//! 堆分配的指针类型。
//!
//! [`Box<T>`], 随意称为 'box'，它在 Rust 中提供了最简单的堆分配形式。Boxes 为这个分配提供所有权，并在离开作用域时丢弃它们的内容。Boxes 还确保它们分配的字节数永远不会超过 `isize::MAX` 字节。
//!
//! # Examples
//!
//! 通过创建 [`Box`]，将值从栈移动到堆:
//!
//! ```
//! let val: u8 = 5;
//! let boxed: Box<u8> = Box::new(val);
//! ```
//!
//! 通过 [解引用][dereferencing] 将值从 [`Box`] 移回栈:
//!
//! ```
//! let boxed: Box<u8> = Box::new(5);
//! let val: u8 = *boxed;
//! ```
//!
//! 创建递归数据结构体:
//!
//! ```
//! #[derive(Debug)]
//! enum List<T> {
//!     Cons(T, Box<List<T>>),
//!     Nil,
//! }
//!
//! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
//! println!("{:?}", list);
//! ```
//!
//! 这将打印 `Cons(1, Cons(2, Nil))`。
//!
//! 递归结构必须为 boxed，因为如果 `Cons` 的定义如下所示:
//!
//! ```compile_fail,E0072
//! # enum List<T> {
//! Cons(T, List<T>),
//! # }
//! ```
//!
//! 这行不通。这是因为 `List` 的大小取决于列表中有多少个元素，因此我们不知道为 `Cons` 分配多少内存。通过引入具有定义大小的 [`Box<T>`]，我们知道 `Cons` 的大小。
//!
//! # 内存布局
//!
//! 对于非零大小的值，[`Box`] 将使用 [`Global`] 分配器进行分配。假定与分配器一起使用的 [`Layout`] 对于该类型是正确的，则在 [`Box`] 和使用 [`Global`] 分配器分配的裸指针之间进行双向转换是有效的。
//!
//! 更准确地说，已使用 `Layout::for_value(&*value)` 与 [`Global`] 分配器一起分配的 `value:* mut T` 可以使用 [`Box::<T>::from_raw(value)`] 转换为 box。
//! 相反，可以使用带有 [`Layout::for_value(&*value)`] 的 [`Global`] 分配器重新分配支持从 [`Box::<T>::into_raw`] 获得的 `value:* mut T` 的内存。
//!
//! 对于零大小的值，`Box` 指针对于读取和写入仍必须为 [有效的][valid]，并且必须充分对齐。
//! 特别是，将任何对齐的非零整数字面量强制转换为裸指针都会产生有效的指针，但是指向先前分配的内存 (由于释放后的指针) 的指针无效。
//! 如果不能使用 `Box::new`，建议将 Box 生成到 ZST 的推荐方法是使用 [`ptr::NonNull::dangling`]。
//!
//! 只要 `T: Sized`，就可以保证将 `Box<T>` 表示为单个指针，并且还与 C 指针 ABI 兼容 (即 C 类型 `T*`)。
//! 这意味着，如果您有从 C 调用的外部 "C" Rust 函数，则可以使用 `Box<T>` 类型定义那些 Rust 函数，并在 C 侧使用 `T*` 作为对应类型。
//! 例如，考虑下面的 C 头文件，该标头声明创建和销毁某种 `Foo` 值的函数:
//!
//! ```c
//! /* C 头文件 */
//!
//! /* 将所有权归还给调用者 */
//! struct Foo* foo_new(void);
//!
//! /* 取得调用者的所有权; 使用 null 调用时无操作 */
//! void foo_delete(struct Foo*);
//! ```
//!
//! 这两个函数可以在 Rust 中实现，如下所示。在这里，来自 C 的 `struct Foo*` 类型被转换为 `Box<Foo>`，它捕获了所有权约束。
//! 还要注意，由于 `Box<Foo>` 不能为 null，因此 `foo_delete` 的 nullable 参数在 Rust 中表示为 `Option<Box<Foo>>`。
//!
//! ```
//! #[repr(C)]
//! pub struct Foo;
//!
//! #[no_mangle]
//! pub extern "C" fn foo_new() -> Box<Foo> {
//!     Box::new(Foo)
//! }
//!
//! #[no_mangle]
//! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
//! ```
//!
//! 即使 `Box<T>` 具有与 C 指针相同的表示形式和 C ABI，但这并不意味着您可以将任意 `T*` 转换为 `Box<T>` 并期望一切正常。
//! `Box<T>` 值将始终是完全对齐的非空指针。此外，`Box<T>` 的析构函数将尝试使用分配器释放该值。通常，最佳实践是仅对来自分配器的指针使用 `Box<T>`。
//!
//! **重要** 至少目前，您应该避免对在 C 中定义但从 Rust 调用的函数使用 `Box<T>` 类型。在这些情况下，您应该尽可能直接地镜像 C 类型。
//! 如 [rust-lang/unsafe-code-guidelines#198][ucg#198] 中所述，使用 C 定义仅使用 `T*` 的 `Box<T>` 这样的类型可能导致未定义的行为。
//!
//! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
//! [dereferencing]: core::ops::Deref
//! [`Box::<T>::from_raw(value)`]: Box::from_raw
//! [`Global`]: crate::alloc::Global
//! [`Layout`]: crate::alloc::Layout
//! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
//! [valid]: ptr#safety
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!
//!

