newline_style = "Unix" has no effect for unchanged files #3939
Description
I have a project where many files have Windows line endings. I set newline_style to Unix and ran cargo fmt, expecting the files to be reformatted, but they are not unless there is some other non-line-ending change being made.
Interestingly, the opposite is not true: if I explicitly set newline_style to Windows, then any Unix-formatted files are rewritten even if there is no other reformatting to do.
To repro this:
cargo init
git add .
cargo fmt -- --config newline_style=Windows
git diff # see that src/main.rs has been reformatted with Windows line endings
git add -u
cargo fmt -- --config newline_style=Unix
git diff # see that there are no changes--src/main.rs still has Windows line endings