Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2429 insertions, 0 deletions

diff --git a/rust/alloc/boxed.rs b/rust/alloc/boxed.rs
new file mode 100644
index 0000000000..c8173cea83
--- /dev/null
+++ b/rust/alloc/boxed.rs
@@ -0,0 +1,2429 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! The `Box<T>` type for heap allocation.
+//!
+//! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
+//! heap allocation in Rust. Boxes provide ownership for this allocation, and
+//! drop their contents when they go out of scope. Boxes also ensure that they
+//! never allocate more than `isize::MAX` bytes.
+//!
+//! # Examples
+//!
+//! Move a value from the stack to the heap by creating a [`Box`]:
+//!
+//! ```
+//! let val: u8 = 5;
+//! let boxed: Box<u8> = Box::new(val);
+//! ```
+//!
+//! Move a value from a [`Box`] back to the stack by [dereferencing]:
+//!
+//! ```
+//! let boxed: Box<u8> = Box::new(5);
+//! let val: u8 = *boxed;
+//! ```
+//!
+//! Creating a recursive data structure:
+//!
+//! ```
+//! #[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:?}");
+//! ```
+//!
+//! This will print `Cons(1, Cons(2, Nil))`.
+//!
+//! Recursive structures must be boxed, because if the definition of `Cons`
+//! looked like this:
+//!
+//! ```compile_fail,E0072
+//! # enum List<T> {
+//! Cons(T, List<T>),
+//! # }
+//! ```
+//!
+//! It wouldn't work. This is because the size of a `List` depends on how many
+//! elements are in the list, and so we don't know how much memory to allocate
+//! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
+//! big `Cons` needs to be.
+//!
+//! # Memory layout
+//!
+//! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
+//! its allocation. It is valid to convert both ways between a [`Box`] and a
+//! raw pointer allocated with the [`Global`] allocator, given that the
+//! [`Layout`] used with the allocator is correct for the type. More precisely,
+//! a `value: *mut T` that has been allocated with the [`Global`] allocator
+//! with `Layout::for_value(&*value)` may be converted into a box using
+//! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
+//! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
+//! [`Global`] allocator with [`Layout::for_value(&*value)`].
+//!
+//! For zero-sized values, the `Box` pointer still has to be [valid] for reads
+//! and writes and sufficiently aligned. In particular, casting any aligned
+//! non-zero integer literal to a raw pointer produces a valid pointer, but a
+//! pointer pointing into previously allocated memory that since got freed is
+//! not valid. The recommended way to build a Box to a ZST if `Box::new` cannot
+//! be used is to use [`ptr::NonNull::dangling`].
+//!
+//! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
+//! as a single pointer and is also ABI-compatible with C pointers
+//! (i.e. the C type `T*`). This means that if you have extern "C"
+//! Rust functions that will be called from C, you can define those
+//! Rust functions using `Box<T>` types, and use `T*` as corresponding
+//! type on the C side. As an example, consider this C header which
+//! declares functions that create and destroy some kind of `Foo`
+//! value:
+//!
+//! ```c
+//! /* C header */
+//!
+//! /* Returns ownership to the caller */
+//! struct Foo* foo_new(void);
+//!
+//! /* Takes ownership from the caller; no-op when invoked with null */
+//! void foo_delete(struct Foo*);
+//! ```
+//!
+//! These two functions might be implemented in Rust as follows. Here, the
+//! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
+//! the ownership constraints. Note also that the nullable argument to
+//! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
+//! cannot be null.
+//!
+//! ```
+//! #[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>>) {}
+//! ```
+//!
+//! Even though `Box<T>` has the same representation and C ABI as a C pointer,
+//! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
+//! and expect things to work. `Box<T>` values will always be fully aligned,
+//! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
+//! free the value with the global allocator. In general, the best practice
+//! is to only use `Box<T>` for pointers that originated from the global
+//! allocator.
+//!
+//! **Important.** At least at present, you should avoid using
+//! `Box<T>` types for functions that are defined in C but invoked
+//! from Rust. In those cases, you should directly mirror the C types
+//! as closely as possible. Using types like `Box<T>` where the C
+//! definition is just using `T*` can lead to undefined behavior, as
+//! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
+//!
+//! # Considerations for unsafe code
+//!
+//! **Warning: This section is not normative and is subject to change, possibly
+//! being relaxed in the future! It is a simplified summary of the rules
+//! currently implemented in the compiler.**
+//!
+//! The aliasing rules for `Box<T>` are the same as for `&mut T`. `Box<T>`
+//! asserts uniqueness over its content. Using raw pointers derived from a box
+//! after that box has been mutated through, moved or borrowed as `&mut T`
+//! is not allowed. For more guidance on working with box from unsafe code, see
+//! [rust-lang/unsafe-code-guidelines#326][ucg#326].
+//!
+//!
+//! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
+//! [ucg#326]: https://github.com/rust-lang/unsafe-code-guidelines/issues/326
+//! [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::async_iter::AsyncIterator;
+use core::borrow;
+use core::cmp::Ordering;
+use core::error::Error;
+use core::fmt;
+use core::future::Future;
+use core::hash::{Hash, Hasher};
+use core::iter::FusedIterator;
+use core::marker::Tuple;
+use core::marker::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::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;
+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::string::String;
+#[cfg(not(no_global_oom_handling))]
+use crate::vec::Vec;
+
+#[cfg(not(no_thin))]
+#[unstable(feature = "thin_box", issue = "92791")]
+pub use thin::ThinBox;
+
+#[cfg(not(no_thin))]
+mod thin;
+
+/// A pointer type that uniquely owns a heap allocation of type `T`.
+///
+/// See the [module-level documentation](../../std/boxed/index.html) for more.
+#[lang = "owned_box"]
+#[fundamental]
+#[stable(feature = "rust1", since = "1.0.0")]
+// The declaration of the `Box` struct must be kept in sync with the
+// `alloc::alloc::box_free` function or ICEs will happen. See the comment
+// on `box_free` for more details.
+pub struct Box<
+    T: ?Sized,
+    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
+>(Unique<T>, A);
+
+impl<T> Box<T> {
+    /// Allocates memory on the heap and then places `x` into it.
+    ///
+    /// This doesn't actually allocate if `T` is zero-sized.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let five = Box::new(5);
+    /// ```
+    #[cfg(all(not(no_global_oom_handling)))]
+    #[inline(always)]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
+    #[rustc_diagnostic_item = "box_new"]
+    pub fn new(x: T) -> Self {
+        #[rustc_box]
+        Box::new(x)
+    }
+
+    /// Constructs a new box with uninitialized contents.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// let mut five = Box::<u32>::new_uninit();
+    ///
+    /// let five = unsafe {
+    ///     // Deferred initialization:
+    ///     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")]
+    #[must_use]
+    #[inline]
+    pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
+        Self::new_uninit_in(Global)
+    }
+
+    /// Constructs a new `Box` with uninitialized contents, with the memory
+    /// being filled with `0` bytes.
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # 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")]
+    #[must_use]
+    pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
+        Self::new_zeroed_in(Global)
+    }
+
+    /// Constructs a new `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
+    /// `x` will be pinned in memory and unable to be moved.
+    ///
+    /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin(x)`
+    /// does the same as <code>[Box::into_pin]\([Box::new]\(x))</code>. Consider using
+    /// [`into_pin`](Box::into_pin) if you already have a `Box<T>`, or if you want to
+    /// construct a (pinned) `Box` in a different way than with [`Box::new`].
+    #[cfg(not(no_global_oom_handling))]
+    #[stable(feature = "pin", since = "1.33.0")]
+    #[must_use]
+    #[inline(always)]
+    pub fn pin(x: T) -> Pin<Box<T>> {
+        Box::new(x).into()
+    }
+
+    /// Allocates memory on the heap then places `x` into it,
+    /// returning an error if the allocation fails
+    ///
+    /// This doesn't actually allocate if `T` is zero-sized.
+    ///
+    /// # 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)
+    }
+
+    /// Constructs a new box with uninitialized contents on the heap,
+    /// returning an error if the allocation fails
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    /// let mut five = Box::<u32>::try_new_uninit()?;
+    ///
+    /// let five = unsafe {
+    ///     // Deferred initialization:
+    ///     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)
+    }
+
+    /// Constructs a new `Box` with uninitialized contents, with the memory
+    /// being filled with `0` bytes on the heap
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # 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> {
+    /// Allocates memory in the given allocator then places `x` into it.
+    ///
+    /// This doesn't actually allocate if `T` is zero-sized.
+    ///
+    /// # 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")]
+    #[must_use]
+    #[inline]
+    pub fn new_in(x: T, alloc: A) -> Self
+    where
+        A: Allocator,
+    {
+        let mut boxed = Self::new_uninit_in(alloc);
+        unsafe {
+            boxed.as_mut_ptr().write(x);
+            boxed.assume_init()
+        }
+    }
+
+    /// Allocates memory in the given allocator then places `x` into it,
+    /// returning an error if the allocation fails
+    ///
+    /// This doesn't actually allocate if `T` is zero-sized.
+    ///
+    /// # 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>
+    where
+        A: Allocator,
+    {
+        let mut boxed = Self::try_new_uninit_in(alloc)?;
+        unsafe {
+            boxed.as_mut_ptr().write(x);
+            Ok(boxed.assume_init())
+        }
+    }
+
+    /// Constructs a new box with uninitialized contents in the provided allocator.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    /// use std::alloc::System;
+    ///
+    /// let mut five = Box::<u32, _>::new_uninit_in(System);
+    ///
+    /// let five = unsafe {
+    ///     // Deferred initialization:
+    ///     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))]
+    #[must_use]
+    // #[unstable(feature = "new_uninit", issue = "63291")]
+    pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
+    where
+        A: Allocator,
+    {
+        let layout = Layout::new::<mem::MaybeUninit<T>>();
+        // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
+        // That would make code size bigger.
+        match Box::try_new_uninit_in(alloc) {
+            Ok(m) => m,
+            Err(_) => handle_alloc_error(layout),
+        }
+    }
+
+    /// Constructs a new box with uninitialized contents in the provided allocator,
+    /// returning an error if the allocation fails
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    /// use std::alloc::System;
+    ///
+    /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
+    ///
+    /// let five = unsafe {
+    ///     // Deferred initialization:
+    ///     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>
+    where
+        A: Allocator,
+    {
+        let layout = Layout::new::<mem::MaybeUninit<T>>();
+        let ptr = alloc.allocate(layout)?.cast();
+        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
+    }
+
+    /// Constructs a new `Box` with uninitialized contents, with the memory
+    /// being filled with `0` bytes in the provided allocator.
