feat!: Update shouldUseFlatConfig and CLI so flat config is default

[nzakas]

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)

Updates shouldUseFlatConfig() to return true unless the ESLINT_USE_FLAT_CONFIG environment variable is "false".

Also ensures the CLI emits a warning when eslintrc is used.

Refs #13481

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

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[nzakas]

Tests are failing because rules are being called with syntax that isn't supported in those older Node.js versions. Need to investigate why that's happening.

[klebba]

This PR timely! I was just about to open a related docs issue: https://eslint.org/docs/latest/use/configure/configuration-files-new#configuration-file-resolution

You can prevent this search for eslint.config.js by setting the ESLINT_USE_FLAT_CONFIG environment variable to true [...]

I think this should guide developers to set the variable to false

[nzakas]

@klebba documentation updates are being tracked in #13481, and that is one of the documentation changes we'll be making.

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[mdjermanovic]

After these changes, npm test will be logging more than 100 deprecation warnings until we drop eslintrc in v10.

Can we avoid that?

allowFlatConfig = false is passed only from tests, so we could update the condition in lib/cli.js to avoid logging warnings while running tests:

-if (!usingFlatConfig) {
+if (allowFlatConfig && !usingFlatConfig) {
    process.emitWarning("You are using an eslintrc configuration file, which is deprecated and support will be removed in v10.0.0. Please migrate to an eslint.config.js file. See https://eslint.org/docs/latest/use/configure/migration-guide for details.", "ESLintRCWarning");
}

Then, we could update this test this way to make it work:

Suggested change

	await cli.execute(`--config ${configPath} ${filePath}`, null, false);
	const localCLI = proxyquire("../../lib/cli", {
	"./eslint/flat-eslint": { shouldUseFlatConfig: () => Promise.resolve(false) }
	});
	await localCLI.execute(`--config ${configPath} ${filePath}`);

Or, perhaps even better, merge this test into the test for ESLINT_USE_FLAT_CONFIG=false as that's anyway the most realistic test:

it(`should not use it when ESLINT_USE_FLAT_CONFIG=false even if an eslint.config.js is present:${configType}`, async () => {
    process.env.ESLINT_USE_FLAT_CONFIG = "false";
    process.cwd = getFixturePath;

    const exitCode = await cli.execute(`--no-ignore --ext .js ${getFixturePath("files")}`, null, useFlatConfig);

    assert.strictEqual(exitCode, 0);

+   if (useFlatConfig) {
+       assert.strictEqual(processStub.callCount, 1, "calls `process.emitWarning()` once");
+       assert.strictEqual(processStub.getCall(0).args[1], "ESLintRCWarning");
+   }
});

That would leave npm test with zero eslintrc deprecation warnings (assuming #17797 will be merged; those tests pass allowFlatConfig = true but mistakenly still work in eslintc mode, so they're mistakenly logging deprecation warnings).

[nzakas]

Good suggestion, I went with the second approach.

[github-actions]

Hi everyone, it looks like we lost track of this pull request. Please review and see what the next steps are. This pull request will auto-close in 7 days without an update.

[mdjermanovic]

LGTM, thanks!

[mdjermanovic]

This might fix merge conflicts.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: nzakas / name: Nicholas C. Zakas (d0dea8f, df69890, 208d2bc, 99e5f75, 8ac03fa, 728b4bc, 7b359b0, 4fc794c, bbf0119, 8b30806, 265105f, 35ccf30, 4816e25)
• ✅ login: mdjermanovic / name: Milos Djermanovic (4fc794c, bbf0119, 8b30806, 35ccf30, 4816e25)

[nzakas]

Only CLA issues are with commits from CI, so this should be safe to merge.

[nzakas]

Will need to revisit once #17889 is merged