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use crate::marker::Unsize;

/// 一个 trait,指示这是一个指针或一个包装器,其中可以对指针调整大小。
///
/// 有关更多详细信息,请参见 [DST 强制 RFC][dst-coerce] 和 [关于强制的 nomicon 入口][nomicon-coerce]。
///
/// 对于内置指针类型,如果 `T: Unsize<U>` 通过从精简指针转换为胖指针,则指向 `T` 的指针将强制指向指向 `U` 的指针。
///
/// 对于自定义类型,这里的强制通过将 `Foo<T>` 强制为 `Foo<U>` 来工作 (如果存在 `CoerceUnsized<Foo<U>> for Foo<T>` 的实现)。
/// 仅当 `Foo<T>` 仅具有涉及 `T` 的单个非虚拟数据字段时,才可以写这样的 impl。
/// 如果该字段的类型为 `Bar<T>`,则必须存在 `CoerceUnsized<Bar<U>> for Bar<T>` 的实现。
/// coercion 将通过将 `Bar<T>` 字段强制转换为 `Bar<U>` 并填充 `Foo<T>` 的其余字段以创建 `Foo<U>` 来起作用。
/// 这将有效地向下钻取指针字段并将其强制。
///
/// 通常,对于智能指针,您将实现 `CoerceUnsized<Ptr<U>> for Ptr<T> where T: Unsize<U>, U: ?Sized`,并在 `T` 本身上绑定了可选的 `?Sized`。
/// 对于直接嵌入 `T` 的包装器类型 (例如 `Cell<T>` 和 `RefCell<T>`),您可以直接实现 `CoerceUnsized<Wrap<U>> for Wrap<T> where T: CoerceUnsized<U>`。
///
/// 这将使像 `Cell<Box<T>>` 这样的强制类型起作用。
///
/// [`Unsize`][unsize] 用于标记在指针后面可以强制转换为 DST 的类型。它由编译器自动实现。
///
/// [dst-coerce]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md
/// [unsize]: crate::marker::Unsize
/// [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 {}

/// 这用于对象安全,以检查是否可以分派方法的接收者类型。
///
/// trait 的一个示例实现:
///
/// ```
/// # #![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 = "none")]
#[lang = "dispatch_from_dyn"]
pub trait DispatchFromDyn<T> {
    // Empty.
}

// &T -> &U
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<&'a U> for &'a T {}
// &mut T -> &mut U
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
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 = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<*const U> for *const T {}
// *mut T -> *mut U
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<*mut U> for *mut T {}