722 files changed, 19450 insertions, 0 deletions

diff --git a/tvix/eval/.skip-subtree b/tvix/eval/.skip-subtree
new file mode 100644
index 000000000000..05f9fc116f11
--- /dev/null
+++ b/tvix/eval/.skip-subtree
@@ -0,0 +1,2 @@
+Do not traverse further, readTree will encounter Nix language tests
+and such and fail.
diff --git a/tvix/eval/Cargo.toml b/tvix/eval/Cargo.toml
new file mode 100644
index 000000000000..99c4fc1a1106
--- /dev/null
+++ b/tvix/eval/Cargo.toml
@@ -0,0 +1,60 @@
+[package]
+name = "tvix-eval"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[lib]
+name = "tvix_eval"
+
+[dependencies]
+builtin-macros = { path = "./builtin-macros", package = "tvix-eval-builtin-macros" }
+codemap = "0.1.3"
+codemap-diagnostic = "0.1.1"
+dirs = "4.0.0"
+genawaiter = { version = "0.99.1", default_features = false }
+imbl = { version = "2.0", features = [ "serde" ] }
+lazy_static = "1.4.0"
+lexical-core = { version = "0.8.5", features = ["format", "parse-floats"] }
+path-clean = "0.1"
+proptest = { version = "1.0.0", default_features = false, features = ["std", "alloc", "break-dead-code", "tempfile"], optional = true }
+regex = "1.6.0"
+rnix = "0.11.0"
+rowan = "*" # pinned by rnix
+serde = { version = "1.0", features = [ "rc", "derive" ] }
+serde_json = "1.0"
+smol_str = "0.2.0"
+tabwriter = "1.2"
+test-strategy = { version = "0.2.1", optional = true }
+toml = "0.6.0"
+xml-rs = "0.8.4"
+
+[dev-dependencies]
+criterion = "0.4"
+pretty_assertions = "1.2.1"
+itertools = "0.10.3"
+tempdir = "0.3.7"
+
+[dev-dependencies.test-generator]
+# This fork of test-generator adds support for cargo workspaces, see
+# also https://github.com/frehberg/test-generator/pull/14
+git = "https://github.com/JamesGuthrie/test-generator.git"
+rev = "82e799979980962aec1aa324ec6e0e4cad781f41"
+
+[features]
+default = ["impure", "arbitrary", "nix_tests"]
+
+# Enables running the Nix language test suite from the original C++
+# Nix implementation (at version 2.3) against Tvix.
+nix_tests = []
+
+# Enables operations in the VM which depend on the ability to perform I/O
+impure = []
+
+# Enables Arbitrary impls for internal types (required to run tests)
+arbitrary = ["proptest", "test-strategy", "imbl/proptest"]
+
+[[bench]]
+name = "eval"
+harness = false
diff --git a/tvix/eval/README.md b/tvix/eval/README.md
new file mode 100644
index 000000000000..04bc6c3eb82b
--- /dev/null
+++ b/tvix/eval/README.md
@@ -0,0 +1,58 @@
+Tvix Evaluator
+==============
+
+This project implements an interpreter for the Nix programming
+language. You can experiment with an online version of the evaluator:
+[tvixbolt][].
+
+The interpreter aims to be compatible with `nixpkgs`, on the
+foundation of Nix 2.3.
+
+**Important note:** The evaluator is not yet feature-complete, and
+while the core mechanisms (compiler, runtime, ...) have stabilised
+somewhat, a lot of components are still changing rapidly.
+
+Please contact [TVL](https://tvl.fyi) with any questions you might
+have.
+
+## Building tvix-eval
+
+Please check the `README.md` one level up for instructions on how to build this.
+
+The evaluator itself can also be built with standard Rust tooling (i.e. `cargo
+build`).
+
+If you would like to clone **only** the evaluator and build it
+directly with Rust tooling, you can do:
+
+```bash
+git clone https://code.tvl.fyi/depot.git:/tvix/eval.git tvix-eval
+
+cd tvix-eval && cargo build
+```
+
+## Nix test suite
+
+C++ Nix implements a language test suite in the form of Nix source
+code files, and their expected output. The coverage of this test suite
+is not complete, but we intend to be compatible with it.
+
+We have ported the test suite to Tvix, but do not run it by default as
+we are not yet compatible with it.
+
+You can run the test suite by enabling the `nix_tests` feature in
+Cargo:
+
+    cargo test --features nix_tests
+
+## rnix-parser
+
+Tvix is written in memory of jD91mZM2, the author of [rnix-parser][]
+who sadly [passed away][rip].
+
+Tvix makes heavy use of rnix-parser in its bytecode compiler. The
+parser is now maintained by Nix community members.
+
+[rnix-parser]: https://github.com/nix-community/rnix-parser
+[rip]: https://www.redox-os.org/news/open-source-mental-health/
+[tvixbolt]: https://tazj.in/blobs/tvixbolt/
diff --git a/tvix/eval/benches/eval.rs b/tvix/eval/benches/eval.rs
new file mode 100644
index 000000000000..f85e3585f017
--- /dev/null
+++ b/tvix/eval/benches/eval.rs
@@ -0,0 +1,39 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use itertools::Itertools;
+
+fn interpret(code: &str) {
+    tvix_eval::Evaluation::new(code, None).evaluate();
+}
+
+fn eval_literals(c: &mut Criterion) {
+    c.bench_function("int", |b| {
+        b.iter(|| {
+            interpret("42");
+            black_box(())
+        })
+    });
+}
+
+fn eval_merge_attrs(c: &mut Criterion) {
+    c.bench_function("merge small attrs", |b| {
+        b.iter(|| {
+            interpret("{ a = 1; b = 2; } // { c = 3; }");
+            black_box(())
+        })
+    });
+
+    c.bench_function("merge large attrs with small attrs", |b| {
+        let large_attrs = format!(
+            "{{{}}}",
+            (0..10000).map(|n| format!("a{n} = {n};")).join(" ")
+        );
+        let expr = format!("{large_attrs} // {{ c = 3; }}");
+        b.iter(move || {
+            interpret(&expr);
+            black_box(())
+        })
+    });
+}
+
+criterion_group!(benches, eval_literals, eval_merge_attrs);
+criterion_main!(benches);
diff --git a/tvix/eval/build.rs b/tvix/eval/build.rs
new file mode 100644
index 000000000000..a9c9a78b060d
--- /dev/null
+++ b/tvix/eval/build.rs
@@ -0,0 +1,9 @@
+use std::env;
+
+fn main() {
+    println!(
+        "cargo:rustc-env=TVIX_CURRENT_SYSTEM={}",
+        &env::var("TARGET").unwrap()
+    );
+    println!("cargo:rerun-if-changed-env=TARGET")
+}
diff --git a/tvix/eval/builtin-macros/.gitignore b/tvix/eval/builtin-macros/.gitignore
new file mode 100644
index 000000000000..eb5a316cbd19
--- /dev/null
+++ b/tvix/eval/builtin-macros/.gitignore
@@ -0,0 +1 @@
+target
diff --git a/tvix/eval/builtin-macros/Cargo.toml b/tvix/eval/builtin-macros/Cargo.toml
new file mode 100644
index 000000000000..3a35ea12a0c0
--- /dev/null
+++ b/tvix/eval/builtin-macros/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "tvix-eval-builtin-macros"
+version = "0.0.1"
+authors = [ "Griffin Smith <root@gws.fyi>" ]
+edition = "2021"
+
+[dependencies]
+syn = { version = "1.0.57", features = ["full", "parsing", "printing", "visit", "visit-mut", "extra-traits"] }
+quote = "1.0.8"
+proc-macro2 = "1"
+
+[lib]
+proc-macro = true
+
+[dev-dependencies]
+tvix-eval = { path = "../" }
diff --git a/tvix/eval/builtin-macros/src/lib.rs b/tvix/eval/builtin-macros/src/lib.rs
new file mode 100644
index 000000000000..dfd0948c7d9f
--- /dev/null
+++ b/tvix/eval/builtin-macros/src/lib.rs
@@ -0,0 +1,334 @@
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use proc_macro2::Span;
+use quote::{quote, quote_spanned, ToTokens};
+use syn::parse::Parse;
+use syn::spanned::Spanned;
+use syn::{
+    parse2, parse_macro_input, parse_quote, parse_quote_spanned, Attribute, FnArg, Ident, Item,
+    ItemMod, LitStr, Meta, Pat, PatIdent, PatType, Token, Type,
+};
+
+/// Description of a single argument passed to a builtin
+struct BuiltinArgument {
+    /// The name of the argument, to be used in docstrings and error messages
+    name: Ident,
+
+    /// Type of the argument.
+    ty: Box<Type>,
+
+    /// Whether the argument should be forced before the underlying builtin
+    /// function is called.
+    strict: bool,
+
+    /// Span at which the argument was defined.
+    span: Span,
+}
+
+fn extract_docstring(attrs: &[Attribute]) -> Option<String> {
+    // Rust docstrings are transparently written pre-macro expansion into an attribute that looks
+    // like:
+    //
+    // #[doc = "docstring here"]
+    //
+    // Multi-line docstrings yield multiple attributes in order, which we assemble into a single
+    // string below.
+
+    #[allow(dead_code)]
+    #[derive(Debug)]
+    struct Docstring {
+        eq: Token![=],
+        doc: LitStr,
+    }
+
+    impl Parse for Docstring {
+        fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
+            Ok(Self {
+                eq: input.parse()?,
+                doc: input.parse()?,
+            })
+        }
+    }
+
+    attrs
+        .iter()
+        .filter(|attr| attr.path.get_ident().into_iter().any(|id| id == "doc"))
+        .filter_map(|attr| parse2::<Docstring>(attr.tokens.clone()).ok())
+        .map(|docstring| docstring.doc.value())
+        .reduce(|mut fst, snd| {
+            if snd.is_empty() {
+                // An empty string represents a spacing newline that was added in the
+                // original doc comment.
+                fst.push_str("\n\n");
+            } else {
+                fst.push_str(&snd);
+            }
+
+            fst
+        })
+}
+
+/// Parse arguments to the `builtins` macro itself, such as `#[builtins(state = Rc<State>)]`.
+fn parse_module_args(args: TokenStream) -> Option<Type> {
+    if args.is_empty() {
+        return None;
+    }
+
+    let meta: Meta = syn::parse(args).expect("could not parse arguments to `builtins`-attribute");
+    let name_value = match meta {
+        Meta::NameValue(nv) => nv,
+        _ => panic!("arguments to `builtins`-attribute must be of the form `name = value`"),
+    };
+
+    if name_value.path.get_ident().unwrap().to_string() != "state" {
+        return None;
+    }
+
+    if let syn::Lit::Str(type_name) = name_value.lit {
+        let state_type: Type =
+            syn::parse_str(&type_name.value()).expect("failed to parse builtins state type");
+        return Some(state_type);
+    }
+
+    panic!("state attribute must be a quoted Rust type");
+}
+
+/// Mark the annotated module as a module for defining Nix builtins.
+///
+/// An optional type definition may be specified as an argument (e.g. `#[builtins(Rc<State>)]`),
+/// which will add a parameter to the `builtins` function of that type which is passed to each
+/// builtin upon instantiation. Using this, builtins that close over some external state can be
+/// written.
+///
+/// The type of each function is rewritten to receive a `Vec<Value>`, containing each `Value`
+/// argument that the function receives. The body of functions is accordingly rewritten to "unwrap"
+/// values from this vector and bind them to the correct names, so unless a static error occurs this
+/// transformation is mostly invisible to users of the macro.
+///
+/// A function `fn builtins() -> Vec<Builtin>` will be defined within the annotated module,
+/// returning a list of [`tvix_eval::Builtin`] for each function annotated with the `#[builtin]`
+/// attribute within the module. If a `state` type is specified, the `builtins` function will take a
+/// value of that type.
+///
+/// Each invocation of the `#[builtin]` annotation within the module should be passed a string
+/// literal for the name of the builtin.
+///
+/// # Examples
+/// ```ignore
+/// # use tvix_eval_builtin_macros::builtins;
+/// # mod value {
+/// #     pub use tvix_eval::Builtin;
+/// # }
+///
+/// #[builtins]
+/// mod builtins {
+///     use tvix_eval::{GenCo, ErrorKind, Value};
+///
+///     #[builtin("identity")]
+///     pub async fn builtin_identity(co: GenCo, x: Value) -> Result<Value, ErrorKind> {
+///         Ok(x)
+///     }
+///
+///     // Builtins can request their argument not be forced before being called by annotating the
+///     // argument with the `#[lazy]` attribute
+///
+///     #[builtin("tryEval")]
+///     pub async fn builtin_try_eval(co: GenCo, #[lazy] x: Value) -> Result<Value, ErrorKind> {
+///         todo!()
+///     }
+/// }
+/// ```
+#[proc_macro_attribute]
+pub fn builtins(args: TokenStream, item: TokenStream) -> TokenStream {
+    let mut module = parse_macro_input!(item as ItemMod);
+
+    // parse the optional state type, which users might want to pass to builtins
+    let state_type = parse_module_args(args);
+
+    let (_, items) = match &mut module.content {
+        Some(content) => content,
+        None => {
+            return (quote_spanned!(module.span() =>
+                compile_error!("Builtin modules must be defined in-line")
+            ))
+            .into();
+        }
+    };
+
+    let mut builtins = vec![];
+    for item in items.iter_mut() {
+        if let Item::Fn(f) = item {
+            if let Some(builtin_attr_pos) = f
+                .attrs
+                .iter()
+                .position(|attr| attr.path.get_ident().iter().any(|id| *id == "builtin"))
+            {
+                let builtin_attr = f.attrs.remove(builtin_attr_pos);
+                let name: LitStr = match builtin_attr.parse_args() {
+                    Ok(args) => args,
+                    Err(err) => return err.into_compile_error().into(),
+                };
+
+                if f.sig.inputs.len() <= 1 {
+                    return (quote_spanned!(
+                        f.sig.inputs.span() =>
+                            compile_error!("Builtin functions must take at least two arguments")
+                    ))
+                    .into();
+                }
+
+                // Inspect the first argument to determine if this function is
+                // taking the state parameter.
+                // TODO(tazjin): add a test in //tvix/eval that covers this
+                let mut captures_state = false;
+                if let FnArg::Typed(PatType { pat, .. }) = &f.sig.inputs[0] {
+                    if let Pat::Ident(PatIdent { ident, .. }) = pat.as_ref() {
+                        if ident.to_string() == "state" {
+                            if state_type.is_none() {
+                                panic!("builtin captures a `state` argument, but no state type was defined");
+                            }
+
+                            captures_state = true;
+                        }
+                    }
+                }
+
+                let mut rewritten_args = std::mem::take(&mut f.sig.inputs)
+                    .into_iter()
+                    .collect::<Vec<_>>();
+
+                // Split out the value arguments from the static arguments.
+                let split_idx = if captures_state { 2 } else { 1 };
+                let value_args = rewritten_args.split_off(split_idx);
+
+                let builtin_arguments = value_args
+                    .into_iter()
+                    .map(|arg| {
+                        let span = arg.span();
+                        let mut strict = true;
+                        let (name, ty) = match arg {
+                            FnArg::Receiver(_) => {
+                                return Err(quote_spanned!(span => {
+                                    compile_error!("unexpected receiver argument in builtin")
+                                }))
+                            }
+                            FnArg::Typed(PatType {
+                                mut attrs, pat, ty, ..
+                            }) => {
+                                attrs.retain(|attr| {
+                                    attr.path.get_ident().into_iter().any(|id| {
+                                        if id == "lazy" {
+                                            strict = false;
+                                            false
+                                        } else {
+                                            true
+                                        }
+                                    })
+                                });
+                                match pat.as_ref() {
+                                    Pat::Ident(PatIdent { ident, .. }) => {
+                                        (ident.clone(), ty.clone())
+                                    }
+                                    _ => panic!("ignored value parameters must be named, e.g. `_x` and not just `_`"),
+                                }
+                            }
+                        };
+
+                        Ok(BuiltinArgument {
+                            strict,
+                            span,
+                            name,
+                            ty,
+                        })
+                    })
+                    .collect::<Result<Vec<BuiltinArgument>, _>>();
+
+                let builtin_arguments = match builtin_arguments {
+                    Err(err) => return err.into(),
+
+                    // reverse argument order, as they are popped from the stack
+                    // slice in opposite order
+                    Ok(args) => args,
+                };
+
+                // Rewrite the argument to the actual function to take a
+                // `Vec<Value>`, which is then destructured into the
+                // user-defined values in the function header.
+                let sig_span = f.sig.span();
+                rewritten_args.push(parse_quote_spanned!(sig_span=> mut values: Vec<Value>));
+                f.sig.inputs = rewritten_args.into_iter().collect();
+
+                // Rewrite the body of the function to do said argument forcing.
+                //
+                // This is done by creating a new block for each of the
+                // arguments that evaluates it, and wraps the inner block.
+                for arg in &builtin_arguments {
+                    let block = &f.block;
+                    let ty = &arg.ty;
+                    let ident = &arg.name;
+
+                    if arg.strict {
+                        f.block = Box::new(parse_quote_spanned! {arg.span=> {
+                            let #ident: #ty = generators::request_force(&co, values.pop()
+                              .expect("Tvix bug: builtin called with incorrect number of arguments")).await;
+
+                            #block
+                        }});
+                    } else {
+                        f.block = Box::new(parse_quote_spanned! {arg.span=> {
+                            let #ident: #ty = values.pop()
+                              .expect("Tvix bug: builtin called with incorrect number of arguments");
+
+                            #block
+                        }})
+                    }
+                }
+
+                let fn_name = f.sig.ident.clone();
+                let arg_count = builtin_arguments.len();
+                let docstring = match extract_docstring(&f.attrs) {
+                    Some(docs) => quote!(Some(#docs)),
+                    None => quote!(None),
+                };
+
+                if captures_state {
+                    builtins.push(quote_spanned! { builtin_attr.span() => {
+                        let inner_state = state.clone();
+                        crate::Builtin::new(
+                            #name,
+                            #docstring,
+                            #arg_count,
+                            move |values| Gen::new(|co| generators::pin_generator(#fn_name(inner_state.clone(), co, values))),
+                        )
+                    }});
+                } else {
+                    builtins.push(quote_spanned! { builtin_attr.span() => {
+                        crate::Builtin::new(
+                            #name,
+                            #docstring,
+                            #arg_count,
+                            |values| Gen::new(|co| generators::pin_generator(#fn_name(co, values))),
+                        )
+                    }});
+                }
+            }
+        }
+    }
+
+    if let Some(state_type) = state_type {
+        items.push(parse_quote! {
+            pub fn builtins(state: #state_type) -> Vec<(&'static str, Value)> {
+                vec![#(#builtins),*].into_iter().map(|b| (b.name(), Value::Builtin(b))).collect()
+            }
+        });
+    } else {
+        items.push(parse_quote! {
+            pub fn builtins() -> Vec<(&'static str, Value)> {
+                vec![#(#builtins),*].into_iter().map(|b| (b.name(), Value::Builtin(b))).collect()
+            }
+        });
+    }
+
+    module.into_token_stream().into()
+}
diff --git a/tvix/eval/builtin-macros/tests/tests.rs b/tvix/eval/builtin-macros/tests/tests.rs
new file mode 100644
index 000000000000..735ff4672007
--- /dev/null
+++ b/tvix/eval/builtin-macros/tests/tests.rs
@@ -0,0 +1,45 @@
+pub use tvix_eval::{Builtin, Value};
+use tvix_eval_builtin_macros::builtins;
+
+#[builtins]
+mod builtins {
+    use tvix_eval::generators::{self, Gen, GenCo};
+    use tvix_eval::{ErrorKind, Value};
+
+    /// Test docstring.
+    ///
+    /// It has multiple lines!
+    #[builtin("identity")]
+    pub async fn builtin_identity(co: GenCo, x: Value) -> Result<Value, ErrorKind> {
+        Ok(todo!())
+    }
+
+    #[builtin("tryEval")]
+    pub async fn builtin_try_eval(co: GenCo, #[lazy] _x: Value) -> Result<Value, ErrorKind> {
+        todo!()
+    }
+}
+
+#[test]
+fn builtins() {
+    let builtins = builtins::builtins();
+    assert_eq!(builtins.len(), 2);
+
+    let (_, identity) = builtins
+        .iter()
+        .find(|(name, _)| *name == "identity")
+        .unwrap();
+
+    match identity {
+        Value::Builtin(identity) => assert_eq!(
+            identity.documentation(),
+            Some(
+                r#" Test docstring.
+
+ It has multiple lines!"#
+            )
+        ),
+
+        _ => panic!("builtin was not a builtin"),
+    }
+}
diff --git a/tvix/eval/default.nix b/tvix/eval/default.nix
new file mode 100644
index 000000000000..91661291f7b6
--- /dev/null
+++ b/tvix/eval/default.nix
@@ -0,0 +1,9 @@
+# TODO: find a way to build the benchmarks via crate2nix
+{ depot, pkgs, ... }:
+
+depot.tvix.crates.workspaceMembers.tvix-eval.build.override {
+  runTests = true;
+
+  # Make C++ Nix available, to compare eval results against.
+  testInputs = [ pkgs.nix ];
+}
diff --git a/tvix/eval/docs/abandoned/thread-local-vm.md b/tvix/eval/docs/abandoned/thread-local-vm.md
new file mode 100644
index 000000000000..c6a2d5e07e5c
--- /dev/null
+++ b/tvix/eval/docs/abandoned/thread-local-vm.md
@@ -0,0 +1,233 @@
+# We can't have nice things because IFD
+
+The thread-local VM work below was ultimately not merged because it
+was decided that it would be harmful for `tvix::eval::Value` to
+implement `Eq`, `Hash`, or any of the other `std` traits.
+
+Implementing `std` traits on `Value` was deemed harmful because IFD
+can cause arbitrary amounts of compilation to occur, including
+network transactions with builders.  Obviously it would be
+unexpected and error-prone to have a `PartialEq::eq()` which does
+something like this.  This problem does not manifest within the
+"nixpkgs compatibility only" scope, or in any undeprecated language
+feature other than IFD.  Although IFD is outside the "nixpkgs
+compatibility scope", it [has been added to the TVL compatibility
+scope](https://cl.tvl.fyi/c/depot/+/7193/comment/3418997b_0dbd0b65/).
+
+This was the sole reason for not merging.
+
+The explanation below may be useful in case future circumstances
+affect the relevance of the reasoning above.
+
+The implementation can be found in these CLs:
+
+- [refactor(tvix/eval): remove lifetime parameter from VM<'o>](https://cl.tvl.fyi/c/depot/+/7194)
+- [feat(tvix/eval): [FOUNDLING] thread-local VM](https://cl.tvl.fyi/c/depot/+/7195)
+- [feat(tvix/eval): [FOUNDLING] VM::vm_xxx convenience methods](https://cl.tvl.fyi/c/depot/+/7196)
+- [refactor(tvix/eval): [FOUNDLING]: drop explicit `&mut vm` parameter](https://cl.tvl.fyi/c/depot/+/7197)
+
+# Thread-local storage for tvix::eval::vm::VM
+
+## The problem
+
+`Value::force()` takes a `&mut VM` argument, since forcing a value
+requires executing opcodes.  This means that `Value::nix_eq()` too
+must take a `&mut VM`, since any sensible definition of equality
+will have to force thunks.
+
+Unfortunately Rust's `PartialEq::eq()` function does not accept any
+additional arguments like this, so `Value` cannot implement
+`PartialEq`.  Worse, structs which *contain* `Value`s can't
+implement `PartialEq` either.  This means `Value`, and anything
+containing it, cannot be the key for a `BTreeMap` or `HashMap`.  We
+can't even insert `Value`s into a `HashSet`!
+
+There are other situations like this that don't involve `PartialEq`,
+but it's the most glaring one.  The main problem is that you need a
+`VM` in order to force thunks, and thunks can be anywhere in a
+`Value`.
+
+## Solving the problem with thread-locals
+
+We could avoid threading the `&mut VM` through the entire codebase
+by making it a thread-local.
+
+To do this without a performance hit, we need to use LLVM
+thread-locals, which are the same cost as references to `static`s
+but load relative to
+[`llvm.threadlocal.address`][threadlocal-intrinsic] instead of
+relative to the data segment.  Unfortunately `#[thread_local]` [is
+unstable][thread-local-unstable] and [unsafe in
+general][thread-local-unsafe] for most of the cases where we would
+want to use it.  There is one [exception][tls-const-init], however:
+if a `!thread_local()` has a `const` initializer, the compiler will
+insert a `#[thread_local]`; this special case is both safe and
+stable.
+
+The difficult decision is what the type of the thread-local should
+be.  Since you can't get a mutable reference to a `thread_local!()`
+it will have to be some interior-mutability-bestowing wrapper around
+our current `struct VM`.  Here are the choices:
+
+### `RefCell<VM>`
+
+This is the obvious first choice, since it lets you borrow a
+`RefMut<Target=VM>`.  The problem here is that we want to keep the
+codebase written such that all the functions in `impl VM` still take
+a `&mut self`.  This means that there will be an active mutable
+borrow for the duration of `VM::call_builtin()`.  So if we implement
+`PartialEq` by having `eq()` attempt a second mutable borrow from
+the thread-local storage, it will fail since there is already an
+active borrow.
+
+The problem here is that you can't "unborrow" a `RefMut` except by
+dropping it.  There's no way around this.
+
+#### Problem: Uglification
+
+The only solution here is to rewrite all the functions in `impl VM`
+so they don't take any kind of `self` argument, and then have them
+do a short-lived `.borrow_mut()` from the thread-local `RefCell`
+*separately, each time* they want to modify one of the fields of
+`VM` (currently `frames`, `stack`, `with_stack`, `warnings`).  This
+means that if you had a code sequence like this:
+
+```
+impl VM {
+  fn foo(&mut self, ...) {
+    ...
+    self.frame().ip += 1;
+    self.some_other_method();
+    self.frame().ip += 1;
+```
+
+You would need to add *two separate `borrow_mut()`s*, one for each
+of the `self.frame().ip+=1` statements.  You can't just do one big
+`borrow_mut()` because `some_other_method()` will call
+`borrow_mut()` and panic.
+
+#### Problem: Performance
+
+The `RefCell<VM>` approach also has a fairly huge performance hit,
+because every single modification to any part of `VM` will require a
+reference count increment/decrement, and a conditional branch based
+on the check (which will never fail) that the `RefCell` isn't
+already mutably borrowed.  It will also impede a lot of rustc's
+optimizations.
+
+### `Cell<VM>`
+
+This is a non-starter because it means that in order to mutate any
+field of `VM`, you have to move the entire `struct VM` out of the
+`Cell`, mutate it, and move it back in.
+
+### `Cell<Box<VM>>`
+
+Now we're getting warmer.  Here, we can move the `Box<VM>` out of
+the cell with a single pointer-sized memory access.
+
+We don't want to do the "uglification" described in the previous
+section.  We are very fortunate that, sometime in mid-2019, the Rust
+dieties [decreed by fiat][fiat-decree] that `&Cell<T>` and `&mut T`
+are bit-for-bit identical, and even gave us mortals safe wrappers
+[`from_mut()`][from_mut] and [`get_mut()`][get_mut] around
+`mem::transmute()`.
+
+So now, when a `VM` method (which takes `&mut self`) calls out to
+some external code (like a builtin), instead of passing the `&mut
+self` to the external code it can call `Cell::from_mut(&mut self)`,
+and then `Cell::swap()` that into the thread-local storage cell for
+the duration of the external code.  After the external code returns,
+it can `Cell::swap()` it back.  This whole dance gets wrapped in a
+lexical block, and the borrow checker sees that the `&Cell<Box<VM>>`
+returned by `Cell::from_mut()` lives only until the end of the
+lexical block, *so we get the `&mut self` back after the close-brace
+for that block*.  NLL FTW.  This sounds like a lot of work, but it
+should compile down to two pointer-sized loads and two pointer-sized
+stores, and it is incurred basically only for `OpBuiltin`.
+
+This all works, with only two issues:
+
+1. `vm.rs` needs to be very careful to do the thread-local cell swap
+   dance before calling anything that might call `PartialEq::eq()`
+   (or any other method that expects to be able to pull the `VM` out
+   of thread-local storage).  There is no compile-time check that we
+   did the dance in all the right places.  If we forget to do the
+   dance somewhere we'll get a runtime panic from `Option::expect()`
+   (see next section).
+
+2. Since we need to call `Cell::from_mut()` on a `Box<VM>` rather
+   than a bare `VM`, we still need to rewrite all of `vm.rs` so that
+   every function takes a `&mut Box<VM>` instead of a `&mut self`.
+   This creates a huge amount of "noise" in the code.
+
+Fortunately, it turns out that nearly all the "noise" that arises
+from the second point can be eliminated by taking advantage of
+[deref coercions][deref-coercions]!  This was the last "shoe to
+drop".
+
+There is still the issue of having to be careful about calls from
+`vm.rs` to things outside that file, but it's manageable.
+
+### `Cell<Option<Box<VM>>>`
+
+In order to get the "safe and stable `#[thread_local]`"
+[exception][tls-const-init] we need a `const` initializer, which
+means we need to be able to put something into the `Cell` that isn't
+a `VM`.  So the type needs to be `Cell<Option<Box<VM>>>`.
+
+Recall that you can't turn an `Option<&T>` into an `&Option<T>`.
+The latter type has the "is this a `Some` or `None`" bit immediately
+adjacent to the bits representing `T`.  So if I hand you a `t:&T`
+and you wrap it as `Some(t)`, those bits aren't adjacent in memory.
