rollup merge of #19642: aaronweiss74/master

This was discussed in [#rust-internals](
https://botbot.me/mozilla/rust-internals/2014-12-08/?msg=27077624&page=5). It's a small change.

src/liballoc/rc.rs

@@ -141,6 +141,7 @@
 
 #![stable]
 
+use core::borrow::BorrowFrom;
 use core::cell::Cell;
 use core::clone::Clone;
 use core::cmp::{PartialEq, PartialOrd, Eq, Ord, Ordering};
@@ -349,6 +350,12 @@ impl<T: Clone> Rc<T> {
     }
 }
 
+impl<T> BorrowFrom<Rc<T>> for T {
+    fn borrow_from(owned: &Rc<T>) -> &T {
+        &**owned
+    }
+}
+
 #[experimental = "Deref is experimental."]
 impl<T> Deref<T> for Rc<T> {
     #[inline(always)]