`cargo fmt` doesn't format files in sub-directories in `tests`

[steveklabnik]

I have a big integration test suite with sub-directories inside of the tests directory; running cargo fmt doesn't format these files.

[brauliobz]

cargo fmt only formats test code that is reached by cargo test, which basically follows the mod inclusions, from what I could see.

I tested the following scenario:

tests/
    root_test.rs
    subdir/
        mod.rs
        sub_test.rs

sub_test.rs is formatted only when:

• root_test.rs includes subdir by mod subdir;;
• and subdir/mod.rs includes sub_test.rs by mod sub_test;.

Is this behavior really desirable?

[nrc]

I think there are two kinds of cargo test - unit tests, which cargo fmt should reach if they are reachable via module includes (I think this is fine); and integration tests, which are not formatted at all (which we should address, though I'm not really sure how).

[calebcartwright]

Just a heads up that I've taken a pass at this and now have a working implementation locally.

Will open a PR for discussion/review/etc. within the next day or two