+This means that all the VM methods need to operate on an
+`Option<Box<VM>>` -- we can't just wrap a `Some()` around `&mut
+self` "at the last minute" before inserting it into the thread-local
+storage cell.  Fortunately deref coercions save the day here too --
+the coercion is inferred through both layers (`Box` and `Option`) of
+wrapper, so there is no additional noise in the code.
+
+Note that Rust is clever and can find some sequence of bits that
+aren't a valid `T`, so `sizeof(Option<T>)==sizeof(T)`.  And in fact,
+`Box<T>` is one of these cases (and this is guaranteed).  So the
+`Option` has no overhead.
+
+# Closing thoughts, language-level support
+
+This would have been easier with language-level support.
+
+## What wouldn't help
+
+Although it [it was decreed][fiat-decree] that `Cell<T>` and `&mut
+T` are interchangeable, a `LocalKey<Cell<T>>` isn't quite the same
+thing as a `Cell<T>`, so it wouldn't be safe for the standard
+library to contain something like this:
+
+```
+impl<T> LocalKey<Cell<T>> {
+  fn get_mut(&self) -> &mut T {
+    unsafe {
+      // ... mem::transmute() voodoo goes here ...
+```
+
+The problem here is that you can call `LocalKey<Cell<T>>::get_mut()` twice and
+end up with two `&mut T`s that point to the same thing (mutable aliasing) which
+results in undefined behavior.
+
+## What would help
+
+The ideal solution is for Rust to let you call arbitrary methods
+`T::foo(&mut self...)` on a `LocalKey<Cell<T>>`.  This way you can
+have one (and only one) `&mut T` at any syntactical point in your
+program -- the `&mut self`.
+
+
+[tls-const-init]: https://github.com/rust-lang/rust/pull/90774
+[thread-local-unstable]: https://github.com/rust-lang/rust/issues/29594
+[thread-local-unsafe-generally]: https://github.com/rust-lang/rust/issues/54366
+[fiat-decree]: https://github.com/rust-lang/rust/issues/43038
+[from_mut]: https://doc.rust-lang.org/stable/std/cell/struct.Cell.html#method.from_mut
+[get_mut]: https://doc.rust-lang.org/stable/std/cell/struct.Cell.html#method.get_mut
+[thread-local-unsafe]: [https://github.com/rust-lang/rust/issues/54366]
+[deref-coercions]: https://doc.rust-lang.org/book/ch15-02-deref.html#implicit-deref-coercions-with-functions-and-methods
+[threadlocal-intrinsic]: https://llvm.org/docs/LangRef.html#llvm-threadlocal-address-intrinsic
diff --git a/tvix/eval/docs/build-references.md b/tvix/eval/docs/build-references.md
new file mode 100644
index 000000000000..788b25c2be6c
--- /dev/null
+++ b/tvix/eval/docs/build-references.md
@@ -0,0 +1,175 @@
+Build references in derivations
+===============================
+
+This document describes how build references are calculated in Tvix. Build
+references are used to determine which store paths should be available to a
+builder during the execution of a build (i.e. the full build closure of a
+derivation).
+
+## String contexts in C++ Nix
+
+In C++ Nix, each string value in the evaluator carries an optional so-called
+"string context".
+
+These contexts are themselves a list of strings that take one of the following
+formats:
+
+1. `!<output_name>!<drv_path>`
+
+   This format describes a build reference to a specific output of a derivation.
+
+2. `=<drv_path>`
+
+   This format is used for a special case where a derivation attribute directly
+   refers to a derivation path (e.g. by accessing `.drvPath` on a derivation).
+
+   Note: In C++ Nix this case is quite special and actually requires a
+   store-database query during evaluation.
+
+3. `<path>` - a non-descript store path input, usually a plain source file (e.g.
+   from something like `src = ./.` or `src = ./foo.txt`).
+
+   In the case of `unsafeDiscardOutputDependency` this is used to pass a raw
+   derivation file, but *not* pull in its outputs.
+
+Lets introduce names for these (in the same order) to make them easier to
+reference below:
+
+```rust
+enum BuildReference {
+    /// !<output_name>!<drv_path>
+    SingleOutput(OutputName, DrvPath),
+
+    /// =<drv_path>
+    DrvClosure(DrvPath),
+
+    /// <path>
+    Path(StorePath),
+}
+```
+
+String contexts are, broadly speaking, created whenever a string is the result
+of a computation (e.g. string interpolation) that used a *computed* path or
+derivation in any way.
+
+Note: This explicitly does *not* include simply writing a literal string
+containing a store path (whether valid or not). That is only permitted through
+the `storePath` builtin.
+
+## Derivation inputs
+
+Based on the data above, the fields `inputDrvs` and `inputSrcs` of derivations
+are populated in `builtins.derivationStrict` (the function which
+`builtins.derivation`, which isn't actually a builtin, wraps).
+
+`inputDrvs` is represented by a map of derivation paths to the set of their
+outputs that were referenced by the context.
+
+TODO: What happens if the set is empty? Somebody claimed this means all outputs.
+
+`inputSrcs` is represented by a set of paths.
+
+These are populated by the above references as follows:
+
+* `SingleOutput` entries are merged into `inputDrvs`
+* `Path` entries are inserted into `inputSrcs`
+* `DrvClosure` leads to a special store computation (`computeFSClosure`), which
+  finds all paths referenced by the derivation and then inserts all of them into
+  the fields as above (derivations with _all_ their outputs)
+
+This is then serialised in the derivation and passed down the pipe.
+
+## Builtins interfacing with contexts
+
+C++ Nix has several builtins that interface directly with string contexts:
+
+* `unsafeDiscardStringContext`: throws away a string's string context (if
+  present)
+* `hasContext`: returns `true`/`false` depending on whether the string has
+  context
+* `unsafeDiscardOutputDependency`: drops dependencies on the *outputs* of a
+  `.drv` in the context, passing only the literal `.drv` itself
+
+  Note: This is only used for special test-cases in nixpkgs, and deprecated Nix
+  commands like `nix-push`.
+* `getContext`: returns the string context in serialised form as a Nix attribute
+  set
+* `appendContext`: adds a given string context to the string in the same format
+  as returned by `getContext`
+
+## Placeholders
+
+C++ Nix has `builtins.placeholder`, which given the name of an output (e.g.
+`out`) creates a hashed string representation of that output name. If that
+string is used anywhere in input attributes, the builder will replace it with
+the actual name of the corresponding output of the current derivation.
+
+C++ Nix does not use contexts for this, it blindly creates a rewrite map of
+these placeholder strings to the names of all outputs, and runs the output
+replacement logic on all environment variables it creates, attribute files it
+passes etc.
+
+## Tvix & string contexts
+
+Tvix does not track string contexts in its evaluator at all. Instead we are
+investigating implementing a system which allows us to drop string contexts in
+favour of reference scanning derivation attributes.
+
+This means that instead of maintaining and passing around a string context data
+structure in eval, we maintain a data structure of *known paths* from the same
+evaluation elsewhere in Tvix, and scan each derivation attribute against this
+set of known paths when instantiating derivations.
+
+Until proven otherwise, we take the stance that the system of string contexts as
+implemented in C++ Nix is likely an implementation detail that should not be
+leaking to the language surface as it does now.
+
+### Tracking "known paths"
+
+Every time a Tvix evaluation does something that causes a store interaction, a
+"known path" is created. On the language surface, this is the result of one of:
+
+1. Path literals (e.g. `src = ./.`).
+2. Calls to `builtins.derivationStrict` yielding a derivation and its output
+   paths.
+3. Calls to `builtins.path`.
+
+Whenever one of these occurs, some metadata that persists for the duration of
+one evaluation should be created in Nix. This metadata needs to be available in
+`builtins.derivationStrict`, and should be able to respond to these queries:
+
+1. What is the set of all known paths? (used for e.g. instantiating an
+   Aho-Corasick type string searcher)
+2. What is the _type_ of a path? (derivation path, derivation output, source
+   file)
+3. What are the outputs of a derivation?
+4. What is the derivation of an output?
+
+These queries will need to be asked of the metadata when populating the
+derivation fields.
+
+Note: Depending on how we implement `builtins.placeholder`, it might be useful
+to track created placeholders in this metadata, too.
+
+### Context builtins
+
+Context-reading builtins can be implemented in Tvix by adding `hasContext` and
+`getContext` with the appropriate reference-scanning logic. However, we should
+evaluate how these are used in nixpkgs and whether their uses can be removed.
+
+Context-mutating builtins can be implemented by tracking their effects in the
+value representation of Tvix, however we should consider not doing this at all.
+
+`unsafeDiscardOutputDependency` should probably never be used and we should warn
+or error on it.
+
+`unsafeDiscardStringContext` is often used as a workaround for avoiding IFD in
+inconvenient places (e.g. in the TVL depot pipeline generation). This is
+unnecessary in Tvix. We should evaluate which other uses exist, and act on them
+appropriately.
+
+The initial danger with diverging here is that we might cause derivation hash
+discrepancies between Tvix and C++ Nix, which can make initial comparisons of
+derivations generated by the two systems difficult. If this occurs we need to
+discuss how to approach it, but initially we will implement the mutating
+builtins as no-ops.
diff --git a/tvix/eval/docs/builtins.md b/tvix/eval/docs/builtins.md
new file mode 100644
index 000000000000..00af50484903
--- /dev/null
+++ b/tvix/eval/docs/builtins.md
@@ -0,0 +1,138 @@
+Nix builtins
+============
+
+Nix has a lot of built-in functions, some of which are accessible in
+the global scope, and some of which are only accessible through the
+global `builtins` attribute set.
+
+This document is an attempt to track all of these builtins, but
+without documenting their functionality.
+
+See also https://nixos.org/manual/nix/stable/expressions/builtins.html
+
+The `impl` column indicates implementation status in tvix:
+- implemented: "" (empty cell)
+- not yet implemented, but not blocked: `todo`
+- not yet implemented, but blocked by other prerequisites:
+  - `store`: awaiting eval<->store api(s)
+  - `context`: awaiting support for string contexts
+
+| name                          | global | arity | pure  | impl    |
+|-------------------------------|--------|-------|-------|---------|
+| abort                         | true   | 1     |       |         |
+| add                           | false  | 2     | true  |         |
+| addErrorContext               | false  | ?     |       | context |
+| all                           | false  | 2     | true  |         |
+| any                           | false  | 2     | true  |         |
+| appendContext                 | false  | ?     |       | context |
+| attrNames                     | false  | 1     | true  |         |
+| attrValues                    | false  |       | true  |         |
+| baseNameOf                    | true   |       |       |         |
+| bitAnd                        | false  |       |       |         |
+| bitOr                         | false  |       |       |         |
+| bitXor                        | false  |       |       |         |
+| builtins                      | true   |       |       |         |
+| catAttrs                      | false  |       |       |         |
+| compareVersions               | false  |       |       |         |
+| concatLists                   | false  |       |       |         |
+| concatMap                     | false  |       |       |         |
+| concatStringsSep              | false  |       |       |         |
+| currentSystem                 | false  |       |       |         |
+| currentTime                   | false  |       | false |         |
+| deepSeq                       | false  |       |       |         |
+| derivation                    | true   |       |       | store   |
+| derivationStrict              | true   |       |       | store   |
+| dirOf                         | true   |       |       |         |
+| div                           | false  |       |       |         |
+| elem                          | false  |       |       |         |
+| elemAt                        | false  |       |       |         |
+| false                         | true   |       |       |         |
+| fetchGit                      | true   |       |       | store   |
+| fetchMercurial                | true   |       |       | store   |
+| fetchTarball                  | true   |       |       | store   |
+| fetchurl                      | false  |       |       | store   |
+| filter                        | false  |       |       |         |
+| filterSource                  | false  |       |       | store   |
+| findFile                      | false  |       | false | todo    |
+| foldl'                        | false  |       |       |         |
+| fromJSON                      | false  |       |       |         |
+| fromTOML                      | true   |       |       |         |
+| functionArgs                  | false  |       |       |         |
+| genList                       | false  |       |       |         |
+| genericClosure                | false  |       |       | todo    |
+| getAttr                       | false  |       |       |         |
+| getContext                    | false  |       |       | context |
+| getEnv                        | false  |       | false |         |
+| hasAttr                       | false  |       |       |         |
+| hasContext                    | false  |       |       | context |
+| hashFile                      | false  |       | false | todo    |
+| hashString                    | false  |       |       | todo    |
+| head                          | false  |       |       |         |
+| import                        | true   |       |       |         |
+| intersectAttrs                | false  |       |       |         |
+| isAttrs                       | false  |       |       |         |
+| isBool                        | false  |       |       |         |
+| isFloat                       | false  |       |       |         |
+| isFunction                    | false  |       |       |         |
+| isInt                         | false  |       |       |         |
+| isList                        | false  |       |       |         |
+| isNull                        | true   |       |       |         |
+| isPath                        | false  |       |       |         |
+| isString                      | false  |       |       |         |
+| langVersion                   | false  |       |       |         |
+| length                        | false  |       |       |         |
+| lessThan                      | false  |       |       |         |
+| listToAttrs                   | false  |       |       |         |
+| map                           | true   |       |       |         |
+| mapAttrs                      | false  |       |       |         |
+| match                         | false  |       |       |         |
+| mul                           | false  |       |       |         |
+| nixPath                       | false  |       |       | todo    |
+| nixVersion                    | false  |       |       | todo    |
+| null                          | true   |       |       |         |
+| parseDrvName                  | false  |       |       |         |
+| partition                     | false  |       |       |         |
+| path                          | false  |       | sometimes | store |
+| pathExists                    | false  |       | false |         |
+| placeholder                   | true   |       |       | context |
+| readDir                       | false  |       | false |         |
+| readFile                      | false  |       | false |         |
+| removeAttrs                   | true   |       |       |         |
+| replaceStrings                | false  |       |       |         |
+| scopedImport                  | true   |       |       |         |
+| seq                           | false  |       |       |         |
+| sort                          | false  |       |       |         |
+| split                         | false  |       |       |         |
+| splitVersion                  | false  |       |       |         |
+| storeDir                      | false  |       |       | store   |
+| storePath                     | false  |       |       | store   |
+| stringLength                  | false  |       |       |         |
+| sub                           | false  |       |       |         |
+| substring                     | false  |       |       |         |
+| tail                          | false  |       |       |         |
+| throw                         | true   |       |       |         |
+| toFile                        | false  |       |       | store   |
+| toJSON                        | false  |       |       | todo    |
+| toPath                        | false  |       |       |         |
+| toString                      | true   |       |       |         |
+| toXML                         | false  |       |       | todo    |
+| trace                         | false  |       |       |         |
+| true                          | true   |       |       |         |
+| tryEval                       | false  |       |       |         |
+| typeOf                        | false  |       |       |         |
+| unsafeDiscardOutputDependency | false  |       |       | context |
+| unsafeDiscardStringContext    | false  |       |       | context |
+| unsafeGetAttrPos              | false  |       |       | todo    |
+| valueSize                     | false  |       |       | todo    |
+
+## Added after C++ Nix 2.3 (without Flakes enabled)
+
+| name          | global | arity | pure  | impl  |
+|---------------|--------|-------|-------|-------|
+| break         | false  | 1     |       | todo  |
+| ceil          | false  | 1     | true  |       |
+| fetchTree     | true   | 1     |       | todo  |
+| floor         | false  | 1     | true  |       |
+| groupBy       | false  | 2     | true  |       |
+| traceVerbose  | false  | 2     |       | todo  |
+| zipAttrsWith  | false  | 2     | true  | todo  |
diff --git a/tvix/eval/docs/known-optimisation-potential.md b/tvix/eval/docs/known-optimisation-potential.md
new file mode 100644
index 000000000000..f45f1ee6c48a
--- /dev/null
+++ b/tvix/eval/docs/known-optimisation-potential.md
@@ -0,0 +1,130 @@
+Known Optimisation Potential
+============================
+
+There are several areas of the Tvix evaluator code base where
+potentially large performance gains can be achieved through
+optimisations that we are already aware of.
+
+The shape of most optimisations is that of moving more work into the
+compiler to simplify the runtime execution of Nix code. This leads, in
+some cases, to drastically higher complexity in both the compiler
+itself and in invariants that need to be guaranteed between the
+runtime and the compiler.
+
+For this reason, and because we lack the infrastructure to adequately
+track their impact (WIP), we have not yet implemented these
+optimisations, but note the most important ones here.
+
+* Use "open upvalues" [hard]
+
+  Right now, Tvix will immediately close over all upvalues that are
+  created and clone them into the `Closure::upvalues` array.
+
+  Instead of doing this, we can statically determine most locals that
+  are closed over *and escape their scope* (similar to how the
+  `compiler::scope::Scope` struct currently tracks whether locals are
+  used at all).
+
+  If we implement the machinery to track this, we can implement some
+  upvalues at runtime by simply sticking stack indices in the upvalue
+  array and only copy the values where we know that they escape.
+
+* Avoid `with` value duplication [easy]
+
+  If a `with` makes use of a local identifier in a scope that can not
+  close before the with (e.g. not across `LambdaCtx` boundaries), we
+  can avoid the allocation of the phantom value and duplication of the
+  `NixAttrs` value on the stack. In this case we simply push the stack
+  index of the known local.
+
+* Multiple attribute selection [medium]
+
+  An instruction could be introduced that avoids repeatedly pushing an
+  attribute set to/from the stack if multiple keys are being selected
+  from it. This occurs, for example, when inheriting from an attribute
+  set or when binding function formals.
+
+* Split closure/function representation [easy]
+
+  Functions have fewer fields that need to be populated at runtime and
+  can directly use the `value::function::Lambda` representation where
+  possible.
+
+* Apply `compiler::optimise_select` to other set operations [medium]
+
+  In addition to selects, statically known attribute resolution could
+  also be used for things like `?` or `with`. The latter might be a
+  little more complicated but is worth investigating.
+
+* Inline fully applied builtins with equivalent operators [medium]
+
+  Some `builtins` have equivalent operators, e.g. `builtins.sub`
+  corresponds to the `-` operator, `builtins.hasAttr` to the `?`
+  operator etc. These operators additionally compile to a primitive
+  VM opcode, so they should be just as cheap (if not cheaper) as
+  a builtin application.
+
+  In case the compiler encounters a fully applied builtin (i.e.
+  no currying is occurring) and the `builtins` global is unshadowed,
+  it could compile the equivalent operator bytecode instead: For
+  example, `builtins.sub 20 22` would be compiled as `20 - 22`.
+  This would ensure that equivalent `builtins` can also benefit
+  from special optimisations we may implement for certain operators
+  (in the absence of currying). E.g. we could optimise access
+  to the `builtins` attribute set which a call to
+  `builtins.getAttr "foo" builtins` should also profit from.
+
+* Avoid nested `VM::run` calls [hard]
+
+  Currently when encountering Nix-native callables (thunks, closures)
+  the VM's run loop will nest and return the value of the nested call
+  frame one level up. This makes the Rust call stack almost mirror the
+  Nix call stack, which is usually undesirable.
+
+  It is possible to detect situations where this is avoidable and
+  instead set up the VM in such a way that it continues and produces
+  the desired result in the same run loop, but this is kind of tricky
+  to get right - especially while other parts are still in flux.
+
+  For details consult the commit with Gerrit change ID
+  `I96828ab6a628136e0bac1bf03555faa4e6b74ece`, in which the initial
+  attempt at doing this was reverted.
+
+* Avoid thunks if only identifier closing is required [medium]
+
+  Some constructs, like `with`, mostly do not change runtime behaviour
+  if thunked. However, they are wrapped in thunks to ensure that
+  deferred identifiers are resolved correctly.
+
+  This can be avoided, as we statically analyse the scope and should
+  be able to tell whether any such logic was required.
+
+* Intern literals [easy]
+
+  Currently, the compiler emits a separate entry in the constant
+  table for each literal.  So the program `1 + 1 + 1` will have
+  three entries in its `Chunk::constants` instead of only one.
+
+* Do some list and attribute set operations in place [hard]
+
+  Algorithms that can not do a lot of work inside `builtins` like `map`,
+  `filter` or `foldl'` usually perform terribly if they use data structures like
+  lists and attribute sets.
+
+  `builtins` can do work in place on a copy of a `Value`, but naïvely expressed
+  recursive algorithms will usually use `//` and `++` to do a single change to a
+  `Value` at a time, requiring a full copy of the data structure each time.
+  It would be a big improvement if we could do some of these operations in place
+  without requiring a new copy.
+
+  There are probably two approaches: We could determine statically if a value is
+  reachable from elsewhere and emit a special in place instruction if not. An
+  easier alternative is probably to rely on reference counting at runtime: If no
+  other reference to a value exists, we can extend the list or update the
+  attribute set in place.
+
+  An **alternative** to this is using [persistent data
+  structures](https://en.wikipedia.org/wiki/Persistent_data_structure) or at the
+  very least [immutable data structures](https://docs.rs/im/latest/im/) that can
+  be copied more efficiently than the stock structures we are using at the
+  moment.
diff --git a/tvix/eval/docs/language-issues.md b/tvix/eval/docs/language-issues.md
new file mode 100644
index 000000000000..26401665bbb5
--- /dev/null
+++ b/tvix/eval/docs/language-issues.md
@@ -0,0 +1,46 @@
+# Nix language issues
+
+In the absence of a language standard, what Nix (the language) is, is prescribed
+by the behavior of the C++ Nix implementation. Still, there are reasons not to
+accept some behavior:
+
+* Tvix aims for nixpkgs compatibility only. This means we can ignore behavior in
+  edge cases nixpkgs doesn't trigger as well as obscure features it doesn't use
+  (e.g. `__overrides`).
+* Some behavior of the Nix evaluator seems to be unintentional or an
+  implementation detail leaking out into language behavior.
+
+Especially in the latter case, it makes sense to raise the respective issue and
+maybe to get rid of the behavior in all implementations for good. Below is an
+(incomplete) list of such issues:
+
+* [Behaviour of nested attribute sets depends on definition order][i7111]
+* [Partially constructed attribute sets are observable during dynamic attr names construction][i7012]
+* [Nix parsers merges multiple attribute set literals for the same key incorrectly depending on definition order](i7115)
+
+On the other hand, there is behavior that seems to violate one's expectation
+about the language at first, but has good enough reasons from an implementor's
+perspective to keep them:
+
+* Dynamic keys are forbidden in `let` and `inherit`. This makes sure that we
+  only need to do runtime identifier lookups for `with`. More dynamic (i.e.
+  runtime) lookups would make the scoping system even more complicated as well
+  as hurt performance.
+* Dynamic attributes of `rec` sets are not added to its scope. This makes sense
+  for the same reason.
+* Dynamic and nested attributes in attribute sets don't get merged. This is a
+  tricky one, but avoids doing runtime (recursive) merges of attribute sets.
+  Instead all necessary merging can be inferred statically, i.e. the C++ Nix
+  implementation already merges at parse time, making nested attribute keys
+  syntactic sugar effectively.
+
+Other behavior is just odd, surprising or underdocumented:
+
+* `builtins.foldl'` doesn't force the initial accumulator (but all other
+  intermediate accumulator values), differing from e.g. Haskell, see
+  the [relevant PR discussion](p7158).
+
+[i7111]: https://github.com/NixOS/nix/issues/7111
+[i7012]: https://github.com/NixOS/nix/issues/7012
+[i7115]: https://github.com/NixOS/nix/issues/7115
+[p7158]: https://github.com/NixOS/nix/pull/7158
diff --git a/tvix/eval/docs/opcodes-attrsets.md b/tvix/eval/docs/opcodes-attrsets.md
new file mode 100644
index 000000000000..7026f3319dda
--- /dev/null
+++ b/tvix/eval/docs/opcodes-attrsets.md
@@ -0,0 +1,122 @@
+# attrset-opcodes
+
+The problem with attrset literals is twofold:
+
+1. The keys of attribute sets may be dynamically evaluated.
+
+   Access:
+
+   ```nix
+   let
+     k = "foo";
+     attrs = { /* etc. */ };
+   in attrs."${k}"
+   ```
+
+   Literal:
+   ```nix
+   let
+     k = "foo";
+   in {
+     "${k}" = 42;
+   }
+   ```
+
+   The problem with this is that the attribute set key is not known at
+   compile time, and needs to be dynamically evaluated by the VM as an
+   expression.
+
+   For the most part this should be pretty simple, assuming a
+   theoretical instruction set:
+
+   ```
+   0000  OP_CONSTANT(0) # key "foo"
+   0001  OP_CONSTANT(1) # value 42
+   0002  OP_ATTR_SET(1) # construct attrset from 2 stack values
+   ```
+
+   The operation pushing the key needs to be replaced with one that
+   leaves a single value (the key) on the stack, i.e. the code for the
+   expression, e.g.:
+
+   ```
+   0000..000n <operations leaving a string value on the stack>
+   000n+1     OP_CONSTANT(1) # value 42
+   000n+2     OP_ATTR_SET(1) # construct attrset from 2 stack values
+   ```
+
+   This is fairly easy to do by simply recursing in the compiler when
+   the key expression is encountered.
+
+2. The keys of attribute sets may be nested.
+
+   This is the non-trivial part of dealing with attribute set
+   literals. Specifically, the nesting can be arbitrarily deep and the
+   AST does not guarantee that related set keys are located
+   adjacently.
+
+   Furthermore, this frequently occurs in practice in Nix. We need a
+   bytecode representation that makes it possible to construct nested
+   attribute sets at runtime.
+
+   Proposal: AttrPath values
+
+   If we can leave a value representing an attribute path on the
+   stack, we can offload the construction of nested attribute sets to
+   the `OpAttrSet` operation.
+
+   Under the hood, OpAttrSet in practice constructs a `Map<NixString,
+   Value>` attribute set in most cases. This means it expects to pop
+   the value of the key of the stack, but is otherwise free to do
+   whatever it wants with the underlying map.
+
+   In a simple example, we could have code like this:
+
+   ```nix
+   {
+     a.b = 15;
+   }
+   ```
+
+   This would be compiled to a new `OpAttrPath` instruction that
+   constructs and pushes an attribute path from a given number of
+   fragments (which are popped off the stack).
+
+   For example,
+
+   ```
+   0000 OP_CONSTANT(0)  # key "a"
+   0001 OP_CONSTANT(1)  # key "b"
+   0002 OP_ATTR_PATH(2) # construct attrpath from 2 fragments
+   0003 OP_CONSTANT(2)  # value 42
+   0004 OP_ATTRS(1)     # construct attrset from one pair
+   ```
+
+   Right before `0004` the stack would be left like this:
+
+   [ AttrPath[a,b], 42 ]
+
+   Inside of the `OP_ATTRS` instruction we could then begin
+   construction of the map and insert the nested attribute sets as
+   required, as well as validate that there are no duplicate keys.
+
+3. Both of these cases can occur simultaneously, but this is not a
+   problem as the opcodes combine perfectly fine, e.g.:
+
+   ```nix
+   let
+     k = "a";
+   in {
+     "${k}".b = 42;
+   }
+   ```
+
+   results in
+
+   ```
+   0000..000n <operations leaving a string value on the stack>
+   000n+1     OP_CONSTANT(1)  # key "b"
+   000n+2     OP_ATTR_PATH(2) # construct attrpath from 2 fragments
+   000n+3     OP_CONSTANT(2)  # value 42
+   000n+4     OP_ATTR_SET(1)  # construct attrset from 2 stack values
+   ```
diff --git a/tvix/eval/docs/recursive-attrs.md b/tvix/eval/docs/recursive-attrs.md
new file mode 100644
index 000000000000..c30cfd33e6c7
--- /dev/null
+++ b/tvix/eval/docs/recursive-attrs.md
@@ -0,0 +1,68 @@
+Recursive attribute sets
+========================
+
+The construction behaviour of recursive attribute sets is very
+specific, and a bit peculiar.
+
+In essence, there are multiple "phases" of scoping that take place
+during attribute set construction:
+
+1. Every inherited value without an explicit source is inherited only
+   from the **outer** scope in which the attribute set is enclosed.
+
+2. A new scope is opened in which all recursive keys are evaluated.
+   This only considers **statically known keys**, attributes can
+   **not** recurse into dynamic keys in `self`!
+
+   For example, this code is invalid in C++ Nix:
+
+   ```
+   nix-repl> rec { ${"a"+""} = 2; b = a * 10; }
+   error: undefined variable 'a' at (string):1:26
+   ```
+
+3. Finally, a third scope is opened in which dynamic keys are
+   evaluated.
+
+This behaviour, while possibly a bit strange and unexpected, actually
+simplifies the implementation of recursive attribute sets in Tvix as
+well.
+
+Essentially, a recursive attribute set like this:
+
+```nix
+rec {
+  inherit a;
+  b = a * 10;
+  ${"c" + ""} = b * 2;
+}
+```
+
+Can be compiled like the following expression:
+
+```nix
+let
+  inherit a;
+in let
+  b = a * 10;
+  in {
+    inherit a b;
+    ${"c" + ""} = b * 2;
+  }
+```
+
+Completely deferring the resolution of recursive identifiers to the
+existing handling of recursive scopes (i.e. deferred access) in let
+bindings.
+
+In practice, we can further specialise this and compile each scope
+directly into the form expected by `OpAttrs` (that is, leaving
+attribute names on the stack) before each value's position.
