Make CloneToUninit dyn-compatible

[zachs18]

Make CloneToUninit dyn-compatible, by making clone_to_uninit's dst parameter *mut u8 instead of *mut Self, so the method does not reference Self except in the self parameter and is thus dispatchable from a trait object.

This allows, among other things, adding CloneToUninit as a supertrait bound for trait Foo to allow cloning dyn Foo in some containers. Currently, this means that Rc::make_mut and Arc::make_mut can work with dyn Foo where trait Foo: CloneToUninit.

Example

#![feature(clone_to_uninit)]
use std::clone::CloneToUninit;
use std::rc::Rc;
use std::fmt::Debug;
use std::borrow::BorrowMut;

trait Foo: BorrowMut<u32> + CloneToUninit + Debug {}

impl<T: BorrowMut<u32> + CloneToUninit + Debug> Foo for T {}

fn main() {
    let foo: Rc<dyn Foo> = Rc::new(42_u32);
    let mut bar = foo.clone();
    *Rc::make_mut(&mut bar).borrow_mut() = 37;
    dbg!(foo, bar); // 42, 37
}

Eventually, Box::<T>::clone is planned to be converted to use T::clone_to_uninit, which when combined with this change, will allow cloning Box<dyn Foo> where trait Foo: CloneToUninit without any additional unsafe code for the author of trait Foo.¹

This PR should have no stable side-effects, as CloneToUninit is unstable so cannot be mentioned on stable, and CloneToUninit is not used as a supertrait anywhere in the stdlib.

This change removes some length checks that could only fail if library UB was already hit (e.g. calling <[T]>::clone_to_uninit with a too-small-length dst is library UB and was previously detected²; since dst does not have a length anymore, this now cannot be detected³).

r? libs-api

---

I chose to make the parameter *mut u8 instead of *mut () because that might make it simpler to pass the result of alloc to clone_to_uninit, but *mut () would also make sense, and any *mut ConcreteType would work. The original motivation for using specifically *mut () appears to be std::ptr::from_raw_parts_mut, but that now takes *mut impl Thin instead of *mut (). I have another branch where the parameter is *mut (), if that is preferred.

It could also take something like &mut [MaybeUninit<u8>] to be dyn-compatible but still allow size-checking and in some cases safe writing, but this is already an unsafe API where misuse is UB, so I'm not sure how many guardrails it's worth adding here, and &mut [MaybeUninit<u8>] might be overly cumbersome to construct for callers compared to *mut u8

Footnotes

1. Note that impl<T: CloneToUninit + ?Sized> Clone for Box must be added before or at the same time as when CloneToUninit becomes stable, due to Box being #[fundamental], as if there is any stable gap between the stabilization of CloneToUninit and impl<T: CloneToUninit + ?Sized> Clone for Box, then users could implement both CloneToUninit for dyn LocalTrait and separately Clone for Box<dyn LocalTrait> during that gap, and be broken by the introduction of impl<T: CloneToUninit + ?Sized> Clone for Box. ↩

2. Using a debug_assert_eq in core::clone::uninit::CopySpec::clone_slice. ↩

3. This PR just uses the metadata (length) from self to construct the *mut [T] to pass to CopySpec::clone_slice in <[T]>::clone_to_uninit. ↩

[dtolnay]

Thanks!

The discussion of *mut u8 vs *mut () vs *mut ConcreteType makes me wonder whether there is design space for an untyped pointer primitive type in Rust, like what LLVM has recently adopted. https://llvm.org/docs/OpaquePointers.html

[dtolnay]

@bors r+

[bors]

📌 Commit e0c1c8b has been approved by dtolnay

It is now in the queue for this repository.

[dtolnay]

@bors r- delegate+

[bors]

✌️ @zachs18, you can now approve this pull request!

If @dtolnay told you to "r=me" after making some further change, please make that change, then do @bors r=@dtolnay

[dtolnay]

@bors r+

[bors]

📌 Commit 6166b0c has been approved by dtolnay

It is now in the queue for this repository.

[bors]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: tweak attributes for const panic macro #132662

[bors]

📌 Commit 6166b0c has been approved by dtolnay

It is now in the queue for this repository.