Implement proper deep merging for settings, env, and globals in config extension and overrides

[wagenet]

Yes, this was written with AI. However, I have manually reviewed all of it and it seems correct and high quality to me. That said, I am new to this codebase.

Problem

When extending configs or using overrides, settings, env, and globals were completely replaced instead of being merged. This meant you had to redeclare entire configuration objects even if you only wanted to change one value.

Solution

Implemented deep merge semantics (ESLint compatible) for config extension and added settings support to overrides:

Config Extension (extends)

• env & globals: Deep merge - nested properties are merged, child values override parent values
• settings: Deep merge at all levels:
  • Objects (like components, attributes): Merged recursively - both parent and child entries are preserved, child overrides on conflict
  • Arrays (like formComponents, rootDir): Replaced, not merged (ESLint v9 behavior)
  • Primitives (like polymorphicPropName, booleans): Child overrides parent

File Overrides (overrides)

• settings: Now supported - can specify different settings for specific file patterns using same deep merge logic
• env & globals: Already worked, now uses consistent deep merge behavior

Changes

• Added merge() methods to all plugin settings types (JSXA11yPluginSettings, ReactPluginSettings, NextPluginSettings, JSDocPluginSettings, VitestPluginSettings)
• Updated OxlintSettings::merge() to orchestrate deep merging across all plugin settings
• Updated OxlintEnv::merge() and OxlintGlobals::merge() for deep merge
• Updated Oxlintrc::merge() to use new merge methods
• Added settings field to OxlintOverride struct
• Applied settings in override resolution logic with deep merge
• Added PartialEq to settings structs for comparison optimization

Testing

• 5 tests covering env, globals, and settings merging in both extension and overrides
• All 816 tests pass
• Test fixtures added in fixtures/extends_config/merge/

Example (Deep Merge)

// parent.json
{
  "env": { "browser": true },
  "settings": {
    "jsx-a11y": {
      "polymorphicPropName": "as",
      "components": { "Link": "a" }
    }
  }
}

// child.json
{
  "extends": ["./parent.json"],
  "env": { "node": true },
  "settings": {
    "jsx-a11y": {
      "polymorphicPropName": "component",
      "components": { "Button": "button" }
    },
    "react": { "formComponents": ["CustomForm"] }
  }
}

Result (Deep Merge):

• env: { "browser": true, "node": true } — Both merged
• settings.jsx-a11y:
  • polymorphicPropName: "component" — Child overrides
  • components: { "Link": "a", "Button": "button" } — Both merged
• settings.react: { "formComponents": ["CustomForm"] } — Added from child

This matches ESLint's deep merge behavior where nested objects are recursively merged but arrays are replaced.

[graphite-app[bot]]

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[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #14940 will not alter performance

_{Comparing wagenet:improved-config-merging (c1edd11) with main (c167dfa)[^unexpected-base]} [^unexpected-base]: 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.

Summary

✅ 4 untouched
⏩ 33 skipped[^skipped]

[^skipped]: 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.

[wagenet]

CI Tests pass for me locally (macOS) so I'm not sure what's going on there. Looks like maybe just whitespace.

[connorshea]

CI Tests pass for me locally (macOS) so I'm not sure what's going on there. Looks like maybe just whitespace.

ah I actually ran into the same problem in the last few weeks with that snapshot, if you revert the changes to that snapshot it should pass

[overlookmotel]

Have we discussed whether this would be considered a breaking change?