FURB142: Parenthesize generator expression arguments in fixer to preserve scoping

[sohamukute]

Summary

Fixes a bug in the FURB142 (for-loop-set-mutations) fixer where an unparenthesized generator expression used as the .add(...) argument would be inlined into the rewritten comprehension without parentheses, changing variable scoping and causing a runtime NameError.

Before (problematic after fix):

s = set()
for x in ("abc", "def"):
    s.add(c for c in x)
# fixed to:
s.update(c for c in x for x in ("abc", "def"))  # NameError: x

After (correct, parenthesized):

s = set()
for x in ("abc", "def"):
    s.add(c for c in x)
# fixed to:
s.update((c for c in x) for x in ("abc", "def"))

Implementation details:

• In the “comprehension” rewrite path of FURB142, detect Expr::Generator arguments that are not already parenthesized and wrap them with parentheses in the generated replacement string.
• Leaves already-parenthesized generator expressions unchanged.
• Leaves non-generator arguments unchanged.
• Maintains existing applicability handling and comment-safety behavior.

References:

• Fixes FURB142 fix should parenthesize generator expressions #21098
• Rule docs: FURB142 (for-loop-set-mutations)

Test Plan

• Repro the original failing case and confirm the new fix:

  1. Create the file:

     s = set()
     for x in ("abc", "def"):
         s.add(c for c in x)

  2. Verify Ruff flags it:

     cargo run -p ruff_cli -- check --isolated --select FURB142 --preview furb142.py
     

  3. Apply the fix:

     cargo run -p ruff_cli -- check --isolated --select FURB142 --preview --fix furb142.py
     

  4. Inspect output (should contain parentheses):

     s.update((c for c in x) for x in ("abc", "def"))
     

  5. Run the file:

     python furb142.py
     

     Expect no NameError.