ESLintUtils.getParserServices should take a Readonly<RuleContext>

[danielnixon]

Repro

I'm writing an ESLint rule to ban code that leads to the surprising mutability described in TypeScript issue #13347.

I'd like to forbid passing an immutable value to a function that expects a mutable (but otherwise compatible) type.

I've noticed that RuleModule.create gives us an Readonly<RuleContext> but ESLintUtils.getParserServices expects a (mutable) RuleContext. This triggers my rule (see below).

It looks like ESLintUtils.getParserServices doesn't mutate the passed in context, so please consider updating it to take a Readonly<RuleContext>. This would stop my rule from complaining.

const rule: RuleModule<...> = {
  meta: {
      ...
  },
  create: (context) => { // Context is a Readonly<RuleContext>
    const parserServices = ESLintUtils.getParserServices(context); // but getParserServices takes a (mutable) RuleContext

Expected Result

ESLintUtils.getParserServices takes Readonly<RuleContext>

Actual Result

ESLintUtils.getParserServices takes (mutable) RuleContext

Additional Info

• New rule: unsafe assignment / declaration / return / call danielnixon/eslint-plugin-total-functions#21
• https://github.com/danielnixon/eslint-plugin-total-functions
• Interface with readonly property is assignable to interface with mutable property microsoft/TypeScript#13347
• Unsafe type-incompatible assignments should not be allowed microsoft/TypeScript#14150

Versions

package	version
@typescript-eslint/experimental-utils	3.3.0 and earlier
TypeScript	NA
node	NA
npm	NA

[bradzacher]

Happy to accept a PR!