Document Pin memory layout

[Jules-Bertholet]

The fact that Pin is #[repr(transparent)] technically isn't documented anywhere currently. I don't see any reason why Pin's layout would ever change, so this PR codifies it.

@rustbot label +T-libs-api -T-libs +A-docs +A-layout +A-pin

[rustbot]

r? @thomcc

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[thomcc]

Yeah, this is already documented effectively, and can't be changed compatibly. I'm going to use my judgement that this doesn't need discussion in a meeting.

@bors r+ rollup

[bors]

📌 Commit f16acbe has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit f16acbe with merge 48ec50a...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 48ec50a to master...

[rust-timer]

Finished benchmarking commit (48ec50a): comparison URL.

Overall result: ✅ improvements - 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.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.6%	[-0.6%, -0.6%]	1

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)	3.8%	[3.8%, 3.8%]	1
Regressions ❌ (secondary)	1.6%	[1.6%, 1.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	3.8%	[3.8%, 3.8%]	1

Cycles

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

Binary size

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

Bootstrap: 644.433s -> 643.76s (-0.10%)

[CAD97]

... Given this is a comment-only change, we might want to up the significance threshold for the cargo-0.60.0-opt-incr-patched benchmark. Though I don't know who to ping for that, and it might actually be a derived property rather than a set one if I read the docs correctly.