Conflict between `no-fallthrough` rule and `reportUnusedDisableDirectives` option

Tell us about your environment

• ESLint Version: 7.7.0
• Node Version: 14.8.0
• npm Version: 6.14.7

What parser (default, @babel/eslint-parser, @typescript-eslint/parser, etc.) are you using?

default

Please show your full configuration:

Configuration

module.exports = {
	ignorePatterns: [
		'node_modules/'
	],
	parserOptions: {
		ecmaVersion: 2020,
		sourceType: 'module'
	},
	env: {
		browser: true,
		commonjs: true
	},
	reportUnusedDisableDirectives: true,
	rules: {
		'no-fallthrough': 'error'
	}
};

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

const type = 'bar';
let xyz = 'x';

switch (type) {
	case 'foo':
	case 'bar':
		xyz += 'y';

	// eslint-disable-next-line no-fallthrough
	case 'baz':
		xyz += 'z';
		break;

	default:
}

console.log(xyz);

eslint .

What did you expect to happen? What actually happened? Please include the actual, raw output from ESLint.

Without // eslint-disable-next-line no-fallthrough Eslint reports Expected a 'break' statement before 'case', which is absolutely correct. But adding this line, ESLint reports Unused eslint-disable directive (no problems were reported from 'no-fallthrough'), which is not correct, because this directive suppresses the no-fallthrough error.

I’ve created a minimal test setup here: https://github.com/jens-duttke/eslint-bug-test

Are you willing to submit a pull request to fix this bug?

No

[mdjermanovic]

"." makes all non-empty comments being the comments that allow fall-through, so I don’t think it helps to avoid conflicts with eslint-disable.

For example, try to run eslint with --report-unused-disable-directives on the following code:

/* eslint no-fallthrough: ["error", { commentPattern: "." }] */

switch (type) {
	case 'foo':
	case 'bar':
		xyz += 'y';

	// eslint-disable-next-line no-fallthrough
	case 'baz':
		xyz += 'z';
		break;

	default:
}

  8:2  error  Unused eslint-disable directive (no problems were reported from 'no-fallthrough')

It’s still the same issue. We just replaced one pattern (the default one) that matches "eslint-disable-next-line no-fallthrough" with another pattern that also matches.

null and false currently produce schema validation errors, so we can use one of them as a value that disables the “intentional comment” feature of this rule. That still wouldn’t help to avoid confusion with the default behavior of this rule (changing or removing the default pattern would help, but I’m not sure should we do that even in a major version), but at least provides a clear way to disable this feature. A workaround that can be used at the moment is to set a pattern that cannot match anything, like { commentPattern: "[]" }.

[anikethsaha]

Thanks for the report.

This is a bug. old discussion https://github.com/eslint/eslint/issues/13260

here is the details: https://github.com/eslint/eslint/issues/13260#issuecomment-624114328