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]

1 files changed, 5 insertions, 4 deletions

diff --git a/src/libstd/ffi/os_str.rs b/src/libstd/ffi/os_str.rs
index 1d14b141778..8c29b11cb99 100644
--- a/src/libstd/ffi/os_str.rs
+++ b/src/libstd/ffi/os_str.rs
@@ -34,7 +34,7 @@
 
 use core::prelude::*;
 
-use core::borrow::{BorrowFrom, ToOwned};
+use borrow::{Borrow, ToOwned};
 use fmt::{self, Debug};
 use mem;
 use string::{String, CowString};
@@ -266,11 +266,12 @@ impl Debug for OsStr {
     }
 }
 
-impl BorrowFrom<OsString> for OsStr {
-    fn borrow_from(owned: &OsString) -> &OsStr { &owned[] }
+impl Borrow<OsStr> for OsString {
+    fn borrow(&self) -> &OsStr { &self[] }
 }
 
-impl ToOwned<OsString> for OsStr {
+impl ToOwned for OsStr {
+    type Owned = OsString;
     fn to_owned(&self) -> OsString { self.to_os_string() }
 }