Refactor borrowck liveness values

[lqd]

This PR starts cleaning up rustc_borrowck, in particular around liveness values:

• refactors simple names that make no sense anymore: either referring to older structures using region elements, or to bitset containers and values.
• improves comments and fixes others
• removes unused return values and unneeded generic arguments

r? @matthewjasper

[cjgillot]

I'm trying to factor this type out in #115291. This would require undoing part of this PR.

[lqd]

@cjgillot I'm not sure that it would:

Moving RegionValueElements and PointIndex looks good, but the borrowck and dest-prop use LivenessValues in different ways in #115291. The former depends on pretty printing that "shouldn't" be moved out. And I'm also looking into storing the live loans there (because various parts of the borrowck are actually pushing liveness data, and need to be updated to compute the live loans. This causes scope differences between NLLs and polonius and needs to be fixed) and that is not something of interest for dest prop. edit: I've done this in #118175.

The common thing between the two is just a SparseIntervalMatrix<N, PointIndex> with some Location mapping, and as far as I can tell only the contains function in today's LivenessValues is used in dest prop. We could uplift such a point index matrix and use it in both rustc_borrowck and dest prop, it's not clear it'd be a huge improvement but why not. Similar results would probably be achieved by having a dataflow visitor that does the mapping to points, or a function converting MaybeLiveLocals's results to a point index matrix, and so on, where LivenessValues itself wouldn't help.

Summary:

• I think most of the refactoring parts of Save liveness results for DestinationPropagation #115291 should land, but not the LivenessValues move: rustc_borrowck doesn't need it (as-is it also breaks NLL MIR dumps), and dest prop does not use it more than a point matrix. It also causes suboptimal naming like LivenessValues<Local> with union_regions.
• I think this PR should land: it doesn't impact your dest prop plans, and neither do the possible changes rustc_borrowck could need from LivenessValues.
• we could uplift in rustc_mir_dataflow the small data structure shared between borrowck and dest prop.

[jackh726]

LGTM, r=me if you'd like

[lqd]

Thanks Jack! I'd first like to give @cjgillot some time for his thoughts on my last comment.

[cjgillot]

@bors r+

[bors]

📌 Commit d203476 has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit d203476 with merge 274b524...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 274b524 to master...

[rust-timer]

Finished benchmarking commit (274b524): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

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
Regressions ❌ (secondary)	4.3%	[4.3%, 4.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.5%	[-4.4%, -2.5%]	2
All ❌✅ (primary)	-	-	0

Cycles

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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 675.527s -> 675.337s (-0.03%)
Artifact size: 313.36 MiB -> 313.38 MiB (0.01%)