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # 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")]
+    #[must_use]
+    pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
+    where
+        A: Allocator,
+    {
+        let layout = Layout::new::<mem::MaybeUninit<T>>();
+        // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
+        // That would make code size bigger.
+        match Box::try_new_zeroed_in(alloc) {
+            Ok(m) => m,
+            Err(_) => handle_alloc_error(layout),
+        }
+    }
+
+    /// Constructs a new `Box` with uninitialized contents, with the memory
+    /// being filled with `0` bytes in the provided allocator,
+    /// returning an error if the allocation fails,
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # 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>
+    where
+        A: Allocator,
+    {
+        let layout = Layout::new::<mem::MaybeUninit<T>>();
+        let ptr = alloc.allocate_zeroed(layout)?.cast();
+        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
+    }
+
+    /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
+    /// `x` will be pinned in memory and unable to be moved.
+    ///
+    /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin_in(x, alloc)`
+    /// does the same as <code>[Box::into_pin]\([Box::new_in]\(x, alloc))</code>. Consider using
+    /// [`into_pin`](Box::into_pin) if you already have a `Box<T, A>`, or if you want to
+    /// construct a (pinned) `Box` in a different way than with [`Box::new_in`].
+    #[cfg(not(no_global_oom_handling))]
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[must_use]
+    #[inline(always)]
+    pub fn pin_in(x: T, alloc: A) -> Pin<Self>
+    where
+        A: 'static + Allocator,
+    {
+        Self::into_pin(Self::new_in(x, alloc))
+    }
+
+    /// Converts a `Box<T>` into a `Box<[T]>`
+    ///
+    /// This conversion does not allocate on the heap and happens in place.
+    #[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) }
+    }
+
+    /// Consumes the `Box`, returning the wrapped value.
+    ///
+    /// # 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]> {
+    /// Constructs a new boxed slice with uninitialized contents.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
+    ///
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     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")]
+    #[must_use]
+    pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
+        unsafe { RawVec::with_capacity(len).into_box(len) }
+    }
+
+    /// Constructs a new boxed slice with uninitialized contents, with the memory
+    /// being filled with `0` bytes.
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # 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")]
+    #[must_use]
+    pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
+        unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
+    }
+
+    /// Constructs a new boxed slice with uninitialized contents. Returns an error if
+    /// the allocation fails
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     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]);
+    /// # Ok::<(), std::alloc::AllocError>(())
+    /// ```
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[inline]
+    pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
+        unsafe {
+            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
+                Ok(l) => l,
+                Err(_) => return Err(AllocError),
+            };
+            let ptr = Global.allocate(layout)?;
+            Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
+        }
+    }
+
+    /// Constructs a new boxed slice with uninitialized contents, with the memory
+    /// being filled with `0` bytes. Returns an error if the allocation fails
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
+    /// let values = unsafe { values.assume_init() };
+    ///
+    /// assert_eq!(*values, [0, 0, 0]);
+    /// # Ok::<(), std::alloc::AllocError>(())
+    /// ```
+    ///
+    /// [zeroed]: mem::MaybeUninit::zeroed
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[inline]
+    pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
+        unsafe {
+            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
+                Ok(l) => l,
+                Err(_) => return Err(AllocError),
+            };
+            let ptr = Global.allocate_zeroed(layout)?;
+            Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
+        }
+    }
+}
+
+impl<T, A: Allocator> Box<[T], A> {
+    /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    /// use std::alloc::System;
+    ///
+    /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
+    ///
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     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")]
+    #[must_use]
+    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) }
+    }
+
+    /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
+    /// with the memory being filled with `0` bytes.
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # 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")]
+    #[must_use]
+    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> {
+    /// Converts to `Box<T, A>`.
+    ///
+    /// # Safety
+    ///
+    /// As with [`MaybeUninit::assume_init`],
+    /// it is up to the caller to guarantee that the value
+    /// really is in an initialized state.
+    /// Calling this when the content is not yet fully initialized
+    /// causes immediate undefined behavior.
+    ///
+    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// let mut five = Box::<u32>::new_uninit();
+    ///
+    /// let five: Box<u32> = unsafe {
+    ///     // Deferred initialization:
+    ///     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) }
+    }
+
+    /// Writes the value and converts to `Box<T, A>`.
+    ///
+    /// This method converts the box similarly to [`Box::assume_init`] but
+    /// writes `value` into it before conversion thus guaranteeing safety.
+    /// In some scenarios use of this method may improve performance because
+    /// the compiler may be able to optimize copying from stack.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// let big_box = Box::<[usize; 1024]>::new_uninit();
+    ///
+    /// let mut array = [0; 1024];
+    /// for (i, place) in array.iter_mut().enumerate() {
+    ///     *place = i;
+    /// }
+    ///
+    /// // The optimizer may be able to elide this copy, so previous code writes
+    /// // to heap directly.
+    /// let big_box = Box::write(big_box, array);
+    ///
+    /// for (i, x) in big_box.iter().enumerate() {
+    ///     assert_eq!(*x, i);
+    /// }
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    #[inline]
+    pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
+        unsafe {
+            (*boxed).write(value);
+            boxed.assume_init()
+        }
+    }
+}
+
+impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
+    /// Converts to `Box<[T], A>`.
