ///
+    /// 3. An error is encountered.
+    ///
+    /// This function *must* ensure that it leaves the frame stack in the
+    /// correct order, especially when re-enqueuing a frame to execute.
+    ///
+    /// The return value indicates whether the bytecode has been executed to
+    /// completion, or whether it has been suspended in favour of a generator.
+    fn execute_bytecode(&mut self, span: LightSpan, mut frame: CallFrame) -> EvalResult<bool> {
+        loop {
+            let op = frame.inc_ip();
+            self.observer.observe_execute_op(frame.ip, &op, &self.stack);
+
+            match op {
+                OpCode::OpThunkSuspended(idx) | OpCode::OpThunkClosure(idx) => {
+                    let blueprint = match &frame.chunk()[idx] {
+                        Value::Blueprint(lambda) => lambda.clone(),
+                        _ => panic!("compiler bug: non-blueprint in blueprint slot"),
+                    };
+
+                    let upvalue_count = blueprint.upvalue_count;
+                    let thunk = if matches!(op, OpCode::OpThunkClosure(_)) {
+                        debug_assert!(
+                            upvalue_count > 0,
+                            "OpThunkClosure should not be called for plain lambdas"
+                        );
+                        Thunk::new_closure(blueprint)
+                    } else {
+                        Thunk::new_suspended(blueprint, frame.current_light_span())
+                    };
+                    let upvalues = thunk.upvalues_mut();
+                    self.stack.push(Value::Thunk(thunk.clone()));
+
+                    // From this point on we internally mutate the
+                    // upvalues. The closure (if `is_closure`) is
+                    // already in its stack slot, which means that it
+                    // can capture itself as an upvalue for
+                    // self-recursion.
+                    self.populate_upvalues(&mut frame, upvalue_count, upvalues)?;
+                }
+
+                OpCode::OpForce => {
+                    if let Some(Value::Thunk(_)) = self.stack.last() {
+                        let thunk = match self.stack_pop() {
+                            Value::Thunk(t) => t,
+                            _ => unreachable!(),
+                        };
+
+                        let gen_span = frame.current_light_span();
+
+                        self.push_call_frame(span, frame);
+                        self.enqueue_generator("force", gen_span.clone(), |co| {
+                            Thunk::force(thunk, co, gen_span)
+                        });
+
+                        return Ok(false);
+                    }
+                }
+
+                OpCode::OpGetUpvalue(upv_idx) => {
+                    let value = frame.upvalue(upv_idx).clone();
+                    self.stack.push(value);
+                }
+
+                // Discard the current frame.
+                OpCode::OpReturn => {
+                    // TODO(amjoseph): I think this should assert `==` rather
+                    // than `<=` but it fails with the stricter condition.
+                    debug_assert!(self.stack.len() - 1 <= frame.stack_offset);
+                    return Ok(true);
+                }
+
+                OpCode::OpConstant(idx) => {
+                    let c = frame.chunk()[idx].clone();
+                    self.stack.push(c);
+                }
+
+                OpCode::OpCall => {
+                    let callable = self.stack_pop();
+                    self.call_value(frame.current_light_span(), Some((span, frame)), callable)?;
+
+                    // exit this loop and let the outer loop enter the new call
+                    return Ok(true);
+                }
+
+                // Remove the given number of elements from the stack,
+                // but retain the top value.
+                OpCode::OpCloseScope(Count(count)) => {
+                    // Immediately move the top value into the right
+                    // position.
+                    let target_idx = self.stack.len() - 1 - count;
+                    self.stack[target_idx] = self.stack_pop();
+
+                    // Then drop the remaining values.
