Add CoerceSized impls throughout libstd

This will make receiver types like `Rc<Self>` and `Pin<&mut Self>`
object-safe.

src/liballoc/boxed.rs

@@ -77,7 +77,7 @@ use core::iter::FusedIterator;
 use core::marker::{Unpin, Unsize};
 use core::mem;
 use core::pin::Pin;
-use core::ops::{CoerceUnsized, Deref, DerefMut, Generator, GeneratorState};
+use core::ops::{CoerceUnsized, CoerceSized, Deref, DerefMut, Generator, GeneratorState};
 use core::ptr::{self, NonNull, Unique};
 use core::task::{LocalWaker, Poll};
 
@@ -696,6 +696,9 @@ impl<'a, A, R> FnOnce<A> for Box<dyn FnBox<A, Output = R> + Send + 'a> {
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 
+#[unstable(feature = "coerce_sized", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Box<T>> for Box<U> {}
+
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl<T: Clone> Clone for Box<[T]> {
     fn clone(&self) -> Self {

src/liballoc/lib.rs

@@ -86,6 +86,7 @@
 #![feature(box_syntax)]
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
+#![feature(coerce_sized)]
 #![feature(core_intrinsics)]
 #![feature(custom_attribute)]
 #![feature(dropck_eyepatch)]

src/liballoc/rc.rs

@@ -255,7 +255,7 @@ use core::marker;
 use core::marker::{Unpin, Unsize, PhantomData};
 use core::mem::{self, align_of_val, forget, size_of_val};
 use core::ops::Deref;
-use core::ops::CoerceUnsized;
+use core::ops::{CoerceUnsized, CoerceSized};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
 use core::convert::From;
@@ -297,6 +297,9 @@ impl<T: ?Sized> !marker::Sync for Rc<T> {}
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
 
+#[unstable(feature = "coerce_sized", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Rc<T>> for Rc<U> {}
+
 impl<T> Rc<T> {
     /// Constructs a new `Rc<T>`.
     ///
@@ -1176,6 +1179,9 @@ impl<T: ?Sized> !marker::Sync for Weak<T> {}
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
 
+#[unstable(feature = "coerce_sized", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Weak<T>> for Weak<U> {}
+
 impl<T> Weak<T> {
     /// Constructs a new `Weak<T>`, without allocating any memory.
     /// Calling [`upgrade`][Weak::upgrade] on the return value always gives [`None`].

src/liballoc/sync.rs

@@ -25,7 +25,7 @@ use core::cmp::Ordering;
 use core::intrinsics::abort;
 use core::mem::{self, align_of_val, size_of_val};
 use core::ops::Deref;
-use core::ops::CoerceUnsized;
+use core::ops::{CoerceUnsized, CoerceSized};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
 use core::marker::{Unpin, Unsize, PhantomData};
@@ -214,6 +214,9 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
 
+#[unstable(feature = "coerce_sized", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Arc<T>> for Arc<U> {}
+
 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
@@ -254,6 +257,8 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
 
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
+#[unstable(feature = "coerce_sized", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceSized<Weak<T>> for Weak<U> {}
 
 #[stable(feature = "arc_weak", since = "1.4.0")]
 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {

src/libcore/nonzero.rs

@@ -10,7 +10,7 @@
 
 //! Exposes the NonZero lang item which provides optimization hints.
 
-use ops::CoerceUnsized;
+use ops::{CoerceUnsized, CoerceSized};
 
 /// A wrapper type for raw pointers and integers that will never be
 /// NULL or 0 that might allow certain optimizations.
@@ -20,3 +20,5 @@ use ops::CoerceUnsized;
 pub(crate) struct NonZero<T>(pub(crate) T);
 
 impl<T: CoerceUnsized<U>, U> CoerceUnsized<NonZero<U>> for NonZero<T> {}
+
+impl<T: CoerceUnsized<U>, U: CoerceSized<T>> CoerceSized<NonZero<T>> for NonZero<U> {}

src/libcore/pin.rs

@@ -91,7 +91,7 @@
 
 use fmt;
 use marker::Sized;
-use ops::{Deref, DerefMut, CoerceUnsized};
+use ops::{Deref, DerefMut, CoerceUnsized, CoerceSized};
 
 #[doc(inline)]
 pub use marker::Unpin;
@@ -324,5 +324,12 @@ where
     P: CoerceUnsized<U>,
 {}
 
+#[unstable(feature = "pin", issue = "49150")]
+impl<'a, P, U> CoerceSized<Pin<P>> for Pin<U>
+where
+    P: CoerceUnsized<U>,
+    U: CoerceSized<P>,
+{}
+
 #[unstable(feature = "pin", issue = "49150")]
 impl<P> Unpin for Pin<P> {}

src/libcore/ptr.rs

@@ -75,7 +75,7 @@
 
 use convert::From;
 use intrinsics;
-use ops::CoerceUnsized;
+use ops::{CoerceUnsized, CoerceSized};
 use fmt;
 use hash;
 use marker::{PhantomData, Unsize};
@@ -2795,6 +2795,9 @@ impl<T: ?Sized> Copy for Unique<T> { }
 #[unstable(feature = "ptr_internals", issue = "0")]
 impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> { }
 
+#[unstable(feature = "ptr_internals", issue = "0")]
+impl<T: ?Sized, U: ?Sized> CoerceSized<Unique<T>> for Unique<U> where T: Unsize<U> { }
+
 #[unstable(feature = "ptr_internals", issue = "0")]
 impl<T: ?Sized> fmt::Pointer for Unique<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
@@ -2951,6 +2954,9 @@ impl<T: ?Sized> Copy for NonNull<T> { }
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized, U: ?Sized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Unsize<U> { }
 
+#[unstable(feature = "coerce_sized", issue = "0")]
+impl<T: ?Sized, U: ?Sized> CoerceSized<NonNull<T>> for NonNull<U> where T: Unsize<U> { }
+
 #[stable(feature = "nonnull", since = "1.25.0")]
 impl<T: ?Sized> fmt::Debug for NonNull<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {