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.

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[Copilot]

Pull Request Overview

This PR implements ESLint-compatible deep merge semantics for config extension and overrides. Previously, settings, env, and globals were replaced entirely when extending configs or using overrides. Now these fields are properly merged, allowing incremental configuration changes without redeclaring entire objects.

Key Changes:

• Added deep merge methods to all plugin settings types and OxlintSettings
• Implemented merge logic for env and globals
• Added settings field support to overrides with deep merge behavior

Reviewed Changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
npm/oxlint/configuration_schema.json	Added settings field to override schema definition
crates/oxc_linter/src/snapshots/schema_json.snap	Updated schema snapshot with settings in overrides
crates/oxc_linter/src/config/settings/vitest.rs	Added merge method and is_empty helper to VitestPluginSettings
crates/oxc_linter/src/config/settings/react.rs	Added merge method and is_empty helper to ReactPluginSettings
crates/oxc_linter/src/config/settings/next.rs	Added merge method to NextPluginSettings
crates/oxc_linter/src/config/settings/mod.rs	Added merge and merge_into methods to OxlintSettings
crates/oxc_linter/src/config/settings/jsx_a11y.rs	Added merge method to JSXA11yPluginSettings
crates/oxc_linter/src/config/settings/jsdoc.rs	Added merge method and is_empty helper to JSDocPluginSettings
crates/oxc_linter/src/config/oxlintrc.rs	Updated to use merge methods for settings, env, and globals
crates/oxc_linter/src/config/overrides.rs	Added settings field to OxlintOverride struct
crates/oxc_linter/src/config/globals.rs	Added merge method to OxlintGlobals
crates/oxc_linter/src/config/env.rs	Added merge method to OxlintEnv
crates/oxc_linter/src/config/config_store.rs	Integrated settings merging in override resolution and added accessor methods
crates/oxc_linter/src/config/config_builder.rs	Added settings to override building and comprehensive merge tests
crates/oxc_linter/fixtures/extends_config/merge/*.json	Added test fixtures for merge behavior validation

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[codspeed-hq]

CodSpeed Performance Report

Merging #14940 will not alter performance

_{Comparing wagenet:improved-config-merging (c1edd11) 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. ↩

[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

[camc314]

you probably need to run pnpm i and re-run the test cases - this snapshot change looks incorrect.

[overlookmotel]

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