ui tests should just show the stderr diff

[nikomatsakis]

Currently, when a UI test fails, it prints a sort of overwhelming amount of stuff:

rust/src/tools/compiletest/src/runtest.rs

Lines 2712 to 2722 in a3c3245

	println!("normalized {}:\n{}\n", kind, actual);
	println!("expected {}:\n{}\n", kind, expected);
	println!("diff of {}:\n", kind);
	for diff in diff::lines(expected, actual) {
	match diff {
	diff::Result::Left(l) => println!("-{}", l),
	diff::Result::Both(l, _) => println!(" {}", l),
	diff::Result::Right(r) => println!("+{}", r),
	}
	}

I think what I would like is something like:

If the expected text is empty, just dump the actual (normalized) output, so people can copy-and-paste it if they want.

Else, just print the diff. And probably it'd be nice to strip off the "prefix" and "suffix" of unchanged lines, and just show the parts that vary (we could imagine breaking things into hunks).

[nikomatsakis]

cc @oli-obk @petrochenkov -- two people who care about test suite I know, sorry if I'm forgetting to cc others =)

[durka]

And probably it'd be nice to strip off the "prefix" and "suffix" of unchanged lines, and just show the parts that vary (we could imagine breaking things into hunks).

Sounds like reinventing features from existing diff libraries where there might be a crate that can do it nicely!

[omakk]

Hello!

I'm a Rust-learner and was wondering if I could pick up this issue?

[oli-obk]

@omakk great! Feel free to ping me on irc or ask any questions here.

I think an experimental implementation of using a diff crate would be the way to go as @durka suggested.

[ritiek]

@omakk If you're busy at the moment, do you mind if I take on this issue?

[omakk]

@ritiek Yes, go ahead! I unfortunately can not find the time to look at this. Apologies for the late update.

[petrochenkov]

cc @petrochenkov

I never read these diffs, personally, just run update-all-references.sh immediately if there are errors.

[ritiek]

I suppose this can now be closed with #47185.