4 files changed, 18 insertions, 3 deletions
diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index f989e701913..74354f605e5 100644
--- a/src/liballoc/boxed.rs
+++ b/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, DispatchFromDyn, 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 = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
+
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl<T: Clone> Clone for Box<[T]> {
     fn clone(&self) -> Self {
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index 7db6261a01c..ad6e594c884 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -86,6 +86,7 @@
 #![feature(box_syntax)]
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
+#![feature(dispatch_from_dyn)]
 #![feature(core_intrinsics)]
 #![feature(custom_attribute)]
 #![feature(dropck_eyepatch)]
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 3e8dfd105de..be452ebb45a 100644
--- a/src/liballoc/rc.rs
+++ b/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, DispatchFromDyn};
 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 = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
+
 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 = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
+
 impl<T> Weak<T> {
     /// Constructs a new `Weak<T>`, without allocating any memory.
     /// Calling [`upgrade`][Weak::upgrade] on the return value always gives [`None`].
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index bcf5212f1ff..d388f76d8e8 100644
--- a/src/liballoc/sync.rs
+++ b/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, DispatchFromDyn};
 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 = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
+
 /// `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 = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
 
 #[stable(feature = "arc_weak", since = "1.4.0")]
 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {