boxed.rs - source
Description: Source of the Rust file `library/alloc/src/boxed.rs`.
Keywords: rust, rustlang, rust-lang

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    //! A pointer 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)`].
    //!
    //! 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]
    //! #[allow(improper_ctypes_definitions)]
    //! pub extern "C" fn foo_new() -> Box<Foo> {
    //!     Box::new(Foo)
    //! }
    //!
    //! #[no_mangle]
    //! #[allow(improper_ctypes_definitions)]
    //! 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].
    //!
    //! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
    //! [dereferencing]: core::ops::Deref
    //! [`Box<T>`]: Box
    //! [`Box::<T>::from_raw(value)`]: Box::from_raw
    //! [`Box::<T>::into_raw`]: Box::into_raw
    //! [`Global`]: crate::alloc::Global
    //! [`Layout`]: crate::alloc::Layout
    //! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value

    #![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};
    use core::iter::{FromIterator, 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::task::{Context, Poll};

    use crate::alloc::{handle_alloc_error, AllocRef, Global, Layout};
    use crate::borrow::Cow;
    use crate::raw_vec::RawVec;
    use crate::str::from_boxed_utf8_unchecked;
    use crate::vec::Vec;

    /// A pointer type for heap allocation.
    ///
    /// See the [module-level documentation](../../std/boxed/index.html) for more.
    #[lang = "owned_box"]
    #[fundamental]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Box<
        T: ?Sized,
        #[unstable(feature = "allocator_api", issue = "32838")] A: AllocRef = 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);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline(always)]
        pub fn new(x: T) -> Self {
            box 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)
        /// ```
        #[unstable(feature = "new_uninit", issue = "63291")]
        #[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
        #[unstable(feature = "new_uninit", issue = "63291")]
        #[inline]
        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.
        #[stable(feature = "pin", since = "1.33.0")]
        #[inline(always)]
        pub fn pin(x: T) -> Pin<Box<T>> {
            (box x).into()
        }
    }

    impl<T, A: AllocRef> 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);
        /// ```
        #[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()
            }
        }

        /// 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")]
        // #[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>>();
            let ptr = alloc.alloc(layout).unwrap_or_else(|_| handle_alloc_error(layout)).cast();
            unsafe { 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")]
        // #[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>>();
            let ptr = alloc.alloc_zeroed(layout).unwrap_or_else(|_| handle_alloc_error(layout)).cast();
            unsafe { 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.
        #[unstable(feature = "allocator_api", issue = "32838")]
        #[inline(always)]
        pub fn pin_in(x: T, alloc: A) -> Pin<Self> {
            Self::new_in(x, alloc).into()
        }

        /// 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_alloc(boxed);
            unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
        }
    }

    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])
        /// ```
        #[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) }
        }

        /// 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
        #[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: AllocRef> 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])
        /// ```
        #[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) }
        }

        /// 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
        #[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: AllocRef> 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_alloc(self);
            unsafe { Box::from_raw_in(raw as *mut T, alloc) }
        }
    }

    impl<T, A: AllocRef> 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_alloc(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.
        ///
        /// # 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]
        pub unsafe fn from_raw(raw: *mut T) -> Self {
            unsafe { Self::from_raw_in(raw, Global) }
        }
    }

    impl<T: ?Sized, A: AllocRef> 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_alloc`]:
        /// ```
        /// #![feature(allocator_api)]
        ///
        /// use std::alloc::System;
        ///
        /// let x = Box::new_in(5, System);
        /// let (ptr, alloc) = Box::into_raw_with_alloc(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::{AllocRef, Layout, System};
        ///
        /// unsafe {
        ///     let ptr = System.alloc(Layout::new::<i32>())?.as_mut_ptr();
        ///     // 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")]
        #[inline]
        pub 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_alloc(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_alloc(b)` instead of `b.into_raw_with_alloc()`. 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_alloc(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::{AllocRef, Layout, System};
        /// use std::ptr::{self, NonNull};
        ///
        /// let x = Box::new_in(String::from("Hello"), System);
        /// let (ptr, alloc) = Box::into_raw_with_alloc(x);
        /// unsafe {
        ///     ptr::drop_in_place(ptr);
        ///     let non_null = NonNull::new_unchecked(ptr);
        ///     alloc.dealloc(non_null.cast(), Layout::new::<String>());
        /// }
        /// ```
        ///
        /// [memory layout]: self#memory-layout
        #[unstable(feature = "allocator_api", issue = "32838")]
        #[inline]
        pub fn into_raw_with_alloc(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::alloc_ref(&b)` instead of `b.alloc_ref()`. This
        /// is so that there is no conflict with a method on the inner type.
        #[unstable(feature = "allocator_api", issue = "32838")]
        #[inline]
        pub fn alloc_ref(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>>`
        ///
        /// This conversion does not allocate on the heap and happens in place.
        ///
        /// This is also available via [`From`].
        #[unstable(feature = "box_into_pin", issue = "62370")]
        pub fn into_pin(boxed: Self) -> Pin<Self> {
            // 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: AllocRef> Drop for Box<T, A> {
        fn drop(&mut self) {
            // FIXME: Do nothing, drop is currently performed by compiler.
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Default> Default for Box<T> {
        /// Creates a `Box<T>`, with the `Default` value for T.
        fn default() -> Self {
            box T::default()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Default for Box<[T]> {
        fn default() -> Self {
            Box::<[T; 0]>::new([])
        }
    }

