Compute polonius loan scopes over the region graph

[lqd]

In issue #117146 a loan flows into an SCC containing a placeholder, and whose representative is an existential region. Since we currently compute loan scopes by looking at SCCs and their representatives only, polonius would compute kill points for this loan here whereas NLLs would not of course.

There are a few ways to fix this:

• don't try to be efficient by doing the computation over SCCs, and simply look for free regions and placeholders in the successors of the issuing region.
• change how the SCC representatives are picked, biasing towards placeholders over existential regions. They shouldn't matter much, but some downstream code may subtly depend on the current scheme (though no tests fail if we do such a change). This is for unrelated reasons also the way rework opaque type region inference #116891 changes the representative computation. So that PR would also fix issue ice: the loans out of scope must be the same as the borrows out of scope #117146.
• try to remove placeholders from the main path, and contain them to a pre-pass + a post-pass kind of polonius leak check. If possible, it would fix this issue by turning an outlives constraints to a placeholder into a constraint to 'static. This should also fix the issue, as the representative would be the free region in the SCC. We want to prototype this change to see if it's possible to try to simplify the borrowck main path from having to deal with placeholders and higher-ranked subtyping 🤞.

I'd like to take advantage of fuzzing and a crater run sooner rather than later, so that we grow more confidence that the 2 models are indeed equivalent empirically. Therefore this PR implements option 1 to fix the issue now.

We can take care of efficiency later after validation, and once we implement option 3 (which could also impact option 2 and that associated PR, maybe the lack of placeholders could remove the need to change the representative computation) to traverse SCCs and their representative again.

(Or we maybe will have some kind of naive position-dependent outlives propagation by then and this code would have been changed)

Fixes #117146.

r? @matthewjasper

[matthewjasper]

@bors r+

[bors]

📌 Commit 2dff90d has been approved by matthewjasper

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2dff90d with merge 341efb1...

[bors]

☀️ Test successful - checks-actions
Approved by: matthewjasper
Pushing 341efb1 to master...

[rust-timer]

Finished benchmarking commit (341efb1): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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
Regressions ❌ (secondary)	3.6%	[3.6%, 3.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	0.5%	[0.4%, 0.6%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.0%	[-2.0%, -0.5%]	8
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	-0.6%	[-2.0%, 0.6%]	11

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: 662.725s -> 662.579s (-0.02%)
Artifact size: 308.73 MiB -> 308.72 MiB (-0.00%)

[lqd]

This is not marked as a regression but if it comes up during triage: this has got to be noise, this code does not run without a -Z flag.