+
+C++ Nix's Implementation
+------------------------
+
+* [`ExprAttrs`](https://github.com/NixOS/nix/blob/2097c30b08af19a9b42705fbc07463bea60dfb5b/src/libexpr/nixexpr.hh#L241-L268)
+  (AST representation of attribute sets)
+* [`ExprAttrs::eval`](https://github.com/NixOS/nix/blob/075bf6e5565aff9fba0ea02f3333c82adf4dccee/src/libexpr/eval.cc#L1333-L1414)
+* [`addAttr`](https://github.com/NixOS/nix/blob/master/src/libexpr/parser.y#L98-L156) (`ExprAttrs` construction in the parser)
diff --git a/tvix/eval/docs/vm-loop.md b/tvix/eval/docs/vm-loop.md
new file mode 100644
index 000000000000..6266d34709cb
--- /dev/null
+++ b/tvix/eval/docs/vm-loop.md
@@ -0,0 +1,315 @@
+tvix-eval VM loop
+=================
+
+This document describes the new tvix-eval VM execution loop implemented in the
+chain focusing around cl/8104.
+
+## Background
+
+The VM loop implemented in Tvix prior to cl/8104 had several functions:
+
+1. Advancing the instruction pointer for a chunk of Tvix bytecode and
+   executing instructions in a loop until a result was yielded.
+
+2. Tracking Nix call frames as functions/thunks were entered/exited.
+
+3. Catching trampoline requests returned from instructions to force suspended
+   thunks without increasing stack size *where possible*.
+
+4. Handling trampolines through an inner trampoline loop, switching between a
+   code execution mode and execution of subsequent trampolines.
+
+This implementation of the trampoline logic was added on to the existing VM,
+which previously always recursed for thunk forcing. There are some cases (for
+example values that need to be forced *inside* of the execution of a builtin)
+where trampolines could not previously be used, and the VM recursed anyways.
+
+As a result of this trampoline logic being added "on top" of the existing VM
+loop the code became quite difficult to understand. This led to several bugs,
+for example: b/251, b/246, b/245, and b/238.
+
+These bugs were tricky to deal with, as we had to try and make the VM do
+things that are somewhat difficult to fit into its model. We could of course
+keep extending the trampoline logic to accommodate all sorts of concepts (such
+as finalisers), but that seems like it does not solve the root problem.
+
+## New VM loop
+
+In cl/8104, a unified new solution is implemented with which the VM is capable
+of evaluating everything without increasing the call stack size.
+
+This is done by introducing a new frame stack in the VM, on which execution
+frames are enqueued that are either:
+
+1. A bytecode frame, consisting of Tvix bytecode that evaluates compiled Nix
+   code.
+2. A generator frame, consisting of some VM logic implemented in pure Rust
+   code that can be *suspended* when it hits a point where the VM would
+   previously need to recurse.
+
+We do this by making use of the `async` *keyword* in Rust, but notably
+*without* introducing asynchronous I/O or concurrency in tvix-eval (the
+complexity of which is currently undesirable for us).
+
+Specifically, when writing a Rust function that uses the `async` keyword, such
+as:
+
+```rust
+async fn some_builtin(input: Value) -> Result<Value, ErrorKind> {
+  let mut out = NixList::new();
+
+  for element in input.to_list()? {
+    let result = do_something_that_requires_the_vm(element).await;
+    out.push(result);
+  }
+
+  Ok(out)
+}
+```
+
+The compiler actually generates a state-machine under-the-hood which allows
+the execution of that function to be *suspended* whenever it hits an `await`.
+
+We use the [`genawaiter`][] crate that gives us a data structure and simple
+interface for getting instances of these state machines that can be stored in
+a struct (in our case, a *generator frame*).
+
+The execution of the VM then becomes the execution of an *outer loop*, which
+is responsible for selecting the next generator frame to execute, and two
+*inner loops*, which drive the execution of a bytecode frame or generator
+frame forward until it either yields a value or asks to be suspended in favour
+of another frame.
+
+All "communication" between frames happens solely through values left on the
+stack: Whenever a frame of either type runs to completion, it is expected to
+leave a *single* value on the stack. It follows that the whole VM, upon
+completion of the last (or initial, depending on your perspective) frame
+yields its result as the return value.
+
+The core of the VM restructuring is cl/8104, unfortunately one of the largest
+single commit changes we've had to make yet, as it touches pretty much all
+areas of tvix-eval. The introduction of the generators and the
+message/response system we built to request something from the VM, suspend a
+generator, and wait for the return is in cl/8148.
+
+The next sections describe in detail how the three different loops work.
+
+### Outer VM loop
+
+The outer VM loop is responsible for selecting the next frame to run, and
+dispatching it correctly to inner loops, as well as determining when to shut
+down the VM and return the final result.
+
+```
+                          ╭──────────────────╮
+                 ╭────────┤ match frame kind ├──────╮
+                 │        ╰──────────────────╯      │
+                 │                                  │
+    ┏━━━━━━━━━━━━┷━━━━━┓                ╭───────────┴───────────╮
+───►┃ frame_stack.pop()┃                ▼                       ▼
+    ┗━━━━━━━━━━━━━━━━━━┛       ┏━━━━━━━━━━━━━━━━┓      ┏━━━━━━━━━━━━━━━━━┓
+                 ▲             ┃ bytecode frame ┃      ┃ generator frame ┃
+                 │             ┗━━━━━━━━┯━━━━━━━┛      ┗━━━━━━━━┯━━━━━━━━┛
+                 │[yes, cont.]          │                       │
+                 │                      ▼                       ▼
+    ┏━━━━━━━━┓   │             ╔════════════════╗      ╔═════════════════╗
+◄───┨ return ┃   │             ║ inner bytecode ║      ║ inner generator ║
+    ┗━━━━━━━━┛   │             ║      loop      ║      ║      loop       ║
+        ▲        │             ╚════════╤═══════╝      ╚════════╤════════╝
+        │   ╭────┴─────╮                │                       │
+        │   │ has next │                ╰───────────┬───────────╯
+   [no] ╰───┤  frame?  │                            │
+            ╰────┬─────╯                            ▼
+                 │                         ┏━━━━━━━━━━━━━━━━━┓
+                 │                         ┃ frame completed ┃
+                 ╰─────────────────────────┨  or suspended   ┃
+                                           ┗━━━━━━━━━━━━━━━━━┛
+```
+
+Initially, the VM always pops a frame from the frame stack and then inspects
+the type of frame it found. As a consequence the next frame to execute is
+always the frame at the top of the stack, and setting up a VM initially for
+code execution is done by leaving a bytecode frame with the code to execute on
+the stack and passing control to the outer loop.
+
+Control is dispatched to either of the inner loops (depending on the type of
+frame) and the cycle continues once they return.
+
+When an inner loop returns, it has either finished its execution (and left its
+result value on the *value stack*), or its frame has requested to be
+suspended.
+
+Frames request suspension by re-enqueueing *themselves* through VM helper
+methods, and then leaving the frame they want to run *on top* of themselves in
+the frame stack before yielding control back to the outer loop.
+
+The inner control loops inform the outer loops about whether the frame has
+been *completed* or *suspended* by returning a boolean.
+
+### Inner bytecode loop
+
+The inner bytecode loop drives the execution of some Tvix bytecode by
+continously looking at the next instruction to execute, and dispatching to the
+instruction handler.
+
+```
+   ┏━━━━━━━━━━━━━┓
+◄──┨ return true ┃
+   ┗━━━━━━━━━━━━━┛
+          ▲
+     ╔════╧═════╗
+     ║ OpReturn ║
+     ╚══════════╝
+          ▲
+          ╰──┬────────────────────────────╮
+             │                            ▼
+             │                 ╔═════════════════════╗
+    ┏━━━━━━━━┷━━━━━┓           ║ execute instruction ║
+───►┃ inspect next ┃           ╚══════════╤══════════╝
+    ┃  instruction ┃                      │
+    ┗━━━━━━━━━━━━━━┛                      │
+             ▲                      ╭─────┴─────╮
+             ╰──────────────────────┤ suspends? │
+                       [no]         ╰─────┬─────╯
+                                          │
+                                          │
+   ┏━━━━━━━━━━━━━━┓                       │
+◄──┨ return false ┃───────────────────────╯
+   ┗━━━━━━━━━━━━━━┛              [yes]
+```
+
+With this refactoring, the compiler now emits a special `OpReturn` instruction
+at the end of bytecode chunks. This is a signal to the runtime that the chunk
+has completed and that its current value should be returned, without having to
+perform instruction pointer arithmetic.
+
+When `OpReturn` is encountered, the inner bytecode loop returns control to the
+outer loop and informs it (by returning `true`) that the bytecode frame has
+completed.
+
+Any other instruction may also request a suspension of the bytecode frame (for
+example, instructions that need to force a value). In this case the inner loop
+is responsible for setting up the frame stack correctly, and returning `false`
+to inform the outer loop of the suspension
+
+### Inner generator loop
+
+The inner generator loop is responsible for driving the execution of a
+generator frame by continously calling [`Gen::resume`][] until it requests a
+suspension (as a result of which control is returned to the outer loop), or
+until the generator is done and yields a value.
+
+```
+   ┏━━━━━━━━━━━━━┓
+◄──┨ return true ┃ ◄───────────────────╮
+   ┗━━━━━━━━━━━━━┛                     │
+                                       │
+                               [Done]  │
+                    ╭──────────────────┴─────────╮
+                    │ inspect generator response │◄────────────╮
+                    ╰──────────────────┬─────────╯             │
+                            [yielded]  │              ┏━━━━━━━━┷━━━━━━━━┓
+                                       │              ┃ gen.resume(msg) ┃◄──
+                                       ▼              ┗━━━━━━━━━━━━━━━━━┛
+                                 ╭────────────╮                ▲
+                                 │ same-frame │                │
+                                 │  request?  ├────────────────╯
+                                 ╰─────┬──────╯      [yes]
+   ┏━━━━━━━━━━━━━━┓                    │
+◄──┨ return false ┃ ◄──────────────────╯
+   ┗━━━━━━━━━━━━━━┛                [no]
+```
+
+On each execution of a generator frame, `resume_with` is called with a
+[`VMResponse`][] (i.e. a message *from* the VM *to* the generator). For a newly
+created generator, the initial message is just `Empty`.
+
+A generator may then respond by signaling that it has finished execution
+(`Done`), in which case the inner generator loop returns control to the outer
+loop and informs it that this generator is done (by returning `true`).
+
+A generator may also respond by signaling that it needs some data from the VM.
+This is implemented through a request-response pattern, in which the generator
+returns a `Yielded` message containing a [`VMRequest`][]. These requests can be
+very simple ("Tell me the current store path") or more complex ("Call this Nix
+function with these values").
+
+Requests are divided into two classes: Same-frame requests (requests that can be
+responded to *without* returning control to the outer loop, i.e. without
+executing a *different* frame), and multi-frame generator requests. Based on the
+type of request, the inner generator loop will either handle it right away and
+send the response in a new `resume_with` call, or return `false` to the outer
+generator loop after setting up the frame stack.
+
+Most of this logic is implemented in cl/8148.
+
+[`Gen::resume`]: https://docs.rs/genawaiter/0.99.1/genawaiter/rc/struct.Gen.html#method.resume_with
+[`VMRequest`]: https://cs.tvl.fyi/depot@2696839770c1ccb62929ff2575a633c07f5c9593/-/blob/tvix/eval/src/vm/generators.rs?L44
+[`VMResponse`]: https://cs.tvl.fyi/depot@2696839770c1ccb62929ff2575a633c07f5c9593/-/blob/tvix/eval/src/vm/generators.rs?L169
+
+## Advantages & Disadvantages of the approach
+
+This approach has several advantages:
+
+* The execution model is much simpler than before, making it fairly
+  straightforward to build up a mental model of what the VM does.
+
+* All "out of band requests" inside the VM are handled through the same
+  abstraction (generators).
+
+* Implementation is not difficult, albeit a little verbose in some cases (we
+  can argue about whether or not to introduce macros for simplifying it).
+
+* Several parts of the VM execution are now much easier to document,
+  potentially letting us onboard tvix-eval contributors faster.
+
+* The linear VM execution itself is much easier to trace now, with for example
+  the `RuntimeObserver` (and by extension `tvixbolt`) giving much clearer
+  output now.
+
+But it also comes with some disadvantages:
+
+* Even though we "only" use the `async` keyword without a full async-I/O
+  runtime, we still encounter many of the drawbacks of the fragmented Rust
+  async ecosystem.
+
+  The biggest issue with this is that parts of the standard library become
+  unavailable to us, for example the built-in `Vec::sort_by` can no longer be
+  used for sorting in Nix because our comparators themselves are `async`.
+
+  This led us to having to implement some logic on our own, as the design of
+  `async` in Rust even makes it difficult to provide usecase-generic
+  implementations of concepts like sorting.
+
+* We need to allocate quite a few new structures on the heap in order to drive
+  generators, as generators involve storing `Future` types (with unknown
+  sizes) inside of structs.
+
+  In initial testing this seems to make no significant difference in
+  performance (our performance in an actual nixpkgs-eval is still bottlenecked
+  by I/O concerns and reference scanning), but is something to keep in mind
+  later on when we start optimising more after the low-hanging fruits have
+  been reaped.
+
+## Alternatives considered
+
+1. Tacking on more functionality onto the existing VM loop
+   implementation to accomodate problems as they show up. This is not
+   preferred as the code is already getting messy.
+
+2. Making tvix-eval a fully `async` project, pulling in something like Tokio
+   or `async-std` as a runtime. This is not preferred due to the massively
+   increased complexity of those solutions, and all the known issues of fully
+   buying in to the async ecosystem.
+
+   tvix-eval fundamentally should work for use-cases besides building Nix
+   packages (e.g. for `//tvix/serde`), and its profile should be as slim as
+   possible.
+
+3. Convincing the Rust developers that Rust needs a way to guarantee
+   constant-stack-depth tail calls through something like a `tailcall`
+   keyword.
+
+4. ... ?
+
+[`genawaiter`]: https://docs.rs/genawaiter/
diff --git a/tvix/eval/proptest-regressions/value/mod.txt b/tvix/eval/proptest-regressions/value/mod.txt
new file mode 100644
index 000000000000..05b01b4c768b
--- /dev/null
+++ b/tvix/eval/proptest-regressions/value/mod.txt
@@ -0,0 +1,10 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc 241ec68db9f684f4280d4c7907f7105e7b746df433fbb5cbd6bf45323a7f3be0 # shrinks to input = _ReflexiveArgs { x: List(NixList([List(NixList([Path("𑁯")]))])) }
+cc b6ab5fb25f5280f39d2372e951544d8cc9e3fcd5da83351266a0a01161e12dd7 # shrinks to input = _ReflexiveArgs { x: Attrs(NixAttrs(KV { name: Path("𐎸-{\u{a81}lq9Z"), value: Bool(false) })) }
+cc 3656053e7a8dbe1c01dd68a8e06840fb6e693dde942717a7c18173876d9c2cce # shrinks to input = _SymmetricArgs { x: List(NixList([Path("\u{1daa2}:.H🢞ୡ\\🕴7iu𝋮T𝓕\\%i:"), Integer(-7435423970896550032), Float(1.2123587650724335e-5), Integer(5314432620816586712), Integer(-8316092768376026052), Integer(-7632521684027819842), String(NixString(Smol("ᰏ᥀\\\\ȷf8=\u{2003}\"𑁢רּA/%�bQffl<౯Ⱥ\u{1b3a}`T{"))), Bool(true), String(NixString(Smol("Ⱥힳ\"<\\`tZ/�൘🢦Ⱥ=x𑇬"))), Null, Bool(false), String(NixString(Smol(";%'࿎.ೊ🉡𑌊/🕴3Ja"))), Null, String(NixString(Smol("vNᛦ=\\`𝐓P\\"))), Bool(true), Null, String(NixString(Smol("\"zાë\u{11cb6}6%yꟽ𚿾🡖`!"))), Integer(1513983844724992869), Bool(true), Float(-8.036903674864022e114), Path("G࿐�/᠆₫%𝈇P"), Bool(false), Bool(true), Null, Attrs(NixAttrs(Empty)), Float(-6.394835856260315e-46), Null, Path("G?🭙O<🟰𐮭𑤳�*ܥ𞹉`?$/j1=p𑙕h\u{e0147}\u{1cf3b}"), Bool(false), Bool(false), Path("$"), Float(7.76801238087078e-309), Integer(-4304837936532390878), Attrs(NixAttrs(Im({NixString(Smol("")): 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NixString(Heap("eቊഎȺﹰ¾0𐣰/அ🠄r~=𐞌_ఽ")): Integer(-4346221131161118847), NixString(Smol("e𐠪ቘ<$=Q\u{ecc}𚿷*}ଐ$/𐂢Y?y\u{11d3d}fଐ𐋹\u{20ed}H\\�û'g<L{")): Integer(1302899345904266282), NixString(Heap("fJ🉀!6$F𑰤𐎁C&ౚ𞓤T\"\u{d81}«Տ𘴃.𑰍Â﬩*")): Float(-9.550597053049143e239), NixString(Heap("j﮼\"🕴$%'Gૌ3?\u{302d}¥")): Bool(false), NixString(Heap("u~Ⱥ¥𝣤YN𐂜 ¥' `r𐿄/\u{b57}")): Integer(3809742241627978303), NixString(Smol("z%c流ü🕴$`.F*`Ѩଏ<\"𣅅'<3p=r:Y\u{1a60}ড:/]𐾁")): String(NixString(Smol("\\*E￭{:🕴"))), NixString(Heap("{<B9-Y𑣊N61ѨH.¡\\ꮬ'Ãkô")): Path("=𑥘^*¥K𚿾સ*Ðᝰf[:P𐤅*<⻡<\"8"), NixString(Smol("{ຄ±𑼉Ѩ`ⶮ𚿺Í%𞹡`!/")): Integer(-7455997128197210401), NixString(Smol("|🕴ේ0\u{1e131}🡑=ਜ਼_`𝔗.ો1ㄈ𐹣\u{1773}ꭆⵯ\u{1c2e}ࠨ𐧔੫=\u{b42}1\u{bd7}�/�¥$")): Null, NixString(Heap("¥⵰ￏ<*'Z¥\"Ὓ$ᥖￒ𚿱\"ꥳÒ𖿱𞓕Ѩ9MÌ¥u{ୈ<﹨")): Float(6.4374351060383694e243), NixString(Smol("¥🕴=r`z&.<V<.Ⱥ๏\"")): Bool(true), NixString(Smol("Ê9𝐐/JkᡶUl\"🕴{i")): Path("z/¥&Wzꡭ`?Ѩ\\🟰þ၈$ࠓ"), NixString(Heap("Ù𑥘c\"�\u{8ce}𑤸𝆥೮?~")): Bool(true), NixString(Heap("ãU&𐔐/")): Bool(false), NixString(Heap("ѨѨ?<'?࿎ȺՉⷊ$\u{fe09}ꞙ")): String(NixString(Smol("Ό/`j🕴🕴𐞂🕴\u{ce3}ਲ$𘟛ºװ?𞸮J𝍣Q\"}🕴q𐧸ࢺ\u{a48}"))), NixString(Heap("ࡩ꯳%^ౝ")): String(NixString(Smol("{%"))), NixString(Heap("\u{b57}🪃ⷚȺMC?¥\u{11c9a}+:*<B𐖰y~/.1&$𐺭hর\"\"")): Path("ቕ𝌫=(|R{ei\u{1a5b}Z!2\u{c3c}+<rí:\"f"), NixString(Heap("ங¸4ᨔ\u{17cb}ਜ'ಣ¥z&=='c𜽫ࡠﹳ꠲𐨖এ<¥🕴𑙫𐖷")): Integer(-2404377207820357643), NixString(Heap("ቘ🕴{ȺL'.")): Null, NixString(Smol("ኸ\"𖦆d\\&𐞷C🠀k1\u{eb1}KV%🪿\u{1cf35}>:3𐡾P.\"Ѩ.ໆኍ*'")): Bool(true), NixString(Heap("ኸ8")): String(NixString(Smol("�?Ia🀛*\u{10d27}U¥𐖙9ીㇺIW:%&G+?"))), NixString(Heap("ᱰῙ:ஙᤄ.:*𐺰𝄿勉Ѩ&¥𐧾7&`AຂѨ𑗒&¥𛅕/🕴Ѩ")): Float(-0.0), NixString(Heap("ꝏs𞊡<g=𝕃ￒRȺ�u𛱜")): Bool(false), NixString(Heap("ꡩ$𰻍🯷@a\\")): Null, NixString(Smol("𐭻𞸧Ã\\Ⱥ𞸢𘌧%h+|@ꖦ*\"~𑄣")): Bool(false), NixString(Smol("𑐑𞟣Ѩ<''¥Ѩ<ອ᪦\"፵𛄲𞀾ឮo")): Float(1.540295382708916e-173), NixString(Heap("\u{1145e}p\u{ac1}e\u{10a05}𝼩#ந*/?🡗\u{c4d}ͼᲿꟑ")): Integer(-4194082747744625061), NixString(Smol("𑘏𞟨:Ѩ?^ c🃊tๆ.Ⱥ9 𔑳&៵ׯ🬔\u{11f01}𬢪*ﷲȺ`𑼆n")): Path("7�$Ì\u{1e00c}海2&𐝀<"), NixString(Smol("𑚆")): String(NixString(Smol("𝒢\u{11727};க\u{c56}P'𝄎🪬*ⷍ\u{1e08f}59/𑨙𐤿ොZ$Jෳ*𞺓"))), NixString(Heap("𑴋¥#=%")): Integer(1639298533614063138), NixString(Heap("\u{11d3d}𑊌o:}=ਫ਼$໓ￋѨ𑌅6🫄ￓ🛴")): Integer(4745566200697725742), NixString(Heap("\u{16af4}𞲡🡕:ೈ𝒩$𝄀\u{c3e}ࡨ")): Float(2.8652739787522095e-21), NixString(Heap("𖽩$,®'w5ޤ*𖿣H'6¥ੀ")): Path("+ꡕ:/f"), NixString(Heap("𞋿@%\u{20d6}÷MqȺÏ🪧𞟣&𝼆Ⱥn2?ᦽ")): Path("n𞺣Ꮝ*𐤤FU.T"), NixString(Smol("🕴/Ⱥᜄ&{.\u{e0109}]𐣨|ຆ᱓𐃯🂥𐺭𑤬@$r{Ⱥ‿🕴〧\u{c3e}'X")): Null, NixString(Smol("🕴<V𖬑u\u{bd7}ໆC`xῴ𐄂f.M^ਐ𑼂;%.𐠈¥໔ꘄዀѨઑ")): Null, NixString(Heap("🕴¥\u{10a38}/{\\-?🕴'Ýὓ𛱱`XuᾺ𐊎ࠅ|𞹒𐆗𑌲")): Bool(true)}))), NixString(Heap("9;G'.%𑴆")): String(NixString(Heap("🕴8?$=𞺒%V*\u{9bc}𑊄ଡ଼JbѨ=ળ=𑛉\u{a81}É;ￅᲿN=ਫ਼:�`𐁙"))), NixString(Smol(":Ѩ\\Lꝅ5𐤀'uꦙ~*肋J")): List(NixList([String(NixString(Smol(".࡞\\P?*<<\"𐖄9&𘚯%Ѩю1G\\౭𒑃&𞹛=9:𞥐@("))), Path("<𝒗ঽ¥<\u{fa7}ୌD={=ଡ଼\"Ⱥ\u{bd7}ઐ$\'�¥𞹼E𐦙🡓t`\u{11357}"), Float(-3.099020840259708e-255), Path("𞟫᪓h𖭁")])), NixString(Heap("<:^X𞹇𞹷\\\u{f7e}\u{1713}^ⴝQ`.|G𝂜𞹍𑐬𐮅{x𑍇סּ\"`🉤𝚚𰳳Ⱥ")): Integer(6011461020988750685), NixString(Smol("<{פּ%#¥🕴d:�,y")): Float(-2.901654271651391e201), NixString(Heap("=/I\"🮧{z&a�<?`L¥𞋿``.|")): String(NixString(Smol("=𖫉m\"ȺhC_"))), NixString(Smol("=L=X'aਿ9.")): Path("<&]Mভ"), NixString(Smol("Eנּ:b$?JὙs𝒢O?ꥠѨ{\\ਸ𝕎𒾮`𐀢")): Float(3.6870021161947895e-16), NixString(Heap("\\�' ìrᩆ#ລB¥𐖻l%𛅥D3{/.4O¥\\.{𐼓^𒾬")): Path("/íR›d🕴-K\u{ac8}"), NixString(Heap("\\\u{1e028}೫I/ͽ4")): Float(-8.628193252502098e81), NixString(Heap("a𐠈/K33/&𝐵¥𐼰.y/🫢\\𐧨𞟹")): String(NixString(Heap("𞹡W<᱂a=ᡞಇ�%&𑴈ࢳ¥𑊈"))), NixString(Smol("eD�¥𐌋x<`B𞣍H\\`èZ𐐅៥{\\?T*")): Bool(false), NixString(Heap("oP𝒬(ѨJਈ𛀺Ѩ\"?T=O𛄲s�🕴LK🝈ከⶩ+\u{1344f}hu?E")): Bool(false), NixString(Heap("vNꟓȺM]")): Float(-2.9637346989743125e232), NixString(Smol("{!¥\\\u{a47}\u{10a38}.E4Ѩ;=R\u{a48}<=/\\&స𐢩N'?.9𑗊\u{7ec}ਸ਼:𐫃")): Null, NixString(Smol("~+!$M.𑐯ౚ\\>.¥𐖥<Q'໙0.kQ{𞹾༺𑼉sEὕ")): Float(9.45765137266644e-294), NixString(Smol("¥;Z$*&+�&\"🉆`ኲ")): Null, NixString(Heap("ãF&<` \u{64c}-:Ѩ*�")): Integer(-7654340132753689736), NixString(Heap("Ⱥ`\u{a81}Ⱥ𞺥\u{11727}<m\u{a51}{$`\u{11d3d}શR/E")): Bool(true), NixString(Heap("Ѩ%gὋ 4𑶠𑤕Y{Q<")): String(NixString(Heap("𛃛Ꟑꩀ𑊈RｏR<ৌ:4`ⶥ(ೳ>8*𑌟/𛅧<}&꒩᠙\""))), NixString(Heap("ѨನQ𑠃%=i𘓦ශ\u{1939}𞟭7;ﺸ𐼗ꟘW")): Bool(true), NixString(Smol("ਬͿ𝒩./🪡Ѩ𝁾ໆ+ఫJ{𑰏*\u{cd6}ΐѨ'APȺ*")): Path("Q%︹֏Ὓ<$Ꟗ$ᦣ𐖻Ѩ\u{c3c}MѨ"), NixString(Heap("ଢ଼")): List(NixList([Path("?&�=\u{fe0a}z\"વ🟰}ৱᦥ𐕅\"𞹭MⷝDJຄ"), String(NixString(Smol("𐺱`𞱸Ⱥ🜛?ୈ&Ꞣ=j¥`൷@ਐෂ't=7)ꬖన2Dꪩl𐝣ጓ=W"))), Path("`ಲ=𑘀eલ7𖽧Ù&ᜉ:.ﺴ𐳂\'𞹎n🕴𓊚>𝓓ை:\'{`o"), Null, Bool(true), Float(-1.424807473222878e-261), String(NixString(Heap("𑤅w𑌏ኸ¡ਊ/𞺩Ⱥ\\𛅤𞢊?\""))), Null, Bool(true), Float(-4.714901519361897e-299), Null, Integer(2676153683650725840), Null, Integer(3879649205909941200), Bool(false), Integer(-7874695792262285476), String(NixString(Heap("Wᥳ⑁*\\%ᜦ.⮏꫞Q𞹔L|ௌѨdU=*Ѩ\u{20dc}"))), Null, String(NixString(Smol("U𑐎𖺗$𞹡𞅏𝔯﮴<)&𑌫\\{\u{1acd}wѨ!𞸤𖭓º"))), Integer(802198132362652319), Path(":\u{b4d}Fᝊ:w:꯵"), Integer(-8241314039419932440), Null, Bool(true), Float(-0.0), Float(0.0), Integer(-3815417798906879402), Path("\"Dෆૌ෪\u{b01}ૐ%M№"), Null, Path("ab`𐒨\\{÷௶𖮌$=ë"), Bool(true), Null, Float(2.607265033718189e-240), String(NixString(Smol("𐖤𑤷=6$`:0ѨE\"Ѩ\"𗾮ኴ}.𞠸𞺢ߔ𑵡"))), Integer(340535348291582986), String(NixString(Heap("᠑%*&t𐮮':\\\"ꩂລ'\"𞟳|J\\\\V𑧈𛇝췭𐮬$\u{eb4}"))), Null, Float(-0.0), Float(2.533509218227985e-50), Integer(2424692299527350019), Integer(8550372276678005182), Integer(2463774675297034756), Float(-1.5273858905127126e203), String(NixString(Smol("𐕐𐐀ùI"))), String(NixString(Heap("?:🕴𑴞ﹲѨË𐫬Zf{𝋍{{࿀&\"Ô:<CJN?"))), Integer(-916756719790576181), Float(5.300552697992164e116), Path("𞹝�.?𝌼uଢ଼%~"), Bool(true), Float(-4.423451855615858e107), String(NixString(Smol("Ώﮱ¥8gH^ᛨꟖjR𐢮\"S𖩑𞸻🪁"))), Integer(8503745651802746605), Integer(8360793923494146338), Bool(false), Path("𝄞3¨/𑢸mº~𑍋ha𐫮$⹎\u{dd6}*ᲬѨク?𝔔¿"), Null, Float(-1.116670032902463e-188)])), NixString(Smol("ഌ⾞V𑄾7𑚅D𞅎$\"")): String(NixString(Heap("¤<ꡀὝ¥𞸤𞸇𑴄�ⷍa𑁟O\"f&𘴇`*<z𑨧ᤗ"))), NixString(Smol("ዄS𐾆ꟓ\")ኛ7G<oѨ🕴?𛄲×𑿕ᩣb。𖩒w")): Float(-2.1782297015402654e-308), NixString(Heap("ᩌਲ਼Ⱥﹲ<'=ւລמּ�p")): Path("𐀽𐠄"), NixString(Smol("\u{1a7b}$༄XୈÂ`7\"$*=࡞៣𐀸.tw")): String(NixString(Smol("ﬖ<j%𑊊kૐmȺrᎇ𒄹ëল.y*ᤫCｮB½"))), NixString(Smol("Ὸ:𝍷🢰8g>\u{fe0d}RѨꥫ^ῷ&ຈ/'𑌅gው=s𝂀'�𑛂𞴍~𑅮?𝑪")): Float(1.6480784640549027e-202), NixString(Smol("ⴭ8🕴V𞹙2𑶦ං=")): String(NixString(Heap("ਓ𞹗Ѩ𖭨𝓼¥..ᾶ`oU{ₖB\\©:/ѨI𝆍.&🕴:ම𞹉fi*v"))), NixString(Smol("יּN\"@𐦽Ë`𑇑ȺC{¥b$ಭೱp$hS[좵&")): Float(0.0), NixString(Smol("ﺵ🟰\"nj𞺡o&*Oz{Ⱥ\"/\\፨\u{8dd}^v´𞹺`")): Path("𑇞.🕂d"), NixString(Smol("�𞸹מּz?_R'ᥲ<<¥/*﹛Ⱥ�R𑗓𑙥𑜄ೱa'𐀞7")): Float(1.6499558125107454e233), NixString(Heap("𐏊എ🟩ﬔV?Ѩ𝔢𖭡\u{1bc9d}𝌅𑤸ڽ𞹺V.𝕊B⸛*")): Null, NixString(Heap("𐞢u1%<🕴â}𛱷$٭b𛲟ຜv着\"𝕃\\$Ⱥ<ºףּ%ଓ")): Null, NixString(Heap("𐞶𐏃*$🂭eස\u{dd6};𞁝'ᩯ")): String(NixString(Heap("'&ⷘᰎ\"𐠸𑈪"))), NixString(Heap("\u{10a05}/")): String(NixString(Heap("ㄴኲY¥'𑛅Ῥ\"𔕑𐖕נּ🇽$l.=$🠤𛰜-𝒢R{$'$^"))), NixString(Heap("\u{11300}🃆+$�ₜj\"ჇA*r&𑥐𑊙W\u{10a0d}yR<\\Ⱥ׳𞄽C&?")): Integer(6886651566886381061), NixString(Heap("𑍌🕴`'\\G¥<ꬤⶮM%𑵠꒦ૉS\u{9d7}Ⴭඹ8🕴*[Ö")): Float(2.2707656538220278e250), NixString(Smol("𘴀'>$¦C/`M*𞻰¢꩙᠖�*🕴:&è")): String(NixString(Smol("'<ড়ᰦ"))), NixString(Smol("𞊭")): Attrs(NixAttrs(Empty)), NixString(Heap("𞥖🢙=ཏ=�%𛲁$zi¥&꧗𫝒૱G𐧠𝓾$/𐼍𫟨Yhⷕ")): String(NixString(Smol("Us:)?𘘼"))), NixString(Smol("𞹪*𑌳`f𝐋ଐ𝕆j")): String(NixString(Heap("𖭿5^Zq𐨗𑄿𑥄ߤ𑜱2ঐ"))), NixString(Smol("𱾼\u{1e4ef}õ<\u{a0}{ö\\$�ῷq")): Null}))), y: Attrs(NixAttrs(KV { name: String(NixString(Smol("�𛲑ો\\?É'{<?W2𫳫p%"))), value: Integer(134481456438872098) })), z: Attrs(NixAttrs(Empty)) }
diff --git a/tvix/eval/src/builtins/impure.rs b/tvix/eval/src/builtins/impure.rs
new file mode 100644
index 000000000000..adf43e1a2ea9
--- /dev/null
+++ b/tvix/eval/src/builtins/impure.rs
@@ -0,0 +1,81 @@
+use builtin_macros::builtins;
+use genawaiter::rc::Gen;
+
+use std::{
+    env,
+    time::{SystemTime, UNIX_EPOCH},
+};
+
+use crate::{
+    errors::ErrorKind,
+    io::FileType,
+    value::NixAttrs,
+    vm::generators::{self, GenCo},
+    NixString, Value,
+};
+
+#[builtins]
+mod impure_builtins {
+    use super::*;
+    use crate::builtins::coerce_value_to_path;
+
+    #[builtin("getEnv")]
+    async fn builtin_get_env(co: GenCo, var: Value) -> Result<Value, ErrorKind> {
+        Ok(env::var(var.to_str()?).unwrap_or_else(|_| "".into()).into())
+    }
+
+    #[builtin("pathExists")]
+    async fn builtin_path_exists(co: GenCo, path: Value) -> Result<Value, ErrorKind> {
+        let path = coerce_value_to_path(&co, path).await?;
+        Ok(generators::request_path_exists(&co, path).await)
+    }
+
+    #[builtin("readDir")]
+    async fn builtin_read_dir(co: GenCo, path: Value) -> Result<Value, ErrorKind> {
+        let path = coerce_value_to_path(&co, path).await?;
+
+        let dir = generators::request_read_dir(&co, path).await;
+        let res = dir.into_iter().map(|(name, ftype)| {
+            (
+                NixString::from(name.as_str()),
+                Value::String(
+                    match ftype {
+                        FileType::Directory => "directory",
+                        FileType::Regular => "regular",
+                        FileType::Symlink => "symlink",
+                        FileType::Unknown => "unknown",
+                    }
+                    .into(),
+                ),
+            )
+        });
+
+        Ok(Value::attrs(NixAttrs::from_iter(res)))
+    }
+
+    #[builtin("readFile")]
+    async fn builtin_read_file(co: GenCo, path: Value) -> Result<Value, ErrorKind> {
+        let path = coerce_value_to_path(&co, path).await?;
+        Ok(generators::request_read_to_string(&co, path).await)
+    }
+}
+
+/// 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 000000000000..53ad6f3f8e50
--- /dev/null
+++ b/tvix/eval/src/builtins/mod.rs
@@ -0,0 +1,1037 @@
+//! 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 builtin_macros::builtins;
+use genawaiter::rc::Gen;
+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::{
+    errors::ErrorKind,
+    value::{CoercionKind, NixAttrs, NixList, NixString, SharedThunkSet, Thunk, Value},
+};
+
+use self::versions::{VersionPart, VersionPartsIter};
+
+mod to_xml;
+mod versions;
+
+#[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).