#![stable(feature = "rust1", since = "1.0.0")]

use core::any::Any;
use core::borrow;
use core::cmp::Ordering;
use core::convert::{From, TryFrom};
use core::fmt;
use core::future::Future;
use core::hash::{Hash, Hasher};
#[cfg(not(no_global_oom_handling))]
use core::iter::FromIterator;
use core::iter::{FusedIterator, Iterator};
use core::marker::{Unpin, Unsize};
use core::mem;
use core::ops::{
    CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
};
use core::pin::Pin;
use core::ptr::{self, Unique};
use core::stream::Stream;
use core::task::{Context, Poll};

#[cfg(not(no_global_oom_handling))]
use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
use crate::alloc::{AllocError, Allocator, Global, Layout};
#[cfg(not(no_global_oom_handling))]
use crate::borrow::Cow;
#[cfg(not(no_global_oom_handling))]
use crate::raw_vec::RawVec;
#[cfg(not(no_global_oom_handling))]
use crate::str::from_boxed_utf8_unchecked;
#[cfg(not(no_global_oom_handling))]
use crate::vec::Vec;

/// 堆分配的指针类型。
///
/// 有关更多信息，请参见 [模块级文档](../../std/boxed/index.html)。
#[lang = "owned_box"]
#[fundamental]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Box<
    T: ?Sized,
    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
>(Unique<T>, A);

impl<T> Box<T> {
    /// 在堆上分配内存，然后将 `x` 放入其中。
    ///
    /// 如果 `T` 的大小为零，则实际上不会分配。
    ///
    /// # Examples
    ///
    /// ```
    /// let five = Box::new(5);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline(always)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new(x: T) -> Self {
        box x
    }

    /// 创建一个具有未初始化内容的新 box。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let mut five = Box::<u32>::new_uninit();
    ///
    /// let five = unsafe {
    ///     // 延迟初始化:
    ///     five.as_mut_ptr().write(5);
    ///
    ///     five.assume_init()
    /// };
    ///
    /// assert_eq!(*five, 5)
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[inline]
    pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
        Self::new_uninit_in(Global)
    }

    /// 创建一个具有未初始化内容的新 `Box`，并用 `0` 字节填充内存。
    ///
    ///
    /// 有关正确和不正确使用此方法的示例，请参见 [`MaybeUninit::zeroed`][zeroed]。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let zero = Box::<u32>::new_zeroed();
    /// let zero = unsafe { zero.assume_init() };
    ///
    /// assert_eq!(*zero, 0)
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    ///
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
        Self::new_zeroed_in(Global)
    }

    /// 创建一个新的 `Pin<Box<T>>`。
    /// 如果 `T` 未实现 `Unpin`，则 `x` 将被固定在内存中并且无法移动。
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "pin", since = "1.33.0")]
    #[inline(always)]
    pub fn pin(x: T) -> Pin<Box<T>> {
        (box x).into()
    }

    /// 在堆上分配内存，然后将 `x` 放入其中，如果分配失败，则返回错误
    ///
    ///
    /// 如果 `T` 的大小为零，则实际上不会分配。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// let five = Box::try_new(5)?;
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn try_new(x: T) -> Result<Self, AllocError> {
        Self::try_new_in(x, Global)
    }

    /// 在堆上创建一个具有未初始化内容的新 box，如果分配失败，则返回错误
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// let mut five = Box::<u32>::try_new_uninit()?;
    ///
    /// let five = unsafe {
    ///     // 延迟初始化:
    ///     five.as_mut_ptr().write(5);
    ///
    ///     five.assume_init()
    /// };
    ///
    /// assert_eq!(*five, 5);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    #[inline]
    pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
        Box::try_new_uninit_in(Global)
    }

    /// 创建一个具有未初始化内容的新 `Box`，堆中的内存由 `0` 字节填充
    ///
    ///
    /// 有关正确和不正确使用此方法的示例，请参见 [`MaybeUninit::zeroed`][zeroed]。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// let zero = Box::<u32>::try_new_zeroed()?;
    /// let zero = unsafe { zero.assume_init() };
    ///
    /// assert_eq!(*zero, 0);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    ///
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    #[inline]
    pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
        Box::try_new_zeroed_in(Global)
    }
}

impl<T, A: Allocator> Box<T, A> {
    /// 在给定的分配器中分配内存，然后将 `x` 放入其中。
    ///
    /// 如果 `T` 的大小为零，则实际上不会分配。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// let five = Box::new_in(5, System);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn new_in(x: T, alloc: A) -> Self {
        let mut boxed = Self::new_uninit_in(alloc);
        unsafe {
            boxed.as_mut_ptr().write(x);
            boxed.assume_init()
        }
    }

    /// 在给定的分配器中分配内存，然后将 `x` 放入其中，如果分配失败，则返回错误
    ///
    ///
    /// 如果 `T` 的大小为零，则实际上不会分配。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// let five = Box::try_new_in(5, System)?;
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError> {
        let mut boxed = Self::try_new_uninit_in(alloc)?;
        unsafe {
            boxed.as_mut_ptr().write(x);
            Ok(boxed.assume_init())
        }
    }

    /// 在提供的分配器中创建一个具有未初始化内容的新 box。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let mut five = Box::<u32, _>::new_uninit_in(System);
    ///
    /// let five = unsafe {
    ///     // 延迟初始化:
    ///     five.as_mut_ptr().write(5);
    ///
    ///     five.assume_init()
    /// };
    ///
    /// assert_eq!(*five, 5)
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[cfg(not(no_global_oom_handling))]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
        let layout = Layout::new::<mem::MaybeUninit<T>>();
        // NOTE: 优先选择不匹配 unwrap_or_else 的匹配项，因为有时闭包不是内联的。
        // 那将使代码更大。
        match Box::try_new_uninit_in(alloc) {
            Ok(m) => m,
            Err(_) => handle_alloc_error(layout),
        }
    }