+    ///
+    /// # Safety
+    ///
+    /// As with [`MaybeUninit::assume_init`],
+    /// it is up to the caller to guarantee that the values
+    /// really are in an initialized state.
+    /// Calling this when the content is not yet fully initialized
+    /// causes immediate undefined behavior.
+    ///
+    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
+    ///
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     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> {
+    /// Constructs a box from a raw pointer.
+    ///
+    /// After calling this function, the raw pointer is owned by the
+    /// resulting `Box`. Specifically, the `Box` destructor will call
+    /// the destructor of `T` and free the allocated memory. For this
+    /// to be safe, the memory must have been allocated in accordance
+    /// with the [memory layout] used by `Box` .
+    ///
+    /// # Safety
+    ///
+    /// This function is unsafe because improper use may lead to
+    /// memory problems. For example, a double-free may occur if the
+    /// function is called twice on the same raw pointer.
+    ///
+    /// The safety conditions are described in the [memory layout] section.
+    ///
+    /// # Examples
+    ///
+    /// Recreate a `Box` which was previously converted to a raw pointer
+    /// using [`Box::into_raw`]:
+    /// ```
+    /// let x = Box::new(5);
+    /// let ptr = Box::into_raw(x);
+    /// let x = unsafe { Box::from_raw(ptr) };
+    /// ```
+    /// Manually create a `Box` from scratch by using the global allocator:
+    /// ```
+    /// use std::alloc::{alloc, Layout};
+    ///
+    /// unsafe {
+    ///     let ptr = alloc(Layout::new::<i32>()) as *mut i32;
+    ///     // In general .write is required to avoid attempting to destruct
+    ///     // the (uninitialized) previous contents of `ptr`, though for this
+    ///     // simple example `*ptr = 5` would have worked as well.
+    ///     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]
+    #[must_use = "call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`"]
+    pub unsafe fn from_raw(raw: *mut T) -> Self {
+        unsafe { Self::from_raw_in(raw, Global) }
+    }
+}
+
+impl<T: ?Sized, A: Allocator> Box<T, A> {
+    /// Constructs a box from a raw pointer in the given allocator.
+    ///
+    /// After calling this function, the raw pointer is owned by the
+    /// resulting `Box`. Specifically, the `Box` destructor will call
+    /// the destructor of `T` and free the allocated memory. For this
+    /// to be safe, the memory must have been allocated in accordance
+    /// with the [memory layout] used by `Box` .
+    ///
+    /// # Safety
+    ///
+    /// This function is unsafe because improper use may lead to
+    /// memory problems. For example, a double-free may occur if the
+    /// function is called twice on the same raw pointer.
+    ///
+    ///
+    /// # Examples
+    ///
+    /// Recreate a `Box` which was previously converted to a raw pointer
+    /// using [`Box::into_raw_with_allocator`]:
+    /// ```
+    /// #![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) };
+    /// ```
+    /// Manually create a `Box` from scratch by using the system allocator:
+    /// ```
+    /// #![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;
+    ///     // In general .write is required to avoid attempting to destruct
+    ///     // the (uninitialized) previous contents of `ptr`, though for this
+    ///     // simple example `*ptr = 5` would have worked as well.
+    ///     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")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    #[inline]
+    pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
+        Box(unsafe { Unique::new_unchecked(raw) }, alloc)
+    }
+
+    /// Consumes the `Box`, returning a wrapped raw pointer.
+    ///
+    /// The pointer will be properly aligned and non-null.
+    ///
+    /// After calling this function, the caller is responsible for the
+    /// memory previously managed by the `Box`. In particular, the
+    /// caller should properly destroy `T` and release the memory, taking
+    /// into account the [memory layout] used by `Box`. The easiest way to
+    /// do this is to convert the raw pointer back into a `Box` with the
+    /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
+    /// the cleanup.
+    ///
+    /// Note: this is an associated function, which means that you have
+    /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
+    /// is so that there is no conflict with a method on the inner type.
+    ///
+    /// # Examples
+    /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
+    /// for automatic cleanup:
+    /// ```
+    /// let x = Box::new(String::from("Hello"));
+    /// let ptr = Box::into_raw(x);
+    /// let x = unsafe { Box::from_raw(ptr) };
+    /// ```
+    /// Manual cleanup by explicitly running the destructor and deallocating
+    /// the memory:
+    /// ```
+    /// 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
+    }
+
+    /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
+    ///
+    /// The pointer will be properly aligned and non-null.
+    ///
+    /// After calling this function, the caller is responsible for the
+    /// memory previously managed by the `Box`. In particular, the
+    /// caller should properly destroy `T` and release the memory, taking
+    /// into account the [memory layout] used by `Box`. The easiest way to
+    /// do this is to convert the raw pointer back into a `Box` with the
+    /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
+    /// the cleanup.
+    ///
+    /// Note: this is an associated function, which means that you have
+    /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
+    /// is so that there is no conflict with a method on the inner type.
+    ///
+    /// # Examples
+    /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
+    /// for automatic cleanup:
+    /// ```
+    /// #![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) };
+    /// ```
+    /// Manual cleanup by explicitly running the destructor and deallocating
+    /// the memory:
+    /// ```
+    /// #![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 is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
+        // raw pointer for the type system. Turning it directly into a raw pointer would not be
+        // recognized as "releasing" the unique pointer to permit aliased raw accesses,
+        // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
+        // behaves correctly.
