fix(tvix/eval): handle `__toString` when JSON-serialising attrsets r/5976

These must be serialised to a JSON string of the *result* of coercing
the function application to a string.

Change-Id: Ib7f49ccd950503ddbdbf99643cd59565e26b50da
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8204
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

1 files changed, 8 insertions, 0 deletions

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;
+}