    /// 在提供的分配器中创建一个具有未初始化内容的新 box，如果分配失败，则返回错误
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
    ///
    /// let five = unsafe {
    ///     // 延迟初始化:
    ///     five.as_mut_ptr().write(5);
    ///
    ///     five.assume_init()
    /// };
    ///
    /// assert_eq!(*five, 5);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
        let layout = Layout::new::<mem::MaybeUninit<T>>();
        let ptr = alloc.allocate(layout)?.cast();
        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
    }

    /// 创建一个具有未初始化内容的新 `Box`，使用提供的分配器中的 `0` 字节填充内存。
    ///
    ///
    /// 有关正确和不正确使用此方法的示例，请参见 [`MaybeUninit::zeroed`][zeroed]。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let zero = Box::<u32, _>::new_zeroed_in(System);
    /// let zero = unsafe { zero.assume_init() };
    ///
    /// assert_eq!(*zero, 0)
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    ///
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[cfg(not(no_global_oom_handling))]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
        let layout = Layout::new::<mem::MaybeUninit<T>>();
        // NOTE: 优先选择不匹配 unwrap_or_else 的匹配项，因为有时闭包不是内联的。
        // 那将使代码更大。
        match Box::try_new_zeroed_in(alloc) {
            Ok(m) => m,
            Err(_) => handle_alloc_error(layout),
        }
    }

    /// 创建一个具有未初始化内容的新 `Box`，使用提供的分配器中的 `0` 字节填充内存，如果分配失败，则返回错误，
    ///
    ///
    /// 有关正确和不正确使用此方法的示例，请参见 [`MaybeUninit::zeroed`][zeroed]。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
    /// let zero = unsafe { zero.assume_init() };
    ///
    /// assert_eq!(*zero, 0);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    ///
    ///
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
        let layout = Layout::new::<mem::MaybeUninit<T>>();
        let ptr = alloc.allocate_zeroed(layout)?.cast();
        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
    }

    /// 创建一个新的 `Pin<Box<T, A>>`。
    /// 如果 `T` 未实现 `Unpin`，则 `x` 将被固定在内存中并且无法移动。
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline(always)]
    pub fn pin_in(x: T, alloc: A) -> Pin<Self>
    where
        A: 'static,
    {
        Self::new_in(x, alloc).into()
    }

    /// 将 `Box<T>` 转换为 `Box<[T]>`
    ///
    /// 这种转换不会在堆上分配，而是就地进行。
    #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
    pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
        let (raw, alloc) = Box::into_raw_with_allocator(boxed);
        unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
    }

    /// 消耗 `Box`，返回包装的值。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(box_into_inner)]
    ///
    /// let c = Box::new(5);
    ///
    /// assert_eq!(Box::into_inner(c), 5);
    /// ```
    #[unstable(feature = "box_into_inner", issue = "80437")]
    #[inline]
    pub fn into_inner(boxed: Self) -> T {
        *boxed
    }
}

impl<T> Box<[T]> {
    /// 创建一个具有未初始化内容的新 boxed 切片。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
    ///
    /// let values = unsafe {
    ///     // 延迟初始化:
    ///     values[0].as_mut_ptr().write(1);
    ///     values[1].as_mut_ptr().write(2);
    ///     values[2].as_mut_ptr().write(3);
    ///
    ///     values.assume_init()
    /// };
    ///
    /// assert_eq!(*values, [1, 2, 3])
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
        unsafe { RawVec::with_capacity(len).into_box(len) }
    }

    /// 创建一个具有未初始化内容的新 boxed 切片，并用 `0` 字节填充内存。
    ///
    ///
    /// 有关正确和不正确使用此方法的示例，请参见 [`MaybeUninit::zeroed`][zeroed]。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let values = Box::<[u32]>::new_zeroed_slice(3);
    /// let values = unsafe { values.assume_init() };
    ///
    /// assert_eq!(*values, [0, 0, 0])
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    ///
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
        unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
    }
}

impl<T, A: Allocator> Box<[T], A> {
    /// 使用提供的分配器中未初始化的内容创建一个新的 boxed 切片。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
    ///
    /// let values = unsafe {
    ///     // 延迟初始化:
    ///     values[0].as_mut_ptr().write(1);
    ///     values[1].as_mut_ptr().write(2);
    ///     values[2].as_mut_ptr().write(3);
    ///
    ///     values.assume_init()
    /// };
    ///
    /// assert_eq!(*values, [1, 2, 3])
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
        unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
    }

