Add support for `#x in obj` private-in syntax

The version of ESLint you are using.

7.31.0

The problem you want to solve.

Support Ergonomic brand checks for Private Fields (#x in obj syntax) officially, as it has reached stage-4. refs: https://github.com/tc39/proposals/blob/master/finished-proposals.md https://github.com/tc39/proposal-private-fields-in-in

Your take on the correct solution to problem.

• Ecma262 update: tc39/ecma262#2290
• ESTree update: estree/estree#252 Maybe we just merge https://github.com/estree/estree/blob/master/stage3/private-fields-in-in.md into es2022.md.
• Acorn update: acornjs/acorn#1068
• eslint-visitor-keys update:
  Maybe we have no task on it.
• espree update: https://github.com/eslint/espree/pull/521
• eslint-scope update:
  Maybe we have no task on it.
• eslint update:

Are you willing to submit a pull request to implement this change?

It’s difficult for me to change the parser. I may be able to do other necessary work.

[nzakas]

Open source doesn’t follow predictable schedules. 😃

[mdjermanovic]

With #x in obj syntax, #x can appear in expressions without preceding ., and it generally can be adjacent to other tokens on its left side, so this might be seen as a false negative:

/* eslint space-unary-ops: ["error", { "words": false }] */

class C {
    #x;
    *foo(bar) {
        yield #x in bar;
    }
}

These are very edge cases, though.