Add default trait implementations for "c-unwind" ABI function pointers

[lopopolo]

Following up on #92964, only add default trait implementations for the c-unwind family of function pointers. The previous attempt in #92964 added trait implementations for many more ABIs and ran into concerns regarding the increase in size of the libcore rlib.

An attempt to abstract away function pointer types behind a unified trait to reduce the duplication of trait impls is being discussed in #99531 but this change looks to be blocked on a lang MCP.

Following @RalfJung's suggestion in #99531 (comment), this commit is another cut at #92964 but it only adds the impls for extern "C-unwind" fn and unsafe extern "C-unwind" fn.

I am interested in landing this patch to unblock the stabilization of the c_unwind feature.

RFC: rust-lang/rfcs#2945
Tracking Issue: #74990

[rust-highfive]

r? @m-ou-se

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

[lopopolo]

it looks like I need to bless changes to the UI tests. Let me try and figure out how to do that 😄

Edit: done!

[RalfJung]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit 2f7cc2d574b203e1f19b63ee705fe217fefc31ee with merge c97600b8b2ec5ef84e1426b75f8be57bf5450712...

[bors]

☀️ Try build successful - checks-actions
Build commit: c97600b8b2ec5ef84e1426b75f8be57bf5450712 (c97600b8b2ec5ef84e1426b75f8be57bf5450712)

[rust-timer]

Queued c97600b8b2ec5ef84e1426b75f8be57bf5450712 with parent aa857eb, future comparison URL.

[rust-timer]

Finished benchmarking commit (c97600b8b2ec5ef84e1426b75f8be57bf5450712): comparison URL.

Overall result: ❌✅ regressions and improvements - 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.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@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)	1.0%	[0.3%, 2.6%]	14
Regressions ❌ (secondary)	1.9%	[0.6%, 2.7%]	22
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.1%	[-1.4%, -0.8%]	6
All ❌✅ (primary)	1.0%	[0.3%, 2.6%]	14

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)	2.9%	[1.5%, 4.3%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.4%	[1.4%, 1.4%]	1

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)	2.3%	[2.0%, 2.8%]	3
Regressions ❌ (secondary)	2.8%	[2.2%, 4.5%]	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[2.0%, 2.8%]	3

Footnotes

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

2. number of relevant changes ↩ ↩² ↩³

[RalfJung]

Yeah, as expected that extra metadata in the rlib makes everything measurably slower. Not sure if there is a way that we can avoid some of these instances (do we really need all the traits here), but in the end it boils down to judging whether that's a cost we are willing to accept for the benefit of #74990.

Sounds like a @rust-lang/compiler question to me.

[pnkfelix]

Discussed in T-compiler triage meeting.

I think we should move forward with this, despite the performance hit. I don't want to block c-unwind based on the performance regression observed above.

(If it turns out that the performance impact matters, then we can adopt other approaches, such as the one suggested in #99531; though I'm hoping that will not be necessary solely for this issue.)

@rustbot label: -I-compiler-nominated

[pnkfelix]

(also, as a side note: the primary benchmarks only regressed on "doc full" scenarios. Normal compilation (check/debug/opt) all came out looking great I would say.)

@rustbot label: +perf-regression-triaged

[lopopolo]

@RalfJung Given the above triage on the perf regression from the compiler team, what are the next steps for getting this approved and merged?

[RalfJung]

It needs someone from t-libs to review and approve.
r? libs

[thomcc]

Impl looks fine, I think this needs libs-api signoff, or perhaps an ACP.

[lopopolo]

I couldn't figure out the right combination of --stage and --keep-stage flags to pass to x.py to check this locally, but I did rebase to include #103239 and reworked the macro from my proposed patch above. Let's see if CI passes.

[lopopolo]

Looks like the build still fails with an #[unstable] annotation here has no effect error. @m-ou-se do I need to wait for a new bootstrap compiler to be released?

[m-ou-se]

Looks like the build still fails with an #[unstable] annotation here has no effect error. @m-ou-se do I need to wait for a new bootstrap compiler to be released?

Oh. :( Yeah. I suppose you could apply #[cfg(not(bootstrap))] to line 1868. These trait impls aren't used in the compiler, meaning it's fine if they're missing from the first stage std.

[lopopolo]

@thomcc I've made the stability annotation changes. This is ready for review.

@rustbot ready

[thomcc]

LGTM, thanks.

@bors r+

[bors]

📌 Commit efe61da has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit efe61da with merge 5c8bff7...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 5c8bff7 to master...

[rust-timer]

Finished benchmarking commit (5c8bff7): comparison URL.

Overall result: ❌ regressions - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	0.9%	[0.2%, 2.6%]	19
Regressions ❌ (secondary)	1.9%	[0.4%, 3.0%]	23
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.9%	[0.2%, 2.6%]	19

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)	2.8%	[1.5%, 4.9%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.4%	[-1.4%, -1.4%]	1
All ❌✅ (primary)	1.4%	[1.4%, 1.4%]	1

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)	2.5%	[2.1%, 3.0%]	2
Regressions ❌ (secondary)	2.7%	[2.1%, 3.1%]	13
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[2.1%, 3.0%]	2

Footnotes

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

2. number of relevant changes ↩ ↩² ↩³

[nnethercote]

It's all doc perf regressions, and was decided earlier to be acceptable.

@rustbot label: +perf-regression-triaged