+pub async fn coerce_value_to_path(co: &GenCo, v: Value) -> Result<PathBuf, ErrorKind> {
+    let value = generators::request_force(co, v).await;
+    if let Value::Path(p) = value {
+        return Ok(*p);
+    }
+
+    let vs = generators::request_string_coerce(co, value, CoercionKind::Weak).await;
+    let path = PathBuf::from(vs.as_str());
+    if path.is_absolute() {
+        Ok(path)
+    } else {
+        Err(ErrorKind::NotAnAbsolutePath(path))
+    }
+}
+
+#[builtins]
+mod pure_builtins {
+    use crate::value::PointerEquality;
+
+    use super::*;
+
+    #[builtin("abort")]
+    async fn builtin_abort(co: GenCo, message: Value) -> Result<Value, ErrorKind> {
+        Err(ErrorKind::Abort(message.to_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 = 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 = 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::String(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 s = s.coerce_to_string(co, CoercionKind::Weak).await?.to_str()?;
+        let result: String = s.rsplit_once('/').map(|(_, x)| x).unwrap_or(&s).into();
+        Ok(result.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.as_str()) {
+                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.as_str());
+        let s2 = y.to_str()?;
+        let s2 = VersionPartsIter::new_for_cmp(s2.as_str());
+
+        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 = 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 = 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 separator = separator.to_str()?;
+        let list = list.to_list()?;
+        let mut res = String::new();
+        for (i, val) in list.into_iter().enumerate() {
+            if i != 0 {
+                res.push_str(&separator);
+            }
+            let s = generators::request_string_coerce(&co, val, CoercionKind::Weak).await;
+            res.push_str(s.as_str());
+        }
+        Ok(res.into())
+    }
+
+    #[builtin("deepSeq")]
+    async fn builtin_deep_seq(co: GenCo, x: Value, y: Value) -> Result<Value, ErrorKind> {
+        generators::request_deep_force(&co, x, SharedThunkSet::default()).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 str = s.coerce_to_string(co, CoercionKind::Weak).await?.to_str()?;
+        let result = str
+            .rsplit_once('/')
+            .map(|(x, _)| match x {
+                "" => "/",
+                _ => x,
+            })
+            .unwrap_or(".");
+        if is_path {
+            Ok(Value::Path(Box::new(result.into())))
+        } else {
+            Ok(result.into())
+        }
+    }
+
+    #[builtin("elem")]
+    async fn builtin_elem(co: GenCo, x: Value, xs: Value) -> Result<Value, ErrorKind> {
+        for val in xs.to_list()? {
+            if generators::check_equality(&co, x.clone(), val, PointerEquality::AllowAll).await? {
+                return Ok(true.into());
+            }
+        }
+        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;
+
+            if generators::request_force(&co, result).await.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;
+        }
+
+        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_str(&json_str).map_err(|err| err.into())
+    }
+
+    #[builtin("toJSON")]
+    async fn builtin_to_json(co: GenCo, val: Value) -> Result<Value, ErrorKind> {
+        let json_value = val.to_json(&co).await?;
+        let json_str = serde_json::to_string(&json_value)?;
+        Ok(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).map_err(|err| err.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")?;
+
+            if !bgc_insert_key(&co, key.clone(), &mut done_keys).await? {
+                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,
+        generator: Value,
+        length: Value,
+    ) -> Result<Value, ErrorKind> {
+        let mut out = imbl::Vector::<Value>::new();
+        let len = length.as_int()?;
+        // the best span we can get…
+        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.as_str()) {
+            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 = generators::request_force(
+                &co,
+                generators::request_call_with(&co, f.clone(), [val.clone()]).await,
+            )
+            .await
+            .to_str()?;
+
+            res.entry(key)
+                .or_insert_with(imbl::Vector::new)
+                .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.as_str())))
+    }
+
+    #[builtin("head")]
+    async fn builtin_head(co: GenCo, list: Value) -> Result<Value, ErrorKind> {
+        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> {
+        let attrs1 = x.to_attrs()?;
+        let attrs2 = y.to_attrs()?;
+        let res = attrs2.iter().filter_map(|(k, v)| {
+            if attrs1.contains(k) {
+                Some((k.clone(), v.clone()))
+            } else {
+                None
+            }
+        });
+        Ok(Value::attrs(NixAttrs::from_iter(res)))
+    }
+
+    #[builtin("isAttrs")]
+    async fn builtin_is_attrs(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Bool(matches!(value, Value::Attrs(_))))
+    }
+
+    #[builtin("isBool")]
+    async fn builtin_is_bool(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Bool(matches!(value, Value::Bool(_))))
+    }
+
+    #[builtin("isFloat")]
+    async fn builtin_is_float(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Bool(matches!(value, Value::Float(_))))
+    }
+
+    #[builtin("isFunction")]
+    async fn builtin_is_function(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Bool(matches!(
+            value,
+            Value::Closure(_) | Value::Builtin(_)
+        )))
+    }
+
+    #[builtin("isInt")]
+    async fn builtin_is_int(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Bool(matches!(value, Value::Integer(_))))
+    }
+
+    #[builtin("isList")]
+    async fn builtin_is_list(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Bool(matches!(value, Value::List(_))))
+    }
+
+    #[builtin("isNull")]
+    async fn builtin_is_null(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Bool(matches!(value, Value::Null)))
+    }
+
+    #[builtin("isPath")]
+    async fn builtin_is_path(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Bool(matches!(value, Value::Path(_))))
+    }
+
+    #[builtin("isString")]
+    async fn builtin_is_string(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::Bool(matches!(value, Value::String(_))))
+    }
+
+    #[builtin("length")]
+    async fn builtin_length(co: GenCo, list: Value) -> Result<Value, ErrorKind> {
+        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> {
+        Ok(Value::Bool(matches!(
+            x.nix_cmp_ordering(y, co).await?,
+            Some(Ordering::Less)
+        )))
+    }
+
+    #[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 = generators::request_force(&co, val).await.to_attrs()?;
+            let name = 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, f: Value, list: Value) -> Result<Value, ErrorKind> {
+        let mut out = imbl::Vector::<Value>::new();
+
+        // the best span we can get…
+        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, f: Value, attrs: Value) -> Result<Value, ErrorKind> {
+        let attrs = attrs.to_attrs()?;
+        let mut out = imbl::OrdMap::new();
+
+        // the best span we can get…
+        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.to_str()?;
+        let re = regex.to_str()?;
+        let re: Regex = Regex::new(&format!("^{}$", re.as_str())).unwrap();
+        match re.captures(&s) {
+            Some(caps) => Ok(Value::List(
+                caps.iter()
+                    .skip(1)
+                    .map(|grp| 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> {
+        // This replicates cppnix's (mis?)handling of codepoints
+        // above U+007f following 0x2d ('-')
+        let s = s.to_str()?;
+        let slice: &[u8] = s.as_str().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 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 {
+            generators::request_force(&co, val.clone()).await;
+        }
+
+        let to = to.to_list()?;
+        for val in &to {
+            generators::request_force(&co, val.clone()).await;
+        }
+
+        let string = s.to_str()?;
+
+        let mut res = String::new();
+
+        let mut i: usize = 0;
+        let mut empty_string_replace = false;
+
+        // 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_str()?;
+                let to = elem.1.to_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.as_str().is_empty() {
+                    continue;
+                }
+
+                // if we match the `from` string, let's replace
+                if &string[i..i + from.len()] == from.as_str() {
+                    res += &to;
+                    i += from.len();
+
+                    // remember if we applied the empty from->to
+                    empty_string_replace = from.as_str().is_empty();
+
+                    continue 'outer;
+                }
+            }
+
+            // If we don't match any `from`, we simply add a character
+            res += &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_str()?;
+            let to = elem.1.to_str()?;
+
+            if from.as_str().is_empty() {
+                res += &to;
+                break;
+            }
+        }
+        Ok(Value::String(res.into()))
+    }
+
+    #[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> {
+        let s = str.to_str()?;
+        let text = s.as_str();
+        let re = regex.to_str()?;
+        let re: Regex = Regex::new(re.as_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(&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)| 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
+        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 sorted = list.sort_by(&co, comparator).await?;
+        Ok(Value::List(sorted))
+    }
+
+    #[builtin("splitVersion")]
+    async fn builtin_split_version(co: GenCo, s: Value) -> Result<Value, ErrorKind> {
+        let s = s.to_str()?;
+        let s = VersionPartsIter::new(s.as_str());
+
+        let parts = s
+            .map(|s| {
+                Value::String(match s {
+                    VersionPart::Number(n) => n.into(),
+                    VersionPart::Word(w) => w.into(),
+                })
+            })
+            .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 s = s.coerce_to_string(co, CoercionKind::Weak).await?;
+        Ok(Value::Integer(s.to_str()?.as_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 x = s.coerce_to_string(co, CoercionKind::Weak).await?.to_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.as_str().len() {
+            return Ok(Value::String("".into()));
+        }
+
+        if len < 0 {
+            return Err(ErrorKind::NegativeLength { length: len });
+        }
+
+        let len = len as usize;
+        let end = cmp::min(beg + len, x.as_str().len());
+
+        Ok(Value::String(x.as_str()[beg..end].into()))
+    }
+
+    #[builtin("tail")]
+    async fn builtin_tail(co: GenCo, list: Value) -> Result<Value, ErrorKind> {
+        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> {
+        Err(ErrorKind::Throw(message.to_str()?.to_string()))
+    }
+
+    #[builtin("toString")]
+    async fn builtin_to_string(co: GenCo, #[lazy] x: Value) -> Result<Value, ErrorKind> {
+        // coerce_to_string forces for us
+        x.coerce_to_string(co, CoercionKind::Strong).await
+    }
+
+    #[builtin("toXML")]
+    async fn builtin_to_xml(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+        let value = generators::request_deep_force(&co, value, SharedThunkSet::default()).await;
+        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> {
+        let path: Value = crate::value::canon_path(coerce_value_to_path(&co, s).await?).into();
+        Ok(path.coerce_to_string(co, CoercionKind::Weak).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 {
+            Some(value) => [("value", value), ("success", true.into())],
+            None => [("value", false.into()), ("success", false.into())],
+        };
+
+        Ok(Value::attrs(NixAttrs::from_iter(res.into_iter())))
+    }
+
+    #[builtin("typeOf")]
+    async fn builtin_type_of(co: GenCo, x: Value) -> Result<Value, ErrorKind> {
+        Ok(Value::String(x.type_of().into()))
+    }
+}
+
+/// 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<bool, ErrorKind> {
+    for existing in done.iter() {
+        if generators::check_equality(
+            co,
+            existing.clone(),
+            key.clone(),
+            // TODO(tazjin): not actually sure which semantics apply here
+            PointerEquality::ForbidAll,
+        )
+        .await?
+        {
+            return Ok(false);
+        }
+    }
+
+    done.push(key);
+    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::String("2.3-compat-tvix-0.1".into())));
+    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(),
+    ));
+
+    result
+}
+
+#[builtins]
+mod placeholder_builtins {
+    use super::*;
+
+    #[builtin("unsafeDiscardStringContext")]
+    async fn builtin_unsafe_discard_string_context(
+        _: GenCo,
+        #[lazy] s: Value,
+    ) -> Result<Value, ErrorKind> {
+        // Tvix does not manually track contexts, and this is a no-op for us.
+        Ok(s)
+    }
+
+    #[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()),
+            ("col", 42.into()),
+            ("file", Value::Path(Box::new("/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 000000000000..375785b7a63e
--- /dev/null
+++ b/tvix/eval/src/builtins/to_xml.rs
@@ -0,0 +1,139 @@
+//! 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 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(super) 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.as_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.as_str()))?;
+                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) => {
+                    if formals.ellipsis {
+                        w.write(XmlEvent::start_element("attrspat").attr("ellipsis", "1"))?;
+                        w.write(XmlEvent::end_element())?;
+                    }
+
+                    for arg in formals.arguments.iter() {
+                        w.write(XmlEvent::start_element("attr").attr("name", arg.0.as_str()))?;
+                        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(_) => {
+            return Err(ErrorKind::TvixBug {
+                msg: "internal value variant encountered in builtins.toXML",
+                metadata: Some(Rc::new(value.clone())),
+            })
+        }
+    }?;
+
+    Ok(())
+}
diff --git a/tvix/eval/src/builtins/versions.rs b/tvix/eval/src/builtins/versions.rs
new file mode 100644
index 000000000000..79fb82b868fb
--- /dev/null
+++ b/tvix/eval/src/builtins/versions.rs
@@ -0,0 +1,155 @@
+use std::cmp::Ordering;
+use std::iter::{once, Chain, Once};
+use std::ops::RangeInclusive;
+
+/// 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 str),
+    Number(&'a str),
+}
+
+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.parse().unwrap();
+                let n2: u64 = s2.parse().unwrap();
+                n1.cmp(&n2)
+            }
+
+            // `pre` looses unless the other part is also a `pre`
+            (VersionPart::Word("pre"), VersionPart::Word("pre")) => Ordering::Equal,
+            (VersionPart::Word("pre"), _) => Ordering::Less,
+            (_, VersionPart::Word("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: std::str::CharIndices<'a>,
+    version: &'a str,
+}
+
+impl<'a> VersionPartsIter<'a> {
+    pub fn new(version: &'a str) -> 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 str) -> Chain<Self, Once<VersionPart>> {
+        Self::new(version).chain(once(VersionPart::Word("")))
+    }
+}
+
+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 (pos, 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: pos..=pos },
+                );
+                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: pos..=pos };
+                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
new file mode 100644
index 000000000000..9d35b30c64a6
--- /dev/null
+++ b/tvix/eval/src/chunk.rs
@@ -0,0 +1,279 @@
+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>,
+    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 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)
+    }
+
+    /// Get the first span of a chunk, no questions asked.
+    pub fn first_span(&self) -> codemap::Span {
+        self.spans[0].span
+    }
+
+    /// 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)
+    }
+
+    // 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/bindings.rs b/tvix/eval/src/compiler/bindings.rs
new file mode 100644
index 000000000000..a3d7c6fbfb33
--- /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(
+        &self,
+        file: Arc<codemap::File>,
+    ) -> Box<dyn Iterator<Item = (Span, PeekableAttrs, ast::Expr)>>;
+}
+
+impl<N: HasEntry> HasEntryProxy for N {
+    fn inherits(&self) -> Box<dyn Iterator<Item = ast::Inherit>> {
+        Box::new(ast::HasEntry::inherits(self))
+    }
+
+    fn attributes(
+        &self,
+        file: Arc<codemap::File>,
+    ) -> Box<dyn Iterator<Item = (Span, PeekableAttrs, ast::Expr)>> {
+        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(
+        &self,
+        _: Arc<codemap::File>,
+    ) -> Box<dyn Iterator<Item = (Span, PeekableAttrs, ast::Expr)>> {
+        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(Value::String(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.clone()) {
+            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(Value::String(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(Value::String(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(Value::String("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(Value::String(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 000000000000..7e1aabbf1f8b
--- /dev/null
+++ b/tvix/eval/src/compiler/import.rs
@@ -0,0 +1,116 @@
+//! 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> {
+    let mut path = coerce_value_to_path(&co, args.pop().unwrap()).await?;
+
+    if path.is_dir() {
+        path.push("default.nix");
+    }
+
+    if let Some(cached) = generators::request_import_cache_lookup(&co, path.clone()).await {
+        return Ok(cached);
+    }
+
+    // TODO(tazjin): make this return a string directly instead
+    let contents = generators::request_read_to_string(&co, path.clone())
+        .await
+        .to_str()?
+        .as_str()
+        .to_string();
+
+    let parsed = rnix::ast::Root::parse(&contents);
+    let errors = parsed.errors();
+    let file = source.add_file(path.to_string_lossy().to_string(), contents);
+
+    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()),
+        file,
+        // 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"),
+        &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 000000000000..478d132758e8
--- /dev/null
+++ b/tvix/eval/src/compiler/mod.rs
@@ -0,0 +1,1497 @@
+//! 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::HashMap;
+use std::path::{Path, PathBuf};
+use std::rc::{Rc, Weak};
+use std::sync::Arc;
+
+use crate::chunk::Chunk;
+use crate::errors::{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::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,
+        }
+    }
+}
+
+/// 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",
+];
+
+pub struct Compiler<'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>,
+
+    /// File reference in the codemap contains all known source code
+    /// and is used to track the spans from which instructions where
+    /// derived.
+    file: Arc<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<'observer> Compiler<'observer> {
+    pub(crate) fn new(
+        location: Option<PathBuf>,
+        file: Arc<codemap::File>,
+        globals: Rc<GlobalsMap>,
+        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,
+                    )
+                })?;
+                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,
+            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 [compile_dead_code].
+    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::String(u.syntax().text().into())
+            }
+        };
+
+        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('~') {
+            return self.thunk(slot, node, move |c, _| {
+                // 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())];
+                c.emit_constant(
+                    Value::UnresolvedPath(Box::new(home_relative_path.into())),
+                    node,
+                );
+                c.push_op(OpCode::OpResolveHomePath, node);
+            });
+        } else if raw_path.starts_with('<') {
+            // TODO: decide what to do with findFile
+            if raw_path.len() == 2 {
+                return self.emit_error(
+                    node,
+                    ErrorKind::NixPathResolution("Empty <> path not allowed".into()),
+                );
+            }
+            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, ipol);
+                }
+
+                ast::InterpolPart::Literal(lit) => {
+                    self.emit_constant(Value::String(lit.as_str().into()), 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());
+
+        // 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);
+    }
+
+    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());
+
+        // 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);
+    }
+
+    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());
+        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);
+    }
+
+    /// 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_str()) {
+                            *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
+    ///  ┌────────────────────────────┐   ┌─────────────────────────┐
+    ///  │    ...                     │   │ ...                     │
+    ///  │ 5  OP_ATTRS(1)             │ → │ 5  [ { a.b = 1; }     ] │
+    ///  │ 6  OP_CONSTANT("a")        │ → │ 6  [ { a.b = 1; } "a" ] │
+    ///  │ 7  OP_ATTR_OR_NOT_FOUND    │ → │ 7  [ { b = 1; }       ] │
+    ///  │ 8  JUMP_IF_NOT_FOUND(13)   │ → │ 8  [ { b = 1; }       ] │
+    ///  │ 9  OP_CONSTANT("C")        │ → │ 9  [ { b = 1; } "c"   ] │
+    ///  │ 10 OP_ATTR_OR_NOT_FOUND    │ → │ 10 [ NOT_FOUND        ] │
+    ///  │ 11 JUMP_IF_NOT_FOUND(13)   │ → │ 11 [                  ] │
+    ///  │ 12 JUMP(14)                │   │ ..     jumped over      │
+    ///  │ 13 CONSTANT(42)            │ → │ 12 [ 42 ]               │
+    ///  │ 14 ...                     │   │ ..   ....               │
+    ///  └────────────────────────────┘   └─────────────────────────┘
+    /// ```
+    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
+    ///                        ┌─────────────────────┐
+    ///                        │ 0  [ conditional ]  │
+    ///                        │ 1   JUMP_IF_FALSE  →┼─┐
+    ///                        │ 2  [  main body  ]  │ │ Jump to else body if
+    ///                       ┌┼─3─←     JUMP        │ │ condition is false.
+    ///  Jump over else body  ││ 4   OP_ASSERT_FAIL ←┼─┘
+    ///  if condition is true.└┼─5─→     ...         │
+    ///                        └─────────────────────┘
+    /// ```
+    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 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);
+    }
+
+    /// Compile conditional expressions using jumping instructions in the VM.
+    ///
+    /// ```notrust
+    ///                        ┌────────────────────┐
+    ///                        │ 0  [ conditional ] │
+    ///                        │ 1   JUMP_IF_FALSE →┼─┐
+    ///                        │ 2  [  main body  ] │ │ Jump to else body if
+    ///                       ┌┼─3─←     JUMP       │ │ condition is false.
+    ///  Jump over else body  ││ 4  [  else body  ]←┼─┘
+    ///  if condition is true.└┼─5─→     ...        │
+    ///                        └────────────────────┘
+    /// ```
+    fn compile_if_else(&mut self, slot: LocalIdx, node: &ast::IfElse) {
+        self.compile(slot, node.condition().unwrap());
+        self.emit_force(&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
+    }
+
+    /// 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>
+    /// ```
+    ///
+    /// The only tricky bit being that 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.
+    fn compile_param_pattern(&mut self, pattern: &ast::Pattern) -> Formals {
+        let span = self.span_for(pattern);
+        let set_idx = match pattern.pat_bind() {
+            Some(name) => self.declare_local(&name, name.ident().unwrap().to_string()),
+            None => self.scope_mut().declare_phantom(span, true),
+        };
+
+        // 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 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<(LocalIdx, ast::PatEntry)> = vec![];
+        let mut indices: Vec<LocalIdx> = vec![];
+        let mut arguments = HashMap::default();
+
+        for entry in pattern.pat_entries() {
+            let ident = entry.ident().unwrap();
+            let idx = self.declare_local(&ident, ident.to_string());
+            let has_default = entry.default().is_some();
+            entries.push((idx, entry));
+            indices.push(idx);
+            arguments.insert(ident.into(), has_default);
+        }
+
+        // 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 (idx, entry) in entries.into_iter() {
+            self.push_op(OpCode::OpGetLocal(stack_idx), pattern);
+            self.emit_literal_ident(&entry.ident().unwrap());
+
+            // Use the same mechanism as `compile_select_or` if a
+            // default value was provided, or simply select otherwise.
+            if let Some(default_expr) = entry.default() {
+                self.push_op(OpCode::OpAttrsTrySelect, &entry.ident().unwrap());
+
+                let jump_to_default =
+                    self.push_op(OpCode::OpJumpIfNotFound(JumpOffset(0)), &default_expr);
+
+                let jump_over_default = self.push_op(OpCode::OpJump(JumpOffset(0)), &default_expr);
+
+                self.patch_jump(jump_to_default);
+
+                // Thunk the default expression, but only if it is something
+                // other than an identifier.
+                if let ast::Expr::Ident(_) = &default_expr {
+                    self.compile(idx, default_expr);
+                } else {
+                    self.thunk(idx, &self.span_for(&default_expr), move |c, s| {
+                        c.compile(s, default_expr)
+                    });
+                }
+
+                self.patch_jump(jump_over_default);
+            } else {
+                self.push_op(OpCode::OpAttrsSelect, &entry.ident().unwrap());
+            }
+
+            self.scope_mut().mark_initialised(idx);
+        }
+
+        for idx in indices {
+            if self.scope()[idx].needs_finaliser {
+                let stack_idx = self.scope().stack_index(idx);
+                self.push_op(OpCode::OpFinalise(stack_idx), pattern);
+            }
+        }
+
+        Formals {
+            arguments,
+            ellipsis,
+            span,
+        }
+    }
+
+    fn compile_lambda(&mut self, slot: LocalIdx, node: &ast::Lambda) {
+        // 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());
+        self.context_mut().lambda.formals = formals;
+    }
+
+    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, content)
+    }
+
+    /// 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),
+    {
+        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();
+
+        content(self, slot);
+        self.cleanup_scope(node);
+
+        // 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::OpJumpIfNotFound(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) {
+        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))
+    }
+}
+
+/// 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,
+            file.clone(),
+            weak.upgrade().unwrap(),
+            &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.into_iter());
+
+        // 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, 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().into_iter())),
+        );
+
+        // 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>,
+    file: Arc<codemap::File>,
+    globals: Rc<GlobalsMap>,
+    observer: &mut dyn CompilerObserver,
+) -> EvalResult<CompilationOutput> {
+    let mut c = Compiler::new(location, file, globals.clone(), 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 000000000000..48960d355cc6
--- /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 000000000000..892727c107c9
--- /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
new file mode 100644
index 000000000000..2fbb6496ceea
--- /dev/null
+++ b/tvix/eval/src/errors.rs
@@ -0,0 +1,1054 @@
+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};
+
+#[derive(Clone, Debug)]
+pub enum ErrorKind {
+    /// These are user-generated errors through builtins.
