[pyupgrade] Add sub-diagnostics showing type variable definitions (UP046)

[ntBre]

Summary

Now that we've handled caching for the new diagnostics, I went looking for diagnostics that could use additional annotations or sub-diagnostics that weren't already tracked in #17203. UP046 (and UP040 and UP047 if we like this one) seemed like a reasonable candidate because we check for separate type variable definitions in the same file before emitting a diagnostic. In this case, I added an info-level sub-diagnostic pointing to this type variable definition. I'm not actually sure if this is super helpful information, so I wouldn't mind closing this if others don't think it's useful.

I also added a helper DiagnosticGuard::info method that I think will be useful for adding this kind of sub-diagnostic in general.

I spent a while debugging why the snapshots were showing a longer path in the sub-diagnostics before remembering that we overwrite the source file for the main diagnostic here:

ruff/crates/ruff_linter/src/test.rs

Lines 280 to 288 in 79c949f

	// Not strictly necessary but adds some coverage for this code path by overriding the
	// noqa offset and the source file
	let range = diagnostic.expect_range();
	diagnostic.set_noqa_offset(directives.noqa_line_for.resolve(range.start()));
	if let Some(annotation) = diagnostic.primary_annotation_mut() {
	annotation.set_span(
	ruff_db::diagnostic::Span::from(source_code.clone()).with_range(range),
	);
	}

As shown below, the sub-diagnostics look as expected in the CLI. I think we could delete this part of the test code, but it results in using the longer test file names (like our new sub-diagnostics) in every snapshot. So alternatively, we may want to apply the same transformation to the sub-diagnostics (and secondary annotations) too.

Test Plan

Existing UP046 tests with newly expanded snapshots, as well as some manual testing in the CLI:

Alternative Idea

I don't really love the help: and info: lines stacked so closely together, so I also tried this as a secondary annotation:

That's not implemented in the PR but could be a different route with a small change to DiagnosticGuard::info (and a rename of the method).

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[ntBre]

Ah, I guess we need to replace all the paths to make the tests work on Windows anyway.

Also, I'm starting to think the secondary annotation might be a lot better, especially for multiple type variables:

[MichaReiser]

I don't really love the help: and info: lines stacked so closely together, so I also tried this as a secondary annotation:

I think what we do in ty is that help texts and notes are always the last sub-diagnostics.

[ntBre]

Ah, that does look better already. Thanks!

I just did this by sorting the sub-diagnostics in our DiagnosticGuard Drop impl. That seemed like the easiest fix since we push the help diagnostic right when creating the Diagnostic from a Violation, but we might want a better solution longer-term.

[ntBre]

I talked with Alex on Discord, and he echoed my concern that this wasn't a very useful piece of information to show to users. I'll go ahead and close this for now and hope to use some of the more general helper code for a different diagnostic.