Use PtrMetadata instead of Len in slice drop shims

[scottmcm]

I tried to do a bigger change in #134297 which didn't work, so here's the part I really wanted: Removing another use of Len, in favour of PtrMetadata.

Split into two commits where the first just adds a test, so you can look at the second commit to see how the drop shim for an array changes with this PR.

Reusing the same reviewer from the last one:
r? BoxyUwU

[lqd]

Let's see if the errors from the previous PR reproduce

[bors]

⌛ Trying commit 8acccdc with merge 214902d...

[bors]

☀️ Try build successful - checks-actions
Build commit: 214902d (214902d83af34ccbac352703f3d75c3a7f4daaa1)

[BoxyUwU]

Seems reasonable to me but I am not super familiar with nuances of MIR so I will assign someone else for a final review

r? @saethlin

[saethlin]

@bors r+ rollup=iffy
I can't tell at a glance if this is perf-sensitive but I think it's more likely that it is not.

[bors]

📌 Commit 8acccdc has been approved by saethlin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 8acccdc with merge d0d464c...

[bors]

💔 Test failed - checks-actions

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[tgross35]

Looks like dist-apple-various just got stuck?

@bors retry

[bors]

⌛ Testing commit 8acccdc with merge b22856d...

[bors]

☀️ Test successful - checks-actions
Approved by: saethlin
Pushing b22856d to master...

[rust-timer]

Finished benchmarking commit (b22856d): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.1%, secondary 3.2%)

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)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.1%	[2.1%, 2.1%]	1

Cycles

Results (secondary -3.7%)

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -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)	0.1%	[0.1%, 0.1%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.3%, -0.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.3%, 0.1%]	4

Bootstrap: 761.148s -> 762.237s (0.14%)
Artifact size: 330.61 MiB -> 330.65 MiB (0.01%)