    /// 使用提供的分配器中未初始化的内容创建一个新的 boxed 切片，并用 `0` 字节填充内存。
    ///
    ///
    /// 有关正确和不正确使用此方法的示例，请参见 [`MaybeUninit::zeroed`][zeroed]。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
    /// let values = unsafe { values.assume_init() };
    ///
    /// assert_eq!(*values, [0, 0, 0])
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    ///
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
        unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
    }
}

impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
    /// 转换为 `Box<T, A>`。
    ///
    /// # Safety
    ///
    /// 与 [`MaybeUninit::assume_init`] 一样，由调用方负责确保该值确实处于初始化状态。
    ///
    /// 在内容尚未完全初始化时调用此方法会立即导致未定义的行为。
    ///
    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let mut five = Box::<u32>::new_uninit();
    ///
    /// let five: Box<u32> = unsafe {
    ///     // 延迟初始化:
    ///     five.as_mut_ptr().write(5);
    ///
    ///     five.assume_init()
    /// };
    ///
    /// assert_eq!(*five, 5)
    /// ```
    ///
    ///
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[inline]
    pub unsafe fn assume_init(self) -> Box<T, A> {
        let (raw, alloc) = Box::into_raw_with_allocator(self);
        unsafe { Box::from_raw_in(raw as *mut T, alloc) }
    }
}

impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
    /// 转换为 `Box<[T], A>`。
    ///
    /// # Safety
    ///
    /// 与 [`MaybeUninit::assume_init`] 一样，由调用方负责确保值确实处于初始化状态。
    ///
    /// 在内容尚未完全初始化时调用此方法会立即导致未定义的行为。
    ///
    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
    ///
    /// let values = unsafe {
    ///     // 延迟初始化:
    ///     values[0].as_mut_ptr().write(1);
    ///     values[1].as_mut_ptr().write(2);
    ///     values[2].as_mut_ptr().write(3);
    ///
    ///     values.assume_init()
    /// };
    ///
    /// assert_eq!(*values, [1, 2, 3])
    /// ```
    ///
    ///
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[inline]
    pub unsafe fn assume_init(self) -> Box<[T], A> {
        let (raw, alloc) = Box::into_raw_with_allocator(self);
        unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
    }
}

impl<T: ?Sized> Box<T> {
    /// 从裸指针构造 box。
    ///
    /// 调用此函数后，结果 `Box` 拥有裸指针。
    /// 具体来说，`Box` 析构函数将调用 `T` 的析构函数并释放分配的内存。
    /// 为了安全起见，必须根据 `Box` 所使用的 [memory layout] 分配内存。
    ///
    ///
    /// # Safety
    ///
    /// 此函数不安全，因为使用不当可能会导致内存问题。
    /// 例如，如果在同一裸指针上两次调用该函数，则可能会出现 double-free。
    ///
    /// 安全条件在 [memory layout] 部分中进行了描述。
    ///
    /// # Examples
    ///
    /// 重新创建以前使用 [`Box::into_raw`] 转换为裸指针的 `Box`:
    ///
    /// ```
    /// let x = Box::new(5);
    /// let ptr = Box::into_raw(x);
    /// let x = unsafe { Box::from_raw(ptr) };
    /// ```
    ///
    /// 使用二进制分配器从头开始手动创建 `Box`:
    ///
    /// ```
    /// use std::alloc::{alloc, Layout};
    ///
    /// unsafe {
    ///     let ptr = alloc(Layout::new::<i32>()) as *mut i32;
    ///     // 通常，需要 .write 以避免尝试销毁 `ptr` 以前的内容，尽管对于这个简单的示例 `*ptr = 5` 也可以工作。
    /////
    /////
    ///     ptr.write(5);
    ///     let x = Box::from_raw(ptr);
    /// }
    /// ```
    ///
    /// [memory layout]: self#memory-layout
    /// [`Layout`]: crate::Layout
    #[stable(feature = "box_raw", since = "1.4.0")]
    #[inline]
    pub unsafe fn from_raw(raw: *mut T) -> Self {
        unsafe { Self::from_raw_in(raw, Global) }
    }
}

impl<T: ?Sized, A: Allocator> Box<T, A> {
    /// 从给定分配器中的裸指针构造 box。
    ///
    /// 调用此函数后，结果 `Box` 拥有裸指针。
    /// 具体来说，`Box` 析构函数将调用 `T` 的析构函数并释放分配的内存。
    /// 为了安全起见，必须根据 `Box` 所使用的 [memory layout] 分配内存。
    ///
    ///
    /// # Safety
    ///
    /// 此函数不安全，因为使用不当可能会导致内存问题。
    /// 例如，如果在同一裸指针上两次调用该函数，则可能会出现 double-free。
    ///
    /// # Examples
    ///
    /// 重新创建以前使用 [`Box::into_raw_with_allocator`] 转换为裸指针的 `Box`:
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// let x = Box::new_in(5, System);
    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
    /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
    /// ```
    ///
    /// 使用系统分配器从头开始手动创建 `Box`:
    ///
    /// ```
    /// #![feature(allocator_api, slice_ptr_get)]
    ///
    /// use std::alloc::{Allocator, Layout, System};
    ///
    /// unsafe {
    ///     let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
    ///     // 通常，需要 .write 以避免尝试销毁 `ptr` 以前的内容，尽管对于这个简单的示例 `*ptr = 5` 也可以工作。
    /////
    /////
    ///     ptr.write(5);
    ///     let x = Box::from_raw_in(ptr, System);
    /// }
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    ///
    /// [memory layout]: self#memory-layout
    /// [`Layout`]: crate::Layout
    ///
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
        Box(unsafe { Unique::new_unchecked(raw) }, alloc)
    }

