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

5 files changed, 29 insertions, 0 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index e19de961588d..a9d04ba7c3fb 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -359,6 +359,15 @@ mod pure_builtins {
 
     #[builtin("toJSON")]
     async fn builtin_to_json(co: GenCo, val: Value) -> Result<Value, ErrorKind> {
+        if let Value::Attrs(attrs) = &val {
+            // Attribute sets with a callable `__toString` attribute
+            // serialise to the string-coerced version of the result of
+            // calling that.
+            if let Some(s) = attrs.try_to_string(&co, CoercionKind::Weak).await {
+                return Ok(Value::String(serde_json::to_string(&s)?.into()));
+            }
+        }
+
         // All thunks need to be evaluated before serialising, as the
         // data structure is fully traversed by the Serializer.
         let val = generators::request_deep_force(&co, val, SharedThunkSet::default()).await;
diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-tojson-tostring-notcallable.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-builtins-tojson-tostring-notcallable.nix
new file mode 100644
index 000000000000..345b76fde037
--- /dev/null
+++ b/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;
+}
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;
+}
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-tostring.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-tostring.exp
new file mode 100644
index 000000000000..2661fd257bf9
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-tostring.exp
@@ -0,0 +1 @@
+"\"it's 42\""
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;
+}