Stabilize std::borrow

This commit stabilizes `std::borrow`, making the following modifications
to catch up the API with language changes:

* It renames `BorrowFrom` to `Borrow`, as was originally intended (but
  blocked for technical reasons), and reorders the parameters
  accordingly.

* It moves the type parameter of `ToOwned` to an associated type. This
  is somewhat less flexible, in that each borrowed type must have a
  unique owned type, but leads to a significant simplification for
  `Cow`. Flexibility can be regained by using newtyped slices, which is
  advisable for other reasons anyway.

* It removes the owned type parameter from `Cow`, making the type much
  less verbose.

* Deprecates the `is_owned` and `is_borrowed` predicates in favor of
  direct matching.

The above API changes are relatively minor; the basic functionality
remains the same, and essentially the whole module is now marked
`#[stable]`.

[breaking-change]

2 files changed, 0 insertions, 14 deletions

diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index 3830d7fe295..a8d6c3ff2b0 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -73,7 +73,6 @@ use core::prelude::*;
 
 use core::atomic;
 use core::atomic::Ordering::{Relaxed, Release, Acquire, SeqCst};
-use core::borrow::BorrowFrom;
 use core::fmt;
 use core::cmp::{Ordering};
 use core::default::Default;
@@ -244,12 +243,6 @@ impl<T> Clone for Arc<T> {
     }
 }
 
-impl<T> BorrowFrom<Arc<T>> for T {
-    fn borrow_from(owned: &Arc<T>) -> &T {
-        &**owned
-    }
-}
-
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Deref for Arc<T> {
     type Target = T;
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index f361c36ec8f..3e52886280b 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -144,7 +144,6 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use core::borrow::BorrowFrom;
 use core::cell::Cell;
 use core::clone::Clone;
 use core::cmp::{PartialEq, PartialOrd, Eq, Ord, Ordering};
@@ -349,12 +348,6 @@ impl<T: Clone> Rc<T> {
     }
 }
 
-impl<T> BorrowFrom<Rc<T>> for T {
-    fn borrow_from(owned: &Rc<T>) -> &T {
-        &**owned
-    }
-}
-
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Deref for Rc<T> {
     type Target = T;