feat: support ESLint v9

[JoshuaKGoldberg]

👉 NOTE: This is into our v8 branch. Once it's merged, it'll only be included in our v8 versions.

PR Checklist

• Addresses an existing open issue: fixes ESLint v9 Support #8211
• That issue was marked as accepting prs
• Steps in Contributing were taken

Overview

Bumps our dependency ranges to include ESLint 9 and its associated packages. I added a few more resolutions because some of our lint dependencies still are on older versions of those associated packages. I think we can tackle updating dependencies as a followup.

Updates most integration tests to the new flat config format. Adds in a new legacy-config integration test so we still have explicit coverage of eslintrc.

Otherwise uses configType: 'eslintrc' for all instances of new Linter so that we don't have to change them to supporting flat config logic yet. That's actually a non-trivial change. For example RuleTester would need to refactor when it adds rules to a Linter instance. I think we can tackle refactoring to flat config for internal Linter instances as a followup. It'll be necessary for ESLint 10.

Comments out usage of a few plugins in our own internal lint config pending their support of ESLint 9:

• eslint-plugin-deprecation: ESLint v9 compatibility gund/eslint-plugin-deprecation#78
• eslint-plugin-react: ESLint v9 contains breaking API changes jsx-eslint/eslint-plugin-react#3699
• eslint-plugin-react-hooks: eslint-plugin-react-hooks & "Flat Config" (ESLint 9) facebook/react#28313

[typescript-eslint]

Thanks for the PR, @JoshuaKGoldberg!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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nx run integration-tests:test

✅ Successfully ran 30 targets

• nx test eslint-plugin --coverage=false
• nx test eslint-plugin
• nx test typescript-estree --coverage=false
• nx test utils --coverage=false
• nx test visitor-keys --coverage=false
• nx run types:build
• nx test type-utils --coverage=false
• nx test typescript-eslint --coverage=false
• nx run repo-tools:postinstall-script
• nx test rule-schema-to-typescript-types --coverage=false
• nx test repo-tools --coverage=false
• nx test eslint-plugin-internal --coverage=false
• nx test scope-manager --coverage=false
• nx run-many --target=build --parallel --exclude=website --exclude=website-eslint
• nx test parser --coverage=false
• nx test ast-spec --coverage=false
• nx run-many --target=lint --parallel --exclude eslint-plugin
• nx test typescript-estree
• nx test type-utils
• nx test scope-manager
• nx test rule-schema-to-typescript-types
• nx test utils
• nx test typescript-eslint
• nx test visitor-keys
• nx test parser
• nx test repo-tools
• nx test eslint-plugin-internal
• nx run-many --target=typecheck --parallel
• nx test ast-spec
• nx run repo-tools:generate-configs

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[JoshuaKGoldberg]

From https://github.com/typescript-eslint/typescript-eslint/actions/runs/8849469567/job/24301889067?pr=9002:

> nx run eslint-plugin:lint

Linting "eslint-plugin"...
Error: You have attempted to use the lint rule deprecation/deprecation which requires the full TypeScript type-checker to be available, but you do not have `parserOptions.project` configured to point at your project tsconfig.json files in the relevant TypeScript file "overrides" block of your project ESLint config `packages/eslint-plugin/.eslintrc.json`
Please see https://nx.dev/guides/eslint for full guidance on how to resolve this issue.

...I haven't dug in to know exactly why this is happening, but my hunch is that it's a dependency version mismatch. Something around gund/eslint-plugin-deprecation#79 / gund/eslint-plugin-deprecation#86. Alternately, it'd be solved on our end by #8988.

[JoshuaKGoldberg]

It looks like this was previously reported to Nx in nrwl/nx#20731. I tried the remediations there but couldn't find the magic combo to get this to work locally. Filed nrwl/nx#23088 to request more informative errors.

cc @JamesHenry

[auvred]

Awesome! LGTM 🏆

[bradzacher]

question: do we really want to do the bump to v9 if it completely breaks our lint setup like this?
would we be better served by instead just adding a decent integration test which runs against v9, and leaving our setup on v8 for now?

[bradzacher]

Do all these rules hard fail in v9? That's a big oof for sure.
How can anyone even begin to migrate to v9 if this all crashes?

[JoshuaKGoldberg]

big ole oof

[bradzacher]

the filename is wrong if this is a flat config?!

[bradzacher]

also - this fixture is called "legacy-config"
so should probably use a legacy config?

[JoshuaKGoldberg]

That's what I was going for with the .eslintrc.js. As in, using that as the legacy config.

eslintIntegrationTest(__filename, '*.ts', true);

...did I miss something?

[bradzacher]

I meant - this file is named .eslintrc but the contents are a flat config.

So things are mixed up - the contents should be a legacy config instead, right?

[bradzacher]

this should still be null based on how the rule works?

[JoshuaKGoldberg]

Ah, might be a merge thing - merging everything in...

[JoshuaKGoldberg]

Threading @bradzacher's #9002 (review):

question: do we really want to do the bump to v9 if it completely breaks our lint setup like this?
would we be better served by instead just adding a decent integration test which runs against v9, and leaving our setup on v8 for now?

I think the plugins not supporting ESLint v9 aren't super damaging to us:

• deprecation/deprecation: is something we keep a close eye on anyway, and will be resolved in Rule proposal: Warn when deprecated APIs are used (eslint-plugin-deprecation) #8988 (which IIRC you'd previously mentioned a 👍 on somewhere?)
• import/: is mostly stylistic, and imo not exactly the most impactful set of rules we have - expect for no-mutable-exports, which we generally look out for in review anyway
• react is something we didn't even have enabled for a while anyway, and our playground isn't a high traffic code area
• react-hooks will be updated with React 19

🤷 I don't love that we're losing this bit of our linting, but for me it's less valuable than us more thoroughly testing ESLint v9.

[JoshuaKGoldberg]

https://github.com/typescript-eslint/typescript-eslint/actions/runs/8978126700/job/24658124387?pr=9002

Finished packing local packages.
 FAIL  tests/flat-config-types.test.ts (59.943 s)

...
Tests:       3 failed, 7 passed, 10 total

I broke this in the latest update / merge, and am out of coding time for now. 😂