fix: Update no-loop-func to not overlap with no-undef

[matwilko]

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
[X] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Tell us about your environment (npx eslint --env-info):

Node version: v18.16.0
npm version: v9.5.1
Local ESLint version: v8.44.0 (Currently used)
Global ESLint version: Not found
Operating System: win32 10.0.23493

What parser are you using (place an "X" next to just one item)?

[X] Default (Espree)
[ ] @typescript-eslint/parser
[ ] @babel/eslint-parser
[ ] vue-eslint-parser
[ ] @angular-eslint/template-parser
[ ] Other

Please show your full configuration:
eslint repo's configuration.

What did you do? Please include the actual source code causing the issue.
Any loop form with a function declaration in it, where there is a reference to an undeclared variable in the function body.

e.g.

const arr = [];

for (let i = 0; i < 10; i++) {
    arr.push(() => a);
             ^^^^^^^ Function declared in a loop contains unsafe references to variable(s) 'a'. eslint(no-loop-func)
                   ^ 'a' is not defined. eslint(no-undef)
}

What did you expect to happen?
no-loop-func shouldn't report on this variable reference, it's not an unsafe loop variable capture, but rather an undeclared variable access that should solely be picked up by no-undef.

What actually happened? Please include the actual, raw output from ESLint.

ESLint Output

npm run lint

> eslint@8.44.0 lint
> node Makefile.js lint

Validating JavaScript files

C:\Code\eslint\tests\lib\rules\no-loop-func.js
  24:14  error  Function declared in a loop contains unsafe references to variable(s) 'a'  no-loop-func
  24:20  error  'a' is not defined                                                         no-undef

✖ 2 problems (2 errors, 0 warnings)

C:\Code\eslint\node_modules\shelljs\src\common.js:401
      if (config.fatal) throw e;
                        ^

Error: exec:
    at Object.error (C:\Code\eslint\node_modules\shelljs\src\common.js:110:27)
    at execSync (C:\Code\eslint\node_modules\shelljs\src\exec.js:120:12)
    at _exec (C:\Code\eslint\node_modules\shelljs\src\exec.js:223:12)
    at C:\Code\eslint\node_modules\shelljs\src\common.js:335:23
    at target.lint (C:\Code\eslint\Makefile.js:537:18)
    at global.target.<computed> [as lint] (C:\Code\eslint\node_modules\shelljs\make.js:36:40)
    at C:\Code\eslint\node_modules\shelljs\make.js:48:27
    at Array.forEach (<anonymous>)
    at Timeout._onTimeout (C:\Code\eslint\node_modules\shelljs\make.js:46:10)
    at listOnTimeout (node:internal/timers:569:17)

Node.js v18.16.0
NativeCommandExitException: Program "npm.cmd" ended with non-zero exit code: 1.

What changes did you make? (Give an overview)

Moved a couple of while/do..while loop tests from invalid to valid (the variable they test is undeclared, but looks unsafe because it's the condition of the while).

Added some new valid test cases for loops where the function accesses an undeclared variable.

Updated no-loop-func to ignore any variable references that are unresolved. These will then be solely picked up by no-undef if enabled.

Why?

My understanding from the Core Rule Guidelines is that no two rules should overlap, and in this case, both no-loop-func and no-undef are warning for the same issue (though with different interpretations of the problem.)

Given that no-loop-func as documented is intended to find captures of loop variables that are shared by all iterations of the loop, it seems like it shouldn't warn on a variable reference that isn't a loop variable at all.

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[mdjermanovic]

I agree that this rule should ignore undefined variables since their usage cannot be analyzed, and they're themselves a problem reported by another rule.

[nzakas]

@mdjermanovic I also agree.

[matwilko]

Comment on tests updated to reflect how the rule operates at the moment.

[mdjermanovic]

LGTM, thanks!