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

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

tvix/eval/src/tests/tvix_tests/eval-fail-builtins-tojson-tostring-notcallable.nix

@@ -0,0 +1,5 @@
+# attribute sets with a non-callable `__toString` can not be
+# serialised to JSON.
+builtins.toJSON {
+  __toString = 42;
+}

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;
+}

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

@@ -0,0 +1 @@
+"\"it's 42\""

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;
+}