Currently some tests use Project::change_file with an empty content string just to update the modified time of the file e.g.:

However, updating a zero length file to be zero length isn't guaranteed to change the modified time (it might be a no-op). Cargo's tests should explicitly set the modified time using the new File::set_times API.

There are at least a couple of ways this could be done. An explicit method could be added and used in place of calling change_file with an empty string. Or else change_file (or the underlying FileBuilder) could be updated to always set the modified time.