Rust rewrite check_diff (Skeleton)

[benluiwj]

Closes first task from #6170
Help screen usage

Sample usage with all fields present

Sample error when 1 compulsory field is missing

Update: Screenshots

[ytmimi]

Let's take a different approach here. Instead of creating a new binary target for rustfmt, lets create a check_diff crate within this repo. Similar to how config_proc_macro is its own crate.

[benluiwj]

Thanks for the insightful discussion on Zulip. It cleared up alot of doubts and gave a clearer picture of what is the end goal. :)

[benluiwj]

@rustbot ready

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[ytmimi]

After squashing the commits and rebasing we'll be good to on this one

After thinking about it a little more I think we need to make some changes to the CliInputs struct.

[ytmimi]

Take a look at the most recent review when you have a chance. Overall I think we're heading in the right direction, but I'd like us to make some further changes.

[ytmimi]

Can we make sure to run rustfmt built from source on these files cargo run --bin rustfmt -- check_diff/src/main.rs I believe there are certain lines that exceed the max_width.

It would probably be worth adding the check_diff crate to rustfmt's self_test to ensure that these files stay formatted:

rustfmt/src/test/mod.rs

Lines 382 to 411 in 871113e

	#[test]
	fn self_tests() {
	init_log();
	let mut files = get_test_files(Path::new("tests"), false);
	let bin_directories = vec!["cargo-fmt", "git-rustfmt", "bin", "format-diff"];
	for dir in bin_directories {
	let mut path = PathBuf::from("src");
	path.push(dir);
	path.push("main.rs");
	files.push(path);
	}
	files.push(PathBuf::from("src/lib.rs"));
	let (reports, count, fails) = check_files(files, &Some(PathBuf::from("rustfmt.toml")));
	let mut warnings = 0;
	// Display results.
	println!("Ran {count} self tests.");
	assert_eq!(fails, 0, "{fails} self tests failed");
	for format_report in reports {
	println!(
	"{}",
	FormatReportFormatterBuilder::new(&format_report).build()
	);
	warnings += format_report.warning_count();
	}
	assert_eq!(warnings, 0, "Rustfmt's code generated {warnings} warnings");
	}

[benluiwj]

Added some code here to deal with external crates instead of modifying the existing loop.

[ytmimi]

Just wanted to note that if we add a lib.rs at some point then we'll need to remember to add that here too

[ytmimi]

Thanks for working through all the changes I requested. I think we're good to go here!