EM101 fix rebinds or shadows a variable named `msg`

[dscorbett]

Summary

The fix for raw-string-in-exception (EM101) can change the program’s behavior when a variable named msg is already in scope. The function fresh_binding_name added in #24316 may help.

Example of rebinding:

$ cat >em101_1.py <<'# EOF'
def f():
    msg = "."
    try:
        raise RuntimeError("!")
    except RuntimeError:
        return msg
print(f())
# EOF

$ python em101_1.py
.

$ ruff --isolated check em101_1.py --select EM101 --unsafe-fixes --fix
Found 1 error (1 fixed, 0 remaining).

$ cat em101_1.py
def f():
    msg = "."
    try:
        msg = "!"
        raise RuntimeError(msg)
    except RuntimeError:
        return msg
print(f())

$ python em101_1.py
!

Example of shadowing:

$ cat >em101_2.py <<'# EOF'
msg = "."
def f():
    try:
        raise RuntimeError("!")
    except RuntimeError:
        return msg
print(f())
# EOF

$ python em101_2.py
.

$ ruff --isolated check em101_2.py --select EM101 --unsafe-fixes --fix
Found 1 error (1 fixed, 0 remaining).

$ cat em101_2.py
msg = "."
def f():
    try:
        msg = "!"
        raise RuntimeError(msg)
    except RuntimeError:
        return msg
print(f())

$ python em101_2.py
!

Version

ruff 0.15.8 (c2a8815 2026-03-26)