fix(tvix/eval): handle toJSON on attribute sets with outPath

These are serialised as the serialisation of the value of that field.

Change-Id: Ida51708b1f43ce09b0ec835f4e265918aa31dd09
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8205
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -359,6 +359,8 @@ mod pure_builtins {
 
     #[builtin("toJSON")]
     async fn builtin_to_json(co: GenCo, val: Value) -> Result<Value, ErrorKind> {
+        let mut val = val; // shadow mutably, not supported by macro
+        loop {
             if let Value::Attrs(attrs) = &val {
                 // Attribute sets with a callable `__toString` attribute
                 // serialise to the string-coerced version of the result of
@@ -366,6 +368,21 @@ mod pure_builtins {
                 if let Some(s) = attrs.try_to_string(&co, CoercionKind::Weak).await {
                     return Ok(Value::String(serde_json::to_string(&s)?.into()));
                 }
+
+                // 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") {
+                    val = out_path.clone();
+                    continue;
+                }
+
+                // Attribute set should be serialised normally (by
+                // traversing it and serialising keys/values).
+                break;
+            }
+
+            break;
         }
 
         // All thunks need to be evaluated before serialising, as the

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath-nested.exp

@@ -0,0 +1 @@
+"{\"a\":40,\"b\":2}"

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;
+  };
+}

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-outpath.exp

@@ -0,0 +1 @@
+"\"/nix/store/jzka5ndnygkkfjfvpqwjipqp75lhz138-emacs-28.2\""

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";
+}