RUF027 creates invalid f-string interpolations with backslashes before Python 3.12

[dscorbett]

Summary

missing-f-string-syntax (RUF027) applies to putative interpolations containing backslashes, which are not valid until Python 3.12. The fix can therefore introduce syntax errors. RUF027 should probably be suppressed for such strings before Python 3.12. Example:

$ cat >ruf027.py <<'# EOF'
x = "!"
print("{'\''}{x\
}")
# EOF

$ python3.11 ruf027.py
{'''}{x}

$ ruff --isolated check ruf027.py --select RUF027 --preview --target-version py311 --unsafe-fixes --fix --output-format concise
ruf027.py:2:11: invalid-syntax: Cannot use an escape sequence (backslash) in f-strings on Python 3.11 (syntax was added in Python 3.12)
ruf027.py:2:17: invalid-syntax: Cannot use an escape sequence (backslash) in f-strings on Python 3.11 (syntax was added in Python 3.12)
Found 3 errors (1 fixed, 2 remaining).

$ python3.11 ruf027.py 2>&1 | tail -n 1
SyntaxError: f-string expression part cannot include a backslash

Version

ruff 0.14.1 (2bffef5 2025-10-16)