+        let alloc = unsafe { ptr::read(&b.1) };
+        (Unique::from(Box::leak(b)), alloc)
+    }
+
+    /// Returns a reference to the underlying allocator.
+    ///
+    /// Note: this is an associated function, which means that you have
+    /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
+    /// is so that there is no conflict with a method on the inner type.
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    #[inline]
+    pub const fn allocator(b: &Self) -> &A {
+        &b.1
+    }
+
+    /// Consumes and leaks the `Box`, returning a mutable reference,
+    /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
+    /// `'a`. If the type has only static references, or none at all, then this
+    /// may be chosen to be `'static`.
+    ///
+    /// This function is mainly useful for data that lives for the remainder of
+    /// the program's life. Dropping the returned reference will cause a memory
+    /// leak. If this is not acceptable, the reference should first be wrapped
+    /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
+    /// then be dropped which will properly destroy `T` and release the
+    /// allocated memory.
+    ///
+    /// Note: this is an associated function, which means that you have
+    /// to call it as `Box::leak(b)` instead of `b.leak()`. This
+    /// is so that there is no conflict with a method on the inner type.
+    ///
+    /// # Examples
+    ///
+    /// Simple usage:
+    ///
+    /// ```
+    /// let x = Box::new(41);
+    /// let static_ref: &'static mut usize = Box::leak(x);
+    /// *static_ref += 1;
+    /// assert_eq!(*static_ref, 42);
+    /// ```
+    ///
+    /// Unsized data:
+    ///
+    /// ```
+    /// 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() }
+    }
+
+    /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
+    /// `*boxed` will be pinned in memory and unable to be moved.
+    ///
+    /// This conversion does not allocate on the heap and happens in place.
+    ///
+    /// This is also available via [`From`].
+    ///
+    /// Constructing and pinning a `Box` with <code>Box::into_pin([Box::new]\(x))</code>
+    /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
+    /// This `into_pin` method is useful if you already have a `Box<T>`, or you are
+    /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
+    ///
+    /// # Notes
+    ///
+    /// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
+    /// as it'll introduce an ambiguity when calling `Pin::from`.
+    /// A demonstration of such a poor impl is shown below.
+    ///
+    /// ```compile_fail
+    /// # use std::pin::Pin;
+    /// struct Foo; // A type defined in this crate.
+    /// impl From<Box<()>> for Pin<Foo> {
+    ///     fn from(_: Box<()>) -> Pin<Foo> {
+    ///         Pin::new(Foo)
+    ///     }
+    /// }
+    ///
+    /// let foo = Box::new(());
+    /// let bar = Pin::from(foo);
+    /// ```
+    #[stable(feature = "box_into_pin", since = "1.63.0")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    pub const fn into_pin(boxed: Self) -> Pin<Self>
+    where
+        A: 'static,
+    {
+        // It's not possible to move or replace the insides of a `Pin<Box<T>>`
+        // when `T: !Unpin`, so it's safe to pin it directly without any
+        // additional requirements.
+        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: Do nothing, drop is currently performed by compiler.
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Default> Default for Box<T> {
+    /// Creates a `Box<T>`, with the `Default` value for T.
+    #[inline]
+    fn default() -> Self {
+        Box::new(T::default())
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> Default for Box<[T]> {
+    #[inline]
+    fn default() -> Self {
+        let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
+        Box(ptr, Global)
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "default_box_extra", since = "1.17.0")]
+impl Default for Box<str> {
+    #[inline]
+    fn default() -> Self {
+        // SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
+        let ptr: Unique<str> = unsafe {
+            let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
+            Unique::new_unchecked(bytes.as_ptr() as *mut str)
+        };
+        Box(ptr, Global)
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
+    /// Returns a new box with a `clone()` of this box's contents.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let x = Box::new(5);
+    /// let y = x.clone();
+    ///
+    /// // The value is the same
+    /// assert_eq!(x, y);
+    ///
+    /// // But they are unique objects
+    /// assert_ne!(&*x as *const i32, &*y as *const i32);
+    /// ```
+    #[inline]
+    fn clone(&self) -> Self {
+        // Pre-allocate memory to allow writing the cloned value directly.
+        let mut boxed = Self::new_uninit_in(self.1.clone());
+        unsafe {
+            (**self).write_clone_into_raw(boxed.as_mut_ptr());
+            boxed.assume_init()
+        }
+    }
+
+    /// Copies `source`'s contents into `self` without creating a new allocation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let x = Box::new(5);
+    /// let mut y = Box::new(10);
+    /// let yp: *const i32 = &*y;
+    ///
+    /// y.clone_from(&x);
+    ///
+    /// // The value is the same
+    /// assert_eq!(x, y);
+    ///
+    /// // And no allocation occurred
+    /// 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 {
+        // this makes a copy of the data
+        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)
+    }
+    fn write_length_prefix(&mut self, len: usize) {
+        (**self).write_length_prefix(len)
+    }
+    fn write_str(&mut self, s: &str) {
+        (**self).write_str(s)
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "from_for_ptrs", since = "1.6.0")]
+impl<T> From<T> for Box<T> {
+    /// Converts a `T` into a `Box<T>`
+    ///
+    /// The conversion allocates on the heap and moves `t`
+    /// from the stack into it.
