stabilize const_swap

[RalfJung]

libs-api FCP passed in #83163.

However, I only just realized that this actually involves an intrinsic. The intrinsic could be implemented entirely with existing stable const functionality, but we choose to make it a primitive to be able to detect more UB. So nominating for @rust-lang/lang to make sure they are aware; I leave it up to them whether they want to FCP this.

While at it I also renamed the intrinsic to make the "nonoverlapping" constraint more clear.

Fixes #83163

[rustbot]

r? @scottmcm

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[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @rust-lang/wg-const-eval

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @rust-lang/wg-const-eval

[RalfJung]

I think this comment is outdated ever since we changed how statics get interned and when exactly validation runs... but if there's still a risk of an ICE here I am sure matthiaskrgr will find it in no time and then we'll have a testcase. ;)

[scottmcm]

I have zero concerns with stabilizing this intrinsic. I added it in #122582 with no specific intention to check things better in const, just a want to be able to detect layouts in a way that doesn't work in libcore source code. If I were remaking that PR today it might just have been #[miri::intrinsic_fallback_is_spec], but AFAICT that didn't exist at the time.

So r=me once CI finishes, unless you really want broader lang approval on it.

[RalfJung]

If I were remaking that PR today it might just have been #[miri::intrinsic_fallback_is_spec],

I would object to that, given that the intrinsic docs clearly say that it is a typed swap, and the fallback implementation uses an untyped swap.

[RalfJung]

@bors r=scottmcm

[bors]

📌 Commit 3c0c138 has been approved by scottmcm

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3c0c138 with merge 4e5fec2...

[bors]

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing 4e5fec2 to master...

[scottmcm]

I would object to that, given that the intrinsic docs clearly say that it is a typed swap

I agree -- would have needed to be a different phrasing to make that reasonable, unsure exactly how.

[rust-timer]

Finished benchmarking commit (4e5fec2): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -4.4%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.4%	[-4.4%, -4.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 3.2%)

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)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 758.809s -> 761.022s (0.29%)
Artifact size: 325.55 MiB -> 325.49 MiB (-0.02%)