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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 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 }
    }

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