1 files changed, 173 insertions, 0 deletions
diff --git a/tvix/eval/tests/nix_oracle.rs b/tvix/eval/tests/nix_oracle.rs
new file mode 100644
index 000000000000..3d3abc73635f
--- /dev/null
+++ b/tvix/eval/tests/nix_oracle.rs
@@ -0,0 +1,173 @@
+//! Tests which use upstream nix as an oracle to test evaluation against
+
+use std::{env, path::PathBuf, process::Command};
+
+use pretty_assertions::assert_eq;
+
+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 = tempfile::tempdir().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()
+                    .canonicalize()
+                    .expect("valid 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]
+#[cfg(feature = "impure")]
+fn compare_eval(expr: &str, strictness: Strictness) {
+    let nix_result = nix_eval(expr, strictness);
+    let mut eval = tvix_eval::Evaluation::new_pure();
+    eval.strict = matches!(strictness, Strictness::Strict);
+    eval.io_handle = Box::new(tvix_eval::StdIO);
+
+    let tvix_result = eval
+        .evaluate(expr, None)
+        .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)*}) => {
+        #[cfg(feature = "impure")]
+        #[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::Strict, { $($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; }) ]");
+    unthunked_relative_path("[ ./foo ]");
+    unthunked_home_relative_path("[ ~/foo ]");
+    unthunked_absolute_path("[ /foo ]");
+
+    unthunked_formals_fallback_literal("({ foo ? 12 }: [ foo ]) { }");
+    unthunked_formals_fallback_string_literal("({ foo ? \"wiggly\" }: [ foo ]) { }");
+    thunked_formals_fallback_application("({ foo ? builtins.add 1 2 }: [ foo ]) { }");
+    thunked_formals_fallback_name_resolution_literal("({ foo ? bar, bar ? 12 }: [ foo ]) { }");
+}