RUF059 fix renames variables without checking existing variable names

[dscorbett]

Summary

The fix for unused-unpacked-variable (RUF059) can rename a variable to shadow or rebind an existing variable.

$ cat >ruf059.py <<'# EOF'
def f(_x):
    x, = "1"
    print(_x)
f("0")
# EOF

$ python ruf059.py
0

$ ruff --isolated check ruf059.py --select RUF059 --preview --unsafe-fixes --fix
Found 1 error (1 fixed, 0 remaining).

$ cat ruf059.py
def f(_x):
    _x, = "1"
    print(_x)
f("0")

$ python ruf059.py
1

Version

ruff 0.11.13 (5faf72a 2025-06-05)