Implement Flat Config

[nzakas]

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
• Change Request: Matching behavior in flat config #15661
• Change Request: Config base path in flat config #15683
• Overrides of ignore patterns #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
• Bug: [new config system] Patterns cannot re-ignore #16416
• Bug: [new config system] Directories cannot be unignored #16415
• Bug: [new config system] Inconsistent behavior with ignored directory #16414
• Bug: [new config system] Files outside cwd were not found #16413
• Bug: FlatESLint getRulesMetaForResults fails on anonymous files when option cwd is set #16410
• Bug: FlatESLint getRulesMetaForResults fails on messages without a ruleId #16402
• Bug: [new config system] baseConfig option is unused in FlatESLint #16341
• Bug: [new config system] Unexpected glob behavior in FlatESLint #16340
• Bug: [new config system] path to a directory can still lint files outside that directory #16299
• Bug: [new config system] unmatched individual patterns are not reported #16275
• Bug: [new config system] globs are ignoring dot files #16265
• Bug: [new config system] .eslintignore doesn't work like .gitignore #16264
• Change Request: Specifying files and ignores for eslint:recommended/eslint:all #16537
• Bug: [flat config] Caching is not supported when parser is an object #16875

Phase 3: Compatibility testing

• Work with typescript-eslint to switch to new format and ensure it works correctly (pull request)
• Work with eslint-plugin-import to switch to new format and ensure it works correctly (@mdjermanovic) [Fix] pass languageOptions through in child context import-js/eslint-plugin-import#2829 Support eslint flat config import-js/eslint-plugin-import#2873
• Work with eslint-config-airbnb to switch to new format and ensure it works correctly (@mdjermanovic)
• Work with eslint-config-standard to switch to new format and ensure it works correctly (pull request]
• Work with eslint-plugin-n to switch configs to new format and ensure it works correctly (@mdjermanovic) (pull request)
• Work with eslint-plugin-vue to switch configs to new format and ensure it works correctly (pull request)
• Work with eslint-plugin-react to switch configs to new format and ensure it works correctly (@mdjermanovic) (pull request)
• Switch ESLint config to load external configs/plugins without FlatCompat
• Release ESLint with extended compatibility

Phase 4: General availability (v9.0.0)

• 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 Linter to use flat config by default
• 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)
• Omiting any file/dir positional parameter to CLI does not give an error #14308
• Rename ESLint class to LegacyESLint and rename FlatESLint to ESLint
• Delete RuleTester class and rename FlatRuleTester to RuleTester
• Update api.js to export new values for ESLint and RuleTester
• Switch shouldUseFlatConfig() to return true except if ESLINT_USE_FLAT_CONFIG is false.
• Release ESLint with GA flat config

Phase 5: Remove eslintrc (v10.0.0)

• Remove old CLI flags
• Remove context.parserPath, context.parserOptions, context.globals in Linter (fix!: Remove deprecated rule context methods #20086, fix!: Remove deprecated RuleContext methods rewrite#263)
• Remove configType constructor option for Linter; throw error if found.
• Report an error when eslint-env config comment is used (feat!: report eslint-env comments as errors #20128)
• Remove eslintrc support (feat!: remove eslintrc support #20037)
• Remove eslintrc documentation
• Delete FlatESLint class
• Delete LegacyESLint class
• Switch shouldUseFlatConfig() to always return true
• Release ESLint without eslintrc 🎉