+    ///
+    /// # 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,
+{
+    /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
+    /// `*boxed` will be pinned in memory and unable to be moved.
+    ///
+    /// This conversion does not allocate on the heap and happens in place.
+    ///
+    /// This is also available via [`Box::into_pin`].
+    ///
+    /// Constructing and pinning a `Box` with <code><Pin<Box\<T>>>::from([Box::new]\(x))</code>
+    /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
+    /// This `From` implementation is useful if you already have a `Box<T>`, or you are
+    /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
+    fn from(boxed: Box<T, A>) -> Self {
+        Box::into_pin(boxed)
+    }
+}
+
+/// Specialization trait used for `From<&[T]>`.
+#[cfg(not(no_global_oom_handling))]
+trait BoxFromSlice<T> {
+    fn from_slice(slice: &[T]) -> Self;
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T: Clone> BoxFromSlice<T> for Box<[T]> {
+    #[inline]
+    default fn from_slice(slice: &[T]) -> Self {
+        slice.to_vec().into_boxed_slice()
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T: Copy> BoxFromSlice<T> for Box<[T]> {
+    #[inline]
+    fn from_slice(slice: &[T]) -> Self {
+        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_slice", since = "1.17.0")]
+impl<T: Clone> From<&[T]> for Box<[T]> {
+    /// Converts a `&[T]` into a `Box<[T]>`
+    ///
+    /// This conversion allocates on the heap
+    /// and performs a copy of `slice` and its contents.
+    ///
+    /// # Examples
+    /// ```rust
+    /// // create a &[u8] which will be used to create a Box<[u8]>
+    /// let slice: &[u8] = &[104, 101, 108, 108, 111];
+    /// let boxed_slice: Box<[u8]> = Box::from(slice);
+    ///
+    /// println!("{boxed_slice:?}");
+    /// ```
+    #[inline]
+    fn from(slice: &[T]) -> Box<[T]> {
+        <Self as BoxFromSlice<T>>::from_slice(slice)
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "box_from_cow", since = "1.45.0")]
+impl<T: Clone> From<Cow<'_, [T]>> for Box<[T]> {
+    /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
+    ///
+    /// When `cow` is the `Cow::Borrowed` variant, this
+    /// conversion allocates on the heap and copies the
+    /// underlying slice. Otherwise, it will try to reuse the owned
+    /// `Vec`'s allocation.
+    #[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> {
+    /// Converts a `&str` into a `Box<str>`
+    ///
+    /// This conversion allocates on the heap
+    /// and performs a copy of `s`.
+    ///
+    /// # 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> {
+    /// Converts a `Cow<'_, str>` into a `Box<str>`
+    ///
+    /// When `cow` is the `Cow::Borrowed` variant, this
+    /// conversion allocates on the heap and copies the
+    /// underlying `str`. Otherwise, it will try to reuse the owned
+    /// `String`'s allocation.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use std::borrow::Cow;
+    ///
+    /// let unboxed = Cow::Borrowed("hello");
+    /// let boxed: Box<str> = Box::from(unboxed);
+    /// println!("{boxed}");
+    /// ```
+    ///
+    /// ```rust
+    /// # use std::borrow::Cow;
+    /// let unboxed = Cow::Owned("hello".to_string());
+    /// let boxed: Box<str> = Box::from(unboxed);
+    /// println!("{boxed}");
+    /// ```
+    #[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> {
+    /// Converts a `Box<str>` into a `Box<[u8]>`
+    ///
+    /// This conversion does not allocate on the heap and happens in place.
+    ///
+    /// # Examples
+    /// ```rust
+    /// // create a Box<str> which will be used to create a Box<[u8]>
+    /// let boxed: Box<str> = Box::from("hello");
+    /// let boxed_str: Box<[u8]> = Box::from(boxed);
+    ///
+    /// // create a &[u8] which will be used to create a 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) }
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "box_from_array", since = "1.45.0")]
+impl<T, const N: usize> From<[T; N]> for Box<[T]> {
+    /// Converts a `[T; N]` into a `Box<[T]>`
+    ///
+    /// This conversion moves the array to newly heap-allocated memory.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// let boxed: Box<[u8]> = Box::from([4, 2]);
+    /// println!("{boxed:?}");
+    /// ```
+    fn from(array: [T; N]) -> Box<[T]> {
+        Box::new(array)
+    }
+}
+
+/// Casts a boxed slice to a boxed array.
+///
+/// # Safety
+///
+/// `boxed_slice.len()` must be exactly `N`.
+unsafe fn boxed_slice_as_array_unchecked<T, A: Allocator, const N: usize>(
+    boxed_slice: Box<[T], A>,
+) -> Box<[T; N], A> {
+    debug_assert_eq!(boxed_slice.len(), N);
+
+    let (ptr, alloc) = Box::into_raw_with_allocator(boxed_slice);
+    // SAFETY: Pointer and allocator came from an existing box,
+    // and our safety condition requires that the length is exactly `N`
+    unsafe { Box::from_raw_in(ptr as *mut [T; N], alloc) }
+}
+
+#[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]>;
+
+    /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
+    ///
+    /// The conversion occurs in-place and does not require a
+    /// new memory allocation.
+    ///
+    /// # Errors
+    ///
+    /// Returns the old `Box<[T]>` in the `Err` variant if
+    /// `boxed_slice.len()` does not equal `N`.
