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]

5 files changed, 33 insertions, 28 deletions

diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index 1b9f8b99017..9d3ffb114c1 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -14,7 +14,7 @@ use self::Entry::*;
 use self::SearchResult::*;
 use self::VacantEntryState::*;
 
-use borrow::BorrowFrom;
+use borrow::Borrow;
 use clone::Clone;
 use cmp::{max, Eq, PartialEq};
 use default::Default;
@@ -453,18 +453,18 @@ impl<K, V, S, H> HashMap<K, V, S>
     /// If you already have the hash for the key lying around, use
     /// search_hashed.
     fn search<'a, Q: ?Sized>(&'a self, q: &Q) -> Option<FullBucketImm<'a, K, V>>
-        where Q: BorrowFrom<K> + Eq + Hash<H>
+        where K: Borrow<Q>, Q: Eq + Hash<H>
     {
         let hash = self.make_hash(q);
-        search_hashed(&self.table, hash, |k| q.eq(BorrowFrom::borrow_from(k)))
+        search_hashed(&self.table, hash, |k| q.eq(k.borrow()))
             .into_option()
     }
 
     fn search_mut<'a, Q: ?Sized>(&'a mut self, q: &Q) -> Option<FullBucketMut<'a, K, V>>
-        where Q: BorrowFrom<K> + Eq + Hash<H>
+        where K: Borrow<Q>, Q: Eq + Hash<H>
     {
         let hash = self.make_hash(q);
-        search_hashed(&mut self.table, hash, |k| q.eq(BorrowFrom::borrow_from(k)))
+        search_hashed(&mut self.table, hash, |k| q.eq(k.borrow()))
             .into_option()
     }
 
@@ -1037,7 +1037,7 @@ impl<K, V, S, H> HashMap<K, V, S>
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V>
-        where Q: Hash<H> + Eq + BorrowFrom<K>
+        where K: Borrow<Q>, Q: Hash<H> + Eq
     {
         self.search(k).map(|bucket| bucket.into_refs().1)
     }
@@ -1060,7 +1060,7 @@ impl<K, V, S, H> HashMap<K, V, S>
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn contains_key<Q: ?Sized>(&self, k: &Q) -> bool
-        where Q: Hash<H> + Eq + BorrowFrom<K>
+        where K: Borrow<Q>, Q: Hash<H> + Eq
     {
         self.search(k).is_some()
     }
@@ -1086,7 +1086,7 @@ impl<K, V, S, H> HashMap<K, V, S>
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn get_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut V>
-        where Q: Hash<H> + Eq + BorrowFrom<K>
+        where K: Borrow<Q>, Q: Hash<H> + Eq
     {
         self.search_mut(k).map(|bucket| bucket.into_mut_refs().1)
     }
@@ -1138,7 +1138,7 @@ impl<K, V, S, H> HashMap<K, V, S>
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn remove<Q: ?Sized>(&mut self, k: &Q) -> Option<V>
-        where Q: Hash<H> + Eq + BorrowFrom<K>
+        where K: Borrow<Q>, Q: Hash<H> + Eq
     {
         if self.table.size() == 0 {
             return None
@@ -1247,8 +1247,8 @@ impl<K, V, S, H> Default for HashMap<K, V, S>
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K, Q: ?Sized, V, S, H> Index<Q> for HashMap<K, V, S>
-    where K: Eq + Hash<H>,
-          Q: Eq + Hash<H> + BorrowFrom<K>,
+    where K: Eq + Hash<H> + Borrow<Q>,
+          Q: Eq + Hash<H>,
           S: HashState<Hasher=H>,
           H: hash::Hasher<Output=u64>
 {
@@ -1262,8 +1262,8 @@ impl<K, Q: ?Sized, V, S, H> Index<Q> for HashMap<K, V, S>
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K, V, S, H, Q: ?Sized> IndexMut<Q> for HashMap<K, V, S>
-    where K: Eq + Hash<H>,
-          Q: Eq + Hash<H> + BorrowFrom<K>,
+    where K: Eq + Hash<H> + Borrow<Q>,
+          Q: Eq + Hash<H>,
           S: HashState<Hasher=H>,
           H: hash::Hasher<Output=u64>
 {
diff --git a/src/libstd/collections/hash/set.rs b/src/libstd/collections/hash/set.rs
index 5fbbcb3b347..7befaa8c368 100644
--- a/src/libstd/collections/hash/set.rs
+++ b/src/libstd/collections/hash/set.rs
@@ -10,7 +10,7 @@
 //
 // ignore-lexer-test FIXME #15883
 
-use borrow::BorrowFrom;
+use borrow::Borrow;
 use clone::Clone;
 use cmp::{Eq, PartialEq};
 use core::marker::Sized;
@@ -462,7 +462,7 @@ impl<T, S, H> HashSet<T, S>
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn contains<Q: ?Sized>(&self, value: &Q) -> bool
-        where Q: BorrowFrom<T> + Hash<H> + Eq
+        where T: Borrow<Q>, Q: Hash<H> + Eq
     {
         self.map.contains_key(value)
     }
@@ -572,7 +572,7 @@ impl<T, S, H> HashSet<T, S>
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn remove<Q: ?Sized>(&mut self, value: &Q) -> bool
-        where Q: BorrowFrom<T> + Hash<H> + Eq
+        where T: Borrow<Q>, Q: Hash<H> + Eq
     {
         self.map.remove(value).is_some()
     }
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() }
 }
 
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 7c9a8a7b4b5..fbd403ea593 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -161,7 +161,6 @@ extern crate libc;
 // NB: These reexports are in the order they should be listed in rustdoc
 
 pub use core::any;
-pub use core::borrow;
 pub use core::cell;
 pub use core::clone;
 #[cfg(not(test))] pub use core::cmp;
@@ -184,6 +183,7 @@ pub use core::error;
 #[cfg(not(test))] pub use alloc::boxed;
 pub use alloc::rc;
 
+pub use core_collections::borrow;
 pub use core_collections::fmt;
 pub use core_collections::slice;
 pub use core_collections::str;
diff --git a/src/libstd/path.rs b/src/libstd/path.rs
index 1d992668900..1f7129bf361 100755
--- a/src/libstd/path.rs
+++ b/src/libstd/path.rs
@@ -108,12 +108,11 @@
 use core::prelude::*;
 
 use ascii::*;
-use borrow::BorrowFrom;
+use borrow::{Borrow, ToOwned, Cow};
 use cmp;
 use iter;
 use mem;
 use ops::{self, Deref};
-use string::CowString;
 use vec::Vec;
 use fmt;
 
@@ -982,12 +981,17 @@ impl ops::Deref for PathBuf {
     }
 }
 
-impl BorrowFrom<PathBuf> for Path {
-    fn borrow_from(owned: &PathBuf) -> &Path {
-        owned.deref()
+impl Borrow<Path> for PathBuf {
+    fn borrow(&self) -> &Path {
+        self.deref()
     }
 }
 
+impl ToOwned for Path {
+    type Owned = PathBuf;
+    fn to_owned(&self) -> PathBuf { self.to_path_buf() }
+}
+
 impl cmp::PartialEq for PathBuf {
     fn eq(&self, other: &PathBuf) -> bool {
         self.components() == other.components()
@@ -1066,10 +1070,10 @@ impl Path {
         self.inner.to_str()
     }
 
-    /// Convert a `Path` to a `CowString`.
+    /// Convert a `Path` to a `Cow<str>`.
     ///
     /// Any non-Unicode sequences are replaced with U+FFFD REPLACEMENT CHARACTER.
-    pub fn to_string_lossy(&self) -> CowString {
+    pub fn to_string_lossy(&self) -> Cow<str> {
         self.inner.to_string_lossy()
     }