Canonicalize free regions from inputs as placeholders in root univ

[ShoyuVanilla]

Context: The box named coroutine witness Send lifetime requirements now considered by leakcheck this roadmap

Fixes #106569
Prerequisite of #155749

[ShoyuVanilla]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: e39d9d1 (e39d9d1e2232da07330c87d8314c3587ec4edb1f, parent: 8da2d28cbd5a4e2b93e028e709afe09541671663)

[rust-timer]

Finished benchmarking commit (e39d9d1): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.1%, 0.3%]	4
Improvements ✅ (primary)	-1.1%	[-4.4%, -0.2%]	40
Improvements ✅ (secondary)	-1.1%	[-3.2%, -0.1%]	33
All ❌✅ (primary)	-1.1%	[-4.4%, -0.2%]	40

Max RSS (memory usage)

Results (primary 2.8%, secondary 3.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.8%	[1.0%, 5.5%]	7
Regressions ❌ (secondary)	3.2%	[1.6%, 4.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.8%	[1.0%, 5.5%]	7

Cycles

Results (primary -3.2%, secondary 0.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.1%	[2.1%, 2.1%]	1
Improvements ✅ (primary)	-3.2%	[-4.0%, -2.3%]	7
Improvements ✅ (secondary)	-1.6%	[-1.6%, -1.6%]	1
All ❌✅ (primary)	-3.2%	[-4.0%, -2.3%]	7

Binary size

Results (secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	4
All ❌✅ (primary)	-	-	0

Bootstrap: 493.313s -> 494.404s (0.22%)
Artifact size: 394.36 MiB -> 394.31 MiB (-0.01%)

[ShoyuVanilla]

r? lcnr

[ShoyuVanilla]

We have lots of those ...but it actually implements X for some specific lifetime '?x -> ...but it actually implements X for some specific lifetime diagnostic drifts due to the canonicalized region being placeholders rather than region vars.
I think this is somewhat regressive. Would it make sense to fix them somehow?

View changes since the review