Auto merge of #24619 - nrc:rc-coerce, r=nikomatsakis

r? @nikomatsakis (note a few TODOs left in the code where I wasn't sure about stuff).

src/liballoc/boxed.rs

@@ -62,6 +62,11 @@ use core::ops::{Deref, DerefMut};
 use core::ptr::{Unique};
 use core::raw::{TraitObject};
 
+#[cfg(not(stage0))]
+use core::marker::Unsize;
+#[cfg(not(stage0))]
+use core::ops::CoerceUnsized;
+
 /// A value that represents the heap. This is the default place that the `box`
 /// keyword allocates into when no place is supplied.
 ///
@@ -390,3 +395,6 @@ impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+Send+'a> {
         self.call_box(args)
     }
 }
+
+#[cfg(not(stage0))]
+impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}