libs: make Cow usable, improve documentation

This commit makes `Cow` more usable by allowing it to be applied to
unsized types (as was intended) and providing some basic `ToOwned`
implementations on slice types. It also corrects the documentation for
`Cow` to no longer mention `DerefMut`, and adds an example.

Author: aturon

src/libcollections/slice.rs

@@ -89,7 +89,7 @@
 
 use self::Direction::*;
 use alloc::boxed::Box;
-use core::borrow::{BorrowFrom, BorrowFromMut};
+use core::borrow::{BorrowFrom, BorrowFromMut, ToOwned};
 use core::cmp;
 use core::kinds::Sized;
 use core::mem::size_of;
@@ -658,6 +658,11 @@ impl<T> BorrowFromMut<Vec<T>> for [T] {
     fn borrow_from_mut(owned: &mut Vec<T>) -> &mut [T] { owned[mut] }
 }
 
+#[unstable = "trait is unstable"]
+impl<T: Clone> ToOwned<Vec<T>> for [T] {
+    fn to_owned(&self) -> Vec<T> { self.to_vec() }
+}
+
 /// Unsafe operations
 pub mod raw {
     pub use core::slice::raw::{buf_as_slice, mut_buf_as_slice};

src/libcollections/str.rs

@@ -54,7 +54,7 @@
 pub use self::MaybeOwned::*;
 use self::RecompositionState::*;
 use self::DecompositionType::*;
-use core::borrow::BorrowFrom;
+use core::borrow::{BorrowFrom, ToOwned};
 use core::default::Default;
 use core::fmt;
 use core::cmp;
@@ -67,7 +67,7 @@ use core::prelude::{range};
 
 use hash;
 use ring_buf::RingBuf;
-use string::String;
+use string::{String, ToString};
 use unicode;
 use vec::Vec;
 
@@ -609,6 +609,11 @@ impl BorrowFrom<String> for str {
     fn borrow_from(owned: &String) -> &str { owned[] }
 }
 
+#[unstable = "trait is unstable"]
+impl ToOwned<String> for str {
+    fn to_owned(&self) -> String { self.to_string() }
+}
+
 /// Unsafe string operations.
 pub mod raw {
     pub use core::str::raw::{from_utf8, c_str_to_static_slice, slice_bytes};

src/libcore/borrow.rs

@@ -37,10 +37,10 @@
 //! data lazily when mutation or ownership is required. The type is designed to
 //! work with general borrowed data via the `BorrowFrom` trait.
 //!
-//! `Cow` implements both `Deref` and `DerefMut`, which means that you can call
-//! methods directly on the data it encloses. The first time a mutable reference
-//! is required, the data will be cloned (via `to_owned`) if it is not
-//! already owned.
+//! `Cow` implements both `Deref`, which means that you can call
+//! non-mutating methods directly on the data it encloses. If mutation
+//! is desired, `to_mut` will obtain a mutable references to an owned
+//! value, cloning if necessary.
 
 #![unstable = "recently added as part of collections reform"]
 
@@ -84,15 +84,31 @@ impl<T> ToOwned<T> for T where T: Clone {
 }
 
 /// A clone-on-write smart pointer.
-pub enum Cow<'a, T, B: 'a> where B: ToOwned<T> {
+///
+/// # Example
+///
+/// ```rust
+/// use std::borrow::Cow;
+///
+/// fn abs_all(input: &mut Cow<Vec<int>, [int]>) {
+///     for i in range(0, input.len()) {
+///         let v = input[i];
+///         if v < 0 {
+///             // clones into a vector the first time (if not already owned)
+///             input.to_mut()[i] = -v;
+///         }
+///     }
+/// }
+/// ```
+pub enum Cow<'a, T, Sized? B: 'a> where B: ToOwned<T> {
     /// Borrowed data.
     Borrowed(&'a B),
 
     /// Owned data.
     Owned(T)
 }
 
-impl<'a, T, B> Cow<'a, T, B> where B: ToOwned<T> {
+impl<'a, T, Sized? B> Cow<'a, T, B> where B: ToOwned<T> {
     /// Acquire a mutable reference to the owned form of the data.
     ///
     /// Copies the data if it is not already owned.
@@ -117,7 +133,7 @@ impl<'a, T, B> Cow<'a, T, B> where B: ToOwned<T> {
     }
 }
 
-impl<'a, T, B> Deref<B> for Cow<'a, T, B> where B: ToOwned<T>  {
+impl<'a, T, Sized? B> Deref<B> for Cow<'a, T, B> where B: ToOwned<T>  {
     fn deref(&self) -> &B {
         match *self {
             Borrowed(borrowed) => borrowed,