2 files changed, 110 insertions, 0 deletions
diff --git a/src/libcore/borrow.rs b/src/libcore/borrow.rs
new file mode 100644
index 00000000000..79330d3a61e
--- /dev/null
+++ b/src/libcore/borrow.rs
@@ -0,0 +1,109 @@
+// 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 marker::Sized;
+
+/// 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]>`.
+///
+/// If you are implementing `Borrow` and both `Self` and `Borrowed` implement
+/// `Hash`, `Eq`, and/or `Ord`, they must produce the same result.
+///
+/// `Borrow` is very similar to, but different than, `AsRef`. See
+/// [the book][book] for more.
+///
+/// [book]: ../../book/borrow-and-asref.html
+#[stable(feature = "rust1", since = "1.0.0")]
+pub trait Borrow<Borrowed: ?Sized> {
+    /// Immutably borrows from an owned value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::borrow::Borrow;
+    ///
+    /// fn check<T: Borrow<str>>(s: T) {
+    ///     assert_eq!("Hello", s.borrow());
+    /// }
+    ///
+    /// let s = "Hello".to_string();
+    ///
+    /// check(s);
+    ///
+    /// let s = "Hello";
+    ///
+    /// check(s);
+    /// ```
+    #[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 borrows from an owned value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::borrow::BorrowMut;
+    ///
+    /// fn check<T: BorrowMut<[i32]>>(mut v: T) {
+    ///     assert_eq!(&mut [1, 2, 3], v.borrow_mut());
+    /// }
+    ///
+    /// let v = vec![1, 2, 3];
+    ///
+    /// check(v);
+    /// ```
+    #[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 }
+}
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index 551ea233f76..78a467e3657 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -139,6 +139,7 @@ pub mod cmp;
 pub mod clone;
 pub mod default;
 pub mod convert;
+pub mod borrow;
 
 /* Core types and methods on primitives */