feat(minifier): remove unused variable declarations in dead code

[sapphi-red]

Unused variable declarations in dead code was not removed. For example, the following cases was not removed:

if (false) { var a; console.log(a) }

(function () { return; var a })()

[sapphi-red]

• feat(minifier): remove unused variable declarations in dead code #13754 👈 (View in Graphite)
• main

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[Copilot]

Pull Request Overview

This PR adds functionality to remove unused variable declarations in dead code by extending the existing dead code elimination to handle variable declarations that are defined but never used within unreachable code blocks.

• Extracts common logic for checking if unused declarators can be removed into a reusable helper method
• Adds a new remove_unused_variable_declaration function to filter out unused variable declarations
• Integrates unused variable removal into dead code elimination for if statements and unreachable code blocks

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/oxc_minifier/src/peephole/remove_unused_declaration.rs	Refactors existing logic into helper methods and adds new function to remove unused variable declarations
crates/oxc_minifier/src/peephole/remove_dead_code.rs	Integrates unused variable removal into dead code elimination and adds test cases
crates/oxc_minifier/src/peephole/minimize_statements.rs	Applies unused variable removal when processing kept variables from dead code

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #13754 will not alter performance

_{Comparing 09-14-feat_minifier_remove_unused_variable_declarations_in_dead_code (c868796) with main (db33196)¹}

Summary

✅ 37 untouched

Footnotes

1. No successful run was found on main (c868796) during the generation of this report, so db33196 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[Boshen]

Merge activity

• Sep 14, 12:49 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Sep 14, 12:49 PM UTC: Boshen added this pull request to the Graphite merge queue.
• Sep 14, 12:55 PM UTC: Merged by the Graphite merge queue.