+                    for _ in 0..(count - 1) {
+                        self.stack.pop();
+                    }
+                }
+
+                OpCode::OpClosure(idx) => {
+                    let blueprint = match &frame.chunk()[idx] {
+                        Value::Blueprint(lambda) => lambda.clone(),
+                        _ => panic!("compiler bug: non-blueprint in blueprint slot"),
+                    };
+
+                    let upvalue_count = blueprint.upvalue_count;
+                    debug_assert!(
+                        upvalue_count > 0,
+                        "OpClosure should not be called for plain lambdas"
+                    );
+
+                    let mut upvalues = Upvalues::with_capacity(blueprint.upvalue_count);
+                    self.populate_upvalues(&mut frame, upvalue_count, &mut upvalues)?;
+                    self.stack
+                        .push(Value::Closure(Rc::new(Closure::new_with_upvalues(
+                            Rc::new(upvalues),
+                            blueprint,
+                        ))));
+                }
+
+                OpCode::OpAttrsSelect => lifted_pop! {
+                    self(key, attrs) => {
+                        let key = key.to_str().with_span(&frame, self)?;
+                        let attrs = attrs.to_attrs().with_span(&frame, self)?;
+
+                        match attrs.select(&key) {
+                            Some(value) => self.stack.push(value.clone()),
+
+                            None => {
+                                return frame.error(
+                                    self,
+                                    ErrorKind::AttributeNotFound {
+                                        name: key.to_str_lossy().into_owned()
+                                    },
+                                );
+                            }
+                        }
+                    }
+                },
+
+                OpCode::OpJumpIfFalse(JumpOffset(offset)) => {
+                    debug_assert!(offset != 0);
+                    if !self.stack_peek(0).as_bool().with_span(&frame, self)? {
+                        frame.ip += offset;
+                    }
+                }
+
+                OpCode::OpJumpIfCatchable(JumpOffset(offset)) => {
+                    debug_assert!(offset != 0);
+                    if self.stack_peek(0).is_catchable() {
+                        frame.ip += offset;
+                    }
+                }
+
+                OpCode::OpJumpIfNoFinaliseRequest(JumpOffset(offset)) => {
+                    debug_assert!(offset != 0);
+                    match self.stack_peek(0) {
+                        Value::FinaliseRequest(finalise) => {
+                            if !finalise {
+                                frame.ip += offset;
+                            }
+                        },
+                        val => panic!("Tvix bug: OpJumIfNoFinaliseRequest: expected FinaliseRequest, but got {}", val.type_of()),
+                    }
+                }
+
+                OpCode::OpPop => {
+                    self.stack.pop();
+                }
+
+                OpCode::OpAttrsTrySelect => {
+                    let key = self.stack_pop().to_str().with_span(&frame, self)?;
+                    let value = match self.stack_pop() {
+                        Value::Attrs(attrs) => match attrs.select(&key) {
+                            Some(value) => value.clone(),
+                            None => Value::AttrNotFound,
+                        },
+
+                        _ => Value::AttrNotFound,
+                    };
+
+                    self.stack.push(value);
+                }
+
+                OpCode::OpGetLocal(StackIdx(local_idx)) => {
+                    let idx = frame.stack_offset + local_idx;
+                    self.stack.push(self.stack[idx].clone());
+                }
+
+                OpCode::OpJumpIfNotFound(JumpOffset(offset)) => {
+                    debug_assert!(offset != 0);
+                    if matches!(self.stack_peek(0), Value::AttrNotFound) {
+                        self.stack_pop();
+                        frame.ip += offset;
+                    }
+                }
+
+                OpCode::OpJump(JumpOffset(offset)) => {
+                    debug_assert!(offset != 0);
+                    frame.ip += offset;
+                }
+
+                OpCode::OpEqual => lifted_pop! {
+                    self(b, a) => {
+                        let gen_span = frame.current_light_span();
+                        self.push_call_frame(span, frame);
+                        self.enqueue_generator("nix_eq", gen_span.clone(), |co| {
+                            a.nix_eq_owned_genco(b, co, PointerEquality::ForbidAll, gen_span)
+                        });
+                        return Ok(false);
+                    }
+                },
+
+                // These assertion operations error out if the stack
+                // top is not of the expected type. This is necessary
+                // to implement some specific behaviours of Nix
+                // exactly.
+                OpCode::OpAssertBool => {
+                    let val = self.stack_peek(0);
+                    // TODO(edef): propagate this into is_bool, since bottom values *are* values of any type
+                    if !val.is_catchable() && !val.is_bool() {
+                        return frame.error(
+                            self,
+                            ErrorKind::TypeError {
+                                expected: "bool",
+                                actual: val.type_of(),
+                            },
+                        );
+                    }
+                }
+
+                OpCode::OpAssertAttrs => {
+                    let val = self.stack_peek(0);
+                    // TODO(edef): propagate this into is_attrs, since bottom values *are* values of any type
+                    if !val.is_catchable() && !val.is_attrs() {
+                        return frame.error(
+                            self,
+                            ErrorKind::TypeError {
+                                expected: "set",
+                                actual: val.type_of(),
+                            },
+                        );
+                    }
+                }
+
+                OpCode::OpAttrs(Count(count)) => self.run_attrset(&frame, count)?,
+
+                OpCode::OpAttrsUpdate => lifted_pop! {
+                    self(rhs, lhs) => {
+                        let rhs = rhs.to_attrs().with_span(&frame, self)?;
+                        let lhs = lhs.to_attrs().with_span(&frame, self)?;
+                        self.stack.push(Value::attrs(lhs.update(*rhs)))
+                    }
+                },
+
+                OpCode::OpInvert => lifted_pop! {
+                    self(v) => {
+                        let v = v.as_bool().with_span(&frame, self)?;
+                        self.stack.push(Value::Bool(!v));
+                    }
+                },
+
+                OpCode::OpList(Count(count)) => {
+                    let list =
+                        NixList::construct(count, self.stack.split_off(self.stack.len() - count));
+
+                    self.stack.push(Value::List(list));
+                }
+
+                OpCode::OpJumpIfTrue(JumpOffset(offset)) => {
+                    debug_assert!(offset != 0);
+                    if self.stack_peek(0).as_bool().with_span(&frame, self)? {
+                        frame.ip += offset;
+                    }
+                }
+
+                OpCode::OpHasAttr => lifted_pop! {
+                    self(key, attrs) => {
+                        let key = key.to_str().with_span(&frame, self)?;
+                        let result = match attrs {
+                            Value::Attrs(attrs) => attrs.contains(&key),
+
+                            // Nix allows use of `?` on non-set types, but
+                            // always returns false in those cases.
