Give RPITITs variances, so they can (not) capture lifetimes

[compiler-errors]

This is needed for edition 2021 and the precise_captures feature, so we can specify that we don't want to capture a lifetime implicitly for an RPITIT. I'll elaborate this further and try to motivate its soundness once perf comes back.

r? @ghost

[rustbot]

changes to the core type system

cc @compiler-errors, @lcnr

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 8ba97b6 with merge 957db6d...

[bors]

☀️ Try build successful - checks-actions
Build commit: 957db6d (957db6d1de288cd9a0bf37d3dd379b44a889dc33)

[rust-timer]

Finished benchmarking commit (957db6d): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.2%, 0.2%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.2%	[-1.2%, -1.2%]	1
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	2

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.2%	[-6.2%, -2.3%]	7
All ❌✅ (primary)	2.1%	[2.1%, 2.1%]	1

Cycles

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

Binary size

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

Bootstrap: 679.507s -> 678.77s (-0.11%)
Artifact size: 316.08 MiB -> 316.06 MiB (-0.01%)