x fmt should only format modified files by default

[jyn514]

Since rustfmt is deterministic and idempotent, running it twice on the same file should always have the same output (if it's not both of those things, it's a bug). So we can avoid running it unless the file has changed, which should make x fmt without arguments much faster.

Originally posted by @chenyukang in #105648 (comment)

[jyn514]

This will need to be careful to still rerun rustfmt when we bump its version (which happens about every 6 weeks). Maybe we can generate a .rustfmt-all-files.stamp file and format everything if it doesn't yet exist or has the wrong version.

[albertlarsan68]

Hello,
I want to work on that.
How would the detection of modified files work ?

[jyn514]

@albertlarsan68 you can use git diff-index --name-only HEAD to see modified files. Make sure to fall back to formatting all files if that command fails for some reason, we support building from source tarballs without git installed.

[albertlarsan68]

This means that already-commited files that were not formatted when committed would not be formatted, right ?

[jyn514]

Ah, good point. You can use git merge-base HEAD origin/master to find where the two diverged and compare all files since then. And fmt --check should still look at all files unconditionally.

[albertlarsan68]

Thanks, I will start working on this !

@rustbot claim

[the8472]

You can use git merge-base HEAD origin/master

That assumes that the rust-lang remote is labeled as origin. I call that upstream and my fork origin. Using the local master may also not work if one does git fetch upstream && git rebase upstream/master on branch.

[jyn514]

Yes, #105058 has a more general solution.

[albertlarsan68]

PR is up at #105702