    #[stable(feature = "default_box_extra", since = "1.17.0")]
    impl Default for Box<str> {
        fn default() -> Self {
            unsafe { from_boxed_utf8_unchecked(Default::default()) }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Clone, A: AllocRef + 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);
        /// ```
        #[rustfmt::skip]
        #[inline]
        fn clone(&self) -> Self {
            Self::new_in((**self).clone(), self.1.clone())
        }

        /// 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));
        }
    }

    #[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: AllocRef> 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: AllocRef> 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: AllocRef> 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: AllocRef> Eq for Box<T, A> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Hash, A: AllocRef> 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: AllocRef> 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)
        }
    }

    #[stable(feature = "from_for_ptrs", since = "1.6.0")]
    impl<T> From<T> for Box<T> {
        /// Converts a generic type `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: AllocRef> From<Box<T, A>> for Pin<Box<T, A>> {
        /// Converts a `Box<T>` into a `Pin<Box<T>>`
        ///
        /// This conversion does not allocate on the heap and happens in place.
        fn from(boxed: Box<T, A>) -> Self {
            Box::into_pin(boxed)
        }
    }

    #[stable(feature = "box_from_slice", since = "1.17.0")]
    impl<T: Copy> From<&[T]> for Box<[T]> {
        /// Converts a `&[T]` into a `Box<[T]>`
        ///
        /// This conversion allocates on the heap
        /// and performs a copy of `slice`.
        ///
        /// # 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);
        /// ```
        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()
            }
        }
    }

    #[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),
            }
        }
    }

    #[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())) }
        }
    }

    #[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: AllocRef> 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_alloc(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]> {
        /// 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 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: AllocRef> Box<dyn Any, A> {
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        /// 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));
        /// ```
        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_alloc(self);
                    Ok(Box::from_raw_in(raw as *mut T, alloc))
                }
            } else {
                Err(self)
            }
        }
    }

    impl<A: AllocRef> Box<dyn Any + Send, A> {
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        /// 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));
        /// ```
        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_alloc(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: AllocRef> 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: AllocRef> 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: AllocRef> 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: AllocRef> Deref for Box<T, A> {
        type Target = T;

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

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

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator + ?Sized, A: AllocRef> 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: AllocRef> 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: AllocRef> 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: AllocRef> 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: AllocRef> 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: AllocRef> FusedIterator for Box<I, A> {}

    #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
    impl<Args, F: FnOnce<Args> + ?Sized, A: AllocRef> 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: AllocRef> 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: AllocRef> 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: AllocRef> 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> {}

    #[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()
        }
    }

    #[stable(feature = "box_slice_clone", since = "1.3.0")]
    impl<T: Clone> Clone for Box<[T]> {
        fn clone(&self) -> Self {
            self.to_vec().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: AllocRef> borrow::Borrow<T> for Box<T, A> {
        fn borrow(&self) -> &T {
            &**self
        }
    }

    #[stable(feature = "box_borrow", since = "1.1.0")]
    impl<T: ?Sized, A: AllocRef> 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: AllocRef> 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: AllocRef> 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: AllocRef> Unpin for Box<T, A> {}

    #[unstable(feature = "generator_trait", issue = "43122")]
    impl<G: ?Sized + Generator<R> + Unpin, R, A: AllocRef> Generator<R> for Box<G, A> {
        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: AllocRef> Generator<R> for Pin<Box<G, A>> {
        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: AllocRef> Future for Box<F, A> {
        type Output = F::Output;

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