layout_of: T: Thin implies sizeof(&T) == sizeof(usize)

[compiler-errors]

Use the <T as Pointee>::Metadata associated type to calculate the layout of a pointee's metadata, instead of hard-coding rules about certain types.

Maybe this approach is overkill -- we could instead hard-code this approach as a fallback, with the matching on Slice/Dynamic/etc. happening first

Fixes this issue here #104338 (comment) .. But is also useful with transmutes, for example, given the UI test I added below.

[rustbot]

r? @wesleywiser

(rustbot has picked a reviewer for you, use r? to override)

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 7e7fefe1829e2a101bcf1ea405fe54ea28cf25fe with merge ab804231e1a8be74b6d8bf87c2500ec91374b892...

[bors]

☀️ Try build successful - checks-actions
Build commit: ab804231e1a8be74b6d8bf87c2500ec91374b892 (ab804231e1a8be74b6d8bf87c2500ec91374b892)

[rust-timer]

Finished benchmarking commit (ab804231e1a8be74b6d8bf87c2500ec91374b892): comparison URL.

Overall result: no relevant changes - 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-review -S-waiting-on-perf -perf-regression

Instruction count

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

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)	-	-	0
Regressions ❌ (secondary)	3.4%	[0.9%, 6.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

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

[wesleywiser]

r=me with nit

[compiler-errors]

@bors r=wesleywiser

[bors]

📌 Commit a5d39cf has been approved by wesleywiser

It is now in the queue for this repository.

[compiler-errors]

Ended up not being perf sensitive anyways, @bors rollup-

[bors]

⌛ Testing commit a5d39cf with merge 16db584a73dcbee32144101bf20483303098cb2f...

[compiler-errors]

Let's let the rollup try again

@bors retry

[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)

[bors]

⌛ Testing commit a5d39cf with merge ddad1e1...

[bors]

☀️ Test successful - checks-actions
Approved by: wesleywiser
Pushing ddad1e1 to master...

[rust-timer]

Finished benchmarking commit (ddad1e1): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

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

Cycles

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