+    Throw(String),
+    Abort(String),
+    AssertionFailed,
+
+    DivisionByZero,
+
+    DuplicateAttrsKey {
+        key: String,
+    },
+
+    /// 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 angle brackets path literal failed in some way.
+    NixPathResolution(String),
+
+    /// 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),
+
+    /// A negative integer was used as a value representing length.
+    NegativeLength {
+        length: i64,
+    },
+
+    // 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 converting JSON to a value
+    FromJsonError(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,
+    },
+
+    /// 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>,
+    },
+}
+
+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<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 ErrorKind {
+    /// Returns `true` if this error can be caught by `builtins.tryEval`
+    pub fn is_catchable(&self) -> bool {
+        match self {
+            Self::Throw(_) | Self::AssertionFailed | Self::NixPathResolution(_) => true,
+            Self::NativeError { err, .. } | Self::BytecodeError(err) => err.kind.is_catchable(),
+            _ => false,
+        }
+    }
+}
+
+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::FromJsonError(format!("error in JSON serialization: {err}"))
+    }
+}
+
+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>,
+}
+
+impl Error {
+    pub fn new(mut kind: ErrorKind, span: Span) -> Self {
+        let mut contexts = vec![];
+        while let ErrorKind::WithContext {
+            context,
+            underlying,
+        } = kind
+        {
+            kind = *underlying;
+            contexts.push(context);
+        }
+
+        Error {
+            kind,
+            span,
+            contexts,
+        }
+    }
+}
+
+impl Display for ErrorKind {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match &self {
+            ErrorKind::Throw(msg) => write!(f, "error thrown: {}", msg),
+            ErrorKind::Abort(msg) => write!(f, "evaluation aborted: {}", msg),
+            ErrorKind::AssertionFailed => write!(f, "assertion failed"),
+
+            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::NixPathResolution(err) | 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 = match kind {
+                    CoercionKind::Strong => "strongly",
+                    CoercionKind::Weak => "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::NegativeLength { length } => {
+                write!(
+                    f,
+                    "cannot use a negative integer, {}, for a value representing length",
+                    length
+                )
+            }
+
+            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::FromJsonError(msg) => {
+                write!(f, "Error converting JSON to a Nix value: {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 `{}` supplied to function",
+                    arg.as_str()
+                )
+            }
+
+            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")
+            }
+        }
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        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, source: &SourceCode) -> String {
+        let mut out = vec![];
+        Emitter::vec(&mut out, Some(&*source.codemap())).emit(&self.diagnostics(source));
+        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, source: &SourceCode) {
+        Emitter::stderr(ColorConfig::Auto, Some(&*source.codemap()))
+            .emit(&self.diagnostics(source));
+    }
+
+    /// 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::NixPathResolution(_) | ErrorKind::RelativePathResolution(_) => {
+                "in this path literal"
+            }
+            ErrorKind::UnexpectedArgument { .. } => "in this function call",
+
+            // The spans for some errors don't have any more descriptive stuff
+            // in them, or we don't utilise it yet.
+            ErrorKind::Throw(_)
+            | ErrorKind::Abort(_)
+            | ErrorKind::AssertionFailed
+            | 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::NegativeLength { .. }
+            | ErrorKind::UnmergeableInherit { .. }
+            | ErrorKind::UnmergeableValue
+            | ErrorKind::ImportParseError { .. }
+            | ErrorKind::ImportCompilerError { .. }
+            | ErrorKind::IO { .. }
+            | ErrorKind::FromJsonError(_)
+            | ErrorKind::NotSerialisableToJson(_)
+            | ErrorKind::FromTomlError(_)
+            | ErrorKind::Xml(_)
+            | ErrorKind::TvixError(_)
+            | ErrorKind::TvixBug { .. }
+            | ErrorKind::NotImplemented(_)
+            | ErrorKind::WithContext { .. } => 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::Throw(_) => "E001",
+            ErrorKind::Abort(_) => "E002",
+            ErrorKind::AssertionFailed => "E003",
+            ErrorKind::InvalidAttributeName { .. } => "E004",
+            ErrorKind::AttributeNotFound { .. } => "E005",
+            ErrorKind::TypeError { .. } => "E006",
+            ErrorKind::Incomparable { .. } => "E007",
+            ErrorKind::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::NegativeLength { .. } => "E022",
+            ErrorKind::TailEmptyList { .. } => "E023",
+            ErrorKind::UnmergeableInherit { .. } => "E024",
+            ErrorKind::UnmergeableValue => "E025",
+            ErrorKind::ImportParseError { .. } => "E027",
+            ErrorKind::ImportCompilerError { .. } => "E028",
+            ErrorKind::IO { .. } => "E029",
+            ErrorKind::FromJsonError { .. } => "E030",
+            ErrorKind::UnexpectedArgument { .. } => "E031",
+            ErrorKind::RelativePathResolution(_) => "E032",
+            ErrorKind::DivisionByZero => "E033",
+            ErrorKind::Xml(_) => "E034",
+            ErrorKind::FromTomlError(_) => "E035",
+            ErrorKind::NotSerialisableToJson(_) => "E036",
+
+            // 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::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, source: &SourceCode) -> Vec<SpanLabel> {
+        let mut spans = match &self.kind {
+            ErrorKind::ImportParseError { errors, file, .. } => {
+                spans_for_parse_errors(file, errors)
+            }
+
+            ErrorKind::ParseErrors(errors) => {
+                let file = 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, source: &SourceCode) -> Diagnostic {
+        Diagnostic {
+            level: Level::Error,
+            message: self.to_string(),
+            spans: self.spans(source),
+            code: Some(self.code().into()),
+        }
+    }
+
+    /// Return the primary diagnostic and all further associated diagnostics (if
+    /// any) of an error.
+    fn diagnostics(&self, source: &SourceCode) -> Vec<Diagnostic> {
+        match &self.kind {
+            ErrorKind::ImportCompilerError { errors, .. } => {
+                let mut out = vec![self.diagnostic(source)];
+                out.extend(errors.iter().map(|e| e.diagnostic(source)));
+                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(source);
+
+                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(source);
+
+                    match next.kind {
+                        ErrorKind::NativeError { err: inner, .. }
+                        | ErrorKind::BytecodeError(inner) => {
+                            next = *inner;
+                            continue;
+                        }
+                        _ => {
+                            diagnostics.extend(next.diagnostics(source));
+                            break;
+                        }
+                    }
+                }
+
+                diagnostics
+            }
+
+            _ => vec![self.diagnostic(source)],
+        }
+    }
+}
+
+// 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/io.rs b/tvix/eval/src/io.rs
new file mode 100644
index 000000000000..708c36153c47
--- /dev/null
+++ b/tvix/eval/src/io.rs
@@ -0,0 +1,138 @@
+//! 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 smol_str::SmolStr;
+use std::{
+    io,
+    path::{Path, PathBuf},
+};
+
+/// Types of files as represented by `builtins.readDir` in Nix.
+#[derive(Debug)]
+pub enum FileType {
+    Directory,
+    Regular,
+    Symlink,
+    Unknown,
+}
+
+/// Defines how filesystem interaction occurs inside of tvix-eval.
+pub trait EvalIO {
+    /// Verify whether the file at the specified path exists.
+    fn path_exists(&self, path: &Path) -> Result<bool, io::Error>;
+
+    /// Read the file at the specified path to a string.
+    fn read_to_string(&self, path: &Path) -> Result<String, io::Error>;
+
+    /// Read the directory at the specified path and return the names
+    /// of its entries associated with their [`FileType`].
+    fn read_dir(&self, path: &Path) -> Result<Vec<(SmolStr, FileType)>, io::Error>;
+
+    /// 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 primarily used in the context of things like coercing
+    /// a local path to a string, or builtins like `path`.
+    fn import_path(&self, path: &Path) -> Result<PathBuf, io::Error>;
+
+    /// Returns the root of the store directory, if such a thing
+    /// exists in the evaluation context.
+    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;
+
+#[cfg(feature = "impure")]
+impl EvalIO for StdIO {
+    fn path_exists(&self, path: &Path) -> Result<bool, io::Error> {
+        path.try_exists()
+    }
+
+    fn read_to_string(&self, path: &Path) -> Result<String, io::Error> {
+        std::fs::read_to_string(&path)
+    }
+
+    fn read_dir(&self, path: &Path) -> Result<Vec<(SmolStr, FileType)>, io::Error> {
+        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((SmolStr::new(entry.file_name().to_string_lossy()), 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) -> Result<PathBuf, io::Error> {
+        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) -> Result<bool, io::Error> {
+        Err(io::Error::new(
+            io::ErrorKind::Unsupported,
+            "I/O methods are not implemented in DummyIO",
+        ))
+    }
+
+    fn read_to_string(&self, _: &Path) -> Result<String, io::Error> {
+        Err(io::Error::new(
+            io::ErrorKind::Unsupported,
+            "I/O methods are not implemented in DummyIO",
+        ))
+    }
+
+    fn read_dir(&self, _: &Path) -> Result<Vec<(SmolStr, FileType)>, io::Error> {
+        Err(io::Error::new(
+            io::ErrorKind::Unsupported,
+            "I/O methods are not implemented in DummyIO",
+        ))
+    }
+
+    fn import_path(&self, _: &Path) -> Result<PathBuf, io::Error> {
+        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 000000000000..deb48b50078e
--- /dev/null
+++ b/tvix/eval/src/lib.rs
@@ -0,0 +1,346 @@
+//! `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, 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::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<'code, 'co, 'ro> {
+    /// The Nix source code to be evaluated.
+    code: &'code str,
+
+    /// 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.
+    location: Option<PathBuf>,
+
+    /// Source code map used for error reporting.
+    source_map: SourceCode,
+
+    /// Top-level file reference for this code inside the source map.
+    file: Arc<codemap::File>,
+
+    /// 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: Box<dyn EvalIO>,
+
+    /// 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<'code, 'co, 'ro> Evaluation<'code, 'co, 'ro> {
+    /// Initialise an `Evaluation` for the given Nix source code snippet, and
+    /// an optional code location.
+    pub fn new(code: &'code str, location: Option<PathBuf>) -> Self {
+        let source_map = SourceCode::new();
+
+        let location_str = location
+            .as_ref()
+            .map(|p| p.to_string_lossy().to_string())
+            .unwrap_or_else(|| "[code]".into());
+
+        let file = source_map.add_file(location_str, code.into());
+
+        let mut builtins = builtins::pure_builtins();
+        builtins.extend(builtins::placeholders()); // these are temporary
+
+        Evaluation {
+            code,
+            location,
+            source_map,
+            file,
+            builtins,
+            src_builtins: vec![],
+            io_handle: Box::new(DummyIO {}),
+            enable_import: false,
+            strict: false,
+            nix_path: None,
+            compiler_observer: None,
+            runtime_observer: None,
+        }
+    }
+
+    #[cfg(feature = "impure")]
+    /// Initialise an `Evaluation` for the given snippet, with all
+    /// impure features turned on by default.
+    pub fn new_impure(code: &'code str, location: Option<PathBuf>) -> Self {
+        let mut eval = Self::new(code, location);
+        eval.enable_import = true;
+        eval.builtins.extend(builtins::impure_builtins());
+        eval.io_handle = Box::new(StdIO);
+        eval
+    }
+
+    /// 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. This does not *run* the
+    /// code, it only provides analysis (errors and warnings) of the
+    /// compiler.
+    pub fn compile_only(mut self) -> EvaluationResult {
+        let mut result = EvaluationResult::default();
+        let source = self.source_map();
+
+        let mut noop_observer = observer::NoOpObserver::default();
+        let compiler_observer = self.compiler_observer.take().unwrap_or(&mut noop_observer);
+
+        parse_compile_internal(
+            &mut result,
+            self.code,
+            self.file.clone(),
+            self.location,
+            source,
+            self.builtins,
+            self.src_builtins,
+            self.enable_import,
+            compiler_observer,
+        );
+
+        result
+    }
+
+    /// Evaluate the provided source code.
+    pub fn evaluate(mut self) -> EvaluationResult {
+        let mut result = EvaluationResult::default();
+        let source = self.source_map();
+
+        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.store_dir() {
+            self.builtins.push(("storeDir", store_dir.into()));
+        }
+
+        let (lambda, globals) = match parse_compile_internal(
+            &mut result,
+            self.code,
+            self.file.clone(),
+            self.location,
+            source,
+            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: self.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,
+            globals,
+            lambda,
+            self.strict,
+        );
+
+        match vm_result {
+            Ok(mut runtime_result) => {
+                result.warnings.append(&mut runtime_result.warnings);
+                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.
+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,
+        ));
+        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, enable_import);
+
+    let compiler_result = match compiler::compile(
+        result.expr.as_ref().unwrap(),
+        location,
+        file,
+        builtins,
+        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/nix_search_path.rs b/tvix/eval/src/nix_search_path.rs
new file mode 100644
index 000000000000..79c19752f6c1
--- /dev/null
+++ b/tvix/eval/src/nix_search_path.rs
@@ -0,0 +1,243 @@
+use path_clean::PathClean;
+use std::convert::Infallible;
+use std::path::{Path, PathBuf};
+use std::str::FromStr;
+
+use crate::errors::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
+    /// └── subdir
+    ///     └── 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
+    /// └── subdir
+    ///     └── 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(
+        &self,
+        io: &mut dyn EvalIO,
+        lookup_path: &Path,
+    ) -> Result<Option<PathBuf>, ErrorKind> {
+        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.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>(&self, io: &mut dyn EvalIO, path: P) -> Result<PathBuf, ErrorKind>
+    where
+        P: AsRef<Path>,
+    {
+        let path = path.as_ref();
+        for entry in &self.entries {
+            if let Some(p) = entry.resolve(io, path)? {
+                return Ok(p);
+            }
+        }
+        Err(ErrorKind::NixPathResolution(format!(
+            "path '{}' was not found in the Nix search path",
+            path.display()
+        )))
+    }
+}
+
+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 mut io = StdIO {};
+            let res = nix_search_path.resolve(&mut io, "src").unwrap();
+            assert_eq!(res, current_dir().unwrap().join("src").clean());
+        }
+
+        #[test]
+        fn failed_resolution() {
+            let nix_search_path = NixSearchPath::from_str("./.").unwrap();
+            let mut io = StdIO {};
+            let err = nix_search_path.resolve(&mut io, "nope").unwrap_err();
+            assert!(
+                matches!(err, ErrorKind::NixPathResolution(..)),
+                "err = {err:?}"
+            );
+        }
+
+        #[test]
+        fn second_in_path() {
+            let nix_search_path = NixSearchPath::from_str("./.:/").unwrap();
+            let mut io = StdIO {};
+            let res = nix_search_path.resolve(&mut io, "etc").unwrap();
+            assert_eq!(res, Path::new("/etc"));
+        }
+
+        #[test]
+        fn prefix() {
+            let nix_search_path = NixSearchPath::from_str("/:tvix=.").unwrap();
+            let mut io = StdIO {};
+            let res = nix_search_path.resolve(&mut io, "tvix/src").unwrap();
+            assert_eq!(res, current_dir().unwrap().join("src").clean());
+        }
+
+        #[test]
+        fn matching_prefix() {
+            let nix_search_path = NixSearchPath::from_str("/:tvix=.").unwrap();
+            let mut io = StdIO {};
+            let res = nix_search_path.resolve(&mut io, "tvix").unwrap();
+            assert_eq!(res, current_dir().unwrap().clean());
+        }
+    }
+}
diff --git a/tvix/eval/src/observer.rs b/tvix/eval/src/observer.rs
new file mode 100644
index 000000000000..cdc6ca16ff97
--- /dev/null
+++ b/tvix/eval/src/observer.rs
@@ -0,0 +1,287 @@
+//! 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 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> {
+    writer: TabWriter<W>,
+}
+
+impl<W: Write> TracingObserver<W> {
+    pub fn new(writer: W) -> Self {
+        Self {
+            writer: TabWriter::new(writer),
+        }
+    }
+
+    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() => Ok(self.write_value(&t.value())),
+
+            // 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,
+    ) {
+        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]) {
+        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]) {
+        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]) {
+        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]) {
+        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]) {
+        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) {
+        let _ = writeln!(
+            &mut self.writer,
+            "=== generator '{}' requested {} ===",
+            name, msg
+        );
+    }
+
+    fn observe_enter_builtin(&mut self, name: &'static str) {
+        let _ = writeln!(&mut self.writer, "=== entering builtin {} ===", name);
+    }
+
+    fn observe_exit_builtin(&mut self, name: &'static str, stack: &[Value]) {
+        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>) {
+        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]) {
+        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
new file mode 100644
index 000000000000..130e242668d6
--- /dev/null
+++ b/tvix/eval/src/opcode.rs
@@ -0,0 +1,186 @@
+//! This module implements the instruction set running on the abstract
+//! machine implemented by tvix.
+
+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);
+
+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.
+#[warn(variant_size_differences)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum OpCode {
+    /// Push a constant onto the stack.
+    OpConstant(ConstantIdx),
+
+    /// Discard a value from the stack.
+    OpPop,
+
+    // Unary operators
+    OpInvert,
+    OpNegate,
+
+    // Arithmetic binary operators
+    OpAdd,
+    OpSub,
+    OpMul,
+    OpDiv,
+
+    // Comparison operators
+    OpEqual,
+    OpLess,
+    OpLessOrEq,
+    OpMore,
+    OpMoreOrEq,
+
+    // Logical operators & generic jumps
+    OpJump(JumpOffset),
+    OpJumpIfTrue(JumpOffset),
+    OpJumpIfFalse(JumpOffset),
+    OpJumpIfNotFound(JumpOffset),
+
+    // Attribute sets
+    /// 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),
+    OpAttrsUpdate,
+    OpAttrsSelect,
+    OpAttrsTrySelect,
+    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
+    OpPushWith(StackIdx),
+    OpPopWith,
+    OpResolveWith,
+
+    // Lists
+    OpList(Count),
+    OpConcat,
+
+    // Strings
+    OpInterpolate(Count),
+    /// Force the Value on the stack and coerce it to a string, always using
+    /// `CoercionKind::Weak`.
+    OpCoerceToString,
+
+    // 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
+    OpAssertBool,
+
+    /// 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
+    OpCall,
+    OpGetUpvalue(UpvalueIdx),
+    /// A Closure which has upvalues but no self-references
+    OpClosure(ConstantIdx),
+    /// A Closure which has self-references (direct or via upvalues)
+    OpThunkClosure(ConstantIdx),
+    /// A suspended thunk, used to ensure laziness
+    OpThunkSuspended(ConstantIdx),
+    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 000000000000..5ac115e21c89
--- /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 000000000000..45c1cdfce911
--- /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 000000000000..649679536080
--- /dev/null
+++ b/tvix/eval/src/source.rs
@@ -0,0 +1,63 @@
+//! 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 {
+    /// Create a new SourceCode instance.
+    pub fn new() -> Self {
+        SourceCode(Rc::new(RefCell::new(CodeMap::new())))
+    }
+
+    /// 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
+    }
+}
diff --git a/tvix/eval/src/spans.rs b/tvix/eval/src/spans.rs
new file mode 100644
index 000000000000..c0130a665428
--- /dev/null
+++ b/tvix/eval/src/spans.rs
@@ -0,0 +1,121 @@
+//! Utilities for dealing with span tracking in the compiler and in
+//! error reporting.
+
+use crate::opcode::CodeIdx;
+use crate::value::Lambda;
+use codemap::{File, Span};
+use rnix::ast;
+use rowan::ast::AstNode;
+use std::rc::Rc;
+
+/// 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 },
+
+    /// The span needs to be computed from the provided data, but only
+    /// when it is required.
+    Delayed { lambda: Rc<Lambda>, offset: CodeIdx },
+}
+
+impl LightSpan {
+    pub fn new_delayed(lambda: Rc<Lambda>, offset: CodeIdx) -> Self {
+        Self::Delayed { lambda, offset }
+    }
+
+    pub fn new_actual(span: Span) -> Self {
+        Self::Actual { span }
+    }
+
+    pub fn span(&self) -> Span {
+        match self {
+            LightSpan::Actual { span } => *span,
+            LightSpan::Delayed { lambda, offset } => lambda.chunk.get_span(*offset),
+        }
+    }
+}
+
+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 000000000000..16386cb9e0ad
--- /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 000000000000..a7d1c3f96899
--- /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 000000000000..f800baf05018
--- /dev/null
+++ b/tvix/eval/src/tests/mod.rs
@@ -0,0 +1,181 @@
+use builtin_macros::builtins;
+use pretty_assertions::assert_eq;
+use test_generator::test_resources;
+
+/// 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::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: &str, expect_success: bool) {
+    let base = code_path
+        .strip_suffix("nix")
+        .expect("test files always end in .nix");
+    let exp_path = format!("{}exp", base);
+    let exp_xml_path = std::path::PathBuf::from(format!("{}exp.xml", base));
+
+    let code = std::fs::read_to_string(code_path).expect("should be able to read test code");
+
+    if exp_xml_path.exists() {
+        // We can't test them at the moment because we don't have XML output yet.
+        // Checking for success / failure only is a bit disingenious.
+        return;
+    }
+
+    let mut eval = crate::Evaluation::new_impure(&code, Some(code_path.into()));
+    eval.strict = true;
+    eval.builtins.extend(mock_builtins::builtins());
+
+    let result = eval.evaluate();
+
+    if expect_success && !result.errors.is_empty() {
+        panic!(
+            "{code_path}: evaluation of eval-okay test should succeed, but failed with {:?}",
+            result.errors,
+        );
+    }
+
+    if !expect_success && !result.errors.is_empty() {
+        return;
+    }
+
+    let result_str = result.value.unwrap().to_string();
+
+    if let Ok(exp) = std::fs::read_to_string(exp_path) {
+        if expect_success {
+            assert_eq!(
+                result_str,
+                exp.trim(),
+                "{code_path}: result value representation (left) must match expectation (right)"
+            );
+        } else {
+            assert_ne!(
+                result_str,
+                exp.trim(),
+                "{code_path}: test passed unexpectedly!  consider moving it out of notyetpassing"
+            );
+        }
+    } else if expect_success {
+        panic!("{code_path}: should be able to read test expectation");
+    } else {
+        panic!(
+            "{code_path}: test should have failed, but succeeded with output {}",
+            result_str
+        );
+    }
+}
+
+// identity-* tests contain Nix code snippets which should evaluate to
+// themselves exactly (i.e. literals).
+#[test_resources("src/tests/tvix_tests/identity-*.nix")]
+fn identity(code_path: &str) {
+    let code = std::fs::read_to_string(code_path).expect("should be able to read test code");
+
+    let mut eval = crate::Evaluation::new(&code, None);
+    eval.strict = true;
+    eval.io_handle = Box::new(crate::StdIO);
+
+    let result = eval.evaluate();
+    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.
+#[test_resources("src/tests/tvix_tests/eval-okay-*.nix")]
+fn eval_okay(code_path: &str) {
+    eval_test(code_path, true)
+}
+
+// eval-okay-* tests from the original Nix test suite.
+#[cfg(feature = "nix_tests")]
+#[test_resources("src/tests/nix_tests/eval-okay-*.nix")]
+fn nix_eval_okay(code_path: &str) {
+    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 :) Until then, to run these tests, use `cargo test
+// --features expected_failures`.
+//
+// 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.
+//
+// Unfortunately test_generator is unmaintained, so the PRs to make
+// it understand #[ignored] has been sitting for two years, so we
+// can't use `cargo test --include-ignored`, which is the normal way
+// of handling this situation.
+//
+//   https://github.com/frehberg/test-generator/pull/10
+//   https://github.com/frehberg/test-generator/pull/8
+#[test_resources("src/tests/nix_tests/notyetpassing/eval-okay-*.nix")]
+fn nix_eval_okay_currently_failing(code_path: &str) {
+    eval_test(code_path, false)
+}
+
+#[test_resources("src/tests/tvix_tests/notyetpassing/eval-okay-*.nix")]
+fn eval_okay_currently_failing(code_path: &str) {
+    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.
+#[test_resources("src/tests/tvix_tests/eval-fail-*.nix")]
+fn eval_fail(code_path: &str) {
+    eval_test(code_path, false)
+}
+
+// eval-fail-* tests from the original Nix test suite.
+#[cfg(feature = "nix_tests")]
+#[test_resources("src/tests/nix_tests/eval-fail-*.nix")]
+fn nix_eval_fail(code_path: &str) {
+    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 000000000000..357f3547da18
--- /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 000000000000..06d740502001
--- /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 000000000000..257cc5642cb1
--- /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 000000000000..231f150c579c
--- /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 000000000000..170df520ab68
--- /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 000000000000..19010cc35ca6
--- /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 000000000000..170df520ab68
--- /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 000000000000..170df520ab68
--- /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 000000000000..cdf158597eef
--- /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 000000000000..170df520ab68
--- /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 000000000000..170df520ab68
--- /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 000000000000..75c51bceb540
--- /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 000000000000..3b7a1e8bf0c2
--- /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 000000000000..ffe9c983c26b
--- /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 000000000000..65b9d4f505b1
--- /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-disabled b/tvix/eval/src/tests/nix_tests/eval-fail-blackhole.nix-disabled
new file mode 100644
index 000000000000..81133b511c95
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-blackhole.nix-disabled
@@ -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 000000000000..9baa49b063ec
--- /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-division-by-zero-float.nix b/tvix/eval/src/tests/nix_tests/eval-fail-division-by-zero-float.nix
new file mode 100644
index 000000000000..82dd6873218e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-division-by-zero-float.nix
@@ -0,0 +1 @@
+1.0 / 0.0
diff --git a/tvix/eval/src/tests/nix_tests/eval-fail-division-by-zero-int.nix b/tvix/eval/src/tests/nix_tests/eval-fail-division-by-zero-int.nix
new file mode 100644
index 000000000000..72dca4d5e478
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-fail-division-by-zero-int.nix
@@ -0,0 +1 @@
+1 / 0
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 000000000000..ce098b82380a
--- /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 000000000000..c4be9797c534
--- /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 000000000000..8c2e104c788f
--- /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 000000000000..539e0eb0a6f6
--- /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 000000000000..cddbbfd3261e
--- /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 000000000000..f37c2bc0a160
--- /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 000000000000..5e322bc31369
--- /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 000000000000..eb273f45b2a6
--- /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 000000000000..a3f26ea2aa83
--- /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 000000000000..5c54d10b7b47
--- /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 000000000000..7e9e6a0b666e
--- /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 000000000000..b4aa387e07b8
--- /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 000000000000..e5b26e9f2e39
--- /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 000000000000..45b0f829eb33
--- /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 000000000000..810b31a5da96
--- /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 000000000000..45b0f829eb33
--- /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 000000000000..9e06b83ac1fd
--- /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 000000000000..19de4fdf79f7
--- /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 000000000000..f29de11fe660
--- /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 000000000000..1851731442d3
--- /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 000000000000..43ec81210f38
--- /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 000000000000..ce0430d78081
--- /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 000000000000..a4584cd3b398
--- /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 000000000000..3e754364cc9c
--- /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 000000000000..7fef3dddd4dd
--- /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 000000000000..3e754364cc9c
--- /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 000000000000..35ddf495c63b
--- /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 000000000000..0350b518a7ec
--- /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 000000000000..c841816222a5
--- /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 000000000000..0661686d611d
--- /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 000000000000..e9d65e88a817
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..310a030df004
--- /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 000000000000..b4a1e66d6b8a
--- /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 000000000000..2c3dc10da528
--- /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 000000000000..e7dbf978160d
--- /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.nix b/tvix/eval/src/tests/nix_tests/eval-okay-closure.nix
new file mode 100644
index 000000000000..cccd4dc35730
--- /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 000000000000..7182dc2f9b8e
--- /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 000000000000..cb2cce218029
--- /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 000000000000..bb4bbd577410
--- /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 000000000000..d158a9bf05b9
--- /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 000000000000..3b8be7739deb
--- /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 000000000000..97da5d37a412
--- /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 000000000000..93987647ffe6
--- /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 000000000000..adc4c41bd551
--- /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 000000000000..8d38505c1686
--- /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 000000000000..53aa4b1dc251
--- /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 000000000000..eaacb55c1aff
--- /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 000000000000..84b388c27130
--- /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 $out" ];
+    };
+  };
+
+  pkgs = pkgs_ // (packageOverrides pkgs_);
+in pkgs.a.b.name
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.exp b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.exp
new file mode 100644
index 000000000000..27ba77ddaf61
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.exp
@@ -0,0 +1 @@
+true
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.nix b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.nix
new file mode 100644
index 000000000000..6d57bf854908
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs-2.nix
@@ -0,0 +1 @@
+{ a."${"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 000000000000..df8750afc036
--- /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 000000000000..0dbe15e6384c
--- /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; ${bString} = false; }.a;
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.exp b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.exp
new file mode 100644
index 000000000000..df8750afc036
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.exp
@@ -0,0 +1 @@
+{ binds = true; hasAttrs = true; multiAttrs = true; recBinds = true; selectAttrs = true; selectOrAttrs = true; }
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.nix b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.nix
new file mode 100644
index 000000000000..ee02ac7e6579
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-dynamic-attrs.nix
@@ -0,0 +1,17 @@
+let
+  aString = "a";
+
+  bString = "b";
+in {
+  hasAttrs = { a.b = null; } ? "${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; "${bString}" = false; }.a;
+}
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-elem.exp b/tvix/eval/src/tests/nix_tests/eval-okay-elem.exp
new file mode 100644
index 000000000000..3cf6c0e962f0
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-elem.exp
@@ -0,0 +1 @@
+[ true false 30 ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-elem.nix b/tvix/eval/src/tests/nix_tests/eval-okay-elem.nix
new file mode 100644
index 000000000000..71ea7a4ed03d
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-elem.nix
@@ -0,0 +1,6 @@
+with import ./lib.nix;
+
+let xs = range 10 40; in
+
+[ (builtins.elem 23 xs) (builtins.elem 42 xs) (builtins.elemAt xs 20) ]
+
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.exp b/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.exp
new file mode 100644
index 000000000000..cb5537d5d7ce
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.exp
@@ -0,0 +1 @@
+"ab"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.nix b/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.nix
new file mode 100644
index 000000000000..78c133afdd94
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-empty-args.nix
@@ -0,0 +1 @@
+({}: {x,y,}: "${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 000000000000..27ba77ddaf61
--- /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 000000000000..73d200b38141
--- /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 000000000000..355d51c27d8f
--- /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 000000000000..85109b0d0eb8
--- /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 000000000000..b979b2b8b9bc
--- /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 000000000000..fe911e9683e2
--- /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 000000000000..3c50a8adce86
--- /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 000000000000..b2702c7b1668
--- /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.exp b/tvix/eval/src/tests/nix_tests/eval-okay-foldlStrict.exp
new file mode 100644
index 000000000000..837e12b406f6
--- /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 000000000000..3b87188d243b
--- /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 000000000000..d0dd3af2c814
--- /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 = { "ʞ" = { 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\tJosé\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; }; }; }; }; }; "ʎǝʞ" = "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 000000000000..963932689942
--- /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"
+    "ʎǝʞ" = "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 . "ʞ" . '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 000000000000..add5505a8287
--- /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 é 2 char unicode encoded s with caron š 3 char unicode encoded rightwards arrow →"
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 000000000000..f00713507732
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..e1c0f86cc4e4
--- /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-hash.exp b/tvix/eval/src/tests/nix_tests/eval-okay-hash.exp
new file mode 100644
index 000000000000..e69de29bb2d1
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-hash.exp
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 000000000000..00750edc07d6
--- /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 000000000000..23e4c74d5016
--- /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 000000000000..7862331fa551
--- /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 000000000000..1669dc0648ea
--- /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 {: ''${
+    And finally to interpret \n etc. as in a string: ''\n, ''\r, ''\t.
