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    // Copyright 2012 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.

    /// Used for immutable dereferencing operations, like `*v`.
    ///
    /// In addition to being used for explicit dereferencing operations with the
    /// (unary) `*` operator in immutable contexts, `Deref` is also used implicitly
    /// by the compiler in many circumstances. This mechanism is called
    /// ['`Deref` coercion'][more]. In mutable contexts, [`DerefMut`] is used.
    ///
    /// Implementing `Deref` for smart pointers makes accessing the data behind them
    /// convenient, which is why they implement `Deref`. On the other hand, the
    /// rules regarding `Deref` and [`DerefMut`] were designed specifically to
    /// accommodate smart pointers. Because of this, **`Deref` should only be
    /// implemented for smart pointers** to avoid confusion.
    ///
    /// For similar reasons, **this trait should never fail**. Failure during
    /// dereferencing can be extremely confusing when `Deref` is invoked implicitly.
    ///
    /// # More on `Deref` coercion
    ///
    /// If `T` implements `Deref<Target = U>`, and `x` is a value of type `T`, then:
    ///
    /// * In immutable contexts, `*x` on non-pointer types is equivalent to
    ///   `*Deref::deref(&x)`.
    /// * Values of type `&T` are coerced to values of type `&U`
    /// * `T` implicitly implements all the (immutable) methods of the type `U`.
    ///
    /// For more details, visit [the chapter in *The Rust Programming Language*]
    /// [book] as well as the reference sections on [the dereference operator]
    /// [ref-deref-op], [method resolution] and [type coercions].
    ///
    /// [book]: ../../book/second-edition/ch15-02-deref.html
    /// [`DerefMut`]: trait.DerefMut.html
    /// [more]: #more-on-deref-coercion
    /// [ref-deref-op]: ../../reference/expressions/operator-expr.html#the-dereference-operator
    /// [method resolution]: ../../reference/expressions/method-call-expr.html
    /// [type coercions]: ../../reference/type-coercions.html
    ///
    /// # Examples
    ///
    /// A struct with a single field which is accessible by dereferencing the
    /// struct.
    ///
    /// ```
    /// use std::ops::Deref;
    ///
    /// struct DerefExample<T> {
    ///     value: T
    /// }
    ///
    /// impl<T> Deref for DerefExample<T> {
    ///     type Target = T;
    ///
    ///     fn deref(&self) -> &T {
    ///         &self.value
    ///     }
    /// }
    ///
    /// let x = DerefExample { value: 'a' };
    /// assert_eq!('a', *x);
    /// ```
    #[lang = "deref"]
    #[doc(alias = "*")]
    #[doc(alias = "&*")]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Deref {
        /// The resulting type after dereferencing.
        #[stable(feature = "rust1", since = "1.0.0")]
        type Target: ?Sized;

        /// Dereferences the value.
        #[must_use]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn deref(&self) -> &Self::Target;
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized> Deref for &'a T {
        type Target = T;

        fn deref(&self) -> &T { *self }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized> Deref for &'a mut T {
        type Target = T;

        fn deref(&self) -> &T { *self }
    }

    /// Used for mutable dereferencing operations, like in `*v = 1;`.
    ///
    /// In addition to being used for explicit dereferencing operations with the
    /// (unary) `*` operator in mutable contexts, `DerefMut` is also used implicitly
    /// by the compiler in many circumstances. This mechanism is called
    /// ['`Deref` coercion'][more]. In immutable contexts, [`Deref`] is used.
    ///
    /// Implementing `DerefMut` for smart pointers makes mutating the data behind
    /// them convenient, which is why they implement `DerefMut`. On the other hand,
    /// the rules regarding [`Deref`] and `DerefMut` were designed specifically to
    /// accommodate smart pointers. Because of this, **`DerefMut` should only be
    /// implemented for smart pointers** to avoid confusion.
    ///
    /// For similar reasons, **this trait should never fail**. Failure during
    /// dereferencing can be extremely confusing when `DerefMut` is invoked
    /// implicitly.
    ///
    /// # More on `Deref` coercion
    ///
    /// If `T` implements `DerefMut<Target = U>`, and `x` is a value of type `T`,
    /// then:
    ///
    /// * In mutable contexts, `*x` on non-pointer types is equivalent to
    ///   `*Deref::deref(&x)`.
    /// * Values of type `&mut T` are coerced to values of type `&mut U`
    /// * `T` implicitly implements all the (mutable) methods of the type `U`.
    ///
    /// For more details, visit [the chapter in *The Rust Programming Language*]
    /// [book] as well as the reference sections on [the dereference operator]
    /// [ref-deref-op], [method resolution] and [type coercions].
    ///
    /// [book]: ../../book/second-edition/ch15-02-deref.html
    /// [`Deref`]: trait.Deref.html
    /// [more]: #more-on-deref-coercion
    /// [ref-deref-op]: ../../reference/expressions/operator-expr.html#the-dereference-operator
    /// [method resolution]: ../../reference/expressions/method-call-expr.html
    /// [type coercions]: ../../reference/type-coercions.html
    ///
    /// # Examples
    ///
    /// A struct with a single field which is modifiable by dereferencing the
    /// struct.
    ///
    /// ```
    /// use std::ops::{Deref, DerefMut};
    ///
    /// struct DerefMutExample<T> {
    ///     value: T
    /// }
    ///
    /// impl<T> Deref for DerefMutExample<T> {
    ///     type Target = T;
    ///
    ///     fn deref(&self) -> &T {
    ///         &self.value
    ///     }
    /// }
    ///
    /// impl<T> DerefMut for DerefMutExample<T> {
    ///     fn deref_mut(&mut self) -> &mut T {
    ///         &mut self.value
    ///     }
    /// }
    ///
    /// let mut x = DerefMutExample { value: 'a' };
    /// *x = 'b';
    /// assert_eq!('b', *x);
    /// ```
    #[lang = "deref_mut"]
    #[doc(alias = "*")]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait DerefMut: Deref {
        /// Mutably dereferences the value.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn deref_mut(&mut self) -> &mut Self::Target;
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized> DerefMut for &'a mut T {
        fn deref_mut(&mut self) -> &mut T { *self }
    }

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