feat(tvix/eval): support builtins implemented in Nix itself

This makes it possible to inject builtins into the builtin set that
are written in Nix code, and which at runtime are represented by a
thunk that will compile them the first time they are used.

Change-Id: Ia632367328f66fb2f26cb64ae464f8f3dc9c6d30
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7891
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/tests/mod.rs

@@ -2,6 +2,10 @@ 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

tvix/eval/src/tests/one_offs.rs

@@ -0,0 +1,19 @@
+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
+    );
+
+    assert!(matches!(result.value.unwrap(), Value::Integer(42)));
+}