+    fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
+        if boxed_slice.len() == N {
+            Ok(unsafe { boxed_slice_as_array_unchecked(boxed_slice) })
+        } else {
+            Err(boxed_slice)
+        }
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "boxed_array_try_from_vec", since = "1.66.0")]
+impl<T, const N: usize> TryFrom<Vec<T>> for Box<[T; N]> {
+    type Error = Vec<T>;
+
+    /// Attempts to convert a `Vec<T>` into a `Box<[T; N]>`.
+    ///
+    /// Like [`Vec::into_boxed_slice`], this is in-place if `vec.capacity() == N`,
+    /// but will require a reallocation otherwise.
+    ///
+    /// # Errors
+    ///
+    /// Returns the original `Vec<T>` in the `Err` variant if
+    /// `boxed_slice.len()` does not equal `N`.
+    ///
+    /// # Examples
+    ///
+    /// This can be used with [`vec!`] to create an array on the heap:
+    ///
+    /// ```
+    /// let state: Box<[f32; 100]> = vec![1.0; 100].try_into().unwrap();
+    /// assert_eq!(state.len(), 100);
+    /// ```
+    fn try_from(vec: Vec<T>) -> Result<Self, Self::Error> {
+        if vec.len() == N {
+            let boxed_slice = vec.into_boxed_slice();
+            Ok(unsafe { boxed_slice_as_array_unchecked(boxed_slice) })
+        } else {
+            Err(vec)
+        }
+    }
+}
+
+impl<A: Allocator> Box<dyn Any, A> {
+    /// Attempt to downcast the box to a concrete type.
+    ///
+    /// # 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));
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
+        if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
+    }
+
+    /// Downcasts the box to a concrete type.
+    ///
+    /// For a safe alternative see [`downcast`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let x: Box<dyn Any> = Box::new(1_usize);
+    ///
+    /// unsafe {
+    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
+    /// }
+    /// ```
+    ///
+    /// # Safety
+    ///
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
+    ///
+    /// [`downcast`]: Self::downcast
+    #[inline]
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
+        debug_assert!(self.is::<T>());
+        unsafe {
+            let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
+            Box::from_raw_in(raw as *mut T, alloc)
+        }
+    }
+}
+
+impl<A: Allocator> Box<dyn Any + Send, A> {
+    /// Attempt to downcast the box to a concrete type.
+    ///
+    /// # 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));
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
+        if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
+    }
+
+    /// Downcasts the box to a concrete type.
+    ///
+    /// For a safe alternative see [`downcast`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let x: Box<dyn Any + Send> = Box::new(1_usize);
+    ///
+    /// unsafe {
+    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
+    /// }
+    /// ```
+    ///
+    /// # Safety
+    ///
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
+    ///
+    /// [`downcast`]: Self::downcast
+    #[inline]
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
+        debug_assert!(self.is::<T>());
+        unsafe {
+            let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
+            Box::from_raw_in(raw as *mut T, alloc)
+        }
+    }
+}
+
+impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
+    /// Attempt to downcast the box to a concrete type.
+    ///
+    /// # 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));
+    /// ```
+    #[inline]
+    #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
+    pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
+        if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
+    }
+
+    /// Downcasts the box to a concrete type.
+    ///
+    /// For a safe alternative see [`downcast`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downcast_unchecked)]
+    ///
+    /// use std::any::Any;
+    ///
+    /// let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);
+    ///
+    /// unsafe {
+    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
+    /// }
+    /// ```
+    ///
+    /// # Safety
+    ///
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
+    ///
+    /// [`downcast`]: Self::downcast
+    #[inline]
+    #[unstable(feature = "downcast_unchecked", issue = "90850")]
+    pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
+        debug_assert!(self.is::<T>());
+        unsafe {
+            let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
+                Box::into_raw_with_allocator(self);
+            Box::from_raw_in(raw as *mut T, alloc)
+        }
+    }
+}
+
+#[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 {
+        // It's not possible to extract the inner Uniq directly from the Box,
+        // instead we cast it to a *const which aliases the Unique
+        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)
+    }
+}
+
+/// Specialization for sized `I`s that uses `I`s implementation of `last()`
+/// instead of the default.
+#[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: Tuple, 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: Tuple, 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: Tuple, 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 = "18598")]
+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_iterator", issue = "79024")]
+impl<S: ?Sized + AsyncIterator + Unpin> AsyncIterator 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()
+    }
+}
+
+impl dyn Error {
+    #[inline]
+    #[stable(feature = "error_downcast", since = "1.3.0")]
+    #[rustc_allow_incoherent_impl]
+    /// Attempts to downcast the box to a concrete type.
+    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error>> {
+        if self.is::<T>() {
+            unsafe {
+                let raw: *mut dyn Error = Box::into_raw(self);
+                Ok(Box::from_raw(raw as *mut T))
+            }
+        } else {
+            Err(self)
+        }
+    }
+}
+
+impl dyn Error + Send {
+    #[inline]
+    #[stable(feature = "error_downcast", since = "1.3.0")]
+    #[rustc_allow_incoherent_impl]
+    /// Attempts to downcast the box to a concrete type.
+    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error + Send>> {
+        let err: Box<dyn Error> = self;
+        <dyn Error>::downcast(err).map_err(|s| unsafe {
+            // Reapply the `Send` marker.
