fix(linter): no-unused-vars: allow updates in for loop test/update

[camchenry]

• Fixes linter: no-unused-vars does not follow tail-first loop #14765

Allows update expressions when they are within a for-loop update/test.

[camchenry]

• fix(linter): no-unused-vars: allow updates in for loop test/update #14766 👈 (View in Graphite)
• main

---

How to use the Graphite Merge Queue

Add either label to this PR to merge it via the merge queue:

• 0-merge - adds this PR to the back of the merge queue
• hotfix - for urgent hot fixes, skip the queue and merge this PR next

You must have a Graphite account in order to use the merge queue. Sign up using this link.

An organization admin has enabled the Graphite Merge Queue in this repository.

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.

This stack of pull requests is managed by Graphite. Learn more about stacking.

[Copilot]

Pull Request Overview

This PR fixes the no-unused-vars linter rule to correctly handle variables used in for-loop test and update expressions. Previously, variables that were only incremented/decremented in for-loop conditions were incorrectly flagged as unused.

Key changes:

• Added logic to detect when update expressions occur within for-loop test or update sections
• Implemented helper method is_in_for_loop_test_or_update to check node ancestry
• Added test cases covering various for-loop patterns with update expressions

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/oxc_linter/src/rules/eslint/no_unused_vars/usage.rs	Added check to allow update expressions in for-loop test/update sections and helper method to detect this context
crates/oxc_linter/src/rules/eslint/no_unused_vars/tests/oxc.rs	Added test cases for valid for-loop patterns and a failing case for truly unused variables

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[codspeed-hq]

CodSpeed Performance Report

Merging #14766 will not alter performance

_{Comparing 10-18-fix_linter_no-unused-vars_allow_updates_in_for_loop_test_update (00010e0) with main (c6395c7)¹}

Summary

✅ 4 untouched
⏩ 33 skipped²

Footnotes

1. No successful run was found on main (cd266b4) during the generation of this report, so c6395c7 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 33 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[camc314]

Merge activity

• Oct 20, 8:10 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Oct 20, 8:10 AM UTC: camc314 added this pull request to the Graphite merge queue.
• Oct 20, 8:30 AM UTC: Merged by the Graphite merge queue.