cargo-fix: Dry-run functionality

[ozkriff]

Is something like --dry-run planned? Or cargo fmt will always require a VCS?

Manishearth:
It would print out a diff of what it would change. with cargo check you still have to wonder about which suggestions would get applied, and how

killercup:
Ah, I see, writing the diff is an interesting idea. In theory you can write it to a .patch file, too, then.

Previous issue in the rustfix repo for some context: rust-lang/rustfix#146

[epage]

We could print out a first pass of files that would change or of the diffs of what will be changed but that can't fully match the behavior of a non-dry-run because we have to loop over fixes in case one fix causes another fix to change.