+            mem::transmute::<Box<dyn Error>, Box<dyn Error + Send>>(s)
+        })
+    }
+}
+
+impl dyn Error + Send + Sync {
+    #[inline]
+    #[stable(feature = "error_downcast", since = "1.3.0")]
+    #[rustc_allow_incoherent_impl]
+    /// Attempts to downcast the box to a concrete type.
+    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
+        let err: Box<dyn Error> = self;
+        <dyn Error>::downcast(err).map_err(|s| unsafe {
+            // Reapply the `Send + Sync` marker.
+            mem::transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)
+        })
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
+    /// Converts a type of [`Error`] into a box of dyn [`Error`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::fmt;
+    /// use std::mem;
+    ///
+    /// #[derive(Debug)]
+    /// struct AnError;
+    ///
+    /// impl fmt::Display for AnError {
+    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         write!(f, "An error")
+    ///     }
+    /// }
+    ///
+    /// impl Error for AnError {}
+    ///
+    /// let an_error = AnError;
+    /// assert!(0 == mem::size_of_val(&an_error));
+    /// let a_boxed_error = Box::<dyn Error>::from(an_error);
+    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(err: E) -> Box<dyn Error + 'a> {
+        Box::new(err)
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
+    /// Converts a type of [`Error`] + [`Send`] + [`Sync`] into a box of
+    /// dyn [`Error`] + [`Send`] + [`Sync`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::fmt;
+    /// use std::mem;
+    ///
+    /// #[derive(Debug)]
+    /// struct AnError;
+    ///
+    /// impl fmt::Display for AnError {
+    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         write!(f, "An error")
+    ///     }
+    /// }
+    ///
+    /// impl Error for AnError {}
+    ///
+    /// unsafe impl Send for AnError {}
+    ///
+    /// unsafe impl Sync for AnError {}
+    ///
+    /// let an_error = AnError;
+    /// assert!(0 == mem::size_of_val(&an_error));
+    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
+    /// assert!(
+    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(err: E) -> Box<dyn Error + Send + Sync + 'a> {
+        Box::new(err)
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl From<String> for Box<dyn Error + Send + Sync> {
+    /// Converts a [`String`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    ///
+    /// let a_string_error = "a string error".to_string();
+    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
+    /// assert!(
+    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    #[inline]
+    fn from(err: String) -> Box<dyn Error + Send + Sync> {
+        struct StringError(String);
+
+        impl Error for StringError {
+            #[allow(deprecated)]
+            fn description(&self) -> &str {
+                &self.0
+            }
+        }
+
+        impl fmt::Display for StringError {
+            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt::Display::fmt(&self.0, f)
+            }
+        }
+
+        // Purposefully skip printing "StringError(..)"
+        impl fmt::Debug for StringError {
+            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                fmt::Debug::fmt(&self.0, f)
+            }
+        }
+
+        Box::new(StringError(err))
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "string_box_error", since = "1.6.0")]
+impl From<String> for Box<dyn Error> {
+    /// Converts a [`String`] into a box of dyn [`Error`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    ///
+    /// let a_string_error = "a string error".to_string();
+    /// let a_boxed_error = Box::<dyn Error>::from(a_string_error);
+    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(str_err: String) -> Box<dyn Error> {
+        let err1: Box<dyn Error + Send + Sync> = From::from(str_err);
+        let err2: Box<dyn Error> = err1;
+        err2
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
+    /// Converts a [`str`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
+    ///
+    /// [`str`]: prim@str
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    ///
+    /// let a_str_error = "a str error";
+    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
+    /// assert!(
+    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    #[inline]
+    fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a> {
+        From::from(String::from(err))
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "string_box_error", since = "1.6.0")]
+impl From<&str> for Box<dyn Error> {
+    /// Converts a [`str`] into a box of dyn [`Error`].
+    ///
+    /// [`str`]: prim@str
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    ///
+    /// let a_str_error = "a str error";
+    /// let a_boxed_error = Box::<dyn Error>::from(a_str_error);
+    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(err: &str) -> Box<dyn Error> {
+        From::from(String::from(err))
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "cow_box_error", since = "1.22.0")]
+impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
+    /// Converts a [`Cow`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    /// use std::borrow::Cow;
+    ///
+    /// let a_cow_str_error = Cow::from("a str error");
+    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
+    /// assert!(
+    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a> {
+        From::from(String::from(err))
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "cow_box_error", since = "1.22.0")]
+impl<'a> From<Cow<'a, str>> for Box<dyn Error> {
+    /// Converts a [`Cow`] into a box of dyn [`Error`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::mem;
+    /// use std::borrow::Cow;
+    ///
+    /// let a_cow_str_error = Cow::from("a str error");
+    /// let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
+    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
+    /// ```
+    fn from(err: Cow<'a, str>) -> Box<dyn Error> {
+        From::from(String::from(err))
+    }
+}
+
+#[stable(feature = "box_error", since = "1.8.0")]
+impl<T: core::error::Error> core::error::Error for Box<T> {
+    #[allow(deprecated, deprecated_in_future)]
+    fn description(&self) -> &str {
+        core::error::Error::description(&**self)
+    }
+
+    #[allow(deprecated)]
+    fn cause(&self) -> Option<&dyn core::error::Error> {
+        core::error::Error::cause(&**self)
+    }
+
+    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
+        core::error::Error::source(&**self)
+    }
+}