Implement Flat Config

This issue describes the implementation plan for eslint/rfcs#9, which will take place in several phases:

Phase 1: Extract current config system

• Create new GitHub repository (eslint/eslintrc)
• Copyeslintrc source files and tests into new GitHub repository
• Create Jenkins release job for @eslint/eslintrc
• Publish @eslint/eslintrc to npm
• Update ESLint to use @eslint/eslintrc
• Remove in-memory filesystem from tests
• Update CLIEngine to use CascadingConfigArrayFactory from @eslint/eslintrc

Phase 2: Implement flat config with eslintrc compatibility

• Create the FlatCompat class in eslint/eslintrc repository
• Create FlatConfigArray to represent simple configs
• Update Linter class to understand FlatConfigArray objects
• Ensure Linter#defineRule and Linter#definePlugin throw errors when using FlatConfigArray
• Add context.languageOptions in backwards-compatible way to context in Linter
• Ensure context.parserPath still works in Linter for FlatConfigArray (for now)
• Create FlatRuleTester to allow rule testing with FlatConfigArray format
• Create FlatESLint class to mirror ESLint class but use eslint.config.js instead
• #15661
• #15683
• #15687
• Implement caching
• Update cli.js to search for eslint.config.js file and use FlatESLint if found (ESLint if not)
• Ensure invalid CLI flags cause errors when using flat config (for example, --resolve-plugins-relative-to)
• Switch eslint/eslint repo to use flat config
• Document flat config
• Release ESLint with eslintrc compatibility
• #16416
• #16415
• #16414
• #16413
• #16410
• #16402
• #16341
• #16340
• #16299
• #16275
• #16265
• #16264
• #16537

Phase 3: Compatibility testing

• Work with eslint-config-airbnb to switch to new format and ensure it works correctly
• Work with eslint-config-standard to switch to new format and ensure it works correctly
• Work with eslint-plugin-node to switch configs to new format and ensure it works correctly
• Work with eslint-plugin-vue to switch configs to new format and ensure it works correctly
• Work with eslint-plugin-react to switch configs to new format and ensure it works correctly
• Switch ESLint config to load external configs/plugins without FlatCompat
• Release ESLint with extended compatibility

Phase 4: General availability

• Write blog post about flat config format
• Output warning when people use eslintrc files
• Switch documentation so eslint.config.js is the default and recommended format (link to legacy eslintrc docs from the same page)
• Switch documentation for shareable configs to flat config format (links to legacy docs included)
• Switch documentation for plugins to flat config format (links to legacy docs included)
• Release ESLint with GA flat config

Phase 5: Remove eslintrc

• Remove old CLI flags
• Remove context.parserPath, context.parserOptions, context.globals in Linter
• Throw error when eslint-env config comment is used
• Remove eslintrc support
• Remove eslintrc documentation
• Delete ESLint class and rename FlatESLint to ESLint
• Delete RuleTester class and rename FlatRuleTester to RuleTester
• Release ESLint without eslintrc 🎉

[nzakas]

Phase 1 is now compete. The old config system has been separated out into a separate package. That means I can now move on to phase 2. 🎉

[bradzacher]

I think it would be a good idea to include @typescript-eslint in the Phase 3 list.

IMO onboarding the project as early as possible is a hard requirement for the workstream. Without ts-eslint’s onboarding the majority of ESLint users won’t able to transition to flat configs without needing to use the compatibility tooling.

For reference - by weekly download volume @typescript-eslint/eslint-plugin users make up over 65% of ESLint users and @typescript-eslint/parser users make up over 70% of ESLint’s users. (plugin = 19.2m / parser = 21m / eslint = 28.8m)

It would be a real shame if most users’ first experience with new config system involves reaching for the compatibility APIs. Making sure that doesn’t happen should be a top priority!

Feel free to chat with us (cc @JoshuaKGoldberg and @JamesHenry)!

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Separately but very related to the above - I’d also suggest including eslint-plugin-import for the same reasons given that it is similar in size (17.1m - ~60%).