feat(linter): Follow-up to config extends fixes

[wagenet]

Follow up to #14939.

[graphite-app]

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[Copilot]

Pull Request Overview

This PR refines the configuration merging logic for Oxlintrc files, improving clarity around merge priority and eliminating unnecessary clones for better performance. The changes follow up on previous config extends fixes (#14939).

Key Changes:

• Improved documentation for the merge method to clearly explain merge priority
• Changed merge method signature to take ownership of other parameter instead of borrowing
• Added comprehensive test coverage for extends order precedence

Reviewed Changes

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

File	Description
crates/oxc_linter/src/config/oxlintrc.rs	Enhanced documentation and optimized merge method to take ownership of other parameter
crates/oxc_linter/src/config/config_builder.rs	Updated merge call site and added test for extends array precedence
crates/oxc_linter/fixtures/extends_config/rules_multiple/extends_order_test.json	Added test fixture for validating extends order behavior

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[codspeed-hq]

CodSpeed Performance Report

Merging #15818 will not alter performance

_{Comparing wagenet:extends-follow-up (b785dac) with main (c167dfa)¹}

Summary

✅ 4 untouched
⏩ 33 skipped²

Footnotes

1. No successful run was found on main (fbf0fd4) during the generation of this report, so c167dfa 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. ↩

[overlookmotel]

Good stuff. Sorry to come back with more suggestions.

Since you've done the work of fixing the problem, I think it's ideal to make this new correct implementation as clear as possible, to make sure it's used correctly and that we don't accidentally break it again!

[overlookmotel]

The text fixture does make sure that the last wins over the first 2. But it'd be ideal to also check all the other combinations (including no clashes). Something like:

// first.json
{
  "rules": { "rule_A": "off", "rule_C": "off", "rule_D": "off", "rule_G": "off" }
}

// second.json
{
  "rules": { "rule_A": "warn", "rule_B": "warn", "rule_D": "warn", "rule_F": "warn" }
}

// third.json
{
  "rules": { "rule_A": "error", "rule_B": "error", "rule_C": "error", "rule_E": "error" }
}

// main.json
{
  "extends": [
    "./first.json",
    "./second.json",
    "./third.json"
  ]
}

Merged config should be equivalent to:

{
  "rules": {
    "rule_A": "error", // `third` wins over both `first` and `second`
    "rule_B": "error", // `third` wins over `second`
    "rule_C": "error", // `third` wins over `first`
    "rule_D": "warn",  // `second` wins over `first`
    "rule_E": "error", // `third` wins when no clash
    "rule_F": "warn",  // `second` wins when no clash
    "rule_G": "off",   // `first` wins when no clash
  },
}

That's rather convoluted, but since we're trying to fix this for once and for all, I think it's worthwhile covering all cases.

[overlookmotel]

I think this would clarify which is which:

Suggested change

	/// // base.json: { "rules": { "eqeqeq": "error" } }
	/// // current.json: { "rules": { "eqeqeq": "warn", "no-console": "error" } }
	/// // After merge: { "rules": { "eqeqeq": "warn", "no-console": "error" } }
	/// // ^ current.json's "warn" wins over base.json's "error"
	/// // self.json: { "rules": { "eqeqeq": "error", "no-debugger": "error" } }
	/// // other.json: { "rules": { "eqeqeq": "warn", "no-console": "warn" } }
	/// // After merge: { "rules": { "eqeqeq": "error", "no-debugger": "error", "no-console": "warn" } }
	/// // ^ self.json's "error" for `eqeqeq` wins over other.json's "warn"

[overlookmotel]

Shall we rename this method to merge_onto? self.merge_onto(other) I think makes it clearer at the call site that self takes priority.