+                            _ => false,
+                        };
+
+                        self.stack.push(Value::Bool(result));
+                    }
+                },
+
+                OpCode::OpConcat => lifted_pop! {
+                    self(rhs, lhs) => {
+                        let rhs = rhs.to_list().with_span(&frame, self)?.into_inner();
+                        let lhs = lhs.to_list().with_span(&frame, self)?.into_inner();
+                        self.stack.push(Value::List(NixList::from(lhs + rhs)))
+                    }
+                },
+
+                OpCode::OpResolveWith => {
+                    let ident = self.stack_pop().to_str().with_span(&frame, self)?;
+
+                    // Re-enqueue this frame.
+                    let op_span = frame.current_light_span();
+                    self.push_call_frame(span, frame);
+
+                    // Construct a generator frame doing the lookup in constant
+                    // stack space.
+                    let with_stack_len = self.with_stack.len();
+                    let closed_with_stack_len = self
+                        .last_call_frame()
+                        .map(|frame| frame.upvalues.with_stack_len())
+                        .unwrap_or(0);
+
+                    self.enqueue_generator("resolve_with", op_span, |co| {
+                        resolve_with(
+                            co,
+                            ident.as_bstr().to_owned(),
+                            with_stack_len,
+                            closed_with_stack_len,
+                        )
+                    });
+
+                    return Ok(false);
+                }
+
+                OpCode::OpFinalise(StackIdx(idx)) => match &self.stack[frame.stack_offset + idx] {
+                    Value::Closure(_) => panic!("attempted to finalise a closure"),
+                    Value::Thunk(thunk) => thunk.finalise(&self.stack[frame.stack_offset..]),
+                    _ => panic!("attempted to finalise a non-thunk"),
+                },
+
+                OpCode::OpCoerceToString(kind) => {
+                    let value = self.stack_pop();
+                    let gen_span = frame.current_light_span();
+                    self.push_call_frame(span, frame);
+
+                    self.enqueue_generator("coerce_to_string", gen_span.clone(), |co| {
+                        value.coerce_to_string(co, kind, gen_span)
+                    });
+
+                    return Ok(false);
+                }
+
+                OpCode::OpInterpolate(Count(count)) => self.run_interpolate(&frame, count)?,
+
+                OpCode::OpValidateClosedFormals => {
+                    let formals = frame.lambda.formals.as_ref().expect(
+                        "OpValidateClosedFormals called within the frame of a lambda without formals",
+                    );
+
+                    let peeked = self.stack_peek(0);
+                    if peeked.is_catchable() {
+                        continue;
+                    }
+
+                    let args = peeked.to_attrs().with_span(&frame, self)?;
+                    for arg in args.keys() {
+                        if !formals.contains(arg) {
+                            return frame.error(
+                                self,
+                                ErrorKind::UnexpectedArgument {
+                                    arg: arg.clone(),
+                                    formals_span: formals.span,
+                                },
+                            );
+                        }
+                    }
+                }
+
+                OpCode::OpAdd => lifted_pop! {
+                    self(b, a) => {
+                        let gen_span = frame.current_light_span();
+                        self.push_call_frame(span, frame);
+
+                        // OpAdd can add not just numbers, but also string-like
+                        // things, which requires more VM logic. This operation is
+                        // evaluated in a generator frame.
