Consider irrefutable pattern similar to if .. else for C901

[blueraft]

Summary

Follow up to #11521

Removes the extra added complexity for catch all match cases. This matches the implementation of plain else statements.

Test Plan

Added new test cases.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[MichaReiser]

Can we use saturating_sub here to avoid a panic for match a:. This is not valid python but the parser might soon be able to recover from incomplete statements.

Suggested change

	let last_index = cases.len() - 1;
	let last_index = cases.len().saturating_sub(1);

[blueraft]

Went with Dhurv's suggestion here.

[MichaReiser]

What's the reason that we restrict the logic to only when the _ pattern is last? Can't we always subtract the complexity in that case?

[dhruvmanila]

Because an irrefutable pattern can only occur in the last case block otherwise it's a syntax error (checked by the compiler). We don't raise this currently, but we should start doing so. I'm going to make an umbrella issue with such syntax errors raised by the compiler.

[MichaReiser]

That makes sense. But that also implies that it would be safe to ignore the check here and simply assume it is the last?

[dhruvmanila]

I think we should just use cases.last:

if let Some(last_case) = cases.last() {
	// ...
}

[dhruvmanila]

Actually, there are more cases to consider here:

1. Make sure that the guard field is None (last_case.guard.is_none())
2. Consider sequence pattern where if any pattern in the sequence is _ or named capture, the entire sequence is considered irrefutable

[dhruvmanila]

(Sorry for sending this review after the approval)

[blueraft]

Good catch! Done.

[dhruvmanila]

Thank you! I've made is_irrefutable a method on Pattern.