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diff --git a/src/libcollections/borrow.rs b/src/libcollections/borrow.rs
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+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! A module for working with borrowed data.
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+use core::clone::Clone;
+use core::cmp::{Eq, Ord, Ordering, PartialEq, PartialOrd};
+use core::hash::{Hash, Hasher};
+use core::marker::Sized;
+use core::ops::Deref;
+use core::option::Option;
+
+use fmt;
+use alloc::{rc, arc};
+
+use self::Cow::*;
+
+/// A trait for borrowing data.
+///
+/// In general, there may be several ways to "borrow" a piece of data.  The
+/// typical ways of borrowing a type `T` are `&T` (a shared borrow) and `&mut T`
+/// (a mutable borrow). But types like `Vec<T>` provide additional kinds of
+/// borrows: the borrowed slices `&[T]` and `&mut [T]`.
+///
+/// When writing generic code, it is often desirable to abstract over all ways
+/// of borrowing data from a given type. That is the role of the `Borrow`
+/// trait: if `T: Borrow<U>`, then `&U` can be borrowed from `&T`.  A given
+/// type can be borrowed as multiple different types. In particular, `Vec<T>:
+/// Borrow<Vec<T>>` and `Vec<T>: Borrow<[T]>`.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub trait Borrow<Borrowed: ?Sized> {
+    /// Immutably borrow from an owned value.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn borrow(&self) -> &Borrowed;
+}
+
+/// A trait for mutably borrowing data.
+///
+/// Similar to `Borrow`, but for mutable borrows.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub trait BorrowMut<Borrowed: ?Sized> : Borrow<Borrowed> {
+    /// Mutably borrow from an owned value.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn borrow_mut(&mut self) -> &mut Borrowed;
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Borrow<T> for T {
+    fn borrow(&self) -> &T { self }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> BorrowMut<T> for T {
+    fn borrow_mut(&mut self) -> &mut T { self }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T: ?Sized> Borrow<T> for &'a T {
+    fn borrow(&self) -> &T { &**self }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T: ?Sized> Borrow<T> for &'a mut T {
+    fn borrow(&self) -> &T { &**self }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T: ?Sized> BorrowMut<T> for &'a mut T {
+    fn borrow_mut(&mut self) -> &mut T { &mut **self }
+}
+
+impl<T> Borrow<T> for rc::Rc<T> {
+    fn borrow(&self) -> &T { &**self }
+}
+
+impl<T> Borrow<T> for arc::Arc<T> {
+    fn borrow(&self) -> &T { &**self }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B> where B: ToOwned, <B as ToOwned>::Owned: 'a {
+    fn borrow(&self) -> &B {
+        &**self
+    }
+}
+
+/// A generalization of Clone to borrowed data.
+///
+/// Some types make it possible to go from borrowed to owned, usually by
+/// implementing the `Clone` trait. But `Clone` works only for going from `&T`
+/// to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
+/// from any borrow of a given type.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub trait ToOwned {
+    #[stable(feature = "rust1", since = "1.0.0")]
+    type Owned: Borrow<Self>;
+
+    /// Create owned data from borrowed data, usually by copying.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn to_owned(&self) -> Self::Owned;
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> ToOwned for T where T: Clone {
+    type Owned = T;
+    fn to_owned(&self) -> T { self.clone() }
+}
+
+/// A clone-on-write smart pointer.
+///
+/// The type `Cow` is a smart pointer providing clone-on-write functionality: it
+/// can enclose and provide immutable access to borrowed data, and clone the
+/// data lazily when mutation or ownership is required. The type is designed to
+/// work with general borrowed data via the `Borrow` trait.
+///
+/// `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.
+///
+/// # Example
+///
+/// ```rust
+/// use std::borrow::Cow;
+///
+/// fn abs_all(input: &mut Cow<[int]>) {
+///     for i in 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;
+///         }
+///     }
+/// }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+pub enum Cow<'a, B: ?Sized + 'a> where B: ToOwned {
+    /// Borrowed data.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    Borrowed(&'a B),
+
+    /// Owned data.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    Owned(<B as ToOwned>::Owned)
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, B: ?Sized> Clone for Cow<'a, B> where B: ToOwned {
+    fn clone(&self) -> Cow<'a, B> {
+        match *self {
+            Borrowed(b) => Borrowed(b),
+            Owned(ref o) => {
+                let b: &B = o.borrow();
+                Owned(b.to_owned())
+            },
+        }
+    }
+}
+
+impl<'a, B: ?Sized> Cow<'a, B> where B: ToOwned {
+    /// Acquire a mutable reference to the owned form of the data.
+    ///
+    /// Copies the data if it is not already owned.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned {
+        match *self {
+            Borrowed(borrowed) => {
+                *self = Owned(borrowed.to_owned());
+                self.to_mut()
+            }
+            Owned(ref mut owned) => owned
+        }
+    }
+
+    /// Extract the owned data.
+    ///
+    /// Copies the data if it is not already owned.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn into_owned(self) -> <B as ToOwned>::Owned {
+        match self {
+            Borrowed(borrowed) => borrowed.to_owned(),
+            Owned(owned) => owned
+        }
+    }
+
+    /// Returns true if this `Cow` wraps a borrowed value
+    #[deprecated(since = "1.0.0", reason = "match on the enum instead")]
+    #[unstable(feature = "std_misc")]
+    pub fn is_borrowed(&self) -> bool {
+        match *self {
+            Borrowed(_) => true,
+            _ => false,
+        }
+    }
+
+    /// Returns true if this `Cow` wraps an owned value
+    #[deprecated(since = "1.0.0", reason = "match on the enum instead")]
+    #[unstable(feature = "std_misc")]
+    pub fn is_owned(&self) -> bool {
+        match *self {
+            Owned(_) => true,
+            _ => false,
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, B: ?Sized> Deref for Cow<'a, B> where B: ToOwned {
+    type Target = B;
+
+    fn deref(&self) -> &B {
+        match *self {
+            Borrowed(borrowed) => borrowed,
+            Owned(ref owned) => owned.borrow()
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, B: ?Sized> Eq for Cow<'a, B> where B: Eq + ToOwned {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, B: ?Sized> Ord for Cow<'a, B> where B: Ord + ToOwned {
+    #[inline]
+    fn cmp(&self, other: &Cow<'a, B>) -> Ordering {
+        Ord::cmp(&**self, &**other)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B> where
+    B: PartialEq<C> + ToOwned, C: ToOwned,
+{
+    #[inline]
+    fn eq(&self, other: &Cow<'b, C>) -> bool {
+        PartialEq::eq(&**self, &**other)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, B: ?Sized> PartialOrd for Cow<'a, B> where B: PartialOrd + ToOwned,
+{
+    #[inline]
+    fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering> {
+        PartialOrd::partial_cmp(&**self, &**other)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, B: ?Sized> fmt::Debug for Cow<'a, B> where
+    B: fmt::Debug + ToOwned,
+    <B as ToOwned>::Owned: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Borrowed(ref b) => fmt::Debug::fmt(b, f),
+            Owned(ref o) => fmt::Debug::fmt(o, f),
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, B: ?Sized> fmt::Display for Cow<'a, B> where
+    B: fmt::Display + ToOwned,
+    <B as ToOwned>::Owned: fmt::Display,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Borrowed(ref b) => fmt::Display::fmt(b, f),
+            Owned(ref o) => fmt::Display::fmt(o, f),
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg(stage0)]
+impl<'a, B: ?Sized, S: Hasher> Hash<S> for Cow<'a, B> where B: Hash<S> + ToOwned
+{
+    #[inline]
+    fn hash(&self, state: &mut S) {
+        Hash::hash(&**self, state)
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg(not(stage0))]
+impl<'a, B: ?Sized> Hash for Cow<'a, B> where B: Hash + ToOwned
+{
+    #[inline]
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        Hash::hash(&**self, state)
+    }
+}
+
+/// Trait for moving into a `Cow`
+#[stable(feature = "rust1", since = "1.0.0")]
+pub trait IntoCow<'a, B: ?Sized> where B: ToOwned {
+    /// Moves `self` into `Cow`
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn into_cow(self) -> Cow<'a, B>;
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a,  B: ?Sized> IntoCow<'a, B> for Cow<'a, B> where B: ToOwned {
+    fn into_cow(self) -> Cow<'a, B> {
+        self
+    }
+}