feat: Improve config error messages

[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)

Includes some keys to catch known eslintrc keys when they appear in flat config. This throws a specific error that the ESLint CLI can then output a more helpful message about.

refs #17370

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

Did I miss any top-level keys?

(Note: we can probably do more to do things like catch the different in plugins format, just wanted to get the ball rolling.)

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[matwilko]

Handful of thoughts, can't see any top-level keys you missed.

[matwilko]

Would this look slightly nicer as an object rather than a big switch? A little less syntax noise to my eyes 😄

e.g.

const keyErrors = {
    env: `
A config object is using the "env" key, which is not supported in flat config system.

Flat config uses "languageOptions.globals" to define global variables for your files.

Please see the following page for information on how to convert your config object into the correct format:
https://eslint.org/docs/latest/use/configure/migration-guide#configuring-language-options
`,

    extends: `
A config object is using the "extends" key, which is not supported in flat config system.

Instead of "extends", you can include config objects that you'd like to extend from directly in the flat config array.

Please see the following page for more information:
https://eslint.org/docs/latest/use/configure/configuration-files-new#using-predefined-configurations
`,

...

};

module.exports = key => keyErrors[key].trim();

[nzakas]

ooh nice, I like it.

[matwilko]

Might suggest a more explicit name than createEslintrcSchema - the current name could be confused as adding support for eslintrc config keys at a glance? Perhaps rejectEslintrcKeySchema or similar to spell it out?

[nzakas]

Fair point, making it explicit that this is throwing an error makes sense.

[matwilko]

Even though we're unlikely to add any further keys, I might suggest a slight refactor here to remove the duplication of the key name, and not take up half the space in this schema with keys that we don't actually care about in the end:

function rejectEslintrcKeySchema(key) {
    return {
        merge: "replace",
        validate() {
            throw new IncompatibleKeyError(key);
        }
    };
}

function rejectEslintrcKeySchemas(...keys) {
    return Object.fromEntries(keys.map(key => [key, rejectEslintrcKeySchema(key)]));
}

exports.flatConfigSchema = {

    ...rejectEslintrcKeySchemas("env", "extends", "globals", "ignorePatterns", "noInlineConfig", "overrides", "parser", "parserOptions", "reportUnusedDisableDirectives", "root"),

    settings: ...
    linterOptions: ...
...

[nzakas]

Another nice update, thanks.

[mdjermanovic]

Did I miss any top-level keys?

No, all eslintrc top-level keys that don't exist as flat config top-level keys are included.

[mdjermanovic]

This test should be updated:

eslint/tests/lib/rule-tester/flat-rule-tester.js

Lines 1331 to 1340 in 42faa17

	it("throw an error when env is included in config", () => {
	assert.throws(() => {
	ruleTester.run("no-eval", require("../../fixtures/testers/rule-tester/no-eval"), {
	valid: [
	{ code: "Eval(foo)", env: ["es6"] }
	],
	invalid: []
	});
	}, /Unexpected key "env" found./u);
	});

[mdjermanovic]

LGTM, thanks!