PR Checklist

• Addresses an existing open issue: fixes Enhancement(rule-tester): FlatRuleTester -> RuleTester #9017
• That issue was marked as accepting prs
• Steps in Contributing were taken

Overview

Adds a new FlatRuleTester implementation, which supports flat config and has been completely (from scratch) rewritten in full TypeScript with greater performance and following more modern coding standards (Namely defining your rules as code as opposed to passing a giant JSON-esque array)

Porting should be easy via the FlatRuleConfig.fromObject method (Which takes a mostly RuleTester.run-compatible object)
And should be more performant (Although I have not checked yet)

BREAKING CHANGES COMPARED TO RuleTester:

• Uses ESLint's flat config system
• Supports per-extension options
• Requires type-checking
• Does not support modifying parserOptions on a per-case basis (For performance reasons)

TODO

• Add unit tests
• Maybe make the API callback-based, i.e.

new FlatRuleTester({...}).rule("my-rule", rule, (def) => {
  def.configuration([], (conf) => {
    conf.valid(`console.log('Example');`);
  }));
});

• Make it check TypeScript for errors and report on missing type definitions
• Support auto importing types (as if /// <reference types="...">)
• Add website documentation and migration guide
• Implement missing options from RuleTester
• By default, we should use @typescript-eslint/parser as the parser
• maybe: Add support for a config file in the project root that defines where the fixtureRoot is (Maybe a subclass or configuration option that automatically does that?)
• maybe: Fix internal ESLint Linter types
• maybe: More Jest integrations?
• maybe: Support caching results (Very unlikely to be implemented by me)
• maybe: Support string imports for parsers
• maybe: File-based testing? (Make files in a directory, point the RuleTester at it, and it'll test every single file, would also use comments to describe where errors should occur)
• maybe: Support modifying parserOptions on a per-configuration basis (Would affect performance noticably)
• maybe: Hook filesystem APIs to emulate a project (Should improve compatibility with ESLint plugins which require configuration files or file context)