+                        self.enqueue_generator("add_values", gen_span, |co| add_values(co, a, b));
+                        return Ok(false);
+                    }
+                },
+
+                OpCode::OpSub => lifted_pop! {
+                    self(b, a) => {
+                        let result = arithmetic_op!(&a, &b, -).with_span(&frame, self)?;
+                        self.stack.push(result);
+                    }
+                },
+
+                OpCode::OpMul => lifted_pop! {
+                    self(b, a) => {
+                        let result = arithmetic_op!(&a, &b, *).with_span(&frame, self)?;
+                        self.stack.push(result);
+                    }
+                },
+
+                OpCode::OpDiv => lifted_pop! {
+                    self(b, a) => {
+                        match b {
+                            Value::Integer(0) => return frame.error(self, ErrorKind::DivisionByZero),
+                            Value::Float(b) if b == 0.0_f64 => {
+                                return frame.error(self, ErrorKind::DivisionByZero)
+                            }
+                            _ => {}
+                        };
+
+                        let result = arithmetic_op!(&a, &b, /).with_span(&frame, self)?;
+                        self.stack.push(result);
+                    }
+                },
+
+                OpCode::OpNegate => match self.stack_pop() {
+                    Value::Integer(i) => self.stack.push(Value::Integer(-i)),
+                    Value::Float(f) => self.stack.push(Value::Float(-f)),
+                    Value::Catchable(cex) => self.stack.push(Value::Catchable(cex)),
+                    v => {
+                        return frame.error(
+                            self,
+                            ErrorKind::TypeError {
+                                expected: "number (either int or float)",
+                                actual: v.type_of(),
+                            },
+                        );
+                    }
+                },
+
+                OpCode::OpLess => cmp_op!(self, frame, span, <),
+                OpCode::OpLessOrEq => cmp_op!(self, frame, span, <=),
+                OpCode::OpMore => cmp_op!(self, frame, span, >),
+                OpCode::OpMoreOrEq => cmp_op!(self, frame, span, >=),
+
+                OpCode::OpFindFile => match self.stack_pop() {
+                    Value::UnresolvedPath(path) => {
+                        let resolved = self
+                            .nix_search_path
+                            .resolve(&self.io_handle, *path)
+                            .with_span(&frame, self)?;
+                        self.stack.push(resolved.into());
+                    }
+
+                    _ => panic!("tvix compiler bug: OpFindFile called on non-UnresolvedPath"),
+                },
+
+                OpCode::OpResolveHomePath => match self.stack_pop() {
+                    Value::UnresolvedPath(path) => {
+                        match dirs::home_dir() {
+                            None => {
+                                return frame.error(
+                                    self,
+                                    ErrorKind::RelativePathResolution(
+                                        "failed to determine home directory".into(),
+                                    ),
+                                );
+                            }
+                            Some(mut buf) => {
+                                buf.push(*path);
+                                self.stack.push(buf.into());
+                            }
+                        };
+                    }
+
+                    _ => {
+                        panic!("tvix compiler bug: OpResolveHomePath called on non-UnresolvedPath")
+                    }
+                },
+
+                OpCode::OpPushWith(StackIdx(idx)) => self.with_stack.push(frame.stack_offset + idx),
+
+                OpCode::OpPopWith => {
+                    self.with_stack.pop();
+                }
+
+                OpCode::OpAssertFail => {
+                    self.stack
+                        .push(Value::from(CatchableErrorKind::AssertionFailed));
+                }
+
+                // Data-carrying operands should never be executed,
+                // that is a critical error in the VM/compiler.
+                OpCode::DataStackIdx(_)
+                | OpCode::DataDeferredLocal(_)
+                | OpCode::DataUpvalueIdx(_)
+                | OpCode::DataCaptureWith => {
+                    panic!("Tvix bug: attempted to execute data-carrying operand")
+                }
+            }
+        }
+    }
+}
+
+/// Implementation of helper functions for the runtime logic above.
+impl<'o, IO> VM<'o, IO>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
+    pub(crate) fn stack_pop(&mut self) -> Value {
+        self.stack.pop().expect("runtime stack empty")
+    }
+
+    fn stack_peek(&self, offset: usize) -> &Value {
+        &self.stack[self.stack.len() - 1 - offset]
+    }
+
+    fn run_attrset(&mut self, frame: &CallFrame, count: usize) -> EvalResult<()> {
+        let attrs = NixAttrs::construct(count, self.stack.split_off(self.stack.len() - count * 2))
+            .with_span(frame, self)?
+            .map(Value::attrs)
+            .into();
+
+        self.stack.push(attrs);
+        Ok(())
+    }
+
+    /// Access the last call frame present in the frame stack.
+    fn last_call_frame(&self) -> Option<&CallFrame> {
+        for frame in self.frames.iter().rev() {
+            if let Frame::CallFrame { call_frame, .. } = frame {
+                return Some(call_frame);
+            }
+        }
+
+        None
+    }
+
+    /// Push an already constructed warning.
+    pub fn push_warning(&mut self, warning: EvalWarning) {
+        self.warnings.push(warning);
+    }
+
+    /// Emit a warning with the given WarningKind and the source span
+    /// of the current instruction.
+    pub fn emit_warning(&mut self, kind: WarningKind) {
+        self.push_warning(EvalWarning {
+            kind,
+            span: self.get_span(),
+        });
+    }
+
+    /// Interpolate string fragments by popping the specified number of
+    /// fragments of the stack, evaluating them to strings, and pushing
+    /// the concatenated result string back on the stack.
+    fn run_interpolate(&mut self, frame: &CallFrame, count: usize) -> EvalResult<()> {
+        let mut out = BString::default();
+        // Interpolation propagates the context and union them.
+        let mut context: NixContext = NixContext::new();
+
+        for i in 0..count {
+            let val = self.stack_pop();
+            if val.is_catchable() {
+                for _ in (i + 1)..count {
+                    self.stack.pop();
+                }
+                self.stack.push(val);
+                return Ok(());
+            }
+            let mut nix_string = val.to_contextful_str().with_span(frame, self)?;
+            out.push_str(nix_string.as_bstr());
+            if let Some(nix_string_ctx) = nix_string.context_mut() {
+                context = context.join(nix_string_ctx);
+            }
+        }
+
+        self.stack
+            .push(Value::String(NixString::new_context_from(context, out)));
+        Ok(())
+    }
+
+    /// Returns a reasonable light span for the current situation that the VM is
+    /// in.
+    pub fn reasonable_light_span(&self) -> LightSpan {
+        self.reasonable_span.clone()
+    }
+
+    /// Apply an argument from the stack to a builtin, and attempt to call it.
+    ///
+    /// All calls are tail-calls in Tvix, as every function application is a
+    /// separate thunk and OpCall is thus the last result in the thunk.
+    ///
+    /// Due to this, once control flow exits this function, the generator will
+    /// automatically be run by the VM.
+    fn call_builtin(&mut self, span: LightSpan, mut builtin: Builtin) -> EvalResult<()> {
+        let builtin_name = builtin.name();
+        self.observer.observe_enter_builtin(builtin_name);
+
+        builtin.apply_arg(self.stack_pop());
+
+        match builtin.call() {
+            // Partially applied builtin is just pushed back on the stack.
+            BuiltinResult::Partial(partial) => self.stack.push(Value::Builtin(partial)),
+
+            // Builtin is fully applied and the generator needs to be run by the VM.
+            BuiltinResult::Called(name, generator) => self.frames.push(Frame::Generator {
+                generator,
+                span,
+                name,
+                state: GeneratorState::Running,
+            }),
+        }
+
+        Ok(())
+    }
+
+    fn call_value(
+        &mut self,
+        span: LightSpan,
+        parent: Option<(LightSpan, CallFrame)>,
+        callable: Value,
+    ) -> EvalResult<()> {
+        match callable {
+            Value::Builtin(builtin) => self.call_builtin(span, builtin),
+            Value::Thunk(thunk) => self.call_value(span, parent, thunk.value().clone()),
+
+            Value::Closure(closure) => {
+                let lambda = closure.lambda();
+                self.observer.observe_tail_call(self.frames.len(), &lambda);
+
+                // The stack offset is always `stack.len() - arg_count`, and
+                // since this branch handles native Nix functions (which always
+                // take only a single argument and are curried), the offset is
+                // `stack_len - 1`.
+                let stack_offset = self.stack.len() - 1;
+
+                // Reenqueue the parent frame, which should only have
+                // `OpReturn` left. Not throwing it away leads to more
+                // useful error traces.
+                if let Some((parent_span, parent_frame)) = parent {
+                    self.push_call_frame(parent_span, parent_frame);
+                }
+
+                self.push_call_frame(
+                    span,
+                    CallFrame {
+                        lambda,
+                        upvalues: closure.upvalues(),
+                        ip: CodeIdx(0),
+                        stack_offset,
+                    },
+                );
+
+                Ok(())
+            }
+
+            // Attribute sets with a __functor attribute are callable.
+            val @ Value::Attrs(_) => {
+                if let Some((parent_span, parent_frame)) = parent {
+                    self.push_call_frame(parent_span, parent_frame);
+                }
+
+                self.enqueue_generator("__functor call", span, |co| call_functor(co, val));
+                Ok(())
+            }
+
+            val @ Value::Catchable(_) => {
+                // the argument that we tried to apply a catchable to
+                self.stack.pop();
+                // applying a `throw` to anything is still a `throw`, so we just
+                // push it back on the stack.
+                self.stack.push(val);
+                Ok(())
+            }
+
+            v => Err(ErrorKind::NotCallable(v.type_of())).with_span(&span, self),
+        }
+    }
+
+    /// Populate the upvalue fields of a thunk or closure under construction.
