5 files changed, 121 insertions, 2 deletions
diff --git a/library/core/src/cell.rs b/library/core/src/cell.rs
index 0d66c2b52c8..d860f3415d9 100644
--- a/library/core/src/cell.rs
+++ b/library/core/src/cell.rs
@@ -255,6 +255,7 @@ use crate::fmt::{self, Debug, Display};
 use crate::marker::{PhantomData, Unsize};
 use crate::mem;
 use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn};
+use crate::pin::PinCoerceUnsized;
 use crate::ptr::{self, NonNull};
 
 mod lazy;
@@ -2396,3 +2397,21 @@ fn assert_coerce_unsized(
     let _: Cell<&dyn Send> = c;
     let _: RefCell<&dyn Send> = d;
 }
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for UnsafeCell<T> {}
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for SyncUnsafeCell<T> {}
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for Cell<T> {}
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for RefCell<T> {}
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<'b, T: ?Sized> PinCoerceUnsized for Ref<'b, T> {}
+
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<'b, T: ?Sized> PinCoerceUnsized for RefMut<'b, T> {}
diff --git a/library/core/src/pin.rs b/library/core/src/pin.rs
index d752151d10c..0569b8b7624 100644
--- a/library/core/src/pin.rs
+++ b/library/core/src/pin.rs
@@ -1715,10 +1715,56 @@ 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")]
+/// Trait that indicates that this is a pointer or a wrapper for one, where
+/// unsizing can be performed on the pointee when it is pinned.
+///
+/// # Safety
+///
+/// If this type implements `Deref`, then the concrete type returned by `deref`
+/// and `deref_mut` must not change without a modification. The following
+/// operations are not considered modifications:
+///
+/// * Moving the pointer.
+/// * Performing unsizing coercions on the pointer.
+/// * Performing dynamic dispatch with the pointer.
+/// * Calling `deref` or `deref_mut` on the pointer.
+///
+/// The concrete type of a trait object is the type that the vtable corresponds
+/// to. The concrete type of a slice is an array of the same element type and
+/// the length specified in the metadata. The concrete type of a sized type
+/// is the type itself.
+pub unsafe trait PinCoerceUnsized {}
+
+#[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")]
+unsafe impl<T: PinCoerceUnsized> PinCoerceUnsized for Pin<T> {}
+
+#[stable(feature = "pin", since = "1.33.0")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for *const T {}
 
 #[stable(feature = "pin", since = "1.33.0")]
-impl<Ptr, U> DispatchFromDyn<Pin<U>> for Pin<Ptr> where Ptr: DispatchFromDyn<U> {}
+unsafe impl<T: ?Sized> PinCoerceUnsized for *mut T {}
 
 /// Constructs a <code>[Pin]<[&mut] T></code>, by pinning a `value: T` locally.
 ///
diff --git a/library/core/src/ptr/non_null.rs b/library/core/src/ptr/non_null.rs
index 4a716a75039..44db227b79e 100644
--- a/library/core/src/ptr/non_null.rs
+++ b/library/core/src/ptr/non_null.rs
@@ -3,6 +3,7 @@ use crate::marker::Unsize;
 use crate::mem::{MaybeUninit, SizedTypeProperties};
 use crate::num::NonZero;
 use crate::ops::{CoerceUnsized, DispatchFromDyn};
+use crate::pin::PinCoerceUnsized;
 use crate::ptr::Unique;
 use crate::slice::{self, SliceIndex};
 use crate::ub_checks::assert_unsafe_precondition;
@@ -1724,6 +1725,9 @@ impl<T: ?Sized, U: ?Sized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Uns
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
 
+#[stable(feature = "pin", since = "1.33.0")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for NonNull<T> {}
+
 #[stable(feature = "nonnull", since = "1.25.0")]
 impl<T: ?Sized> fmt::Debug for NonNull<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
diff --git a/library/core/src/ptr/unique.rs b/library/core/src/ptr/unique.rs
index b74d691e454..4810ebe01f9 100644
--- a/library/core/src/ptr/unique.rs
+++ b/library/core/src/ptr/unique.rs
@@ -1,6 +1,7 @@
 use crate::fmt;
 use crate::marker::{PhantomData, Unsize};
 use crate::ops::{CoerceUnsized, DispatchFromDyn};
+use crate::pin::PinCoerceUnsized;
 use crate::ptr::NonNull;
 
 /// A wrapper around a raw non-null `*mut T` that indicates that the possessor
@@ -166,6 +167,9 @@ impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsiz
 #[unstable(feature = "ptr_internals", issue = "none")]
 impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Unique<U>> for Unique<T> where T: Unsize<U> {}
 
+#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
+unsafe impl<T: ?Sized> PinCoerceUnsized for Unique<T> {}
+
 #[unstable(feature = "ptr_internals", issue = "none")]
 impl<T: ?Sized> fmt::Debug for Unique<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
diff --git a/library/core/tests/pin.rs b/library/core/tests/pin.rs
index 6f617c8d0c2..7a6af46a743 100644
--- a/library/core/tests/pin.rs
+++ b/library/core/tests/pin.rs
@@ -29,3 +29,49 @@ fn pin_const() {
 
     pin_mut_const();
 }
+
+#[allow(unused)]
+mod pin_coerce_unsized {
+    use core::cell::{Cell, RefCell, UnsafeCell};
+    use core::pin::Pin;
+    use core::ptr::NonNull;
+
+    pub trait MyTrait {}
+    impl MyTrait for String {}
+
+    // These Pins should continue to compile.
+    // Do note that these instances of Pin types cannot be used
+    // meaningfully because all methods require a Deref/DerefMut
+    // bounds on the pointer type and Cell, RefCell and UnsafeCell
+    // do not implement Deref/DerefMut.
+
+    pub fn cell(arg: Pin<Cell<Box<String>>>) -> Pin<Cell<Box<dyn MyTrait>>> {
+        arg
+    }
+    pub fn ref_cell(arg: Pin<RefCell<Box<String>>>) -> Pin<RefCell<Box<dyn MyTrait>>> {
+        arg
+    }
+    pub fn unsafe_cell(arg: Pin<UnsafeCell<Box<String>>>) -> Pin<UnsafeCell<Box<dyn MyTrait>>> {
+        arg
+    }
+
+    // These sensible Pin coercions are possible.
+    pub fn pin_mut_ref(arg: Pin<&mut String>) -> Pin<&mut dyn MyTrait> {
+        arg
+    }
+    pub fn pin_ref(arg: Pin<&String>) -> Pin<&dyn MyTrait> {
+        arg
+    }
+    pub fn pin_ptr(arg: Pin<*const String>) -> Pin<*const dyn MyTrait> {
+        arg
+    }
+    pub fn pin_ptr_mut(arg: Pin<*mut String>) -> Pin<*mut dyn MyTrait> {
+        arg
+    }
+    pub fn pin_non_null(arg: Pin<NonNull<String>>) -> Pin<NonNull<dyn MyTrait>> {
+        arg
+    }
+    pub fn nesting_pins(arg: Pin<Pin<&String>>) -> Pin<Pin<&dyn MyTrait>> {
+        arg
+    }
+}