    /// 消耗 `Box`，并返回一个包装的裸指针。
    ///
    /// 指针将正确对齐且不为空。
    ///
    /// 调用此函数后，调用者将负责先前由 `Box` 管理的内存。
    /// 特别地，考虑到 `Box` 使用的 [memory layout]，调用者应正确销毁 `T` 并释放内存。
    /// 最简单的方法是使用 [`Box::from_raw`] 函数将裸指针转换回 `Box`，从而允许 `Box` 析构函数执行清理。
    ///
    ///
    /// Note: 这是一个关联函数，这意味着您必须将其称为 `Box::into_raw(b)` 而不是 `b.into_raw()`。
    /// 这样就不会与内部类型的方法发生冲突。
    ///
    /// # Examples
    /// 使用 [`Box::from_raw`] 将裸指针转换回 `Box` 以进行自动清理:
    ///
    /// ```
    /// let x = Box::new(String::from("Hello"));
    /// let ptr = Box::into_raw(x);
    /// let x = unsafe { Box::from_raw(ptr) };
    /// ```
    ///
    /// 通过显式运行析构函数并释放内存来进行手动清理:
    ///
    /// ```
    /// use std::alloc::{dealloc, Layout};
    /// use std::ptr;
    ///
    /// let x = Box::new(String::from("Hello"));
    /// let p = Box::into_raw(x);
    /// unsafe {
    ///     ptr::drop_in_place(p);
    ///     dealloc(p as *mut u8, Layout::new::<String>());
    /// }
    /// ```
    ///
    /// [memory layout]: self#memory-layout
    ///
    ///
    ///
    #[stable(feature = "box_raw", since = "1.4.0")]
    #[inline]
    pub fn into_raw(b: Self) -> *mut T {
        Self::into_raw_with_allocator(b).0
    }

    /// 消耗 `Box`，返回包装的裸指针和分配器。
    ///
    /// 指针将正确对齐且不为空。
    ///
    /// 调用此函数后，调用者将负责先前由 `Box` 管理的内存。
    /// 特别地，考虑到 `Box` 使用的 [memory layout]，调用者应正确销毁 `T` 并释放内存。
    /// 最简单的方法是使用 [`Box::from_raw_in`] 函数将裸指针转换回 `Box`，从而允许 `Box` 析构函数执行清理。
    ///
    ///
    /// Note: 这是一个关联函数，这意味着您必须将其称为 `Box::into_raw_with_allocator(b)` 而不是 `b.into_raw_with_allocator()`。
    /// 这样就不会与内部类型的方法发生冲突。
    ///
    /// # Examples
    /// 使用 [`Box::from_raw_in`] 将裸指针转换回 `Box` 以进行自动清理:
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// let x = Box::new_in(String::from("Hello"), System);
    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
    /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
    /// ```
    ///
    /// 通过显式运行析构函数并释放内存来进行手动清理:
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::{Allocator, Layout, System};
    /// use std::ptr::{self, NonNull};
    ///
    /// let x = Box::new_in(String::from("Hello"), System);
    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
    /// unsafe {
    ///     ptr::drop_in_place(ptr);
    ///     let non_null = NonNull::new_unchecked(ptr);
    ///     alloc.deallocate(non_null.cast(), Layout::new::<String>());
    /// }
    /// ```
    ///
    /// [memory layout]: self#memory-layout
    ///
    ///
    ///
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
        let (leaked, alloc) = Box::into_unique(b);
        (leaked.as_ptr(), alloc)
    }

    #[unstable(
        feature = "ptr_internals",
        issue = "none",
        reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
    )]
    #[inline]
    #[doc(hidden)]
    pub fn into_unique(b: Self) -> (Unique<T>, A) {
        // Box 被 Stacked 借用识别为 "unique pointer"，但在内部它是类型系统的裸指针。
        // 将其直接转换为裸指针将不会被视为 "releasing" 允许别名原始访问的唯一指针，因此所有裸指针方法都必须通过 `Box::leak`。
        //
        // 将其转换为裸指针的行为是正确的。
        //
        let alloc = unsafe { ptr::read(&b.1) };
        (Unique::from(Box::leak(b)), alloc)
    }

    /// 返回基础分配器的引用。
    ///
    /// Note: 这是一个关联函数，这意味着您必须将其称为 `Box::allocator(&b)` 而不是 `b.allocator()`。
    /// 这样就不会与内部类型的方法发生冲突。
    ///
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn allocator(b: &Self) -> &A {
        &b.1
    }

    /// 消耗并泄漏 `Box`，返回变量引用， `&'a mut T`.
    /// 请注意，类型 `T` 必须超过所选的生命周期 `'a`。
    /// 如果类型仅具有静态引用，或者根本没有静态引用，则可以将其选择为 `'static`。
    ///
    /// 该函数主要用于在程序的剩余生命期内保留的数据。
    /// 丢弃返回的引用将导致内存泄漏。
    /// 如果这是不可接受的，则应首先将引用与 [`Box::from_raw`] 函数包装在一起，生成 `Box`。
    ///
    /// 这个 `Box` 可以被丢弃，这将正确销毁 `T` 并释放分配的内存。
    ///
    /// Note: 这是一个关联函数，这意味着您必须将其称为 `Box::leak(b)` 而不是 `b.leak()`。
    /// 这样就不会与内部类型的方法发生冲突。
    ///
    /// # Examples
    ///
    /// 简单用法:
    ///
    /// ```
    /// let x = Box::new(41);
    /// let static_ref: &'static mut usize = Box::leak(x);
    /// *static_ref += 1;
    /// assert_eq!(*static_ref, 42);
    /// ```
    ///
    /// 未定义大小的数据:
    ///
    /// ```
    /// let x = vec![1, 2, 3].into_boxed_slice();
    /// let static_ref = Box::leak(x);
    /// static_ref[0] = 4;
    /// assert_eq!(*static_ref, [4, 2, 3]);
    /// ```
    ///
    ///
    ///
    #[stable(feature = "box_leak", since = "1.26.0")]
    #[inline]
    pub fn leak<'a>(b: Self) -> &'a mut T
    where
        A: 'a,
    {
        unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
    }

