StableDeref trait into core

compiler/rustc_span/src/symbol.rs

@@ -265,6 +265,7 @@ symbols! {
         Path,
         PathBuf,
         Pending,
+        PinCoerceUnsized,
         Pointer,
         Poll,
         ProcMacro,

library/alloc/src/boxed.rs

@@ -163,7 +163,7 @@ use core::ops::{AsyncFn, AsyncFnMut, AsyncFnOnce};
 use core::ops::{
     CoerceUnsized, Coroutine, CoroutineState, Deref, DerefMut, DerefPure, DispatchFromDyn, Receiver,
 };
-use core::pin::Pin;
+use core::pin::{Pin, PinCoerceUnsized};
 use core::ptr::{self, addr_of_mut, NonNull, Unique};
 use core::task::{Context, Poll};
 
@@ -2505,3 +2505,6 @@ impl<T: core::error::Error> core::error::Error for Box<T> {
         core::error::Error::provide(&**self, request);
     }
 }
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Box<T, A> {}

library/alloc/src/lib.rs

@@ -139,6 +139,7 @@
 #![feature(maybe_uninit_uninit_array_transpose)]
 #![feature(panic_internals)]
 #![feature(pattern)]
+#![feature(pin_coerce_unsized_trait)]
 #![feature(ptr_internals)]
 #![feature(ptr_metadata)]
 #![feature(ptr_sub_ptr)]

library/alloc/src/rc.rs

@@ -264,6 +264,7 @@ use core::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, Rece
 use core::panic::{RefUnwindSafe, UnwindSafe};
 #[cfg(not(no_global_oom_handling))]
 use core::pin::Pin;
+use core::pin::PinCoerceUnsized;
 use core::ptr::{self, drop_in_place, NonNull};
 #[cfg(not(no_global_oom_handling))]
 use core::slice::from_raw_parts_mut;
@@ -2127,6 +2128,9 @@ impl<T: ?Sized, A: Allocator> Deref for Rc<T, A> {
     }
 }
 
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Rc<T, A> {}
+
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
 unsafe impl<T: ?Sized, A: Allocator> DerefPure for Rc<T, A> {}
 
@@ -3572,6 +3576,9 @@ impl<T> Deref for UniqueRc<T> {
     }
 }
 
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T> PinCoerceUnsized for UniqueRc<T> {}
+
 #[unstable(feature = "unique_rc_arc", issue = "112566")]
 impl<T> DerefMut for UniqueRc<T> {
     fn deref_mut(&mut self) -> &mut T {

library/alloc/src/sync.rs

@@ -23,7 +23,7 @@ use core::mem::size_of_val;
 use core::mem::{self, align_of_val_raw};
 use core::ops::{CoerceUnsized, Deref, DerefPure, DispatchFromDyn, Receiver};
 use core::panic::{RefUnwindSafe, UnwindSafe};
-use core::pin::Pin;
+use core::pin::{Pin, PinCoerceUnsized};
 use core::ptr::{self, NonNull};
 #[cfg(not(no_global_oom_handling))]
 use core::slice::from_raw_parts_mut;
@@ -2115,6 +2115,9 @@ impl<T: ?Sized, A: Allocator> Deref for Arc<T, A> {
     }
 }
 
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Arc<T, A> {}
+
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
 unsafe impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A> {}
 

library/core/src/pin.rs

@@ -1717,10 +1717,50 @@ impl<Ptr: fmt::Pointer> fmt::Pointer for Pin<Ptr> {
 // for other reasons, though, so we just need to take care not to allow such
 // impls to land in std.
 #[stable(feature = "pin", since = "1.33.0")]
-impl<Ptr, U> CoerceUnsized<Pin<U>> for Pin<Ptr> where Ptr: CoerceUnsized<U> {}
+impl<Ptr, U> CoerceUnsized<Pin<U>> for Pin<Ptr>
+where
+    Ptr: CoerceUnsized<U> + PinCoerceUnsized,
+    U: PinCoerceUnsized,
+{
+}
+
+#[stable(feature = "pin", since = "1.33.0")]
+impl<Ptr, U> DispatchFromDyn<Pin<U>> for Pin<Ptr>
+where
+    Ptr: DispatchFromDyn<U> + PinCoerceUnsized,
+    U: PinCoerceUnsized,
+{
+}
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+/// # Safety
+///
+/// Implementing this unsafe traits requires the guarantee that any two calls
+/// to `Deref::deref` must return the same value at the same address, **even after moves
+/// or unsize-coercions of `self`**, with exceptions of mutations to `self`.
+///
+/// Here, "same value" means that if `deref` returns a trait object, then the actual
+/// concrete type behind that trait object must not change.
+/// Additionally, when you unsize- coerce from `Self` to `Unsized`,
+/// then if you call `deref` on `Unsized` which returns a trait object reference,
+/// the underlying type of that trait object must be `<Self as Deref>::Target`.
+///
+/// Analogous requirements apply to other unsized types. E.g., if `deref` returns
+/// `[T]`, then the length must not change. In other words, the underlying type
+/// must not change from `[T; N]` to `[T; M]` with an `N` different from `M`.
+///
+/// If this type alos implements `DerefMut`, then the same guarantee must be upheld by
+/// calls to `deref_mut`.
+pub unsafe trait PinCoerceUnsized: Deref {}
+
+#[stable(feature = "pin", since = "1.33.0")]
+unsafe impl<'a, T: ?Sized> PinCoerceUnsized for &'a T {}
+
+#[stable(feature = "pin", since = "1.33.0")]
+unsafe impl<'a, T: ?Sized> PinCoerceUnsized for &'a mut T {}
 
 #[stable(feature = "pin", since = "1.33.0")]
-impl<Ptr, U> DispatchFromDyn<Pin<U>> for Pin<Ptr> where Ptr: DispatchFromDyn<U> {}
+unsafe impl<T: PinCoerceUnsized> PinCoerceUnsized for Pin<T> {}
 
 /// Constructs a <code>[Pin]<[&mut] T></code>, by pinning a `value: T` locally.
 ///