Make ZST checks in core/alloc more readable

[scottmcm]

There's a bunch of these checks because of special handing for ZSTs in various unsafe implementations of stuff.

This lets them be T::IS_ZST instead of mem::size_of::<T>() == 0 every time, making them both more readable and more terse.

Not proposed for stabilization. Would be pub(crate) except alloc wants to use it too.

(And while it doesn't matter now, if we ever get something like #85836 making it a const can help codegen be simpler.)

[rust-highfive]

r? @thomcc

(rust-highfive has picked a reviewer for you, use r? to override)

[thomcc]

Hm, neat. I know you said it shouldn't impact codegen at all, but I'm a little curious.

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit cbbcd9f with merge 62c55795633b779a3a1d8085381e862b9104bf4d...

[scottmcm]

I know you said it shouldn't impact codegen at all

All I meant by that was that there's currently no special handling for switchInt on a constant in cg_llvm.

I agree this should have a perf run, but was waiting on the PR build because I'm too good at under-testing locally. Looks like when I fixed some of the warnings(-as-errors) about unused imports I broke the docs 🤦

[bors]

💔 Test failed - checks-actions

[scottmcm]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 96314f9cfb3a489b83e05ce70a48ef045aca6d62 with merge 20e3190ff975b91b5b541a7f5b264084b94ace56...

[bors]

☀️ Try build successful - checks-actions
Build commit: 20e3190ff975b91b5b541a7f5b264084b94ace56 (20e3190ff975b91b5b541a7f5b264084b94ace56)

[rust-timer]

Queued 20e3190ff975b91b5b541a7f5b264084b94ace56 with parent 77e7e88, future comparison URL.

[rust-timer]

Finished benchmarking commit (20e3190ff975b91b5b541a7f5b264084b94ace56): comparison URL.

Overall result: ❌ regressions - 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 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)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.0%	[-5.0%, -5.0%]	1
All ❌✅ (primary)	-	-	0

Cycles

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)	-	-	0
Regressions ❌ (secondary)	11.9%	[11.9%, 11.9%]	1
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.6%	[-2.6%, -2.6%]	1

Footnotes

1. the arithmetic mean of the percent change ↩ ↩² ↩³

2. number of relevant changes ↩ ↩² ↩³

[scottmcm]

Hmm, that's a weird build failure. RustDoc handled it fine, but now it's a linkchecker error?

Local repro time, I guess -- x.py doc library/std was clean :(

[scottmcm]

Ok, filed #102221 for the weird failure when I tried to use cfg(doc), and fixed it with just using an #[allow].

Should be good to go now 🤞

@rustbot ready

[thomcc]

Thank you.

@bors r+

[bors]

📌 Commit ed16dbf has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit ed16dbf with merge e58621a...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing e58621a to master...

[rust-timer]

Finished benchmarking commit (e58621a): 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%	[0.4%, 2.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
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)	-	-	0
Regressions ❌ (secondary)	3.2%	[2.4%, 4.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

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)	-	-	0
Regressions ❌ (secondary)	5.2%	[3.2%, 7.9%]	4
Improvements ✅ (primary)	-2.3%	[-2.7%, -2.1%]	3
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.8%]	1
All ❌✅ (primary)	-2.3%	[-2.7%, -2.1%]	3

Footnotes

1. the arithmetic mean of the percent change ↩ ↩² ↩³

2. number of relevant changes ↩ ↩² ↩³