There are two options:

1. Add a new --diff option to ruff format --check that prints the formatting changes similar to Black
2. --check uses Ruff's diagnostic system and shows diffs for format: human but not for format: compact (which would enable diffs by default)

I prefer option 2.

Open questions

• Should this be integrated into our diagnostic system?
• Should it print the entire diff or collapse very large code frames
• Which diffing library to use. We ran into exponential back-tracking cases in our cargo dev script for large files / large changes

CC @zanieb regarding CLI design