Remove subtyping for object types and replace with an *upcast* coercion.

This upcast coercion currently preserves the vtable for the object, but
eventually it can be used to create a derived vtable. The upcast
coercion is not introduced into method dispatch; see comment on #18737
for information about why. Fixes #18737.

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 {