+  '';
+
+  # Regression test: antiquotation in '${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-let.exp b/tvix/eval/src/tests/nix_tests/eval-okay-let.exp
new file mode 100644
index 000000000000..14e24d419005
--- /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 000000000000..fe118c5282e3
--- /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 000000000000..f784f26d83f4
--- /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 000000000000..d433bcf908ba
--- /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 000000000000..74abef7bc6ed
--- /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 000000000000..4186e029b538
--- /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 000000000000..d00491fd7e5b
--- /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 000000000000..fbb12794401f
--- /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 000000000000..dbb64f717b96
--- /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 000000000000..a76c1d811454
--- /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 000000000000..3f113f17bab1
--- /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 000000000000..f075b6275e5a
--- /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 000000000000..0cfbf08886fc
--- /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 000000000000..ba9d79aa79b1
--- /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 000000000000..f98b388071c2
--- /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 000000000000..738123141508
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..b060c0bc9850
--- /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 000000000000..cd8b8b020c05
--- /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 000000000000..846d2ce49486
--- /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-pathexists.exp b/tvix/eval/src/tests/nix_tests/eval-okay-pathexists.exp
new file mode 100644
index 000000000000..27ba77ddaf61
--- /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 000000000000..50c28ee0cd30
--- /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 000000000000..a4304010fe80
--- /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 000000000000..96fd25a01517
--- /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 000000000000..a2c87d0c439f
--- /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 000000000000..82f7cb17435a
--- /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 000000000000..c508d5366f70
--- /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 000000000000..df9fc3f37d22
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..273e2590713e
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..0073e057787d
--- /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 000000000000..00750edc07d6
--- /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 000000000000..694e9a13b7c5
--- /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 000000000000..00750edc07d6
--- /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 000000000000..48550237394a
--- /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 000000000000..8d38505c1686
--- /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 000000000000..4ad5ba897fa7
--- /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-replacestrings.exp b/tvix/eval/src/tests/nix_tests/eval-okay-replacestrings.exp
new file mode 100644
index 000000000000..72e8274d8c58
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-replacestrings.exp
@@ -0,0 +1 @@
+[ "faabar" "fbar" "fubar" "faboor" "fubar" "XaXbXcX" "X" "a_b" ]
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-replacestrings.nix b/tvix/eval/src/tests/nix_tests/eval-okay-replacestrings.nix
new file mode 100644
index 000000000000..bd8031fc004e
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-replacestrings.nix
@@ -0,0 +1,11 @@
+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")
+]
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 000000000000..00750edc07d6
--- /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 000000000000..fa38a7174e03
--- /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 000000000000..d00491fd7e5b
--- /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 000000000000..eb8b02bc4994
--- /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 000000000000..b8626c4cff28
--- /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 000000000000..10d6bc04d830
--- /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 000000000000..00ff03a46c9b
--- /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 000000000000..dc8243bc8546
--- /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 000000000000..00ff03a46c9b
--- /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 000000000000..0995d4e7e7e0
--- /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 000000000000..d00491fd7e5b
--- /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 000000000000..4da02968f6b7
--- /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 000000000000..0cfbf08886fc
--- /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 000000000000..0a9a21c03b62
--- /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 000000000000..899119e20e38
--- /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 000000000000..50aa78e40325
--- /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 000000000000..153ceb8186a0
--- /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 000000000000..9e5c99d2e7f6
--- /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 000000000000..63f650f73a3a
--- /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$\"$\"$"
diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-string.nix b/tvix/eval/src/tests/nix_tests/eval-okay-string.nix
new file mode 100644
index 000000000000..47cc989ad46a
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-string.nix
@@ -0,0 +1,12 @@
+"foo" + "bar"
+  + toString (/a/b + /c/d)
+  + toString (/foo/bar + "/../xyzzy/." + "/foo.txt")
+  + ("/../foo" + toString /x/y)
+  + "escape: \"quote\" \n \\"
+  + "end
+of
+line"
+  + "foo${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 000000000000..27ba77ddaf61
--- /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 000000000000..5e40928dbe31
--- /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 000000000000..6aace04b0f57
--- /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 000000000000..424af00d9b3b
--- /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 000000000000..f7393e847d34
--- /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 000000000000..a3962ce3fdb5
--- /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 000000000000..e92aae3235f2
--- /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 000000000000..ce67943bead5
--- /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 000000000000..828220890ecd
--- /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 000000000000..068c97a6c1b3
--- /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 000000000000..634a841eb190
--- /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 000000000000..ff1791b30eb5
--- /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 000000000000..2b2e6fa711f4
--- /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 000000000000..629bc440a85a
--- /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 000000000000..92a15329935a
--- /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 000000000000..9b58be5d1dd4
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..e9111f5f4331
--- /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 000000000000..378c8dc80403
--- /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 000000000000..033e8d3aba57
--- /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/imported.nix b/tvix/eval/src/tests/nix_tests/imported.nix
new file mode 100644
index 000000000000..fb39ee4efacd
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/imported.nix
@@ -0,0 +1,3 @@
+# The function ‘range’ 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 000000000000..6d0a2992b732
--- /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 000000000000..028a538314b7
--- /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 000000000000..3745235ce95e
--- /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-nonexist-path.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-nonexist-path.nix
new file mode 100644
index 000000000000..f2f08107b516
--- /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).
+"${./fnord}/xyzzy"
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-scope-5.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-scope-5.nix
new file mode 100644
index 000000000000..f89a65a99be3
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-scope-5.nix
@@ -0,0 +1,10 @@
+let {
+
+  x = "a";
+  y = "b";
+
+  f = {x ? y, y ? x}: x + y;
+
+  body = f {};
+
+}
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-undeclared-arg.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-undeclared-arg.nix
new file mode 100644
index 000000000000..cafdf1636272
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-fail-undeclared-arg.nix
@@ -0,0 +1 @@
+({x, z}: x + z) {x = "foo"; y = "bla"; z = "bar";}
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.exp
new file mode 100644
index 000000000000..b46938032e73
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.exp
@@ -0,0 +1 @@
+{ __overrides = { bar = "qux"; }; bar = "qux"; foo = "bar"; }
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.nix
new file mode 100644
index 000000000000..2e5c85483be6
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-attrs6.nix
@@ -0,0 +1,4 @@
+rec {
+  "${"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 000000000000..7a8391786a09
--- /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 000000000000..217c7a5ae291
--- /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 000000000000..815f51b1d67a
--- /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-closure.exp.xml b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-closure.exp.xml
new file mode 100644
index 000000000000..dffc03a99891
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-context-introspection.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.exp
new file mode 100644
index 000000000000..27ba77ddaf61
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.exp
@@ -0,0 +1 @@
+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 000000000000..43178bd2eef9
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-context-introspection.nix
@@ -0,0 +1,24 @@
+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;
+    };
+    "${builtins.unsafeDiscardStringContext drv.drvPath}" = {
+      outputs = [ "foo" "out" ];
+      allOutputs = true;
+    };
+  };
+
+  legit-context = builtins.getContext "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+
+  constructed-context = builtins.getContext (builtins.appendContext "" desired-context);
+in legit-context == constructed-context
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 000000000000..2f535bdbc454
--- /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 000000000000..7b9531cfe9e1
--- /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 000000000000..65fd65b4d01f
--- /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 000000000000..b79553df0bd3
--- /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 000000000000..8e7c61ab8e77
--- /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 000000000000..3fb023e1cd42
--- /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 000000000000..ec04aab6aeec
--- /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 000000000000..d526cb4a2161
--- /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 000000000000..81f80420b996
--- /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 000000000000..d76a0d86ea78
--- /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-functionargs.exp.xml b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-functionargs.exp.xml
new file mode 100644
index 000000000000..651f54c36341
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-functionargs.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-functionargs.nix
new file mode 100644
index 000000000000..68dca62ee18d
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-getattrpos-functionargs.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getattrpos-functionargs.exp
new file mode 100644
index 000000000000..7f9ac40e81b1
--- /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 000000000000..11d6bb0e3ac9
--- /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 000000000000..19765bd501b6
--- /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 000000000000..14dd38f7734c
--- /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 000000000000..469249bbc646
--- /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 000000000000..ca6b07961547
--- /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-getenv.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getenv.exp
new file mode 100644
index 000000000000..14e24d419005
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getenv.exp
@@ -0,0 +1 @@
+"foobar"
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getenv.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-getenv.nix
new file mode 100644
index 000000000000..4cfec5f553d9
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-groupBy.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-groupBy.exp
new file mode 100644
index 000000000000..bfca5652a59b
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-groupBy.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-groupBy.nix
new file mode 100644
index 000000000000..7e0eab28b036
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-hashfile.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-hashfile.exp
new file mode 100644
index 000000000000..ff1e8293ef22
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-hashfile.exp
@@ -0,0 +1 @@
+[ "d3b07384d113edec49eaa6238ad5ff00" "0f343b0931126a20f133d67c2b018a3b" "f1d2d2f924e986ac86fdf7b36c94bcdf32beec15" "60cacbf3d72e1e7834203da608037b1bf83b40e8" "b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c" "5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef" "0cf9180a764aba863a67b6d72f0918bc131c6772642cb2dce5a34f0a702f9470ddc2bf125c12198b1995c233c34b4afd346c54a2334c350a948a51b6e8b4e6b6" "8efb4f73c5655351c444eb109230c556d39e2c7624e9c11abc9e3fb4b9b9254218cc5085b454a9698d085cfa92198491f07a723be4574adc70617b73eb0b6461" ]
diff --git a/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-hashfile.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-hashfile.nix
new file mode 100644
index 000000000000..8c9de66b7ecf
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-hashstring.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-hashstring.exp
new file mode 100644
index 000000000000..d720a082ddb3
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-hashstring.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-hashstring.nix
new file mode 100644
index 000000000000..b0f62b245ca8
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-import.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-import.exp
new file mode 100644
index 000000000000..c508125b55be
--- /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 000000000000..76213a9541e3
--- /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 000000000000..0cfbf08886fc
--- /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 000000000000..358742b36e22
--- /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.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-path.nix
new file mode 100644
index 000000000000..e67168cf3edf
--- /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 000000000000..bf8d2c14ea4f
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-readDir.exp
@@ -0,0 +1 @@
+{ bar = "regular"; foo = "directory"; }
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 000000000000..a7ec9292aae2
--- /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-search-path.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-search-path.exp
new file mode 100644
index 000000000000..4519bc406db5
--- /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 000000000000..a28e6821004a
--- /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 000000000000..6fe33decc019
--- /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-xml.exp.xml b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-xml.exp.xml
new file mode 100644
index 000000000000..20099326cc96
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-xml.nix b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-xml.nix
new file mode 100644
index 000000000000..9ee9f8a0b4f5
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/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/notyetpassing/eval-okay-zipAttrsWith.exp b/tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-zipAttrsWith.exp
new file mode 100644
index 000000000000..9c0b15d22ba4
--- /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 000000000000..e5d4cdccb74e
--- /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 000000000000..e69de29bb2d1
--- /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 000000000000..e69de29bb2d1
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/notyetpassing/readDir/foo/git-hates-directories
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 000000000000..2c02317d2a19
--- /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 000000000000..864d9865e07d
--- /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 000000000000..114d19779f86
--- /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 000000000000..77417432b347
--- /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 000000000000..bbc3eb08c0f6
--- /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 000000000000..a0edd91a9666
--- /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 000000000000..19775d2ec810
--- /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 000000000000..11e40e66fd1b
--- /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 000000000000..17da82e5f0c7
--- /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 000000000000..7b40616417b8
--- /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 000000000000..b1934f7e1e82
--- /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 000000000000..34948d48aed2
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/parse-fail-uft8.nix
@@ -0,0 +1 @@
+123 é 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 000000000000..c10a52b1ea42
--- /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 000000000000..7b63008110db
--- /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 000000000000..23a58ed109b1
--- /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 000000000000..21518d4c6d80
--- /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 000000000000..f4b9efd0c596
--- /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 000000000000..ae6d7a769305
--- /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 000000000000..fd1001c8cafc
--- /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 000000000000..ad066b680384
--- /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 000000000000..45a33e480373
--- /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 000000000000..ae2e256eeaaa
--- /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 000000000000..05c78b3016d3
--- /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 000000000000..356272815d26
--- /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 000000000000..08de27d0a4ce
--- /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 000000000000..23bc9465d6bb
--- /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("builtins.testSourceBuiltin", None);
+    eval.src_builtins.push(("testSourceBuiltin", "42"));
+
+    let result = eval.evaluate();
+    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(/* code = */ "x", None).evaluate();
+
+    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 000000000000..b493aa81f12f
--- /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 000000000000..d81cc0710eb6
--- /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-thunk-error.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-thunk-error.nix
new file mode 100644
index 000000000000..bb0d5920d757
--- /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 000000000000..345b76fde037
--- /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 000000000000..d1c72dc6783a
--- /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 000000000000..937604c563e9
--- /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 000000000000..b7a758302266
--- /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 000000000000..9baa49b063ec
--- /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-foldlStrict-strict-op-application.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-foldlStrict-strict-op-application.nix
new file mode 100644
index 000000000000..adc029b2f29e
--- /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 000000000000..de6c94325661
--- /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
\ No newline at end of file
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 000000000000..fe48a5690c46
--- /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 000000000000..5e4fd3789cd6
--- /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 000000000000..a8c3cedf61d7
--- /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 000000000000..a1218de3fe58
--- /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 000000000000..539e0eb0a6f6
--- /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;
+}
\ No newline at end of file
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 000000000000..cddbbfd3261e
--- /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-okay-access-strange-identifier.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-access-strange-identifier.exp
new file mode 100644
index 000000000000..d81cc0710eb6
--- /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 000000000000..8e282d1280f1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-access-strange-identifier.nix
@@ -0,0 +1,8 @@
+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 000000000000..94ba9a881ae6
--- /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 000000000000..462f670882a0
--- /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 000000000000..08ef6079f899
--- /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 000000000000..9d12aee061b2
--- /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 000000000000..a5711e8bfe9b
--- /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 000000000000..c53790db099b
--- /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 000000000000..aabe6ec3909c
--- /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 000000000000..8934b3d8cf02
--- /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-attrs-inherit-literal.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-inherit-literal.exp
new file mode 100644
index 000000000000..60d3b2f4a4cd
--- /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 000000000000..587aec893372
--- /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 000000000000..a779fce51abc
--- /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 000000000000..68880bcfd857
--- /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 000000000000..fedf8f25a693
--- /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 000000000000..9596be22b831
--- /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 000000000000..fedf8f25a693
--- /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 000000000000..117c01141357
--- /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 000000000000..c2234a47e2b8
--- /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 000000000000..6f71684902e5
--- /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 000000000000..57f4d541bd85
--- /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 000000000000..735602fe02d5
--- /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.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-basenameof.exp
new file mode 100644
index 000000000000..60a773f4af69
--- /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 000000000000..bc59613f541d
--- /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 000000000000..c3ac813de6b9
--- /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 000000000000..b04b1d1fa6ba
--- /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.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-all.exp
new file mode 100644
index 000000000000..82ca7e6b6db7
--- /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 000000000000..12d62632dd6c
--- /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.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-any.exp
new file mode 100644
index 000000000000..d6846ac3f756
--- /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 000000000000..2c659f130b65
--- /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 000000000000..6521066a8ea5
--- /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 000000000000..67f7dcee5672
--- /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.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-attrvalues.exp
new file mode 100644
index 000000000000..35c369772038
--- /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 000000000000..5eced31f0d2d
--- /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 000000000000..30b348853e44
--- /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 000000000000..af40005ed983
--- /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 000000000000..2556b4183c77
--- /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 000000000000..9c28f6d7acee
--- /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 000000000000..457157d45970
--- /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 000000000000..80e363fb0748
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..cfbcbbb7684d
--- /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 000000000000..f8c0b2de5fba
--- /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 000000000000..edac76d4461a
--- /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-compareVersions.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-compareVersions.exp
new file mode 100644
index 000000000000..e69498c3e1c3
--- /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 000000000000..cce554b4494a
--- /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.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.exp
new file mode 100644
index 000000000000..64ae529ac257
--- /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 000000000000..19ef5eba1145
--- /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-div.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-div.exp
new file mode 100644
index 000000000000..73e9bc33b083
--- /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 ]
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 000000000000..98b8b74bdf2b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-div.nix
@@ -0,0 +1,11 @@
+[
+  (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)
+]
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 000000000000..3701c9d75f94
--- /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 000000000000..762adeebbf0a
--- /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.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-filter.exp
new file mode 100644
index 000000000000..fb94ebaa492c
--- /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 000000000000..b621fdb43e7c
--- /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-getattr.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.exp
new file mode 100644
index 000000000000..89fa6c681064
--- /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 000000000000..87a2adbcd3dd
--- /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-groupby-thunk.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-groupby-thunk.exp
new file mode 100644
index 000000000000..94649819caab
--- /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 000000000000..d62ae628dc71
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-groupby-thunk.nix
@@ -0,0 +1,6 @@
+
+builtins.groupBy
+  (v: v.x)
+  [ (rec { y = x; x = "fred"; }) ]
+
+
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 000000000000..541fe347cbcd
--- /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 000000000000..e87e186b641d
--- /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-head.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-head.exp
new file mode 100644
index 000000000000..afe288459f2e
--- /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 000000000000..1741a7aac4bb
--- /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-length.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length.exp
new file mode 100644
index 000000000000..e80eb6ef1465
--- /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 000000000000..ef1f638cbbdc
--- /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 000000000000..31f4598bb504
--- /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 000000000000..cd2d0c209c3a
--- /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-map.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-map.exp
new file mode 100644
index 000000000000..6cf53040320f
--- /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 000000000000..71b351fd55b0
--- /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-mul.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-mul.exp
new file mode 100644
index 000000000000..e3e0f03a8af5
--- /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 000000000000..2a8d6c4214c3
--- /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-partition.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-partition.exp
new file mode 100644
index 000000000000..d2390db4f528
--- /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 000000000000..0587330ff917
--- /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-replaceStrings.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replaceStrings.exp
new file mode 100644
index 000000000000..c2cb89bac663
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replaceStrings.exp
@@ -0,0 +1 @@
+[ "fabir" "a" "1a1" ]
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 000000000000..b8101c448bb0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-replaceStrings.nix
@@ -0,0 +1,5 @@
+[
+  (builtins.replaceStrings ["oo" "a"] ["a" "i"] "foobar")
+  (builtins.replaceStrings ["o"] ["a"] "a")
+  (builtins.replaceStrings ["" ""] ["1" "2"] "a")
+]
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 000000000000..222a0093f570
--- /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 000000000000..4083e86714fc
--- /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-string-length.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-string-length.exp
new file mode 100644
index 000000000000..b019be4bfd7a
--- /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 000000000000..b7d51db3c578
--- /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 "😀")
+]
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 000000000000..51842eccfac5
--- /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 000000000000..2929c4dddd81
--- /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 000000000000..192548e94978
--- /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 000000000000..626ae1d1be55
--- /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.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-substring.exp
new file mode 100644
index 000000000000..168276022898
--- /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 000000000000..f4ee82e2736f
--- /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.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.exp
new file mode 100644
index 000000000000..b9e3aa1ef79d
--- /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 000000000000..2be9496a98e2
--- /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 000000000000..3d4204d5a83e
--- /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 000000000000..d96ddb3bd16d
--- /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 "Ziemßen";
+    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-toString.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-toString.exp
new file mode 100644
index 000000000000..a148ebc3b53f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-toString.exp
@@ -0,0 +1 @@
+[ "1" "4.200000" "" "" "1" "foo" "/etc" "Hello World" "Hello World" "1" "out" "2" "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 000000000000..e4dc18ac96a7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-toString.nix
@@ -0,0 +1,23 @@
+let
+  toStringableSet = {
+    __toString = self: self.content;
+    content = "Hello World";
+  };
+
+  toStringExamples = [
+    (toString 1)
+    (toString 4.2)
+    (toString null)
+    (toString false)
+    (toString true)
+    (toString "foo")
+    (toString /etc)
+    (toString toStringableSet)
+    (toString { __toString = _: toStringableSet; })
+    (toString { __toString = _: true; })
+    (toString { outPath = "out"; })
+    (toString { outPath = { outPath = { __toString = _: 2; }; }; })
+  ];
+in
+
+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 000000000000..0a274c201fa8
--- /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 000000000000..12e8c03b171d
--- /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 000000000000..69667de5a1c3
--- /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 000000000000..70755c8c6dbe
--- /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 000000000000..82dd08179843
--- /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 000000000000..7f9d95ac60f4
--- /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 000000000000..9ccd94224ba8
--- /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 000000000000..16234ab4514a
--- /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 000000000000..2661fd257bf9
--- /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 000000000000..ec6f8d947cec
--- /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.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-type-of.exp
new file mode 100644
index 000000000000..1ea054fc2d72
--- /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 000000000000..a3cb659ecfde
--- /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 000000000000..724c1f9c34b9
--- /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 000000000000..3d688cb8bd68
--- /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-ceil.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-ceil.exp
new file mode 100644
index 000000000000..dffbbe59f0a0
--- /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 000000000000..5835bf829b03
--- /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 000000000000..c3bb809c9ffb
--- /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 000000000000..7c4333668df8
--- /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 000000000000..be54b4b4e39e
--- /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 000000000000..bda364f42992
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-closure-self.nix
@@ -0,0 +1,4 @@
+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 000000000000..d00491fd7e5b
--- /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 000000000000..305463775217
--- /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 000000000000..95a0e7378b5e
--- /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 000000000000..2b511f56eecb
--- /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 000000000000..9160829dde78
--- /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 000000000000..c505a85b1f3c
--- /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 000000000000..95a0e7378b5e
--- /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 000000000000..7d6b30419fac
--- /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 000000000000..9160829dde78
--- /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 000000000000..0bf474e53fd0
--- /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 000000000000..95a0e7378b5e
--- /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 000000000000..61b206c03376
--- /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 000000000000..9160829dde78
--- /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 000000000000..ad77074710ca
--- /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 000000000000..95a0e7378b5e
--- /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 000000000000..b5773a21d374
--- /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 000000000000..9160829dde78
--- /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 000000000000..172d2237e9e2
--- /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 000000000000..d874518a37ed
--- /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 000000000000..6ddd30e7a189
--- /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 000000000000..3b7fd398198a
--- /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 000000000000..9b73df61d84e
--- /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-concat-lists.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-concat-lists.exp
new file mode 100644
index 000000000000..3bed31f76e3f
--- /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 000000000000..de332cd29f3a
--- /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 000000000000..cd4bc1ab64cc
--- /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 000000000000..1fc708929989
--- /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 000000000000..14d804aa22f5
--- /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 000000000000..149a0722b11d
--- /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 000000000000..93987647ffe6
--- /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 000000000000..adc4c41bd551
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_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/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 000000000000..c508d5366f70
--- /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 000000000000..361ba9144594
--- /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 000000000000..ca00e3c049d6
--- /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 000000000000..c086759f456b
--- /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 000000000000..7cf54d959611
--- /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 000000000000..91649d0c6dd4
--- /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 000000000000..3bed31f76e3f
--- /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 000000000000..7f13f1f27030
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deeply-nested-with-closure.nix
@@ -0,0 +1,18 @@
+# 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 000000000000..3bed31f76e3f
--- /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 000000000000..7f1128b6707b
--- /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 000000000000..8d38505c1686
--- /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 000000000000..53aa4b1dc251
--- /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 000000000000..5993db7ccc5a
--- /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 000000000000..1fbb3e853af4
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..af227ae28e87
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-deferred-with.nix
@@ -0,0 +1,8 @@
+# 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 000000000000..ff464e4c30d4
--- /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 000000000000..13cf47320515
--- /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 000000000000..3cf6c0e962f0
--- /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 000000000000..71ea7a4ed03d
--- /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 000000000000..ffcd4415b08f
--- /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 000000000000..a1181431deca
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..398f4a9dfc55
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..dc52ba112a60
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..5dbcb515295b
--- /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-escapify-integer-keys.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-escapify-integer-keys.exp
new file mode 100644
index 000000000000..aa98a082a8ac
--- /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 000000000000..aa98a082a8ac
--- /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 000000000000..8643cf6debac
--- /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 000000000000..9a22d85ac5f1
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fib.nix
@@ -0,0 +1,7 @@
+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 000000000000..c1581543511f
--- /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 000000000000..27d2fae1042e
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-fix.nix
@@ -0,0 +1,7 @@
+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 000000000000..c55d2be717f1
--- /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 000000000000..447bd5af7f5c
--- /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 000000000000..6f98a7f48f2f
--- /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 000000000000..c6b79c91a1b4
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..fc4129a2543a
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..59fd29b55237
--- /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 000000000000..8d683a20fab7
--- /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 000000000000..44c0349387ff
--- /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-fromjson-escapes.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-fromjson-escapes.exp
new file mode 100644
index 000000000000..add5505a8287
--- /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 é 2 char unicode encoded s with caron š 3 char unicode encoded rightwards arrow →"
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 000000000000..f00713507732
--- /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 000000000000..24aa21d78ff7
--- /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 000000000000..e530789446ef
--- /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 000000000000..c1c9f8ffaf69
--- /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 000000000000..cd95b971f8a9
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_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/tvix_tests/eval-okay-functor-call.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-functor-call.exp
new file mode 100644
index 000000000000..d81cc0710eb6
--- /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 000000000000..80ae345d836b
--- /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 000000000000..cd4ca34f14df
--- /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 000000000000..2c4dfba20324
--- /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-identifier-formatting.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.exp
new file mode 100644
index 000000000000..074f5f07f5f0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.exp
@@ -0,0 +1 @@
+{ "'quoted'" = false; "-20°" = 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 = true; "attr.path" = false; false = true; foldl' = true; normal = true; normal2 = true; null = true; or = true; throw = true; true = true; x = true; x' = true; x'' = true; "😀" = 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 000000000000..8f9aa2823801
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-identifier-formatting.nix
@@ -0,0 +1,30 @@
+{
+  __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" = true; # -ish
+  throw = true;
+  abort = true;
+
+  "9front" = false;
+  "2normal" = false;
+  "-20°" = false;
+  "45 44 43-'3 2 1" = false;
+  "attr.path" = false;
+  "'quoted'" = false;
+  "_'12.5" = false;
+  "😀" = false;
+}
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 000000000000..5ba7f64d78a7
--- /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 000000000000..49cd244f06c1
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..dde81e5a7c30
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-inherit-string-ident.nix
@@ -0,0 +1,7 @@
+# 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 000000000000..d81cc0710eb6
--- /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 000000000000..5c6702120fc4
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..c6dd5e9d54cf
--- /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 000000000000..25001b211fb8
--- /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 000000000000..3534132ed48b
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..f2ee49df8092
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..dae170b06bad
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding-closure.nix
@@ -0,0 +1,4 @@
+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 000000000000..d81cc0710eb6
--- /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 000000000000..41c4c53ea271
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-late-binding.nix
@@ -0,0 +1,4 @@
+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 000000000000..48082f72f087
--- /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 000000000000..5a36964976ca
--- /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 000000000000..1c70d1bcf188
--- /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 000000000000..d19d1213d695
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-lazy-equality.nix
@@ -0,0 +1,15 @@
+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 000000000000..d81cc0710eb6
--- /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 000000000000..22ac14b3f19e
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..8b1a0191dcc0
--- /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 000000000000..5d2955ffd5e1
--- /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 000000000000..87fcffadee5f
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..a5b05426689e
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..850e0252c2b6
--- /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 000000000000..5776134d0e41
--- /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 000000000000..b6c37c34f949
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-identifiers.nix
@@ -0,0 +1,5 @@
+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 000000000000..409940768f2a
--- /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 000000000000..21196f48bcbe
--- /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 000000000000..0cfbf08886fc
--- /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 000000000000..4ec270e3bf43
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-inherit.nix
@@ -0,0 +1,12 @@
+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 000000000000..00750edc07d6
--- /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 000000000000..7a65a5b1cc9f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-let-sibling-access.nix
@@ -0,0 +1,5 @@
+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 000000000000..3bed31f76e3f
--- /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 000000000000..30981099cbde
--- /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 … 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 000000000000..d00491fd7e5b
--- /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 000000000000..3d1c46b10bb2
--- /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-listtoattrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-listtoattrs.exp
new file mode 100644
index 000000000000..74abef7bc6ed
--- /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 000000000000..89888fd56178
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-listtoattrs.nix
@@ -0,0 +1,15 @@
+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-manual-rec.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-manual-rec.exp
new file mode 100644
index 000000000000..d81cc0710eb6
--- /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 000000000000..8e1256d764e3
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-manual-rec.nix
@@ -0,0 +1,9 @@
+# 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 000000000000..911ab51de5ca
--- /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 000000000000..78b28909a29c
--- /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 000000000000..768eaae61cfc
--- /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 000000000000..cea4cb1b4f0d
--- /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 000000000000..9839e480b781
--- /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 000000000000..84beb22ed5c3
--- /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 000000000000..b5c707cf462e
--- /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 000000000000..5d611930ca7a
--- /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 000000000000..edca9baca9c0
--- /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 000000000000..1b3feda432ef
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..e06b571a28bf
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..a83471e500ce
--- /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-closure.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-closure.exp
new file mode 100644
index 000000000000..b6a7d89c68e0
--- /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 000000000000..97bff7f07758
--- /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 000000000000..209e3ef4b624
--- /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 000000000000..358925e992c7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-deferred-upvalue.nix
@@ -0,0 +1,6 @@
+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 000000000000..d2c1c04da3c4
--- /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 000000000000..47dcec7a95f4
--- /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 000000000000..6db47b033efb
--- /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 000000000000..c75b7130e1ba
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-keys-let.nix
@@ -0,0 +1,4 @@
+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 000000000000..77eb325dde19
--- /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 000000000000..797d11108ff4
--- /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 000000000000..e45ef1da2f54
--- /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 000000000000..811bb4361bf7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let-slots.nix
@@ -0,0 +1,17 @@
+# 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 000000000000..7f8f011eb73d
--- /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 000000000000..caaa20211928
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-let.nix
@@ -0,0 +1,7 @@
+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 000000000000..d81cc0710eb6
--- /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 000000000000..8d0280bb8973
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-poisoning.nix
@@ -0,0 +1,4 @@
+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 000000000000..d81cc0710eb6
--- /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 000000000000..f3ad8293540f
--- /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 000000000000..d757cae1f588
--- /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 000000000000..31111d80810c
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..133929dd1961
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-thunks.nix
@@ -0,0 +1,7 @@
+# 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 000000000000..0cfbf08886fc
--- /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 000000000000..ea5ef568edab
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-nested-with.nix
@@ -0,0 +1,4 @@
+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 000000000000..3a2e3f4984a0
--- /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 000000000000..6f353056127c
--- /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-observe-infinite-attrs.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-observe-infinite-attrs.exp
new file mode 100644
index 000000000000..bbb332a5eea0
--- /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 000000000000..684c88f80067
--- /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 000000000000..9d9185fcd155
--- /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 000000000000..650d7f028df2
--- /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 000000000000..d00491fd7e5b
--- /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 000000000000..444f270af637
--- /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 000000000000..0cfbf08886fc
--- /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 000000000000..ceffd0697b28
--- /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 000000000000..d00491fd7e5b
--- /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 000000000000..1a76594546b3
--- /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 000000000000..a833e32892e4
--- /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 000000000000..fd09bfee64c2
--- /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 000000000000..d00491fd7e5b
--- /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 000000000000..ce1e6e67c289
--- /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 000000000000..2483a2718301
--- /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 000000000000..4154ff9da29f
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..fea6e234dcfd
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-parsedrvname.nix
@@ -0,0 +1,12 @@
+
+# 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 000000000000..27ba77ddaf61
--- /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 000000000000..ab3d0369401c
--- /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 000000000000..5776134d0e41
--- /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 000000000000..81f03d9e2b09
--- /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 000000000000..bf8d2c14ea4f
--- /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 000000000000..a7ec9292aae2
--- /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 000000000000..a2c87d0c439f
--- /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 000000000000..82f7cb17435a
--- /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 000000000000..ac8d062a6911
--- /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 000000000000..8d7a8cef8e10
--- /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 000000000000..a1dca9bb6860
--- /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 000000000000..7d037c6b37bc
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..a234705b5e53
--- /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 000000000000..950103539123
--- /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 000000000000..f774e00a215a
--- /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 000000000000..8d38505c1686
--- /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 000000000000..4ad5ba897fa7
--- /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;
+}
\ No newline at end of file
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 000000000000..b4a1e66d6b8a
--- /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 000000000000..2f719dcef5be
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-repeated-list-to-attrs.nix
@@ -0,0 +1,13 @@
+# 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 000000000000..d81cc0710eb6
--- /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 000000000000..fd0806c199a5
--- /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 000000000000..7f8f011eb73d
--- /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 000000000000..56445454fec7
--- /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 000000000000..cd4bc1ab64cc
--- /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 000000000000..125b0859ac42
--- /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 000000000000..00750edc07d6
--- /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 000000000000..b440a220ff6d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-let.nix
@@ -0,0 +1,4 @@
+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 000000000000..6db47b033efb
--- /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 000000000000..a97394d16523
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..c86ff80383c0
--- /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 000000000000..d00491fd7e5b
--- /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 000000000000..64962b50ff63
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-simple-with.nix
@@ -0,0 +1,5 @@
+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 000000000000..9d783762143e
--- /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 000000000000..9969e0a29433
--- /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-thunked-functor.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-functor.exp
new file mode 100644
index 000000000000..d81cc0710eb6
--- /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 000000000000..f8eba2ac2fc0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-functor.nix
@@ -0,0 +1,7 @@
+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 000000000000..ffcd4415b08f
--- /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 000000000000..65e9e66d74dc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-if.nix
@@ -0,0 +1,6 @@
+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 000000000000..fc2f21e9305c
--- /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 000000000000..bd3555bb2412
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..6f32660c4c33
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-thunked-with.nix
@@ -0,0 +1,7 @@
+# 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 000000000000..edca9baca9c0
--- /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 000000000000..4c6884bec3df
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-toplevel-finaliser.nix
@@ -0,0 +1,10 @@
+# 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.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-tryeval.exp
new file mode 100644
index 000000000000..8b6ed7dbac6b
--- /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 000000000000..e2357c798753
--- /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 000000000000..5462431496bc
--- /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 000000000000..30e9667da821
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-unpoison-scope.nix
@@ -0,0 +1,7 @@
+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 000000000000..d00491fd7e5b
--- /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 000000000000..d335e5363000
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.nix
@@ -0,0 +1,15 @@
+# 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-pointer-compare.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-value-pointer-compare.exp
new file mode 100644
index 000000000000..27ba77ddaf61
--- /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 000000000000..f60d27d38157
--- /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 ]
\ No newline at end of file
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 000000000000..aec30f297af9
--- /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 000000000000..206881d7d9c2
--- /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)
+]
\ No newline at end of file
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 000000000000..fa8f08cb6ff8
--- /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 000000000000..7e2f7c073bfc
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..c44455a5bf83
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-dynamic-key.nix
@@ -0,0 +1,12 @@
+# 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 000000000000..5776134d0e41
--- /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 000000000000..bb62fdf31cd7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-with-in-list.nix
@@ -0,0 +1,13 @@
+# 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/identity-bool-false.nix b/tvix/eval/src/tests/tvix_tests/identity-bool-false.nix
new file mode 100644
index 000000000000..c508d5366f70
--- /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 000000000000..27ba77ddaf61
--- /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 000000000000..08951d7637a6
--- /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 000000000000..ffcd4415b08f
--- /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 000000000000..1e3ec7217afb
--- /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 000000000000..e7c2ae18a661
--- /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 000000000000..bf77d549685a
--- /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 000000000000..87f7fb0d0668
--- /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 000000000000..48e6655fb169
--- /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 000000000000..d23a5c38147b
--- /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 000000000000..d78a54e5b002
--- /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 000000000000..d81cc0710eb6
--- /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 000000000000..f1398b8d0592
--- /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 000000000000..6a139452ef28
--- /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 000000000000..19765bd501b6
--- /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-signed-float.nix b/tvix/eval/src/tests/tvix_tests/identity-signed-float.nix
new file mode 100644
index 000000000000..50c9d06aa52c
--- /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 000000000000..6a0e60d48b17
--- /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 000000000000..d71ddbcf823f
--- /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 000000000000..028a538314b7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_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/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 000000000000..967fc858bc3d
--- /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 000000000000..804355f34d10
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-minimal-2.3-builtins.nix
@@ -0,0 +1,35 @@
+# 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/readDir/bar b/tvix/eval/src/tests/tvix_tests/readDir/bar
new file mode 100644
index 000000000000..e69de29bb2d1
--- /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 000000000000..e69de29bb2d1
--- /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 000000000000..687d6850ccfa
--- /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 000000000000..bf53f4fcb28a
--- /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
new file mode 100644
index 000000000000..bacfd22217bd
--- /dev/null
+++ b/tvix/eval/src/value/attrs.rs
@@ -0,0 +1,609 @@
+//! 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::iter::FromIterator;
+
+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::CoercionKind;
+use super::TotalDisplay;
+use super::Value;
+use crate::errors::ErrorKind;
+use crate::generators::{self, GenCo};
+
+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;
+}
+
+#[cfg(test)]
+mod tests;
+
+#[derive(Clone, Debug, Deserialize)]
+pub(super) enum AttrsRep {
+    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 Default for AttrsRep {
+    fn default() -> Self {
+        AttrsRep::Empty
+    }
+}
+
+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 {
+            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: &str) -> Option<&Value> {
+        match self {
+            AttrsRep::Empty => None,
+
+            AttrsRep::KV { name, value } => match key {
+                "name" => Some(name),
+                "value" => Some(value),
+                _ => None,
+            },
+
+            AttrsRep::Im(map) => map.get(&key.into()),
+        }
+    }
+
+    fn contains(&self, key: &str) -> bool {
+        match self {
+            AttrsRep::Empty => false,
+            AttrsRep::KV { .. } => key == "name" || key == "value",
+            AttrsRep::Im(map) => map.contains_key(&key.into()),
+        }
+    }
+}
+
+#[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("; ")?;
+            }
+
+            AttrsRep::Im(map) => {
+                for (name, value) in map {
+                    write!(f, "{} = ", name.ident_str())?;
+                    value.total_fmt(f, set)?;
+                    f.write_str("; ")?;
+                }
+            }
+
+            AttrsRep::Empty => { /* no values to print! */ }
+        }
+
+        f.write_str("}")
+    }
+}
+
+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);
+                }
+
+                NixAttrs::construct(stack_array.len() / 2, stack_array).map_err(A::Error::custom)
+            }
+        }
+
+        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(&self, key: &str) -> Option<&Value> {
+        self.0.select(key)
+    }
+
+    /// Select a required value from an attribute set by key, return
+    /// an `AttributeNotFound` error if it is missing.
+    pub fn select_required(&self, key: &str) -> Result<&Value, ErrorKind> {
+        self.select(key)
+            .ok_or_else(|| ErrorKind::AttributeNotFound { name: key.into() })
+    }
+
+    pub fn contains(&self, key: &str) -> bool {
+        self.0.contains(key)
+    }
+
+    /// 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(),
+            },
+        })
+    }
+
+    pub fn into_iter(self) -> IntoIter {
+        match self.0 {
+            AttrsRep::Empty => IntoIter(IntoIterRepr::Empty),
+            AttrsRep::KV { name, value } => IntoIter(IntoIterRepr::Finite(
+                vec![(NAME_REF.clone(), name), (VALUE_REF.clone(), value)].into_iter(),
+            )),
+            AttrsRep::Im(map) => IntoIter(IntoIterRepr::Im(map.into_iter())),
+        }
+    }
+
+    /// Same as into_iter(), but marks call sites which rely on the
+    /// iteration being lexicographic.
+    pub fn into_iter_sorted(self) -> IntoIter {
+        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<Self, 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(NixAttrs(AttrsRep::Empty));
+        }
+
+        // Optimisation: KV pattern
+        if count == 2 {
+            if let Some(kv) = attempt_optimise_kv(&mut stack_slice) {
+                return 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;
+                }
+
+                other => return Err(ErrorKind::InvalidAttributeName(other)),
+            }
+        }
+
+        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 })
+    }
+
+    /// Attempt to coerce an attribute set with a `__toString`
+    /// attribute to a string.
+    pub(crate) async fn try_to_string(&self, co: &GenCo, kind: CoercionKind) -> Option<NixString> {
+        if let Some(to_string) = self.select("__toString") {
+            let callable = generators::request_force(&co, to_string.clone()).await;
+
+            // Leave the attribute set on the stack as an argument
+            // to the function call.
+            generators::request_stack_push(&co, Value::Attrs(Box::new(self.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.
+            let s = generators::request_string_coerce(&co, result, kind).await;
+
+            return Some(s);
+        }
+
+        None
+    }
+}
+
+/// 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().as_str().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)>),
+}
+
+#[repr(transparent)]
+pub struct IntoIter(IntoIterRepr);
+
+impl Iterator for IntoIter {
+    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 IntoIter {
+    fn len(&self) -> usize {
+        match &self.0 {
+            IntoIterRepr::Empty => 0,
+            IntoIterRepr::Finite(inner) => inner.len(),
+            IntoIterRepr::Im(inner) => inner.len(),
+        }
+    }
+}
diff --git a/tvix/eval/src/value/attrs/tests.rs b/tvix/eval/src/value/attrs/tests.rs
new file mode 100644
index 000000000000..473592c519df
--- /dev/null
+++ b/tvix/eval/src/value/attrs/tests.rs
@@ -0,0 +1,108 @@
+use super::*;
+
+#[test]
+fn test_empty_attrs() {
+    let attrs = NixAttrs::construct(0, vec![]).expect("empty attr construction should succeed");
+
+    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::String("key".into()), Value::String("value".into())],
+    )
+    .expect("simple attr construction should succeed");
+
+    assert!(
+        matches!(attrs, NixAttrs(AttrsRep::Im(_))),
+        "simple attribute set should use map representation",
+    )
+}
+
+#[test]
+fn test_kv_attrs() {
+    let name_val = Value::String("name".into());
+    let value_val = Value::String("value".into());
+    let meaning_val = Value::String("meaning".into());
+    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");
+
+    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();
+    assert!(attrs.iter().next().is_none());
+}
+
+#[test]
+fn test_kv_attrs_iter() {
+    let name_val = Value::String("name".into());
+    let value_val = Value::String("value".into());
+    let meaning_val = Value::String("meaning".into());
+    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");
+
+    let mut iter = kv_attrs
+        .iter()
+        .collect::<Vec<_>>()
+        .into_iter()
+        .map(|(k, v)| (k, v));
+    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::String("key".into()), Value::String("value".into())],
+    )
+    .expect("simple attr construction should succeed");
+
+    let mut iter = attrs.iter().collect::<Vec<_>>().into_iter();
+    let (k, v) = iter.next().unwrap();
+    assert!(k == &NixString::from("key"));
+    assert!(v.to_str().unwrap().as_str() == "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 000000000000..6d08ebf9506d
--- /dev/null
+++ b/tvix/eval/src/value/builtin.rs
@@ -0,0 +1,136 @@
+//! 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, [`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 000000000000..1a32795393bf
--- /dev/null
+++ b/tvix/eval/src/value/function.rs
@@ -0,0 +1,108 @@
+//! This module implements the runtime representation of functions.
+use std::{collections::HashMap, 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: HashMap<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,
+}
+
+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 + 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 [`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 000000000000..33e16ebffcd0
--- /dev/null
+++ b/tvix/eval/src/value/json.rs
@@ -0,0 +1,84 @@
+/// 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::generators::{self, GenCo};
+use crate::ErrorKind;
+
+use serde_json::value::to_value;
+use serde_json::Value as Json;
+use serde_json::{Map, Number}; // name clash with *our* `Value`
+
+impl Value {
+    pub(crate) async fn to_json(self, co: &GenCo) -> Result<Json, ErrorKind> {
+        let self_forced = generators::request_force(co, self).await;
+
+        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) => Json::String(s.as_str().into()),
+
+            Value::Path(p) => {
+                let imported = generators::request_path_import(co, *p).await;
+                Json::String(imported.to_string_lossy().to_string())
+            }
+
+            Value::List(l) => {
+                let mut out = vec![];
+
+                for val in l.into_iter() {
+                    out.push(generators::request_to_json(co, val).await);
+                }
+
+                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 let Some(s) = attrs.try_to_string(&co, CoercionKind::Weak).await {
+                    return Ok(Json::String(s.as_str().to_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.as_str().to_string(),
+                        generators::request_to_json(co, value).await,
+                    );
+                }
+
+                Json::Object(out)
+            }
+
+            val @ Value::Closure(_)
+            | val @ Value::Thunk(_)
+            | val @ Value::Builtin(_)
+            | val @ Value::AttrNotFound
+            | val @ Value::Blueprint(_)
+            | val @ Value::DeferredUpvalue(_)
+            | val @ Value::UnresolvedPath(_)
+            | val @ Value::Json(_) => return Err(ErrorKind::NotSerialisableToJson(val.type_of())),
+        };
+
+        Ok(value)
+    }
+
+    /// Generator version of the above, which wraps responses in
+    /// Value::Json.
+    pub(crate) async fn to_json_generator(self, co: GenCo) -> Result<Value, ErrorKind> {
+        Ok(Value::Json(self.to_json(&co).await?))
+    }
+}
diff --git a/tvix/eval/src/value/list.rs b/tvix/eval/src/value/list.rs
new file mode 100644
index 000000000000..cfaefff82195
--- /dev/null
+++ b/tvix/eval/src/value/list.rs
@@ -0,0 +1,152 @@
+//! This module implements Nix lists.
+use std::ops::Index;
+use std::rc::Rc;
+
+use imbl::{vector, Vector};
+
+use serde::Deserialize;
+
+use crate::generators;
+use crate::generators::GenCo;
+use crate::AddContext;
+use crate::ErrorKind;
+
+use super::thunk::ThunkSet;
+use super::TotalDisplay;
+use super::Value;
+
+#[repr(transparent)]
+#[derive(Clone, Debug, Deserialize)]
+pub struct NixList(Rc<Vector<Value>>);
+
+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 {
+            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.into_iter())))
+    }
+
+    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.into_iter())))
+    }
+
+    /// 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.
+    pub async fn sort_by(self, co: &GenCo, cmp: Value) -> Result<Self, ErrorKind> {
+        let mut len = self.len();
+        let mut data = self.into_inner();
+
+        loop {
+            let mut new_len = 0;
+            for i in 1..len {
+                if generators::request_force(
+                    co,
+                    generators::request_call_with(
+                        co,
+                        cmp.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(data.into())
+    }
+}
+
+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
new file mode 100644
index 000000000000..77d40b0e3906
--- /dev/null
+++ b/tvix/eval/src/value/mod.rs
@@ -0,0 +1,842 @@
+//! 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 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::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 path::canon_path;
+pub use string::NixString;
+pub use thunk::Thunk;
+
+pub use self::thunk::{SharedThunkSet, ThunkSet};
+
+use lazy_static::lazy_static;
+
+#[warn(variant_size_differences)]
+#[derive(Clone, Debug, Deserialize)]
+#[serde(untagged)]
+pub enum Value {
+    Null,
+    Bool(bool),
+    Integer(i64),
+    Float(f64),
+    String(NixString),
+
+    #[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.
+    #[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(serde_json::Value),
+}
+
+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, Debug)]
+pub enum CoercionKind {
+    /// Only coerce already "stringly" types like strings and paths, but also
+    /// coerce sets that have a `__toString` attribute. Equivalent to
+    /// `!coerceMore` in C++ Nix.
+    Weak,
+    /// Coerce all value types included by `Weak`, but also coerce `null`,
+    /// booleans, integers, floats and lists of coercible types. Equivalent to
+    /// `coerceMore` in C++ Nix.
+    Strong,
+}
+
+impl<T> From<T> for Value
+where
+    T: Into<NixString>,
+{
+    fn from(t: T) -> Self {
+        Self::String(t.into())
+    }
+}
+
+/// Constructors
+impl Value {
+    /// 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 {
+    // TODO(amjoseph): de-asyncify this (when called directly by the VM)
+    /// 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,
+        thunk_set: SharedThunkSet,
+    ) -> Result<Value, ErrorKind> {
+        // Get rid of any top-level thunks, and bail out of self-recursive
+        // thunks.
+        let value = if let Value::Thunk(ref t) = &self {
+            if !thunk_set.insert(t) {
+                return Ok(self);
+            }
+            generators::request_force(&co, self).await
+        } else {
+            self
+        };
+
+        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(_) => return Ok(value),
+
+            Value::List(list) => {
+                for val in list {
+                    generators::request_deep_force(&co, val.clone(), thunk_set.clone()).await;
+                }
+            }
+
+            Value::Attrs(attrs) => {
+                for (_, val) in attrs.iter() {
+                    generators::request_deep_force(&co, val.clone(), thunk_set.clone()).await;
+                }
+            }
+
+            Value::Thunk(_) => panic!("Tvix bug: force_value() returned a thunk"),
+
+            Value::AttrNotFound
+            | Value::Blueprint(_)
+            | Value::DeferredUpvalue(_)
+            | Value::UnresolvedPath(_)
+            | Value::Json(_) => panic!(
+                "Tvix bug: internal value left on stack: {}",
+                value.type_of()
+            ),
+        };
+
+        Ok(value)
+    }
+
+    // TODO(amjoseph): de-asyncify this (when called directly by the VM)
+    /// 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) -> Result<Value, ErrorKind> {
+        let value = generators::request_force(&co, self).await;
+
+        match (value, kind) {
+            // coercions that are always done
+            tuple @ (Value::String(_), _) => Ok(tuple.0),
+
+            // 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), _) => {
+                // TODO(tazjin): there are cases where coerce_to_string does not import
+                let imported = generators::request_path_import(&co, *p).await;
+                Ok(imported.to_string_lossy().into_owned().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(s) = attrs.try_to_string(&co, kind).await {
+                    return Ok(Value::String(s));
+                }
+
+                if let Some(out_path) = attrs.select("outPath") {
+                    let s = generators::request_string_coerce(&co, out_path.clone(), kind).await;
+                    return Ok(Value::String(s));
+                }
+
+                Err(ErrorKind::NotCoercibleToString { from: "set", kind })
+            }
+
+            // strong coercions
+            (Value::Null, CoercionKind::Strong) | (Value::Bool(false), CoercionKind::Strong) => {
+                Ok("".into())
+            }
+            (Value::Bool(true), CoercionKind::Strong) => Ok("1".into()),
+
+            (Value::Integer(i), CoercionKind::Strong) => Ok(format!("{i}").into()),
+            (Value::Float(f), CoercionKind::Strong) => {
+                // 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) => {
+                let mut out = String::new();
+
+                for (idx, elem) in list.into_iter().enumerate() {
+                    if idx > 0 {
+                        out.push(' ');
+                    }
+
+                    let s = generators::request_string_coerce(&co, elem, kind).await;
+                    out.push_str(s.as_str());
+                }
+
+                Ok(Value::String(out.into()))
+            }
+
+            (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,
+            }),
+
+            (Value::AttrNotFound, _)
+            | (Value::Blueprint(_), _)
+            | (Value::DeferredUpvalue(_), _)
+            | (Value::UnresolvedPath(_), _)
+            | (Value::Json(_), _) => {
+                panic!("tvix bug: .coerce_to_string() called on internal value")
+            }
+        }
+    }
+
+    // TODO(amjoseph): de-asyncify this (when called directly by the VM)
+    /// 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,
+    ) -> Result<Value, ErrorKind> {
+        let a = match self {
+            Value::Thunk(ref 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) = &other {
+                        if t1.ptr_eq(thunk) {
+                            return Ok(Value::Bool(true));
+                        }
+                    }
+                };
+
+                generators::request_force(&co, self).await
+            }
+
+            _ => self,
+        };
+
+        let b = match other {
+            Value::Thunk(_) => generators::request_force(&co, other).await,
+            _ => other,
+        };
+
+        debug_assert!(!matches!(a, Value::Thunk(_)));
+        debug_assert!(!matches!(b, Value::Thunk(_)));
+
+        let result = match (a, b) {
+            // Trivial comparisons
+            (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) {
+                    return Ok(Value::Bool(true));
+                }
+
+                if l1.len() != l2.len() {
+                    return Ok(Value::Bool(false));
+                }
+
+                for (vi1, vi2) in l1.into_iter().zip(l2.into_iter()) {
+                    if !generators::check_equality(
+                        &co,
+                        vi1,
+                        vi2,
+                        std::cmp::max(ptr_eq, PointerEquality::AllowNested),
+                    )
+                    .await?
+                    {
+                        return Ok(Value::Bool(false));
+                    }
+                }
+
+                true
+            }
+
+            (_, Value::List(_)) | (Value::List(_), _) => false,
+
+            // Attribute set comparisons
+            (Value::Attrs(a1), Value::Attrs(a2)) => {
+                if ptr_eq >= PointerEquality::AllowNested && a1.ptr_eq(&a2) {
+                    return Ok(Value::Bool(true));
+                }
+
+                // Special-case for derivation comparisons: If both attribute sets
+                // have `type = derivation`, compare them by `outPath`.
+                match (a1.select("type"), a2.select("type")) {
+                    (Some(v1), Some(v2)) => {
+                        let s1 = generators::request_force(&co, v1.clone()).await.to_str();
+                        let s2 = generators::request_force(&co, v2.clone()).await.to_str();
+
+                        if let (Ok(s1), Ok(s2)) = (s1, s2) {
+                            if s1.as_str() == "derivation" && s2.as_str() == "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 result = generators::request_force(&co, out1.clone())
+                                    .await
+                                    .to_str()?
+                                    == generators::request_force(&co, out2.clone())
+                                        .await
+                                        .to_str()?;
+                                return Ok(Value::Bool(result));
+                            }
+                        }
+                    }
+                    _ => {}
+                };
+
+                if a1.len() != a2.len() {
+                    return Ok(Value::Bool(false));
+                }
+
+                let iter1 = a1.into_iter_sorted();
+                let iter2 = a2.into_iter_sorted();
+
+                for ((k1, v1), (k2, v2)) in iter1.zip(iter2) {
+                    if k1 != k2 {
+                        return Ok(Value::Bool(false));
+                    }
+
+                    if !generators::check_equality(
+                        &co,
+                        v1,
+                        v2,
+                        std::cmp::max(ptr_eq, PointerEquality::AllowNested),
+                    )
+                    .await?
+                    {
+                        return Ok(Value::Bool(false));
+                    }
+                }
+
+                true
+            }
+
+            (Value::Attrs(_), _) | (_, Value::Attrs(_)) => false,
+
+            (Value::Closure(c1), Value::Closure(c2)) if ptr_eq >= PointerEquality::AllowNested => {
+                Rc::ptr_eq(&c1, &c2)
+            }
+
+            // Everything else is either incomparable (e.g. internal types) or
+            // false.
+            _ => false,
+        };
+
+        Ok(Value::Bool(result))
+    }
+
+    pub fn type_of(&self) -> &'static str {
+        match self {
+            Value::Null => "null",
+            Value::Bool(_) => "bool",
+            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. 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]",
+        }
+    }
+
+    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);
+    gen_cast!(to_str, NixString, "string", Value::String(s), s.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(_));
+
+    // TODO(amjoseph): de-asyncify this (when called directly by the VM)
+    /// Compare `self` against other using (fallible) Nix ordering semantics.
+    ///
+    /// Note that as this returns an `Option<Ordering>` it can not directly be
+    /// used as a generator function in the VM. The exact use depends on the
+    /// callsite, as the meaning is interpreted in different ways e.g. based on
+    /// the comparison operator used.
+    ///
+    /// 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,
+    ) -> Result<Option<Ordering>, ErrorKind> {
+        Self::nix_cmp_ordering_(self, other, co).await
+    }
+
+    async fn nix_cmp_ordering_(
+        mut myself: Self,
+        mut other: Self,
+        co: GenCo,
+    ) -> Result<Option<Ordering>, ErrorKind> {
+        'outer: loop {
+            match (myself, other) {
+                // same types
+                (Value::Integer(i1), Value::Integer(i2)) => return Ok(i1.partial_cmp(&i2)),
+                (Value::Float(f1), Value::Float(f2)) => return Ok(f1.partial_cmp(&f2)),
+                (Value::String(s1), Value::String(s2)) => return Ok(s1.partial_cmp(&s2)),
+                (Value::List(l1), Value::List(l2)) => {
+                    for i in 0.. {
+                        if i == l2.len() {
+                            return Ok(Some(Ordering::Greater));
+                        } else if i == l1.len() {
+                            return Ok(Some(Ordering::Less));
+                        } else if !generators::check_equality(
+                            &co,
+                            l1[i].clone(),
+                            l2[i].clone(),
+                            PointerEquality::AllowAll,
+                        )
+                        .await?
+                        {
+                            // TODO: do we need to control `top_level` here?
+                            myself = generators::request_force(&co, l1[i].clone()).await;
+                            other = generators::request_force(&co, l2[i].clone()).await;
+                            continue 'outer;
+                        }
+                    }
+
+                    unreachable!()
+                }
+
+                // different types
+                (Value::Integer(i1), Value::Float(f2)) => return Ok((i1 as f64).partial_cmp(&f2)),
+                (Value::Float(f1), Value::Integer(i2)) => return Ok(f1.partial_cmp(&(i2 as f64))),
+
+                // unsupported types
+                (lhs, rhs) => {
+                    return Err(ErrorKind::Incomparable {
+                        lhs: lhs.type_of(),
+                        rhs: rhs.type_of(),
+                    })
+                }
+            }
+        }
+    }
+
+    // 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 {
+            return thunk.force(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::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) => format!("the 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::AttrNotFound
+            | Value::Blueprint(_)
+            | Value::DeferredUpvalue(_)
+            | Value::UnresolvedPath(_)
+            | Value::Json(_) => "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 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.
+    else if s.contains(&b'.') {
+        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, "{}", format!("{}", 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) => write!(f, "{}", num),
+            Value::String(s) => s.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),
+            Value::Closure(_) => f.write_str("lambda"), // TODO: print position
+            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::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]"),
+
+            // 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),
+        }
+    }
+}
+
+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))
+    }
+}
+
+fn type_error(expected: &'static str, actual: &Value) -> ErrorKind {
+    ErrorKind::TypeError {
+        expected,
+        actual: actual.type_of(),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    mod floats {
+        use crate::value::total_fmt_float;
+
+        #[test]
+        fn format_float() {
+            let ff = vec![
+                (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 000000000000..ad526a8746f8
--- /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
new file mode 100644
index 000000000000..7144ca360d15
--- /dev/null
+++ b/tvix/eval/src/value/string.rs
@@ -0,0 +1,262 @@
+//! 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 rnix::ast;
+use std::ffi::OsStr;
+use std::hash::Hash;
+use std::ops::Deref;
+use std::path::Path;
+use std::{borrow::Cow, fmt::Display, str::Chars};
+
+use serde::de::{Deserializer, Visitor};
+use serde::{Deserialize, Serialize};
+
+#[repr(transparent)]
+#[derive(Clone, Debug, Serialize)]
+pub struct NixString(Box<str>);
+
+impl PartialEq for NixString {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_str() == other.as_str()
+    }
+}
+
+impl Eq for NixString {}
+
+impl PartialOrd for NixString {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        self.as_str().partial_cmp(other.as_str())
+    }
+}
+
+impl Ord for NixString {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.as_str().cmp(other.as_str())
+    }
+}
+
+impl From<&str> for NixString {
+    fn from(s: &str) -> Self {
+        NixString(Box::from(s))
+    }
+}
+
+impl From<String> for NixString {
+    fn from(s: String) -> Self {
+        NixString(s.into_boxed_str())
+    }
+}
+
+impl From<Box<str>> for NixString {
+    fn from(s: Box<str>) -> Self {
+        Self(s)
+    }
+}
+
+impl From<ast::Ident> for NixString {
+    fn from(ident: ast::Ident) -> Self {
+        ident.ident_token().unwrap().text().into()
+    }
+}
+
+impl Hash for NixString {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.as_str().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)
+    }
+}
+
+#[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 {
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+
+    /// 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 = nix_escape_string(self.as_str());
+
+        match escaped {
+            // A borrowed string is unchanged and can be returned as
+            // is.
+            Cow::Borrowed(_) => {
+                if is_valid_nix_identifier(&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.as_str().to_owned();
+        s.push_str(other.as_str());
+        NixString(s.into_boxed_str())
+    }
+
+    pub fn chars(&self) -> Chars<'_> {
+        self.0.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 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.chars().enumerate().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);
+
+            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("\"")?;
+        f.write_str(&nix_escape_string(self.as_str()))?;
+        f.write_str("\"")
+    }
+}
+
+impl AsRef<str> for NixString {
+    fn as_ref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl AsRef<OsStr> for NixString {
+    fn as_ref(&self) -> &OsStr {
+        self.as_str().as_ref()
+    }
+}
+
+impl AsRef<Path> for NixString {
+    fn as_ref(&self) -> &Path {
+        self.as_str().as_ref()
+    }
+}
+
+impl Deref for NixString {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        self.as_str()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::properties::{eq_laws, hash_laws, ord_laws};
+
+    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 000000000000..9c048d40b499
--- /dev/null
+++ b/tvix/eval/src/value/thunk.rs
@@ -0,0 +1,365 @@
+//! 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>,
+    },
+
+    /// 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),
+        }
+    }
+}
+
+/// 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,
+            },
+        }
+    }
+
+    // TODO(amjoseph): de-asyncify this
+    pub async fn force(self, co: GenCo, span: LightSpan) -> Result<Value, ErrorKind> {
+        // If the current thunk is already fully evaluated, return its evaluated
+        // value. The VM will continue running the code that landed us here.
+        if self.is_forced() {
+            return Ok(self.value().clone());
+        }
+
+        // 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 = self.0.replace(self.prepare_blackhole(span));
+
+        match inner {
+            // If there was already a blackhole in the thunk, this is an
+            // evaluation cycle.
+            ThunkRepr::Blackhole {
+                forced_at,
+                suspended_at,
+                content_span,
+            } => 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()?;
+
+                // Force the returned value again, in case the native call
+                // returned a thunk.
+                let value = generators::request_force(&co, value).await;
+
+                self.0.replace(ThunkRepr::Evaluated(value.clone()));
+                Ok(value)
+            }
+
+            // When encountering a suspended thunk, request that the VM enters
+            // it and produces the result.
+            ThunkRepr::Suspended {
+                lambda,
+                upvalues,
+                light_span,
+            } => {
+                let value =
+                    generators::request_enter_lambda(&co, lambda, upvalues, light_span).await;
+
+                // This may have returned another thunk, so we need to request
+                // that the VM forces this value, too.
+                let value = generators::request_force(&co, value).await;
+
+                self.0.replace(ThunkRepr::Evaluated(value.clone()));
+                Ok(value)
+            }
+
+            // If an inner value is found, force it and then update. This is
+            // most likely an inner thunk, as `Thunk:is_forced` returned false.
+            ThunkRepr::Evaluated(val) => {
+                let value = generators::request_force(&co, val).await;
+                self.0.replace(ThunkRepr::Evaluated(value.clone()));
+                Ok(value)
+            }
+        }
+    }
+
+    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 {
+        match *self.0.borrow() {
+            ThunkRepr::Evaluated(Value::Thunk(_)) => false,
+            ThunkRepr::Evaluated(_) => true,
+            _ => false,
+        }
+    }
+
+    /// 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")
+            }
+        })
+    }
+
+    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 cycle detection.
+///
+/// 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)
+    }
+}
+
+#[derive(Default, Clone)]
+pub struct SharedThunkSet(Rc<RefCell<ThunkSet>>);
+
+impl SharedThunkSet {
+    /// Check whether the given thunk has already been seen. Will mark the thunk
+    /// as seen otherwise.
+    pub fn insert(&self, thunk: &Thunk) -> bool {
+        self.0.borrow_mut().insert(thunk)
+    }
+}
diff --git a/tvix/eval/src/vm/generators.rs b/tvix/eval/src/vm/generators.rs
new file mode 100644
index 000000000000..f86683dff542
--- /dev/null
+++ b/tvix/eval/src/vm/generators.rs
@@ -0,0 +1,786 @@
+//! 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 smol_str::SmolStr;
+use std::fmt::Display;
+use std::future::Future;
+
+use crate::value::{PointerEquality, SharedThunkSet};
+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, SharedThunkSet),
+
+    /// 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 reads the given path to a string.
+    ReadToString(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, kind) => match kind {
+                CoercionKind::Weak => write!(f, "weak_string_coerce({})", v.type_of()),
+                CoercionKind::Strong => write!(f, "strong_string_coerce({})", 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::ReadToString(p) => {
+                write!(f, "read_to_string({})", 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<(SmolStr, FileType)>),
+
+    /// VM response with a span to use at the current point.
+    Span(LightSpan),
+
+    /// Message returned by the VM when a catchable error is encountered during
+    /// the evaluation of `builtins.tryEval`.
+    ForceError,
+}
+
+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::ForceError => write!(f, "force_error"),
+        }
+    }
+}
+
+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> VM<'o> {
+    /// 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(co, span)
+                            });
+                            return Ok(false);
+                        }
+
+                        // Generator has requested a deep-force.
+                        VMRequest::DeepForceValue(value, thunk_set) => {
+                            self.reenqueue_generator(name, span.clone(), generator);
+                            self.enqueue_generator("deep_force", span, |co| {
+                                value.deep_force(co, thunk_set)
+                            });
+                            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(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(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, |co| {
+                                values.0.nix_eq(values.1, co, ptr_eq)
+                            });
+                            return Ok(false);
+                        }
+
+                        VMRequest::StringCoerce(val, kind) => {
+                            self.reenqueue_generator(name, span.clone(), generator);
+                            self.enqueue_generator("coerce_to_string", span, |co| {
+                                val.coerce_to_string(co, kind)
+                            });
+                            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
+                                .import_path(&path)
+                                .map_err(|e| ErrorKind::IO {
+                                    path: Some(path),
+                                    error: e.into(),
+                                })
+                                .with_span(&span, self)?;
+
+                            message = VMResponse::Path(imported);
+                        }
+
+                        VMRequest::ReadToString(path) => {
+                            let content = self
+                                .io_handle
+                                .read_to_string(&path)
+                                .map_err(|e| ErrorKind::IO {
+                                    path: Some(path),
+                                    error: e.into(),
+                                })
+                                .with_span(&span, self)?;
+
+                            message = VMResponse::Value(Value::String(content.into()))
+                        }
+
+                        VMRequest::PathExists(path) => {
+                            let exists = self
+                                .io_handle
+                                .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
+                                .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(co, span)
+                            });
+                            return Ok(false);
+                        }
+
+                        VMRequest::ToJson(value) => {
+                            self.reenqueue_generator(name, span.clone(), generator);
+                            self.enqueue_generator("to_json", span, |co| {
+                                value.to_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, but inform the caller (by returning `None`) if a catchable
+/// error occured.
+pub(crate) async fn request_try_force(co: &GenCo, val: Value) -> Option<Value> {
+    if let Value::Thunk(_) = val {
+        match co.yield_(VMRequest::TryForce(val)).await {
+            VMResponse::Value(value) => Some(value),
+            VMResponse::ForceError => None,
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        }
+    } else {
+        Some(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) -> NixString {
+    match val {
+        Value::String(s) => s,
+        _ => match co.yield_(VMRequest::StringCoerce(val, kind)).await {
+            VMResponse::Value(value) => value
+                .to_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, thunk_set: SharedThunkSet) -> Value {
+    match co.yield_(VMRequest::DeepForceValue(val, thunk_set)).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<bool, ErrorKind> {
+    match co
+        .yield_(VMRequest::NixEquality(Box::new((a, b)), ptr_eq))
+        .await
+    {
+        VMResponse::Value(value) => value.as_bool(),
+        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(crate) 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
+        ),
+    }
+}
+
+pub(crate) async fn request_read_to_string(co: &GenCo, path: PathBuf) -> Value {
+    match co.yield_(VMRequest::ReadToString(path)).await {
+        VMResponse::Value(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<(SmolStr, 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) -> serde_json::Value {
+    match co.yield_(VMRequest::ToJson(value)).await {
+        VMResponse::Value(Value::Json(json)) => json,
+        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 000000000000..969b50807766
--- /dev/null
+++ b/tvix/eval/src/vm/macros.rs
@@ -0,0 +1,70 @@
+/// 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 ) => {{
+        let b = $vm.stack_pop();
+        let a = $vm.stack_pop();
+
+        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 ordering = a.nix_cmp_ordering(b, co).await?;
+            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 == Some(Ordering::Less)
+    };
+
+    (@order > $ordering:expr) => {
+        $ordering == Some(Ordering::Greater)
+    };
+
+    (@order <= $ordering:expr) => {
+        !matches!($ordering, None | Some(Ordering::Greater))
+    };
+
+    (@order >= $ordering:expr) => {
+        !matches!($ordering, None | Some(Ordering::Less))
+    };
+}
diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
new file mode 100644
index 000000000000..4af23a72d73b
--- /dev/null
+++ b/tvix/eval/src/vm/mod.rs
@@ -0,0 +1,1235 @@
+//! 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 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::{Error, ErrorKind, EvalResult},
+    io::EvalIO,
+    nix_search_path::NixSearchPath,
+    observer::RuntimeObserver,
+    opcode::{CodeIdx, Count, JumpOffset, OpCode, StackIdx, UpvalueIdx},
+    spans::LightSpan,
+    upvalues::Upvalues,
+    value::{
+        Builtin, BuiltinResult, Closure, CoercionKind, Lambda, NixAttrs, NixList, PointerEquality,
+        SharedThunkSet, Thunk, Value,
+    },
+    vm::generators::GenCo,
+    warnings::{EvalWarning, WarningKind},
+};
+
+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> GetSpan for &VM<'o> {
+    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> {
+    fn with_span(self, top_span: S, vm: &VM) -> Result<T, Error>;
+}
+
+impl<T, S: GetSpan> WithSpan<T, S> for Result<T, ErrorKind> {
+    fn with_span(self, top_span: S, vm: &VM) -> Result<T, Error> {
+        match self {
+            Ok(something) => Ok(something),
+            Err(kind) => {
+                let mut error = Error::new(kind, top_span.get_span());
+
+                // Short-circuit the wrapping if we're dealing with tryEval, in
+                // which case the error is hidden and does not need to be
+                // exhaustive.
+                if !vm.try_eval_frames.is_empty() && error.kind.is_catchable() {
+                    return Err(error);
+                }
+
+                // 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());
+                        }
+                        Frame::Generator { name, span, .. } => {
+                            error = Error::new(
+                                ErrorKind::NativeError {
+                                    err: Box::new(error),
+                                    gen_type: name,
+                                },
+                                span.span(),
+                            );
+                        }
+                    }
+                }
+
+                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>(&self, vm: &VM, 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_delayed(self.lambda.clone(), self.ip - 1)
+    }
+}
+
+/// 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(),
+        }
+    }
+}
+
+struct VM<'o> {
+    /// 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: Box<HashMap<PathBuf, Value>>,
+
+    /// 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: Box<dyn EvalIO>,
+
+    /// 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
+    /// ┌──┬──────────────────────────┐
+    /// │ 0│ `Result`-producing frame │
+    /// ├──┼──────────────────────────┤
+    /// │-1│ `builtins.tryEval` frame │
+    /// ├──┼──────────────────────────┤
+    /// │..│ ... other frames ...     │
+    /// └──┴──────────────────────────┘
+    /// ```
+    ///
+    /// 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> VM<'o> {
+    pub fn new(
+        nix_search_path: NixSearchPath,
+        io_handle: Box<dyn EvalIO>,
+        observer: &'o mut dyn RuntimeObserver,
+        globals: Rc<GlobalsMap>,
+        reasonable_span: LightSpan,
+    ) -> Self {
+        Self {
+            nix_search_path,
+            io_handle,
+            observer,
+            globals,
+            reasonable_span,
+            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> {
+        let mut catchable_error_occurred = false;
+
+        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) => {
+                            if let Some(catching_frame_idx) = self.try_eval_frames.pop() {
+                                if err.kind.is_catchable() {
+                                    self.observer.observe_exit_call_frame(frame_id, &self.stack);
+                                    catchable_error_occurred = true;
+
+                                    // truncate the frame stack back to the
+                                    // frame that can catch this error
+                                    self.frames.truncate(/* len = */ catching_frame_idx + 1);
+                                    continue;
+                                }
+                            }
+
+                            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);
+
+                    let initial_msg = if catchable_error_occurred {
+                        catchable_error_occurred = false;
+                        Some(VMResponse::ForceError)
+                    } else {
+                        None
+                    };
+
+                    match self.run_generator(name, span, frame_id, state, generator, initial_msg) {
+                        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) => {
+                            if let Some(catching_frame_idx) = self.try_eval_frames.pop() {
+                                if err.kind.is_catchable() {
+                                    self.observer.observe_exit_generator(
+                                        frame_id,
+                                        name,
+                                        &self.stack,
+                                    );
+                                    catchable_error_occurred = true;
+
+                                    // truncate the frame stack back to the
+                                    // frame that can catch this error
+                                    self.frames.truncate(/* len = */ catching_frame_idx + 1);
+                                    continue;
+                                }
+                            }
+
+                            return Err(err);
+                        }
+                    };
+                }
+            }
+        }
+
+        // Once no more frames are present, return the stack's top value as the
+        // result.
+        Ok(RuntimeResult {
+            value: self
+                .stack
+                .pop()
+                .expect("tvix bug: runtime stack empty after execution"),
+
+            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(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 => {
+                    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 => {
+                    let key = self.stack_pop().to_str().with_span(&frame, self)?;
+                    let attrs = self.stack_pop().to_attrs().with_span(&frame, self)?;
+
+                    match attrs.select(key.as_str()) {
+                        Some(value) => self.stack.push(value.clone()),
+
+                        None => {
+                            return frame.error(
+                                self,
+                                ErrorKind::AttributeNotFound {
+                                    name: key.as_str().to_string(),
+                                },
+                            );
+                        }
+                    }
+                }
+
+                OpCode::OpJumpIfFalse(JumpOffset(offset)) => {
+                    debug_assert!(offset != 0);
+                    if !self.stack_peek(0).as_bool().with_span(&frame, self)? {
+                        frame.ip += offset;
+                    }
+                }
+
+                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.as_str()) {
+                            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 => {
+                    let b = self.stack_pop();
+                    let a = self.stack_pop();
+                    let gen_span = frame.current_light_span();
+                    self.push_call_frame(span, frame);
+                    self.enqueue_generator("nix_eq", gen_span, |co| {
+                        a.nix_eq(b, co, PointerEquality::ForbidAll)
+                    });
+                    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);
+                    if !val.is_bool() {
+                        return frame.error(
+                            self,
+                            ErrorKind::TypeError {
+                                expected: "bool",
+                                actual: val.type_of(),
+                            },
+                        );
+                    }
+                }
+
+                OpCode::OpAttrs(Count(count)) => self.run_attrset(&frame, count)?,
+
+                OpCode::OpAttrsUpdate => {
+                    let rhs = self.stack_pop().to_attrs().with_span(&frame, self)?;
+                    let lhs = self.stack_pop().to_attrs().with_span(&frame, self)?;
+
+                    self.stack.push(Value::attrs(lhs.update(*rhs)))
+                }
+
+                OpCode::OpInvert => {
+                    let v = self.stack_pop().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 => {
+                    let key = self.stack_pop().to_str().with_span(&frame, self)?;
+                    let result = match self.stack_pop() {
+                        Value::Attrs(attrs) => attrs.contains(key.as_str()),
+
+                        // Nix allows use of `?` on non-set types, but
+                        // always returns false in those cases.
+                        _ => false,
+                    };
+
+                    self.stack.push(Value::Bool(result));
+                }
+
+                OpCode::OpConcat => {
+                    let rhs = self
+                        .stack_pop()
+                        .to_list()
+                        .with_span(&frame, self)?
+                        .into_inner();
+                    let lhs = self
+                        .stack_pop()
+                        .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_str().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..]),
+
+                        // In functions with "formals" attributes, it is
+                        // possible for `OpFinalise` to be called on a
+                        // non-capturing value, in which case it is a no-op.
+                        //
+                        // TODO: detect this in some phase and skip the finalise; fail here
+                        _ => { /* TODO: panic here again to catch bugs */ }
+                    }
+                }
+
+                OpCode::OpCoerceToString => {
+                    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, |co| {
+                        value.coerce_to_string(co, CoercionKind::Weak)
+                    });
+
+                    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 args = self.stack_peek(0).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 => {
+                    let b = self.stack_pop();
+                    let a = self.stack_pop();
+
+                    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 => {
+                    let b = self.stack_pop();
+                    let a = self.stack_pop();
+                    let result = arithmetic_op!(&a, &b, -).with_span(&frame, self)?;
+                    self.stack.push(result);
+                }
+
+                OpCode::OpMul => {
+                    let b = self.stack_pop();
+                    let a = self.stack_pop();
+                    let result = arithmetic_op!(&a, &b, *).with_span(&frame, self)?;
+                    self.stack.push(result);
+                }
+
+                OpCode::OpDiv => {
+                    let b = self.stack_pop();
+
+                    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 a = self.stack_pop();
+                    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)),
+                    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(&mut *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 => {
+                    frame.error(self, ErrorKind::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> VM<'o> {
+    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)?;
+
+        self.stack.push(Value::attrs(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) {
+        // TODO: put LightSpan in warning, calculate only *after* eval
+        // TODO: what to do with the spans?
+        // self.push_warning(EvalWarning {
+        //     kind,
+        //     span: self.current_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 = String::new();
+
+        for _ in 0..count {
+            out.push_str(self.stack_pop().to_str().with_span(frame, self)?.as_str());
+        }
+
+        self.stack.push(Value::String(out.into()));
+        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(())
+            }
+            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()
+                        .map(Clone::clone)
+                        // ... 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: String,
+    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;
+
+        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;
+
+        match with.to_attrs()?.select(&ident) {
+            None => continue,
+            Some(val) => return Ok(val.clone()),
+        }
+    }
+
+    Err(ErrorKind::UnknownDynamicVariable(ident))
+}
+
+// TODO(amjoseph): de-asyncify this
+async fn add_values(co: GenCo, a: Value, b: Value) -> Result<Value, ErrorKind> {
+    let result = match (a, b) {
+        (Value::Path(p), v) => {
+            let mut path = p.to_string_lossy().into_owned();
+            let vs = generators::request_string_coerce(&co, v, CoercionKind::Weak).await;
+            path.push_str(vs.as_str());
+            crate::value::canon_path(PathBuf::from(path)).into()
+        }
+        (Value::String(s1), Value::String(s2)) => Value::String(s1.concat(&s2)),
+        (Value::String(s1), v) => Value::String(
+            s1.concat(&generators::request_string_coerce(&co, v, CoercionKind::Weak).await),
+        ),
+        (v, Value::String(s2)) => Value::String(
+            generators::request_string_coerce(&co, v, CoercionKind::Weak)
+                .await
+                .concat(&s2),
+        ),
+        (a, b) => arithmetic_op!(&a, &b, +)?,
+    };
+
+    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, SharedThunkSet::default()).await)
+}
+
+pub fn run_lambda(
+    nix_search_path: NixSearchPath,
+    io_handle: Box<dyn EvalIO>,
+    observer: &mut dyn RuntimeObserver,
+    globals: Rc<GlobalsMap>,
+    lambda: Rc<Lambda>,
+    strict: bool,
+) -> EvalResult<RuntimeResult> {
+    // 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,
+        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 000000000000..aa567f302755
--- /dev/null
+++ b/tvix/eval/src/warnings.rs
@@ -0,0 +1,142 @@
+//! 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,
+
+    /// 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::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::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()),
+        }
+    }
+}
diff --git a/tvix/eval/tests/nix_oracle.rs b/tvix/eval/tests/nix_oracle.rs
new file mode 100644
index 000000000000..ecaf53a1b21a
--- /dev/null
+++ b/tvix/eval/tests/nix_oracle.rs
@@ -0,0 +1,157 @@
+//! Tests which use upstream nix as an oracle to test evaluation against
+
+use std::{env, path::PathBuf, process::Command};
+
+use pretty_assertions::assert_eq;
+use tempdir::TempDir;
+
+fn nix_binary_path() -> PathBuf {
+    env::var("NIX_INSTANTIATE_BINARY_PATH")
+        .unwrap_or_else(|_| "nix-instantiate".to_owned())
+        .into()
+}
+
+#[derive(Clone, Copy)]
+enum Strictness {
+    Lazy,
+    Strict,
+}
+
+fn nix_eval(expr: &str, strictness: Strictness) -> String {
+    let store_dir = TempDir::new("store-dir").unwrap();
+
+    let mut args = match strictness {
+        Strictness::Lazy => vec![],
+        Strictness::Strict => vec!["--strict"],
+    };
+    args.extend_from_slice(&["--eval", "-E"]);
+
+    let output = Command::new(nix_binary_path())
+        .args(&args[..])
+        .arg(format!("({expr})"))
+        .env(
+            "NIX_REMOTE",
+            format!("local?root={}", store_dir.path().display()),
+        )
+        .output()
+        .unwrap();
+    if !output.status.success() {
+        panic!(
+            "nix eval {expr} failed!\n    stdout: {}\n    stderr: {}",
+            String::from_utf8_lossy(&output.stdout),
+            String::from_utf8_lossy(&output.stderr)
+        )
+    }
+
+    String::from_utf8(output.stdout).unwrap()
+}
+
+/// Compare the evaluation of the given nix expression in nix (using the
+/// `NIX_INSTANTIATE_BINARY_PATH` env var to resolve the `nix-instantiate` binary) and tvix, and
+/// assert that the result is identical
+#[track_caller]
+fn compare_eval(expr: &str, strictness: Strictness) {
+    let nix_result = nix_eval(expr, strictness);
+    let mut eval = tvix_eval::Evaluation::new(expr, None);
+    eval.strict = matches!(strictness, Strictness::Strict);
+    eval.io_handle = Box::new(tvix_eval::StdIO);
+
+    let tvix_result = eval
+        .evaluate()
+        .value
+        .expect("tvix evaluation should succeed")
+        .to_string();
+
+    assert_eq!(nix_result.trim(), tvix_result);
+}
+
+/// Generate a suite of tests which call [`compare_eval`] on expressions, checking that nix and tvix
+/// return identical results.
+macro_rules! compare_eval_tests {
+    ($strictness:expr, {}) => {};
+    ($strictness:expr, {$(#[$meta:meta])* $test_name: ident($expr: expr); $($rest:tt)*}) => {
+        #[test]
+        $(#[$meta])*
+        fn $test_name() {
+            compare_eval($expr, $strictness);
+        }
+
+        compare_eval_tests!($strictness, { $($rest)* });
+    }
+}
+
+macro_rules! compare_strict_eval_tests {
+    ($($tests:tt)*) => {
+        compare_eval_tests!(Strictness::Lazy, { $($tests)* });
+    }
+}
+
+macro_rules! compare_lazy_eval_tests {
+    ($($tests:tt)*) => {
+        compare_eval_tests!(Strictness::Lazy, { $($tests)* });
+    }
+}
+
+compare_strict_eval_tests! {
+    literal_int("1");
+    add_ints("1 + 1");
+    add_lists("[1 2] ++ [3 4]");
+    add_paths(r#"[
+        (./. + "/")
+        (./foo + "bar")
+        (let name = "bar"; in ./foo + name)
+        (let name = "bar"; in ./foo + "${name}")
+        (let name = "bar"; in ./foo + "/" + "${name}")
+        (let name = "bar"; in ./foo + "/${name}")
+        (./. + ./.)
+    ]"#);
+}
+
+// TODO(sterni): tvix_tests should gain support for something similar in the future,
+// but this requires messing with the path naming which would break compat with
+// C++ Nix's test suite
+compare_lazy_eval_tests! {
+    // Wrap every expression type supported by [Compiler::compile] in a list
+    // with lazy evaluation enabled, so we can check it being thunked or not
+    // against C++ Nix.
+    unthunked_literals_in_list("[ https://tvl.fyi 1 1.2 ]");
+    unthunked_path_in_list("[ ./nix_oracle.rs ]");
+    unthunked_string_literal_in_list("[ \":thonking:\" ]");
+    thunked_unary_ops_in_list("[ (!true) (-1) ]");
+    thunked_bin_ops_in_list(r#"
+      let
+        # Necessary to fool the optimiser for && and ||
+        true' = true;
+        false' = false;
+      in
+      [
+        (true' && false')
+        (true' || false')
+        (false -> true)
+        (40 + 2)
+        (43 - 1)
+        (21 * 2)
+        (126 / 3)
+        ({ } // { bar = null; })
+        (12 == 13)
+        (3 < 2)
+        (4 > 2)
+        (23 >= 42)
+        (33 <= 22)
+        ([ ] ++ [ ])
+        (42 != null)
+      ]
+    "#);
+    thunked_has_attrs_in_list("[ ({ } ? foo) ]");
+    thunked_list_in_list("[ [ 1 2 3 ] ]");
+    thunked_attr_set_in_list("[ { foo = null; } ]");
+    thunked_select_in_list("[ ({ foo = null; }.bar) ]");
+    thunked_assert_in_list("[ (assert false; 12) ]");
+    thunked_if_in_list("[ (if false then 13 else 12) ]");
+    thunked_let_in_list("[ (let foo = 12; in foo) ]");
+    thunked_with_in_list("[ (with { foo = 13; }; fooo) ]");
+    unthunked_identifier_in_list("let foo = 12; in [ foo ]");
+    thunked_lambda_in_list("[ (x: x) ]");
+    thunked_function_application_in_list("[ (builtins.add 1 2) ]");
+    thunked_legacy_let_in_list("[ (let { foo = 12; body = foo; }) ]");
+}