feat: Rule Performance Statistics for flat ESLint

[mnkiefer]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[X] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

• Introduces a new flag --stats to flat ESLint, which enables the user to obtain a series of runtime statistics on top of their final lint results (refer to the table below).
• See RFC

Is there anything you'd like reviewers to focus on?

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[nzakas]

Just converting this to a draft so we know it's not yet time to review it.

[nzakas]

@me4502 getting back around to this now. Can you please rebase on top of main? And if you feel you're ready for review now, please feel free to hit that button. :)

[mnkiefer]

Getting back around to this now. Can you please rebase on top of main? And if you feel you're ready for review now, please feel free to hit that button. :)

Great to hear! I just did a rebase, so the implementation should be up to date with the new ESLint now. All that is left is to add tests for it, but I suppose those would come in a second review round? So, pressing the Ready for Review button now 👍

[nzakas]

Go ahead and add the tests now. We like to review the code and the tests together as oftentimes the tests tell more of the story than the code does.

[nzakas]

This is looking good. I'd just like to avoid passing stats around in so many places. (I left a comment inline about this, as well.) If we can instead pull that parts out of slots and later assign them back, we can make the additional functionality more testable.

[nzakas]

The refactoring around slots looks good to me. Left a couple of suggestions for areas to clean up, but this is looking really good.

[nzakas]

LGTM, thanks! Would like @mdjermanovic to review before merging.

[nzakas]

Latest changes LGTM. Waiting for @mdjermanovic to re-review.

[mnkiefer]

Latest changes LGTM. Waiting for @mdjermanovic to re-review.

@nzakas: Any updates on this? 😇

[mdjermanovic]

@mnkiefer sorry for the delay, I'll review it soon.

[mdjermanovic]

Totals for rules should be the same as with TIMING? I'm getting lower numbers with --stats than with TIMING.

For example:

// eslint.config.js
module.exports = [{
    languageOptions: {
        sourceType: "commonjs"
    },
    linterOptions: {
        reportUnusedDisableDirectives: "off"
    },
    rules: {
        indent: ["error", 4, { SwitchCase: 1 }]
    }
}];

$ TIMING=all node bin/eslint tests/lib/linter/linter.js --stats -f json
(node:7892) ESLintIgnoreWarning: The ".eslintignore" file is no longer supported. Switch to using the "ignores" property in "eslint.config.js": https://eslint.org/docs/latest/use/configure/migration-guide#ignoring-files
(Use `node --trace-warnings ...` to show where the warning was created)
[{"filePath":"C:\\projects\\tmp\\mnkiefer-eslint\\tests\\lib\\linter\\linter.js","messages":[],"suppressedMessages":[],"errorCount":0,"fatalErrorCount":0,"warningCount":0,"fixableErrorCount":0,"fixablea 
rningCount":0,"stats":{"times":{"passes":[{"parse":{"total":251.2441},"rules":{"indent":{"total":182.5374}},"fix":{"total":0},"total":619.8087}]},"fixPasses":0},"usedDeprecatedRules":[{"ruleId":"indent","replacedBy":[]}]}]
Rule   | Time (ms) | Relative
:------|----------:|--------:
indent |   214.375 |   100.0%

Per the stats, indent rule takes 182.5374ms, but TIMING in the same run outputs 214.375ms.

[mdjermanovic]

LGTM, thanks! Leaving open for @nzakas to verify again as this has had a lot of changes since the last approval.

[nzakas]

Just left a few notes of things to clean up before merging.

[nzakas]

LGTM. Once again, thank you for your hard work on such a detailed feature. We really appreciate it. 🙏