Bug: Unable to test rules in v9 in both flat config and eslintrc

[ljharb]

Environment

Node version: n/a
npm version: n/a
Local ESLint version: v9.0.0
Global ESLint version: none
Operating System: n/a

What parser are you using?

Default (Espree)

What did you do?

A contributor ran the RuleTester with ESLINT_USE_FLAT_CONFIG=false, but it seems like there's no way my eslint plugin can maintain compatibility with eslint v9 and eslintrc (since unless it's tested, it's not compatible)

What did you expect to happen?

I expected RuleTester to function just like it did in eslint v8, so I can test rules via eslintrc.

What actually happened?

import-js/eslint-plugin-import#2996 (comment) reports it's an error

Link to Minimal Reproducible Example

import-js/eslint-plugin-import#2996 (comment)

Participation

• I am willing to submit a pull request for this issue.

Additional comments

No response

[nzakas]

As we announced, RuleTester in v9 now tests against flat config because that is the default configuration system.

Functionally, if your rules pass RuleTester in v9, they should work with eslintrc also, as the fundamental parts of rule tester remain the same, it's really just the format of the options that have changed.

If you really need to test eslintrc specifically, then you'll need to use ESLint v8.x for that.

[ljharb]

It makes perfect sense for RuleTester to test it by default, but I don't understand why eslint 9, which still supports two config systems, has failed to provide a way to test both.

[aladdin-add]

I'm 👍 to the change.

Remember: users can still use eslintrc with eslint v9 (ESLINT_USE_FLAT_CONFIG=false), but it's no longer testable.