+    fn populate_upvalues(
+        &mut self,
+        frame: &mut CallFrame,
+        count: usize,
+        mut upvalues: impl DerefMut<Target = Upvalues>,
+    ) -> EvalResult<()> {
+        for _ in 0..count {
+            match frame.inc_ip() {
+                OpCode::DataStackIdx(StackIdx(stack_idx)) => {
+                    let idx = frame.stack_offset + stack_idx;
+
+                    let val = match self.stack.get(idx) {
+                        Some(val) => val.clone(),
+                        None => {
+                            return frame.error(
+                                self,
+                                ErrorKind::TvixBug {
+                                    msg: "upvalue to be captured was missing on stack",
+                                    metadata: Some(Rc::new(json!({
+                                        "ip": format!("{:#x}", frame.ip.0 - 1),
+                                        "stack_idx(relative)": stack_idx,
+                                        "stack_idx(absolute)": idx,
+                                    }))),
+                                },
+                            );
+                        }
+                    };
+
+                    upvalues.deref_mut().push(val);
+                }
+
+                OpCode::DataUpvalueIdx(upv_idx) => {
+                    upvalues.deref_mut().push(frame.upvalue(upv_idx).clone());
+                }
+
+                OpCode::DataDeferredLocal(idx) => {
+                    upvalues.deref_mut().push(Value::DeferredUpvalue(idx));
+                }
+
+                OpCode::DataCaptureWith => {
+                    // Start the captured with_stack off of the
+                    // current call frame's captured with_stack, ...
+                    let mut captured_with_stack = frame
+                        .upvalues
+                        .with_stack()
+                        .cloned()
+                        // ... or make an empty one if there isn't one already.
+                        .unwrap_or_else(|| Vec::with_capacity(self.with_stack.len()));
+
+                    for idx in &self.with_stack {
+                        captured_with_stack.push(self.stack[*idx].clone());
+                    }
+
+                    upvalues.deref_mut().set_with_stack(captured_with_stack);
+                }
+
+                _ => panic!("compiler error: missing closure operand"),
+            }
+        }
+
+        Ok(())
+    }
+}
+
+// TODO(amjoseph): de-asyncify this
+/// Resolve a dynamically bound identifier (through `with`) by looking
+/// for matching values in the with-stacks carried at runtime.
+async fn resolve_with(
+    co: GenCo,
+    ident: BString,
+    vm_with_len: usize,
+    upvalue_with_len: usize,
+) -> Result<Value, ErrorKind> {
+    /// Fetch and force a value on the with-stack from the VM.
+    async fn fetch_forced_with(co: &GenCo, idx: usize) -> Value {
+        match co.yield_(VMRequest::WithValue(idx)).await {
+            VMResponse::Value(value) => value,
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        }
+    }
+
+    /// Fetch and force a value on the *captured* with-stack from the VM.
+    async fn fetch_captured_with(co: &GenCo, idx: usize) -> Value {
+        match co.yield_(VMRequest::CapturedWithValue(idx)).await {
+            VMResponse::Value(value) => value,
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        }
+    }
+
+    for with_stack_idx in (0..vm_with_len).rev() {
+        // TODO(tazjin): is this branch still live with the current with-thunking?
+        let with = fetch_forced_with(&co, with_stack_idx).await;
+
+        if with.is_catchable() {
+            return Ok(with);
+        }
+
+        match with.to_attrs()?.select(&ident) {
+            None => continue,
+            Some(val) => return Ok(val.clone()),
+        }
+    }
+
+    for upvalue_with_idx in (0..upvalue_with_len).rev() {
+        let with = fetch_captured_with(&co, upvalue_with_idx).await;
+
+        if with.is_catchable() {
+            return Ok(with);
+        }
+
+        match with.to_attrs()?.select(&ident) {
+            None => continue,
+            Some(val) => return Ok(val.clone()),
+        }
+    }
+
+    Err(ErrorKind::UnknownDynamicVariable(ident.to_string()))
+}
+
+// TODO(amjoseph): de-asyncify this
+async fn add_values(co: GenCo, a: Value, b: Value) -> Result<Value, ErrorKind> {
+    // What we try to do is solely determined by the type of the first value!
+    let result = match (a, b) {
+        (Value::Path(p), v) => {
+            let mut path = p.into_os_string();
+            match generators::request_string_coerce(
+                &co,
+                v,
+                CoercionKind {
+                    strong: false,
+
+                    // Concatenating a Path with something else results in a
+                    // Path, so we don't need to import any paths (paths
+                    // imported by Nix always exist as a string, unless
+                    // converted by the user). In C++ Nix they even may not
+                    // contain any string context, the resulting error of such a
+                    // case can not be replicated by us.
+                    import_paths: false,
+                    // FIXME(raitobezarius): per https://b.tvl.fyi/issues/364, this is a usecase
+                    // for having a `reject_context: true` option here. This didn't occur yet in
+                    // nixpkgs during my evaluations, therefore, I skipped it.
