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, 6 insertions, 0 deletions

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;
+}