Add slice::DrainRaw for internal use

[scottmcm]

I'd like to separate out this hopefully-non-controversial part of #124421 to have fewer commits to juggle there :)

r? @the8472

[scottmcm]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 3f38556 with merge 2132470...

[bors]

☀️ Try build successful - checks-actions
Build commit: 2132470 (213247063d14960f8b256772f274b288657fb599)

[rust-timer]

Finished benchmarking commit (2132470): comparison URL.

Overall result: ❌✅ regressions and improvements - 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.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@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)	0.5%	[0.3%, 1.0%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.7%, -0.3%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.7%, 1.0%]	6

Max RSS (memory usage)

Results (primary 2.9%, secondary 0.0%)

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)	4.1%	[2.0%, 7.3%]	8
Regressions ❌ (secondary)	0.8%	[0.7%, 0.8%]	2
Improvements ✅ (primary)	-2.3%	[-2.5%, -2.2%]	2
Improvements ✅ (secondary)	-0.7%	[-0.8%, -0.6%]	2
All ❌✅ (primary)	2.9%	[-2.5%, 7.3%]	10

Cycles

Results (primary 1.3%, secondary 0.7%)

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)	1.3%	[1.3%, 1.3%]	1
Regressions ❌ (secondary)	1.4%	[0.6%, 2.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.7%	[-0.7%, -0.7%]	1
All ❌✅ (primary)	1.3%	[1.3%, 1.3%]	1

Binary size

Results (primary 0.0%, secondary -0.3%)

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)	0.2%	[0.0%, 0.7%]	42
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.7%, -0.1%]	17
Improvements ✅ (secondary)	-0.3%	[-0.8%, -0.1%]	6
All ❌✅ (primary)	0.0%	[-0.7%, 0.7%]	59

Bootstrap: 698.879s -> 701.919s (0.43%)
Artifact size: 328.27 MiB -> 328.37 MiB (0.03%)

[the8472]

Looks ok overall. The PR description could contain a bit of motivation, since that goes into the git history.

Also, I was not even aware of array::Drain. Once the alloc types get migrated that'll be more reasonable but right now it looks like quite a rube-goldberg machine to implement array::try_map 😅

[the8472]

This file contains a bunch of references to vec::IntoIter, with the current state that's confusing. Maybe refer to array::Drain instead?

[the8472]

Is the mix of len and clamped intentional?

[the8472]

ping @scottmcm