Generalize `impl<T> Clone for Box<T>` to unsized types by a1phyr · Pull Request #146381 · rust-lang/rust

a1phyr · 2025-09-09T18:19:59Z

I had to add a non-default method to CloneToUninit for this to work, but I think this is fine. (I did not do the renaming to CloneUnsized as suggested in #126799 (comment), as this is quite orthogonal)

This also adds some infrastructure that will be helpful for in-place initialization.

Commits can be reviewed independently.

rustbot · 2025-09-09T18:20:05Z

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workingjubilee · 2025-09-09T19:42:11Z

r? library

rustbot · 2025-09-09T19:42:14Z

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workingjubilee · 2025-09-09T19:42:28Z

r? libs

zachs18 · 2025-09-10T01:56:10Z

+    ///
+    /// # Safety
+    ///
+    /// `dest` must be a valid pointer to a valid value of the same concrete type than `self`.


Given that trait vtable pointers can be deduplicated, I'm not sure if "the same concrete type" is what we want to require here. cc #146381 (comment) . I.e. it's not possible in general to verify for certain that two dyn Trait pointers with the same metadata actually point to the same erased/concrete type.

This seems similar to the issue with Rc::get_mut_unchecked, where either:

if any other pointers exist to this value, we must only write a value that is valid for all those pointers' types/metadatas.

if there are no other pointers to this value, we can write anything that is valid for this pointer's type/metadata.

I think this is the right thing. It be not be possible in general, but it is in some cases (eg with TypeId). Another formulation would be that it is safe to create a mutable reference from dest.with_metadata_of(self).

Hm, I guess unlike clone_to_uninit we are not placing a precondition that the destination is big enough here? ("Concrete type" is not enough for that, [T] can be different lengths while being the same type).

I think it's worth elaborating in the doc comment on how this differs from clone_to_uninit and when one or the other is safe to use. Naively, I would expect that this could be implemented as:

ptr::drop_in_place(dest.cast::<Self>().with_metadata_of(self)); self.clone_to_uninit(dest);

But based on the implementations and the safety comment here, it doesn't seem like that's true. That should be clearly highlighted IMO.

jhpratt · 2025-09-12T23:02:12Z

As CI is failing, @rustbot author

rustbot · 2025-09-12T23:02:16Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

@khyperia

… r=khyperia enable `do_not_recommend` attr for method call errors in current solver This should help with rust-lang#146381 (comment) The logic is mostly copied from the `apply_do_not_recommend` function. r? @khyperia :3

@khyperia

… r=khyperia enable `do_not_recommend` attr for method call errors in current solver This should help with rust-lang#146381 (comment) The logic is mostly copied from the `apply_do_not_recommend` function. r? @khyperia :3

@khyperia

Rollup merge of #158882 - mejrs:do_not_recommend_old_solver, r=khyperia enable `do_not_recommend` attr for method call errors in current solver This should help with #146381 (comment) The logic is mostly copied from the `apply_do_not_recommend` function. r? @khyperia :3

mejrs

@a1phyr #158882 has merged so if you rebase and apply do_not_recommend you can fix the regression pointed out at #146381 (comment)

Can you also update the unique-object-noncopyable and unique-pinned-nocopy tests with next solver revisions similar to tests/ui/diagnostic_namespace/do_not_recommend/method_call.rs?

View changes since this review

This new module contains a type that is like an uninitialized `Box`, but works with any type, not just `Sized` ones. Additionally, this will be useful for future work on in-place initialization (see rust-lang#142518).

This is necessary to properly support `Clone::clone_from`.

rustbot · 2026-07-28T20:44:38Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

a1phyr · 2026-07-28T20:45:38Z

@rustbot ready

Mark-Simulacrum

I think it would be worth engaging libs-api to get their read on whether we want this in general before we continue implementing work, as the internals are a bit hairy and I'm not sure this comes up often enough to justify it.

@rustbot label +I-libs-api-nominated

View changes since this review

Mark-Simulacrum · 2026-08-02T23:10:09Z

+        // SAFETY: guaranteed by caller
+        let layout = unsafe { Layout::for_value_raw(ptr) };
+
+        let ptr = if layout.size() == 0 {


We allow zero-sized allocations in Allocator, so I think this can be removed?

Mark-Simulacrum · 2026-08-02T23:12:08Z

+            // if the concrete types are different, so we never allow them to
+            // go through `clone_from`.
+            #[inline]
+            default fn meta_eq(self, _: Self) -> bool {


This seems still unresolved.

Mark-Simulacrum · 2026-08-02T23:17:04Z

+    /// # Safety
+    ///
+    /// `dest` must be a valid pointer to a valid value of the same concrete type than `self`.
+    unsafe fn clone_to_init(&self, dest: *mut u8);


It seems pretty strange to have clone_to_init on CloneToUninit trait. I guess this is more of a trait DynClone at this point...

Mark-Simulacrum · 2026-08-02T23:23:14Z

+    ///
+    /// # Safety
+    ///
+    /// `dest` must be a valid pointer to a valid value of the same concrete type than `self`.


Hm, I guess unlike clone_to_uninit we are not placing a precondition that the destination is big enough here? ("Concrete type" is not enough for that, [T] can be different lengths while being the same type).

I think it's worth elaborating in the doc comment on how this differs from clone_to_uninit and when one or the other is safe to use. Naively, I would expect that this could be implemented as:

ptr::drop_in_place(dest.cast::<Self>().with_metadata_of(self)); self.clone_to_uninit(dest);

But based on the implementations and the safety comment here, it doesn't seem like that's true. That should be clearly highlighted IMO.

Mark-Simulacrum · 2026-08-02T23:24:04Z

+    #[cfg_attr(debug_assertions, track_caller)]
+    unsafe fn clone_to_init(&self, dest: *mut u8) {
+        // SAFETY: by contract, `dest` is a valid pointer to a `[T]` with the
+        // same length as `self`


I don't think this is true from the contract above (just leaving a note as this is a concrete example of divergence).

Mark-Simulacrum · 2026-08-04T15:47:38Z

libs-api discussed and agreed this is generally the right direction to go in, so removing the nomination. Still some review comments left to iterate through of course, and we'll re-nominate if this ends up having any stable-facing implications.

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zachs18 reviewed Sep 10, 2025

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Comment thread library/alloc/src/boxed.rs Outdated

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Comment thread library/alloc/src/boxed/uninit.rs

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 12, 2025

a1phyr force-pushed the box_clone_more branch from f32cd66 to bdc70ea Compare September 17, 2025 14:15

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a1phyr force-pushed the box_clone_more branch from 9cc7f3f to 84c014a Compare September 25, 2025 23:30

ojeda mentioned this pull request Jul 4, 2026

Rust wanted features Rust-for-Linux/linux#354

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mejrs mentioned this pull request Jul 6, 2026

enable do_not_recommend attr for method call errors in current solver #158882

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Comment thread library/alloc/src/boxed.rs

a1phyr added 3 commits July 28, 2026 22:40

Add alloc::boxed::uninit

8324512

This new module contains a type that is like an uninitialized `Box`, but works with any type, not just `Sized` ones. Additionally, this will be useful for future work on in-place initialization (see rust-lang#142518).

Add CloneToUninit::clone_to_init

0fc1fec

This is necessary to properly support `Clone::clone_from`.

Generalize Box::clone

db7715a

a1phyr force-pushed the box_clone_more branch from e4bbe17 to 72b8534 Compare July 28, 2026 20:44

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 28, 2026

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a1phyr force-pushed the box_clone_more branch from 72b8534 to a7fc515 Compare July 30, 2026 22:04

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a1phyr added 3 commits August 1, 2026 14:26

Update ui tests

a58c975

Update CloneToUninit example to use Box

193c97c

Add regression tests for Box clone impls

f9a25ed

a1phyr force-pushed the box_clone_more branch from a7fc515 to f9a25ed Compare August 1, 2026 12:29

Mark-Simulacrum requested changes Aug 2, 2026

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rustbot added I-libs-api-nominated [DEPRECATED; DO NOT USE] S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 2, 2026

dtolnay mentioned this pull request Aug 4, 2026

Tracking Issue for core::clone::CloneToUninit trait #126799

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Mark-Simulacrum removed the I-libs-api-nominated [DEPRECATED; DO NOT USE] label Aug 4, 2026

clarfonthey added T-libs Relevant to the library team, which will review and decide on the PR/issue. and removed T-libs-api [DEPRECATED; DO NOT USE] labels Aug 12, 2026

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