correctly deal with user type ascriptions in pat

[lcnr]

supersedes #93856

thir::PatKind::AscribeUserType previously resulted in CanonicalUserTypeAnnotations where the inferred type already had a subtyping relation according to variance to the user_ty.

The bug can pretty much be summarized as follows:

• during mir building
  • user_ty -> inferred_ty: considers variance
  • StatementKind::AscribeUserType: inferred_ty is the type of the place, so no variance needed
• during mir borrowck
  • user_ty -> inferred_ty: does not consider variance
  • StatementKind::AscribeUserType: applies variance

This mostly worked fine. The lifetimes in inferred_ty were only bound by its relation to user_ty and to the place of StatementKind::AscribeUserType, so it doesn't matter where exactly the subtyping happens.

It does however matter when having higher ranked subtying. At this point the place where the subtyping happens is forced, causing this mismatch between building and borrowck to result in unintended errors.

cc #96514 which is pretty much the same issue

r? @nikomatsakis

[nikomatsakis]

I don't really understand the PR description, @lcnr :( maybe we can find some time to sync up on this. I'd like to understand better what is going on!

[nikomatsakis]

r=me if you add back in the "inferred type" in the pretty printer and have an open PR against rustc-dev-guide that at least starts to explain wtf is going on here :)

[nikomatsakis]

@bors r+

[bors]

📌 Commit 9b5a0f55a7d08ec900a7ca3e08de1e9ef4b634fa has been approved by nikomatsakis

[bors]

⌛ Testing commit 9b5a0f55a7d08ec900a7ca3e08de1e9ef4b634fa with merge 35e0e69c59029e2659cc6b35c3d1006856e44995...

[bors]

💔 Test failed - checks-actions

[lcnr]

@bors r=nikomatsakis

[bors]

📌 Commit 7637008 has been approved by nikomatsakis

[bors]

⌛ Testing commit 7637008 with merge e52e711...

[bors]

☀️ Test successful - checks-actions
Approved by: nikomatsakis
Pushing e52e711 to master...

[rust-timer]

Finished benchmarking commit (e52e711): comparison url.

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: 😿 relevant regressions found

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count1	0	2	0	0	0
mean2	N/A	2.2%	N/A	N/A	N/A
max	N/A	2.7%	N/A	N/A	N/A

Cycles

Results

• Primary benchmarks: mixed results
• Secondary benchmarks: 🎉 relevant improvements found

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count1	1	0	1	2	2
mean2	1.8%	N/A	-1.2%	-2.7%	0.3%
max	1.8%	N/A	-1.2%	-3.7%	1.8%

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

1. number of relevant changes ↩ ↩²

2. the arithmetic mean of the percent change ↩ ↩²