+                },
+            )
+            .await
+            {
+                Ok(vs) => {
+                    path.push(vs.to_os_str()?);
+                    crate::value::canon_path(PathBuf::from(path)).into()
+                }
+                Err(c) => Value::Catchable(Box::new(c)),
+            }
+        }
+        (Value::String(s1), Value::String(s2)) => Value::String(s1.concat(&s2)),
+        (Value::String(s1), v) => generators::request_string_coerce(
+            &co,
+            v,
+            CoercionKind {
+                strong: false,
+                // Behaves the same as string interpolation
+                import_paths: true,
+            },
+        )
+        .await
+        .map(|s2| Value::String(s1.concat(&s2)))
+        .into(),
+        (a @ Value::Integer(_), b) | (a @ Value::Float(_), b) => arithmetic_op!(&a, &b, +)?,
+        (a, b) => {
+            let r1 = generators::request_string_coerce(
+                &co,
+                a,
+                CoercionKind {
+                    strong: false,
+                    import_paths: false,
+                },
+            )
+            .await;
+            let r2 = generators::request_string_coerce(
+                &co,
+                b,
+                CoercionKind {
+                    strong: false,
+                    import_paths: false,
+                },
+            )
+            .await;
+            match (r1, r2) {
+                (Ok(s1), Ok(s2)) => Value::String(s1.concat(&s2)),
+                (Err(c), _) => return Ok(Value::from(c)),
+                (_, Err(c)) => return Ok(Value::from(c)),
+            }
+        }
+    };
+
+    Ok(result)
+}
+
+/// The result of a VM's runtime evaluation.
+pub struct RuntimeResult {
+    pub value: Value,
+    pub warnings: Vec<EvalWarning>,
+}
+
+// TODO(amjoseph): de-asyncify this
+/// Generator that retrieves the final value from the stack, and deep-forces it
+/// before returning.
+async fn final_deep_force(co: GenCo) -> Result<Value, ErrorKind> {
+    let value = generators::request_stack_pop(&co).await;
+    Ok(generators::request_deep_force(&co, value).await)
+}
+
+pub fn run_lambda<IO>(
+    nix_search_path: NixSearchPath,
+    io_handle: IO,
+    observer: &mut dyn RuntimeObserver,
+    source: SourceCode,
+    globals: Rc<GlobalsMap>,
+    lambda: Rc<Lambda>,
+    strict: bool,
+) -> EvalResult<RuntimeResult>
+where
+    IO: AsRef<dyn EvalIO> + 'static,
+{
+    // Retain the top-level span of the expression in this lambda, as
+    // synthetic "calls" in deep_force will otherwise not have a span
+    // to fall back to.
+    //
+    // We exploit the fact that the compiler emits a final instruction
+    // with the span of the entire file for top-level expressions.
+    let root_span = lambda.chunk.get_span(CodeIdx(lambda.chunk.code.len() - 1));
+
+    let mut vm = VM::new(
+        nix_search_path,
+        io_handle,
+        observer,
+        source,
+        globals,
+        root_span.into(),
+    );
+
+    // When evaluating strictly, synthesise a frame that will instruct
+    // the VM to deep-force the final value before returning it.
+    if strict {
+        vm.enqueue_generator("final_deep_force", root_span.into(), final_deep_force);
+    }
+
+    vm.frames.push(Frame::CallFrame {
+        span: root_span.into(),
+        call_frame: CallFrame {
+            lambda,
+            upvalues: Rc::new(Upvalues::with_capacity(0)),
+            ip: CodeIdx(0),
+            stack_offset: 0,
+        },
+    });
+
+    vm.execute()
+}
diff --git a/tvix/eval/src/warnings.rs b/tvix/eval/src/warnings.rs
new file mode 100644
index 0000000000..f537aa913e
--- /dev/null
+++ b/tvix/eval/src/warnings.rs
@@ -0,0 +1,152 @@
+//! Implements warnings that are emitted in cases where code passed to
+//! Tvix exhibits problems that the user could address.
+
+use codemap_diagnostic::{ColorConfig, Diagnostic, Emitter, Level, SpanLabel, SpanStyle};
+
+use crate::SourceCode;
+
+#[derive(Debug)]
+pub enum WarningKind {
+    DeprecatedLiteralURL,
+    UselessInherit,
+    UnusedBinding,
+    ShadowedGlobal(&'static str),
+    DeprecatedLegacyLet,
+    InvalidNixPath(String),
+    UselessBoolOperation(&'static str),
+    DeadCode,
+    EmptyInherit,
+    EmptyLet,
+    ShadowedOutput(String),
+    SRIHashWrongPadding,
+
+    /// Tvix internal warning for features triggered by users that are
+    /// not actually implemented yet, but do not cause runtime failures.
