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    use marker::Unsize;

    /// Trait that indicates that this is a pointer or a wrapper for one,
    /// where unsizing can be performed on the pointee.
    ///
    /// See the [DST coercion RFC][dst-coerce] and [the nomicon entry on coercion][nomicon-coerce]
    /// for more details.
    ///
    /// For builtin pointer types, pointers to `T` will coerce to pointers to `U` if `T: Unsize<U>`
    /// by converting from a thin pointer to a fat pointer.
    ///
    /// For custom types, the coercion here works by coercing `Foo<T>` to `Foo<U>`
    /// provided an impl of `CoerceUnsized<Foo<U>> for Foo<T>` exists.
    /// Such an impl can only be written if `Foo<T>` has only a single non-phantomdata
    /// field involving `T`. If the type of that field is `Bar<T>`, an implementation
    /// of `CoerceUnsized<Bar<U>> for Bar<T>` must exist. The coercion will work by
    /// coercing the `Bar<T>` field into `Bar<U>` and filling in the rest of the fields
    /// from `Foo<T>` to create a `Foo<U>`. This will effectively drill down to a pointer
    /// field and coerce that.
    ///
    /// Generally, for smart pointers you will implement
    /// `CoerceUnsized<Ptr<U>> for Ptr<T> where T: Unsize<U>, U: ?Sized`, with an
    /// optional `?Sized` bound on `T` itself. For wrapper types that directly embed `T`
    /// like `Cell<T>` and `RefCell<T>`, you
    /// can directly implement `CoerceUnsized<Wrap<U>> for Wrap<T> where T: CoerceUnsized<U>`.
    /// This will let coercions of types like `Cell<Box<T>>` work.
    ///
    /// [`Unsize`][unsize] is used to mark types which can be coerced to DSTs if behind
    /// pointers. It is implemented automatically by the compiler.
    ///
    /// [dst-coerce]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
    /// [unsize]: ../marker/trait.Unsize.html
    /// [nomicon-coerce]: ../../nomicon/coercions.html
    #[unstable(feature = "coerce_unsized", issue = "27732")]
    #[lang = "coerce_unsized"]
    pub trait CoerceUnsized<T: ?Sized> {
        // Empty.
    }

    // &mut T -> &mut U
    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<&'a mut U> for &'a mut T {}
    // &mut T -> &U
    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<'a, 'b: 'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b mut T {}
    // &mut T -> *mut U
    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for &'a mut T {}
    // &mut T -> *const U
    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for &'a mut T {}

    // &T -> &U
    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<'a, 'b: 'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b T {}
    // &T -> *const U
    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for &'a T {}

    // *mut T -> *mut U
    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for *mut T {}
    // *mut T -> *const U
    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *mut T {}

    // *const T -> *const U
    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {}

    /// This is used for object safety, to check that a method's receiver type can be dispatched on.
    ///
    /// example impl:
    ///
    /// ```
    /// # #![feature(dispatch_from_dyn, unsize)]
    /// # use std::{ops::DispatchFromDyn, marker::Unsize};
    /// # struct Rc<T: ?Sized>(::std::rc::Rc<T>);
    /// impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T>
    /// where
    ///     T: Unsize<U>,
    /// {}
    /// ```
    #[unstable(feature = "dispatch_from_dyn", issue = "0")]
    #[lang = "dispatch_from_dyn"]
    pub trait DispatchFromDyn<T> {
        // Empty.
    }

    // &T -> &U
    #[unstable(feature = "dispatch_from_dyn", issue = "0")]
    impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<&'a U> for &'a T {}
    // &mut T -> &mut U
    #[unstable(feature = "dispatch_from_dyn", issue = "0")]
    impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<&'a mut U> for &'a mut T {}
    // *const T -> *const U
    #[unstable(feature = "dispatch_from_dyn", issue = "0")]
    impl<T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<*const U> for *const T {}
    // *mut T -> *mut U
    #[unstable(feature = "dispatch_from_dyn", issue = "0")]
    impl<T: ?Sized+Unsize<U>, U: ?Sized> DispatchFromDyn<*mut U> for *mut T {}

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