Use ptr::metadata in <[T]>::len implementation

[KamilaBorowska]

This avoids duplication of ptr::metadata code.

I believe this is acceptable as the previous approach essentially duplicated ptr::metadata because back then rustc_allow_const_fn_unstable annotation did not exist.

I would like somebody to ping @rust-lang/wg-const-eval as the documentation says:

Always ping @rust-lang/wg-const-eval if you are adding more rustc_allow_const_fn_unstable attributes to any const fn.

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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[rust-highfive]

r? @m-ou-se

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[thomcc]

This is a nice cleanup. I suppose it could impact compile-perf (len() is used a lot...), so I'll mark it as iffy with regards to rollup.

@bors r+ rollup=iffy

[bors]

📌 Commit 155b4c2 has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit 155b4c2 with merge 56f1325...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 56f1325 to master...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 56f1325 to master...

[rust-timer]

Finished benchmarking commit (56f1325): 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.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.0%	[0.7%, 1.3%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	1
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.

	mean1	range	count2
Regressions ❌ (primary)	1.4%	[1.4%, 1.4%]	1
Regressions ❌ (secondary)	4.6%	[4.6%, 4.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.5%	[-4.9%, -4.2%]	2
All ❌✅ (primary)	1.4%	[1.4%, 1.4%]	1

Cycles

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

Footnotes

1. the arithmetic mean of the percent change ↩ ↩²

2. number of relevant changes ↩ ↩²