+    NotImplemented(&'static str),
+}
+
+#[derive(Debug)]
+pub struct EvalWarning {
+    pub kind: WarningKind,
+    pub span: codemap::Span,
+}
+
+impl EvalWarning {
+    /// Render a fancy, human-readable output of this warning and
+    /// return it as a String. Note that this version of the output
+    /// does not include any colours or font styles.
+    pub fn fancy_format_str(&self, source: &SourceCode) -> String {
+        let mut out = vec![];
+        Emitter::vec(&mut out, Some(&*source.codemap())).emit(&[self.diagnostic(source)]);
+        String::from_utf8_lossy(&out).to_string()
+    }
+
+    /// Render a fancy, human-readable output of this warning and
+    /// print it to stderr. If rendered in a terminal that supports
+    /// colours and font styles, the output will include those.
+    pub fn fancy_format_stderr(&self, source: &SourceCode) {
+        Emitter::stderr(ColorConfig::Auto, Some(&*source.codemap()))
+            .emit(&[self.diagnostic(source)]);
+    }
+
+    /// Create the optional span label displayed as an annotation on
+    /// the underlined span of the warning.
+    fn span_label(&self) -> Option<String> {
+        match self.kind {
+            WarningKind::UnusedBinding | WarningKind::ShadowedGlobal(_) => {
+                Some("variable declared here".into())
+            }
+            _ => None,
+        }
+    }
+
+    /// Create the primary warning message displayed to users for a
+    /// warning.
+    fn message(&self, source: &SourceCode) -> String {
+        match self.kind {
+            WarningKind::DeprecatedLiteralURL => {
+                "URL literal syntax is deprecated, use a quoted string instead".to_string()
+            }
+
+            WarningKind::UselessInherit => {
+                "inherit does nothing (this variable already exists with the same value)"
+                    .to_string()
+            }
+
+            WarningKind::UnusedBinding => {
+                format!(
+                    "variable '{}' is declared, but never used:",
+                    source.source_slice(self.span)
+                )
+            }
+
+            WarningKind::ShadowedGlobal(name) => {
+                format!("declared variable '{}' shadows a built-in global!", name)
+            }
+
+            WarningKind::DeprecatedLegacyLet => {
+                "legacy `let` syntax used, please rewrite this as `let .. in ...`".to_string()
+            }
+
+            WarningKind::InvalidNixPath(ref err) => {
+                format!("invalid NIX_PATH resulted in a parse error: {}", err)
+            }
+
+            WarningKind::UselessBoolOperation(msg) => {
+                format!("useless operation on boolean: {}", msg)
+            }
+
+            WarningKind::DeadCode => "this code will never be executed".to_string(),
+
+            WarningKind::EmptyInherit => "this `inherit` statement is empty".to_string(),
+
+            WarningKind::EmptyLet => "this `let`-expression contains no bindings".to_string(),
+
+            WarningKind::ShadowedOutput(ref out) => format!(
+                "this derivation's environment shadows the output name {}",
+                out
+            ),
+            WarningKind::SRIHashWrongPadding => "SRI hash has wrong padding".to_string(),
+
+            WarningKind::NotImplemented(what) => {
+                format!("feature not yet implemented in tvix: {}", what)
+            }
+        }
+    }
+
+    /// Return the unique warning code for this variant which can be
+    /// used to refer users to documentation.
+    fn code(&self) -> &'static str {
+        match self.kind {
+            WarningKind::DeprecatedLiteralURL => "W001",
+            WarningKind::UselessInherit => "W002",
+            WarningKind::UnusedBinding => "W003",
+            WarningKind::ShadowedGlobal(_) => "W004",
+            WarningKind::DeprecatedLegacyLet => "W005",
+            WarningKind::InvalidNixPath(_) => "W006",
+            WarningKind::UselessBoolOperation(_) => "W007",
+            WarningKind::DeadCode => "W008",
+            WarningKind::EmptyInherit => "W009",
+            WarningKind::EmptyLet => "W010",
+            WarningKind::ShadowedOutput(_) => "W011",
+            WarningKind::SRIHashWrongPadding => "W012",
+
+            WarningKind::NotImplemented(_) => "W999",
+        }
+    }
+
+    fn diagnostic(&self, source: &SourceCode) -> Diagnostic {
+        let span_label = SpanLabel {
+            label: self.span_label(),
+            span: self.span,
+            style: SpanStyle::Primary,
+        };
+
+        Diagnostic {
+            level: Level::Warning,
+            message: self.message(source),
+            spans: vec![span_label],
+            code: Some(self.code().into()),
+        }
+    }
+}