Cache causes confusing behavior when running with and without "--unstable"

[twitchard]

Describe the bug

If you run black without --unstable on a file and then run it with --unstable, it won't apply changes that should be applied by --unstable.

To Reproduce

black --version
python_code='long_line = "      20        30        40        50        60        70        80        90        100       110       120       130       140"'
echo "$python_code" > my_file.py
echo "Formatting without --unstable..."
black -l 120 my_file.py
cat my_file.py

echo "Formatting with --unstable..."
black -l 120 --unstable my_file.py
cat my_file.py
echo "Adding a newline to invalidate the cache..."
echo -e "\n" >> my_file.py
echo "Formatting with --unstable again..."
black -l 120 --unstable my_file.py
cat my_file.py

for me outputs

black, 24.8.0 (compiled: yes)
Python (CPython) 3.12.4
Formatting without --unstable...
All done! ✨ 🍰 ✨
1 file left unchanged.
long_line = "      20        30        40        50        60        70        80        90        100       110       120       130       140"
Formatting with --unstable...
All done! ✨ 🍰 ✨
1 file left unchanged.
long_line = "      20        30        40        50        60        70        80        90        100       110       120       130       140"
Adding a newline to invalidate the cache...
Formatting with --unstable again...
reformatted my_file.py

All done! ✨ 🍰 ✨
1 file reformatted.
long_line = (
    "      20        30        40        50        60        70        80        90        100       110       120     "
    "  130       140"
)

Expected behavior

• Adding --unstable to the command should cause black to ignore cache results that were produced without --unstable.
• To me "1 file left unchanged" misleadingly suggests that black looked at the file and determined there were no changes to make. A wording like "1 unchanged file ignored" would have helped me understand a cache was in play.
• Similarly the fact that black --help makes no mention of a cache / has no flag to bypass was a barrier to me understanding that this was a possibility.

Environment

• Black's version: 24.8.0
• OS and Python version: 3.12.4, Mac OS

[JelleZijlstra]

Thanks, we were missing --unstable in the cache key. Submitted a fix in #4466.