This changes rustfmt to return exit code 4
when run with write mode diff and differences between
the formatted code and the original code are found.
Useful for CI to make sure your contributors actually ran rustfmt.
The old behaviour stored everything in memory until we were finished. Now we write as soon as we can.
This gives better behaviour when formatting large programs, since there is some progress indication. It also opens the door to optimising memory use by not storing everything in memory unless it is required (which it still might be). That is left as future work though.
This commit tidies up handling of `write_mode` by setting it in the
config at the start, and removing the `write_mode` parameter threaded
throughout the formatting process.
Also from @marcusklaas:
Refactor code output functions
Specifically, `write_all_files` no longer returns a HashMap. It would sometimes
contain items, and sometimes be empty. When "fixed" newlines are required, this
must now be done with a separate call. The tests use this strategy and should now pass!
Adds support for receiving input from stdin in case no file was
specified. This is useful for editor/IDE integrations and other tooling.
To achieve clean output a new write-mode option called plain was added,
this option is mandatory when using stdin.
