Generalize `{Rc,Arc}::make_mut()` to unsized types. #116113

kpreid · 2023-09-23T23:45:23Z

{Rc,Arc}::make_mut() now accept any type implementing the new unstable trait core::clone::CloneToUninit.
CloneToUninit is implemented for T: Clone and for [T] where T: Clone.
CloneToUninit is a generalization of the existing internal trait alloc::alloc::WriteCloneIntoRaw.
New feature gate: clone_to_uninit

This allows performing make_mut() on Rc<[T]> and Arc<[T]>, which was not previously possible.

Previous PR description, now obsolete:

Add {Rc, Arc}::make_mut_slice()

These functions behave identically to make_mut(), but operate on Arc<[T]> instead of Arc<T>.

This allows performing the operation on slices, which was not previously possible because make_mut() requires T: Clone (and slices, being !Sized, do not and currently cannot implement Clone).

Feature gate: make_mut_slice

try-job: test-various

rustbot · 2023-09-23T23:45:30Z

r? @thomcc

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

kpreid · 2023-09-23T23:45:46Z

@rustbot label +T-libs-api

thomcc · 2023-09-24T00:22:47Z

This is insta-stable, so it needs libs-api FCP.

r? @rust-lang/libs-api

thomcc · 2023-09-24T00:23:28Z

Rerolling because @BurntSushi doesn't do libs reviews.

r? rust-lang/libs-api

dtolnay

How about adjusting the trait bound on make_mut instead? It can be implemented the current way for T: Clone and separately for [T].

impl<T: CanMakeMut, A: Allocator + Clone> Arc<T, A> {
    pub fn make_mut(this: &mut Self) -> &mut T { ... }
}

impl<T: Clone> CanMakeMut for T { ... }

impl<T: Clone> CanMakeMut for [T] { ... }

kpreid · 2023-09-24T00:46:57Z

This is insta-stable, so it needs libs-api FCP.

Is it? The current commit only adds unstable inherent methods. There's no trait implementation involved.

How about adjusting the trait bound on make_mut instead?

I like that idea much better since it doesn't add a new name. (Though it would be insta-stable.) I'm not familiar with how introducing a trait like CanMakeMut should work in std; should the trait be (perma?-)unstable or public-in-private sealed?

Actually, the trait could be of more broad use — it seems like it could be a general "clone ?Sized into uninit", which suggests it should be an unstable trait not tied to Rc/Arc in particular. So maybe it becomes core::mem::CloneUnsized or something like that? Is there any existing work on unsized rvalues that has such a trait?

thomcc · 2023-09-24T00:56:23Z

Oh, it's totally not insta-stable, my bad (sorry, been a draining couple of weeks for me).

Still, I'll leave the review to the assignee at this point.

dtolnay · 2023-09-24T00:57:41Z

The core::clone module has been looking pretty bare for a decade. 🥳

I would definitely be on board with a trait that allows cloning into a pre-allocated &mut MaybeUninit<Self>. I think it would need to be unsafe trait, right? The calling code needs to be able to do assume_init afterwards without the possibility that the trait impl didn't actually initialize the thing.

I am not familiar with the Rc and Arc code but if such a trait existed, would it be straightforward for their make_mut to allocate a value of size header + mem::size_of_val(&**self), cast the spot for the value to &mut MaybeUninit<T>, and call the CloneUnsized to get the value cloned into there?

library/alloc/src/sync.rs

kpreid · 2023-09-24T16:05:54Z

Observation: The new CloneToUninit trait turns out to be identical in signature to the internal trait alloc::alloc::WriteCloneIntoRaw, except for the ?Sizedness. That's a good sign, I think — I'm refining something that already proved useful rather than making up an ad-hoc solution.

kpreid · 2023-09-24T17:41:08Z

Observation: It's not possible to define these operations in terms of MaybeUninit<T> (as Rc currently does inside maek_mut(), and a previous comment suggested), because MaybeUninit<T> requires T: Sized, so you cannot have a MaybeUninit<T> where T might be [U]. I wonder if changing that would help all maybe-unsized code — but presumably it's difficult, and certainly out of scope for this PR.

kpreid · 2023-09-25T00:57:52Z

It was quite an adventure, but I believe I've got it all set now. I had to introduce additional helper types at both the CloneToUninit layer and the make_mut() layer in order to avoid leaks on panic.

Note that make_mut()'s enlarged receiver type bounds are insta-stable. Additionally, you can tell that CloneToUninit is specialized in the documentation, but I assume that's fine because it doesn't affect which programs will compile.

kpreid · 2023-09-25T01:03:40Z

Updated PR description with the new design based on CloneToUninit.

dtolnay · 2024-06-22T16:14:25Z

@bors r+

bors · 2024-06-22T16:14:28Z

📌 Commit 88c3db5 has been approved by dtolnay

It is now in the queue for this repository.

bors · 2024-06-22T16:35:32Z

⌛ Testing commit 88c3db5 with merge f944afe...

bors · 2024-06-22T18:47:03Z

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing f944afe to master...

rust-timer · 2024-06-22T20:02:14Z

Finished benchmarking commit (f944afe): 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.

	mean	range	count
Regressions ❌ (primary)	0.8%	[0.3%, 2.2%]	13
Regressions ❌ (secondary)	0.9%	[0.9%, 0.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.8%	[0.3%, 2.2%]	13

Max RSS (memory usage)

Results (primary -5.7%, secondary -2.2%)

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)	-	-	0
Improvements ✅ (primary)	-5.7%	[-5.7%, -5.7%]	1
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-5.7%	[-5.7%, -5.7%]	1

Cycles

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

Binary size

Results (primary 0.1%, secondary 0.3%)

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)	0.2%	[0.1%, 0.3%]	8
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.0%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.1%, 0.3%]	15

Bootstrap: 694.577s -> 694.429s (-0.02%)
Artifact size: 327.03 MiB -> 326.84 MiB (-0.06%)

Mark-Simulacrum · 2024-06-23T22:08:09Z

Regressions are mostly in doc benchmarks, seem likely to be just new docs due to extra stuff in the standard library.

rustbot assigned thomcc Sep 23, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Sep 23, 2023

rustbot added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Sep 23, 2023

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rustbot assigned BurntSushi and unassigned thomcc Sep 24, 2023

rustbot assigned dtolnay and unassigned BurntSushi Sep 24, 2023

dtolnay removed the T-libs Relevant to the library team, which will review and decide on the PR/issue. label Sep 24, 2023

dtolnay requested changes Sep 24, 2023

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dtolnay reviewed Sep 24, 2023

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library/alloc/src/sync.rs Outdated Show resolved Hide resolved

kpreid force-pushed the arcmut branch from b621d08 to 12c3efe Compare September 25, 2023 00:27

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kpreid force-pushed the arcmut branch from 12c3efe to 8ebd675 Compare September 25, 2023 00:56

kpreid changed the title ~~Add {Rc, Arc}::make_mut_slice().~~ Generalize {Rc,Arc}::make_mut() to unsized types. Sep 25, 2023

kpreid requested a review from dtolnay September 25, 2023 01:03

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kpreid force-pushed the arcmut branch from 8ebd675 to 8923cab Compare September 25, 2023 01:44

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 22, 2024

bors added the merged-by-bors This PR was explicitly merged by bors. label Jun 22, 2024

bors merged commit f944afe into rust-lang:master Jun 22, 2024

rustbot added this to the 1.81.0 milestone Jun 22, 2024

This was referenced Jun 22, 2024

Replace uses of MaybeUninit::uninit_array() with inline const blocks. #125082

Merged

speed up String::push and String::insert #124810

Merged

Add MAX_LEN_UTF8 and MAX_LEN_UTF16 Constants #120580

Merged

rustbot added the perf-regression Performance regression. label Jun 22, 2024

Enselic mentioned this pull request Jun 23, 2024

Bless nightly-2024-06-23 output cargo-public-api/cargo-public-api#585

Merged

Mark-Simulacrum added the perf-regression-triaged The performance regression has been triaged. label Jun 23, 2024

kpreid deleted the arcmut branch June 24, 2024 17:39

apiraino removed the to-announce Announce this issue on triage meeting label Jul 4, 2024

This was referenced Nov 13, 2024

Make CloneToUninit dyn-compatible #133003

Merged

Box::new_from_ref for making a Box<T> from a &T where T: CloneToUninit + ?Sized (and Rc and Arc) rust-lang/libs-team#483

Open

Generalize {Rc,Arc}::make_mut() to unsized types. #116113

Generalize {Rc,Arc}::make_mut() to unsized types. #116113

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rust-timer commented Jun 22, 2024

Overall result: ❌ regressions - ACTION NEEDED

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Generalize `{Rc,Arc}::make_mut()` to unsized types. #116113

Generalize `{Rc,Arc}::make_mut()` to unsized types. #116113

kpreid commented Sep 23, 2023 •

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