Replace last usize -> ptr transmute in alloc with strict provenance API

[jwnrt]

This conversion doesn't trip fuzzy_provenance_casts because it's a transmute, but it still causes CHERI targets to fail compiling because of the size difference.

While I was here I changed the align arg in the (internal) RawVecInner::new_in function to an Alignment type to encode the non-zero constraint of Unique.

I recognise that bit is slightly messy, especially now that there's an Alignment -> usize -> Alignment -> usize chain from Layout::align + Alignment::new_unchecked + Alignment::usize. Happy to drop that commit if it's not helpful.

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[joboet]

Transmuting between usize and pointers is completely fine and is (under the current semantics) equivalent to using the strict-provenance APIs. That's why e.g. addr is actually implemented via a transmute. Regardless, I still think the changes suggested here are worthwhile – it's always a good idea to make internal API safer to use.

RawVec is extremely hot code, so its performance is very sensitive to even small changes. Therefore we run a few tests to check that the code emitted for Vec is optimised enough. Your changes are currently breaking those tests because the helper methods you use insert additional debug assertions that can still inhibit some optimisations even in release builds. Those debug assertions are probably the reason why transmute was used here to begin with, as it doesn't emit those assertions. I do however think there is a way around that, I'll explain in the comments.

[scottmcm]

Cool; I've been tempted to add this for a while, so happy to see it.

[scottmcm]

I'm hopeful this is fine -- after all, GVN will collapse transmute chains now -- but because it touched RawVec and not just RawVecInner, let's run perf just in case

@bors try @rust-timer queue

[bors]

⌛ Trying commit aadfd81 with merge 3230851...

[bors]

☀️ Try build successful - checks-actions
Build commit: 3230851 (3230851bb3beb4974e6914f3ca72ca407d5ff07e)

[rust-timer]

Finished benchmarking commit (3230851): comparison URL.

Overall result: ❌ regressions - 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 the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

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

Max RSS (memory usage)

Results (primary 3.9%)

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)	10.0%	[4.5%, 16.6%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.2%	[-2.6%, -1.4%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.9%	[-2.6%, 16.6%]	6

Cycles

Results (primary 1.4%, secondary -1.6%)

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)	1.4%	[1.4%, 1.4%]	1
Regressions ❌ (secondary)	2.6%	[1.7%, 3.0%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.8%	[-3.8%, -1.9%]	13
All ❌✅ (primary)	1.4%	[1.4%, 1.4%]	1

Binary size

Results (primary 0.1%, secondary 0.1%)

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)	0.1%	[0.0%, 0.5%]	50
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	3
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.0%, 0.5%]	51

Bootstrap: 778.47s -> 777.282s (-0.15%)
Artifact size: 365.96 MiB -> 365.98 MiB (0.01%)