Tracking Issue for `PinCoerceUnsized`

[Darksonn]

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 Pin is unsound due to transitive effects of CoerceUnsized #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.
  • Make PinCoerceUnsized require Deref #149218 (comment)
  • Make PinCoerceUnsized require Deref #149218 (comment)

Implementation history

• PinCoerceUnsized trait into core #125048
• Remove impl PinCoerceUnsized for Pin #144896
• Make PinCoerceUnsized require Deref #149218
• Update tracking issue for PinCoerceUnsized #150113

[theemathas]

Having Deref not be a supertrait of PinCoerceUnsized potentially complicates the soundness reasoning in #152667 (comment).

In particular, I'm unsure what would happen if someone were to impl PinCoereUnsized for Foo, impl Deref for Pin<Foo>, create a Pin<Foo> via an unsize coercion, implement Clone on Pin<Foo> to return that created value, and clone a Pin<Pin<Foo>> to effectively cause std to call Pin::new_unchecked on something that isn't supposed to be pinned.