    /// 将 `Box<T>` 转换为 `Pin<Box<T>>`
    ///
    /// 这种转换不会在堆上分配，而是就地进行。
    ///
    /// 也可以通过 [`From`] 获得。
    #[unstable(feature = "box_into_pin", issue = "62370")]
    pub fn into_pin(boxed: Self) -> Pin<Self>
    where
        A: 'static,
    {
        // `T: !Unpin` 时，不能移动或更换 `Pin<Box<T>>` 的内部，因此可以安全地直接固定 `Pin<Box<T>>`，而无需任何其他要求。
        //
        //
        unsafe { Pin::new_unchecked(boxed) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
    fn drop(&mut self) {
        // FIXME: 不执行任何操作，当前由编译器执行丢弃。
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for Box<T> {
    /// 创建一个 `Box<T>`，其 T 值为 `Default`。
    fn default() -> Self {
        box T::default()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Default for Box<[T]> {
    fn default() -> Self {
        Box::<[T; 0]>::new([])
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "default_box_extra", since = "1.17.0")]
impl Default for Box<str> {
    fn default() -> Self {
        unsafe { from_boxed_utf8_unchecked(Default::default()) }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
    /// 返回带有此 box 的 内容的 `clone()` 的新 box。
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Box::new(5);
    /// let y = x.clone();
    ///
    /// // 值是一样的
    /// assert_eq!(x, y);
    ///
    /// // 但是它们是独特的对象
    /// assert_ne!(&*x as *const i32, &*y as *const i32);
    /// ```
    #[inline]
    fn clone(&self) -> Self {
        // 预分配内存以允许直接写入克隆的值。
        let mut boxed = Self::new_uninit_in(self.1.clone());
        unsafe {
            (**self).write_clone_into_raw(boxed.as_mut_ptr());
            boxed.assume_init()
        }
    }

    /// 将 `source` 的内容复制到 `self`，而不创建新的分配。
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Box::new(5);
    /// let mut y = Box::new(10);
    /// let yp: *const i32 = &*y;
    ///
    /// y.clone_from(&x);
    ///
    /// // 值是一样的
    /// assert_eq!(x, y);
    ///
    /// // 没有分配发生
    /// assert_eq!(yp, &*y);
    /// ```
    #[inline]
    fn clone_from(&mut self, source: &Self) {
        (**self).clone_from(&(**source));
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_slice_clone", since = "1.3.0")]
impl Clone for Box<str> {
    fn clone(&self) -> Self {
        // 这将复制数据
        let buf: Box<[u8]> = self.as_bytes().into();
        unsafe { from_boxed_utf8_unchecked(buf) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        PartialEq::eq(&**self, &**other)
    }
    #[inline]
    fn ne(&self, other: &Self) -> bool {
        PartialEq::ne(&**self, &**other)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
    #[inline]
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        PartialOrd::partial_cmp(&**self, &**other)
    }
    #[inline]
    fn lt(&self, other: &Self) -> bool {
        PartialOrd::lt(&**self, &**other)
    }
    #[inline]
    fn le(&self, other: &Self) -> bool {
        PartialOrd::le(&**self, &**other)
    }
    #[inline]
    fn ge(&self, other: &Self) -> bool {
        PartialOrd::ge(&**self, &**other)
    }
    #[inline]
    fn gt(&self, other: &Self) -> bool {
        PartialOrd::gt(&**self, &**other)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
    #[inline]
    fn cmp(&self, other: &Self) -> Ordering {
        Ord::cmp(&**self, &**other)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        (**self).hash(state);
    }
}

#[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
    fn finish(&self) -> u64 {
        (**self).finish()
    }
    fn write(&mut self, bytes: &[u8]) {
        (**self).write(bytes)
    }
    fn write_u8(&mut self, i: u8) {
        (**self).write_u8(i)
    }
    fn write_u16(&mut self, i: u16) {
        (**self).write_u16(i)
    }
    fn write_u32(&mut self, i: u32) {
        (**self).write_u32(i)
    }
    fn write_u64(&mut self, i: u64) {
        (**self).write_u64(i)
    }
    fn write_u128(&mut self, i: u128) {
        (**self).write_u128(i)
    }
    fn write_usize(&mut self, i: usize) {
        (**self).write_usize(i)
    }
    fn write_i8(&mut self, i: i8) {
        (**self).write_i8(i)
    }
    fn write_i16(&mut self, i: i16) {
        (**self).write_i16(i)
    }
    fn write_i32(&mut self, i: i32) {
        (**self).write_i32(i)
    }
    fn write_i64(&mut self, i: i64) {
        (**self).write_i64(i)
    }
    fn write_i128(&mut self, i: i128) {
        (**self).write_i128(i)
    }
    fn write_isize(&mut self, i: isize) {
        (**self).write_isize(i)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "from_for_ptrs", since = "1.6.0")]
impl<T> From<T> for Box<T> {
    /// 将 `T` 转换为 `Box<T>`
    ///
    /// 转换在堆上分配，并将 `t` 从栈移到堆中。
    ///
    /// # Examples
    ///
    /// ```rust
    /// let x = 5;
    /// let boxed = Box::new(5);
    ///
    /// assert_eq!(Box::from(x), boxed);
    /// ```
    fn from(t: T) -> Self {
        Box::new(t)
    }
}

