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/libcore/any.rs

@@ -71,6 +71,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use marker::Send;
 use mem::transmute;
 use option::Option::{self, Some, None};
 use raw::TraitObject;
@@ -154,6 +155,31 @@ impl Any {
     }
 }
 
+#[cfg(not(stage0))]
+impl Any+Send {
+    /// Forwards to the method defined on the type `Any`.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
+    pub fn is<T: 'static>(&self) -> bool {
+        Any::is::<T>(self)
+    }
+
+    /// Forwards to the method defined on the type `Any`.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
+    pub fn downcast_ref<T: 'static>(&self) -> Option<&T> {
+        Any::downcast_ref::<T>(self)
+    }
+
+    /// Forwards to the method defined on the type `Any`.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
+    pub fn downcast_mut<T: 'static>(&mut self) -> Option<&mut T> {
+        Any::downcast_mut::<T>(self)
+    }
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 // TypeID and its methods
 ///////////////////////////////////////////////////////////////////////////////