Tracking Issue for `PinCoerceUnsized`

Feature gate: #![feature(pin_coerce_unsized_trait)]

This is a tracking issue for PinCoerceUnsized, which is an unsafe trait that determines whether unsizing coercions are possible when the given type is pinned.

Public API

/// # Safety
/// The impl of `Deref`/`DerefMut` for this type is not malicious.
pub unsafe trait PinCoerceUnsized: Deref {}

impl<Ptr, U> CoerceUnsized<Pin<U>> for Pin<Ptr>
where
    Ptr: CoerceUnsized<U> + PinCoerceUnsized,
    U: PinCoerceUnsized,
{}

impl<T: ?Sized> PinCoerceUnsized for &'_ T {}
impl<T: ?Sized> PinCoerceUnsized for &'_ mut T {}
impl<T: ?Sized> PinCoerceUnsized for Box<T> {}
impl<T: ?Sized> PinCoerceUnsized for Rc<T> {}
impl<T: ?Sized> PinCoerceUnsized for Arc<T> {}
// and so on for all smart pointers

Steps

• Implementation
• Stabilize CoerceUnsized first
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Decide whether PinCoerceUnsized is the correct solution to https://github.com/rust-lang/rust/issues/68015
• Decide whether Deref should be a supertrait of PinCoerceUnsized (it closes doors in terms of extending Pin<T> to non-Deref Ts) or whether we should, e.g., use a sealed supertrait with a blanket impl or consider other options.
  • https://github.com/rust-lang/rust/pull/149218#issuecomment-3721507988
  • https://github.com/rust-lang/rust/pull/149218#issuecomment-3720554259

Implementation history

• https://github.com/rust-lang/rust/pull/125048
• https://github.com/rust-lang/rust/pull/144896
• https://github.com/rust-lang/rust/pull/149218
• https://github.com/rust-lang/rust/pull/150113

Start by reviewing the PinCoerceUnsized feature gate, the linked implementation PRs, and the stabilization guide, with CoerceUnsized stabilization as a prerequisite. Done means resolving the listed design questions and completing a stabilization PR for the feature.