#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Pin<Box<T, A>>
where
    A: 'static,
{
    /// 将 `Box<T>` 转换为 `Pin<Box<T>>`
    ///
    /// 这种转换不会在堆上分配，而是就地进行。
    fn from(boxed: Box<T, A>) -> Self {
        Box::into_pin(boxed)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_from_slice", since = "1.17.0")]
impl<T: Copy> From<&[T]> for Box<[T]> {
    /// 将 `&[T]` 转换为 `Box<[T]>`
    ///
    /// 此转换在堆上分配并执行 `slice` 的副本。
    ///
    /// # Examples
    ///
    /// ```rust
    /// // 创建 &[u8] which 将用于创建 Box<[u8]>
    /// let slice: &[u8] = &[104, 101, 108, 108, 111];
    /// let boxed_slice: Box<[u8]> = Box::from(slice);
    ///
    /// println!("{:?}", boxed_slice);
    /// ```
    fn from(slice: &[T]) -> Box<[T]> {
        let len = slice.len();
        let buf = RawVec::with_capacity(len);
        unsafe {
            ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
            buf.into_box(slice.len()).assume_init()
        }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_from_cow", since = "1.45.0")]
impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
    #[inline]
    fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
        match cow {
            Cow::Borrowed(slice) => Box::from(slice),
            Cow::Owned(slice) => Box::from(slice),
        }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_from_slice", since = "1.17.0")]
impl From<&str> for Box<str> {
    /// 将 `&str` 转换为 `Box<str>`
    ///
    /// 此转换在堆上分配并执行 `s` 的副本。
    ///
    /// # Examples
    ///
    /// ```rust
    /// let boxed: Box<str> = Box::from("hello");
    /// println!("{}", boxed);
    /// ```
    #[inline]
    fn from(s: &str) -> Box<str> {
        unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_from_cow", since = "1.45.0")]
impl From<Cow<'_, str>> for Box<str> {
    #[inline]
    fn from(cow: Cow<'_, str>) -> Box<str> {
        match cow {
            Cow::Borrowed(s) => Box::from(s),
            Cow::Owned(s) => Box::from(s),
        }
    }
}

#[stable(feature = "boxed_str_conv", since = "1.19.0")]
impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
    /// 将 `Box<str>` 转换为 `Box<[u8]>`
    ///
    /// 这种转换不会在堆上分配，而是就地进行。
    ///
    /// # Examples
    /// ```rust
    /// // 创建一个 Box<str>，该 Box<str> 将用于创建 Box<[u8]>
    /// let boxed: Box<str> = Box::from("hello");
    /// let boxed_str: Box<[u8]> = Box::from(boxed);
    ///
    /// // 创建 &[u8] which 将用于创建 Box<[u8]>
    /// let slice: &[u8] = &[104, 101, 108, 108, 111];
    /// let boxed_slice = Box::from(slice);
    ///
    /// assert_eq!(boxed_slice, boxed_str);
    /// ```
    #[inline]
    fn from(s: Box<str, A>) -> Self {
        let (raw, alloc) = Box::into_raw_with_allocator(s);
        unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
    }
}

#[stable(feature = "box_from_array", since = "1.45.0")]
impl<T, const N: usize> From<[T; N]> for Box<[T]> {
    /// 将 `[T; N]` 转换为 `Box<[T]>`
    ///
    /// 此转换将数组移动到新的堆分配的内存中。
    ///
    /// # Examples
    /// ```rust
    /// let boxed: Box<[u8]> = Box::from([4, 2]);
    /// println!("{:?}", boxed);
    /// ```
    fn from(array: [T; N]) -> Box<[T]> {
        box array
    }
}

#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
    type Error = Box<[T]>;

    fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
        if boxed_slice.len() == N {
            Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
        } else {
            Err(boxed_slice)
        }
    }
}

impl<A: Allocator> Box<dyn Any, A> {
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    /// 尝试将 box 转换为具体类型。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::any::Any;
    ///
    /// fn print_if_string(value: Box<dyn Any>) {
    ///     if let Ok(string) = value.downcast::<String>() {
    ///         println!("String ({}): {}", string.len(), string);
    ///     }
    /// }
    ///
    /// let my_string = "Hello World".to_string();
    /// print_if_string(Box::new(my_string));
    /// print_if_string(Box::new(0i8));
    /// ```
    pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
        if self.is::<T>() {
            unsafe {
                let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
                Ok(Box::from_raw_in(raw as *mut T, alloc))
            }
        } else {
            Err(self)
        }
    }
}

impl<A: Allocator> Box<dyn Any + Send, A> {
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    /// 尝试将 box 转换为具体类型。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::any::Any;
    ///
    /// fn print_if_string(value: Box<dyn Any + Send>) {
    ///     if let Ok(string) = value.downcast::<String>() {
    ///         println!("String ({}): {}", string.len(), string);
    ///     }
    /// }
    ///
    /// let my_string = "Hello World".to_string();
    /// print_if_string(Box::new(my_string));
    /// print_if_string(Box::new(0i8));
    /// ```
    pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
        if self.is::<T>() {
            unsafe {
                let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
                Ok(Box::from_raw_in(raw as *mut T, alloc))
            }
        } else {
            Err(self)
        }
    }
}

impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
    #[inline]
    #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
    /// 尝试将 box 转换为具体类型。
    ///
    /// # Examples
    ///
    /// ```
    /// use std::any::Any;
    ///
    /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
    ///     if let Ok(string) = value.downcast::<String>() {
    ///         println!("String ({}): {}", string.len(), string);
    ///     }
    /// }
    ///
    /// let my_string = "Hello World".to_string();
    /// print_if_string(Box::new(my_string));
    /// print_if_string(Box::new(0i8));
    /// ```
    pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
        if self.is::<T>() {
            unsafe {
                let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
                    Box::into_raw_with_allocator(self);
                Ok(Box::from_raw_in(raw as *mut T, alloc))
            }
        } else {
            Err(self)
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&**self, f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        // 无法直接从 Box 提取内部 Uniq，而是将其强制转换为 *const which 别名 (唯一)
        //
        let ptr: *const T = &**self;
        fmt::Pointer::fmt(&ptr, f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
    type Target = T;

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

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
    fn deref_mut(&mut self) -> &mut T {
        &mut **self
    }
}

#[unstable(feature = "receiver_trait", issue = "none")]
impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
    type Item = I::Item;
    fn next(&mut self) -> Option<I::Item> {
        (**self).next()
    }
    fn size_hint(&self) -> (usize, Option<usize>) {
        (**self).size_hint()
    }
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        (**self).nth(n)
    }
    fn last(self) -> Option<I::Item> {
        BoxIter::last(self)
    }
}

trait BoxIter {
    type Item;
    fn last(self) -> Option<Self::Item>;
}

impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
    type Item = I::Item;
    default fn last(self) -> Option<I::Item> {
        #[inline]
        fn some<T>(_: Option<T>, x: T) -> Option<T> {
            Some(x)
        }

        self.fold(None, some)
    }
}

/// 使用 `last()` 的 I 实现而不是默认值的大小 I 的专业化。
///
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
    fn last(self) -> Option<I::Item> {
        (*self).last()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
    fn next_back(&mut self) -> Option<I::Item> {
        (**self).next_back()
    }
    fn nth_back(&mut self, n: usize) -> Option<I::Item> {
        (**self).nth_back(n)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
    fn len(&self) -> usize {
        (**self).len()
    }
    fn is_empty(&self) -> bool {
        (**self).is_empty()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}

#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
impl<Args, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
    type Output = <F as FnOnce<Args>>::Output;

    extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
        <F as FnOnce<Args>>::call_once(*self, args)
    }
}

#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
impl<Args, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
        <F as FnMut<Args>>::call_mut(self, args)
    }
}

#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
impl<Args, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
    extern "rust-call" fn call(&self, args: Args) -> Self::Output {
        <F as Fn<Args>>::call(self, args)
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}

#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
impl<I> FromIterator<I> for Box<[I]> {
    fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
        iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_slice_clone", since = "1.3.0")]
impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
    fn clone(&self) -> Self {
        let alloc = Box::allocator(self).clone();
        self.to_vec_in(alloc).into_boxed_slice()
    }

    fn clone_from(&mut self, other: &Self) {
        if self.len() == other.len() {
            self.clone_from_slice(&other);
        } else {
            *self = other.clone();
        }
    }
}

#[stable(feature = "box_borrow", since = "1.1.0")]
impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
    fn borrow(&self) -> &T {
        &**self
    }
}

#[stable(feature = "box_borrow", since = "1.1.0")]
impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
    fn borrow_mut(&mut self) -> &mut T {
        &mut **self
    }
}

#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
    fn as_ref(&self) -> &T {
        &**self
    }
}

#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
    fn as_mut(&mut self) -> &mut T {
        &mut **self
    }
}

/* Nota bene
 *
 *  We could have chosen not to add this impl, and instead have written a
 *  function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
 *  because Box<T> implements Unpin even when T does not, as a result of
 *  this impl.
 *
 *  We chose this API instead of the alternative for a few reasons:
 *      - Logically, it is helpful to understand pinning in regard to the
 *        memory region being pointed to. For this reason none of the
 *        standard library pointer types support projecting through a pin
 *        (Box<T> is the only pointer type in std for which this would be
 *        safe.)
 *      - It is in practice very useful to have Box<T> be unconditionally
 *        Unpin because of trait objects, for which the structural auto
 *        trait functionality does not apply (e.g., Box<dyn Foo> would
 *        otherwise not be Unpin).
 *
 *  Another type with the same semantics as Box but only a conditional
 *  implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
 *  could have a method to project a Pin<T> from it.
 */
#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> where A: 'static {}

#[unstable(feature = "generator_trait", issue = "43122")]
impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
where
    A: 'static,
{
    type Yield = G::Yield;
    type Return = G::Return;

    fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
        G::resume(Pin::new(&mut *self), arg)
    }
}

#[unstable(feature = "generator_trait", issue = "43122")]
impl<G: ?Sized + Generator<R>, R, A: Allocator> Generator<R> for Pin<Box<G, A>>
where
    A: 'static,
{
    type Yield = G::Yield;
    type Return = G::Return;

    fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
        G::resume((*self).as_mut(), arg)
    }
}

#[stable(feature = "futures_api", since = "1.36.0")]
impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
where
    A: 'static,
{
    type Output = F::Output;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        F::poll(Pin::new(&mut *self), cx)
    }
}

#[unstable(feature = "async_stream", issue = "79024")]
impl<S: ?Sized + Stream + Unpin> Stream for Box<S> {
    type Item = S::Item;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        Pin::new(&mut **self).poll_next(cx)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (**self).size_hint()
    }
}