Option for --check to list violating files only

[abonander]

For our CI script we just want to say "cargo fmt needs to be run, here's the files that need it" which doesn't appear to be possible using --check. The diff output is kind of too noisy to be useful unless you expect the committer to actually fix the files by hand.

[calebcartwright]

I was just thinking about this one the other day, kinda like how black for python displays a x file(s) would be reformatted message when the --check flag is included, or x file(s) reformatted when the formatting is actually applied.

There's a request (#3274) to add support for the --message-format flag found in other cargo subcommands, and one of the options for that flag is short

Do you think something like the below would work for to generate the list of files?
cargo fmt --message-format short -- --check

[topecongiro]

We can add a command line flag like the -l,--files-with-matches of rg:

# Run `cargo fmt -- --check` but only prints file names which will be formatted
cargo fmt -- --check -l
# Run `cargo fmt` and print file names that were formatted
cargo fmt -- -l

[calebcartwright]

Even better! I'd be happy to look at implementing this @topecongiro once I've finished up my work with the json and checkstyle emitters

[abonander]

That was quick! Awesome!