Auto merge of #23423 - nikomatsakis:issue-18737-trait-subtyping, r=nrc

This upcast coercion currently never requires vtable changes. It should be generalized. 

This is a [breaking-change] -- if you have an impl on an object type like `impl SomeTrait`, then this will no longer be applicable to object types like `SomeTrait+Send`. In the standard library, this primarily affected `Any`, and this PR adds impls for `Any+Send` as to keep the API the same in practice. An alternate workaround is to use UFCS form or standalone fns. For more details, see <https://github.com/rust-lang/rust/issues/18737#issuecomment-78450798>.

r? @nrc

src/liballoc/boxed.rs

@@ -241,7 +241,7 @@ pub trait BoxAny {
     /// Returns the boxed value if it is of type `T`, or
     /// `Err(Self)` if it isn't.
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn downcast<T: 'static>(self) -> Result<Box<T>, Self>;
+    fn downcast<T: 'static>(self) -> Result<Box<T>, Box<Any>>;
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -264,6 +264,15 @@ impl BoxAny for Box<Any> {
     }
 }
 
+#[cfg(not(stage0))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl BoxAny for Box<Any+Send> {
+    #[inline]
+    fn downcast<T: 'static>(self) -> Result<Box<T>, Box<Any>> {
+        <Box<Any>>::downcast(self)
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {