libcore: add borrow module

Following [the collections reform
RFC](https://github.com/rust-lang/rfcs/pull/235),
this commit adds a new `borrow` module to libcore.

The module contains traits for borrowing data (`BorrowFrom` and
`BorrowFromMut`),
generalized cloning (`ToOwned`), and a clone-on-write smartpointer (`Cow`).

src/libcollections/slice.rs

@@ -89,6 +89,7 @@
 
 use self::Direction::*;
 use alloc::boxed::Box;
+use core::borrow::{BorrowFrom, BorrowFromMut};
 use core::cmp;
 use core::kinds::Sized;
 use core::mem::size_of;
@@ -647,6 +648,16 @@ impl<T> SliceAllocPrelude<T> for [T] {
     }
 }
 
+#[unstable = "trait is unstable"]
+impl<T> BorrowFrom<Vec<T>> for [T] {
+    fn borrow_from(owned: &Vec<T>) -> &[T] { owned[] }
+}
+
+#[unstable = "trait is unstable"]
+impl<T> BorrowFromMut<Vec<T>> for [T] {
+    fn borrow_from_mut(owned: &mut Vec<T>) -> &mut [T] { owned[mut] }
+}
+
 /// 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::default::Default;
 use core::fmt;
 use core::cmp;
@@ -604,6 +604,11 @@ impl<'a> fmt::Show for MaybeOwned<'a> {
     }
 }
 
+#[unstable = "trait is unstable"]
+impl BorrowFrom<String> for str {
+    fn borrow_from(owned: &String) -> &str { owned[] }
+}
+
 /// Unsafe string operations.
 pub mod raw {
     pub use core::str::raw::{from_utf8, c_str_to_static_slice, slice_bytes};

src/libcore/borrow.rs

@@ -0,0 +1,126 @@
+// 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.
+//!
+//! # The `BorrowFrom` traits
+//!
+//! 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 `BorrowFrom`
+//! trait: if `T: BorrowFrom<U>`, then `&T` can be borrowed from `&U`.  A given
+//! type can be borrowed as multiple different types. In particular, `Vec<T>:
+//! BorrowFrom<Vec<T>>` and `[T]: BorrowFrom<Vec<T>>`.
+//!
+//! # The `ToOwned` trait
+//!
+//! 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.
+//!
+//! # The `Cow` (clone-on-write) type
+//!
+//! 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 `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.
+
+#![unstable = "recently added as part of collections reform"]
+
+use clone::Clone;
+use kinds::Sized;
+use ops::Deref;
+
+/// A trait for borrowing data.
+pub trait BorrowFrom<Sized? Owned> for Sized? {
+    /// Immutably borrow from an owned value.
+    fn borrow_from(owned: &Owned) -> &Self;
+}
+
+/// A trait for mutably borrowing data.
+pub trait BorrowFromMut<Sized? Owned> for Sized? : BorrowFrom<Owned> {
+    /// Mutably borrow from an owned value.
+    fn borrow_from_mut(owned: &mut Owned) -> &mut Self;
+}
+
+impl<Sized? T> BorrowFrom<T> for T {
+    fn borrow_from(owned: &T) -> &T { owned }
+}
+
+impl<Sized? T> BorrowFromMut<T> for T {
+    fn borrow_from_mut(owned: &mut T) -> &mut T { owned }
+}
+
+impl BorrowFrom<&'static str> for str {
+    fn borrow_from<'a>(owned: &'a &'static str) -> &'a str { &**owned }
+}
+
+/// A generalization of Clone to borrowed data.
+pub trait ToOwned<Owned> for Sized?: BorrowFrom<Owned> {
+    /// Create owned data from borrowed data, usually by copying.
+    fn to_owned(&self) -> Owned;
+}
+
+impl<T> ToOwned<T> for T where T: Clone {
+    fn to_owned(&self) -> T { self.clone() }
+}
+
+/// A clone-on-write smart pointer.
+pub enum Cow<'a, T, 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> {
+    /// Acquire a mutable reference to the owned form of the data.
+    ///
+    /// Copies the data if it is not already owned.
+    pub fn to_mut(&mut self) -> &mut T {
+        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.
+    pub fn into_owned(self) -> T {
+        match self {
+            Borrowed(borrowed) => borrowed.to_owned(),
+            Owned(owned) => owned
+        }
+    }
+}
+
+impl<'a, T, B> Deref<B> for Cow<'a, T, B> where B: ToOwned<T>  {
+    fn deref(&self) -> &B {
+        match *self {
+            Borrowed(borrowed) => borrowed,
+            Owned(ref owned) => BorrowFrom::borrow_from(owned)
+        }
+    }
+}

src/libcore/lib.rs

@@ -108,6 +108,7 @@ pub mod default;
 pub mod any;
 pub mod atomic;
 pub mod bool;
+pub mod borrow;
 pub mod cell;
 pub mod char;
 pub mod panicking;

src/libstd/lib.rs

@@ -141,6 +141,7 @@ extern crate rustrt;
 
 pub use core::any;
 pub use core::bool;
+pub use core::borrow;
 pub use core::cell;
 pub use core::clone;
 #[cfg(not(test))] pub use core::cmp;