fix: error when a mixin call is used as a property value

[Lfan-ke]

Using a mixin call directly as a property value (without a lookup) silently
emitted [object Object]:

#rgbify(@color) { @rgb: red(@color), green(@color), blue(@color); }
.x { color: #rgbify(#fff); }   // -> color: [object Object];

The mixin call resolves to its ruleset, which reaches the declaration as an
array of rules and renders as [object Object]. Declaration.eval already
rejects a DetachedRuleset used on a property with "Rulesets cannot be
evaluated on a property."; this extends that same guard to the array case, so
the mistake surfaces as a clear error instead of invalid output.

Only non-variable (property) declarations are affected, so valid uses such as
#ns.mixin()[@var] lookups, statement mixin calls, and passing a ruleset to
each() are unchanged. Added an eval-error test; pnpm test passes.

Closes #4305

Summary by CodeRabbit

• Bug Fixes
  • Improved validation for mixins used directly as property values.
  • Prevented invalid mixin results from rendering as [object Object].
  • Added a clearer error indicating that rulesets cannot be evaluated on a property.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: ac841316-5cb7-47a4-9634-0b0dbbe16954

📥 Commits

Reviewing files that changed from the base of the PR and between ebbce6d and 41035ea.

📒 Files selected for processing (3)

• packages/less/lib/less/tree/declaration.js
• packages/test-data/tests-error/eval/mixin-as-value.less
• packages/test-data/tests-error/eval/mixin-as-value.txt

---

📝 Walkthrough

Walkthrough

The declaration evaluator now reports an error when a mixin used as a property value evaluates to an array, and a regression fixture records the expected syntax error.

Changes

Mixin property-value validation

Layer / File(s)	Summary
Reject array mixin results and verify the error packages/less/lib/less/tree/declaration.js, packages/test-data/tests-error/eval/mixin-as-value.less, packages/test-data/tests-error/eval/mixin-as-value.txt	Declaration.eval rejects array results alongside detached rulesets, while the mixin-as-value fixture and expected output cover the resulting SyntaxError.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: raising an error for mixin calls used as property values.
Linked Issues check	✅ Passed	The code and test changes address #4305 by erroring on mixin values that would otherwise render as [object Object].
Out of Scope Changes check	✅ Passed	The PR stays focused on the mixin-as-value error fix and its test coverage, with no evident unrelated changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[greptile-apps]

Greptile Summary

The PR prevents bare mixin calls used as property values from rendering as [object Object].

• Extends Declaration.eval validation to reject array-valued ruleset results for non-variable declarations.
• Adds an evaluation-error fixture covering a mixin call used directly as a property value.

Confidence Score: 5/5

The PR appears safe to merge, with the new guard narrowly covering raw rule arrays produced by mixin calls in property-value position.

The changed validation rejects the invalid array result before rendering, retains the existing detached-ruleset behavior, and excludes variable declarations; no other supported property-value evaluation path was found to produce a raw array.

_{Reviews (1): Last reviewed commit: "fix: error when a mixin call is used as ..." | Re-trigger Greptile}