boxed.rs.html -- source
Description: Source to the Rust file `src/liballoc/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.
    //!
    //! # 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,
    //! }
    //!
    //! fn main() {
    //!     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`, 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)`.
    //!
    //!
    //! [dereferencing]: ../../std/ops/trait.Deref.html
    //! [`Box`]: struct.Box.html
    //! [`Global`]: ../alloc/struct.Global.html
    //! [`Layout`]: ../alloc/struct.Layout.html

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

    use core::any::Any;
    use core::borrow;
    use core::cmp::Ordering;
    use core::convert::From;
    use core::fmt;
    use core::future::Future;
    use core::hash::{Hash, Hasher};
    use core::iter::{Iterator, FromIterator, FusedIterator};
    use core::marker::{Unpin, Unsize};
    use core::mem;
    use core::pin::Pin;
    use core::ops::{
        CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Receiver, Generator, GeneratorState
    };
    use core::ptr::{self, NonNull, Unique};
    use core::task::{Context, Poll};

    use crate::vec::Vec;
    use crate::raw_vec::RawVec;
    use crate::str::from_boxed_utf8_unchecked;

    /// 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>(Unique<T>);

    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) -> Box<T> {
            box x
        }

        /// 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: ?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;
        ///     *ptr = 5;
        ///     let x = Box::from_raw(ptr);
        /// }
        /// ```
        ///
        /// [memory layout]: index.html#memory-layout
        /// [`Layout`]: ../alloc/struct.Layout.html
        /// [`Box::into_raw`]: struct.Box.html#method.into_raw
        #[stable(feature = "box_raw", since = "1.4.0")]
        #[inline]
        pub unsafe fn from_raw(raw: *mut T) -> Self {
            Box(Unique::new_unchecked(raw))
        }

        /// 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]: index.html#memory-layout
        /// [`Box::from_raw`]: struct.Box.html#method.from_raw
        #[stable(feature = "box_raw", since = "1.4.0")]
        #[inline]
        pub fn into_raw(b: Box<T>) -> *mut T {
            Box::into_raw_non_null(b).as_ptr()
        }

        /// Consumes the `Box`, returning the wrapped pointer as `NonNull<T>`.
        ///
        /// 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. The
        /// easiest way to do so is to convert the `NonNull<T>` pointer
        /// into a raw pointer and back into a `Box` with the [`Box::from_raw`]
        /// function.
        ///
        /// Note: this is an associated function, which means that you have
        /// to call it as `Box::into_raw_non_null(b)`
        /// instead of `b.into_raw_non_null()`. This
        /// is so that there is no conflict with a method on the inner type.
        ///
        /// [`Box::from_raw`]: struct.Box.html#method.from_raw
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(box_into_raw_non_null)]
        ///
        /// fn main() {
        ///     let x = Box::new(5);
        ///     let ptr = Box::into_raw_non_null(x);
        ///
        ///     // Clean up the memory by converting the NonNull pointer back
        ///     // into a Box and letting the Box be dropped.
        ///     let x = unsafe { Box::from_raw(ptr.as_ptr()) };
        /// }
        /// ```
        #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
        #[inline]
        pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
            Box::into_unique(b).into()
        }

        #[unstable(feature = "ptr_internals", issue = "0", reason = "use into_raw_non_null instead")]
        #[inline]
        #[doc(hidden)]
        pub fn into_unique(b: Box<T>) -> Unique<T> {
            let mut unique = b.0;
            mem::forget(b);
            // Box is kind-of a library type, but recognized as a "unique pointer" by
            // Stacked Borrows.  This function here corresponds to "reborrowing to
            // a raw pointer", but there is no actual reborrow here -- so
            // without some care, the pointer we are returning here still carries
            // the tag of `b`, with `Unique` permission.
            // We round-trip through a mutable reference to avoid that.
            unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
        }

        /// 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.
        ///
        /// [`Box::from_raw`]: struct.Box.html#method.from_raw
        ///
        /// # Examples
        ///
        /// Simple usage:
        ///
        /// ```
        /// fn main() {
        ///     let x = Box::new(41);
        ///     let static_ref: &'static mut usize = Box::leak(x);
        ///     *static_ref += 1;
        ///     assert_eq!(*static_ref, 42);
        /// }
        /// ```
        ///
        /// Unsized data:
        ///
        /// ```
        /// fn main() {
        ///     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: Box<T>) -> &'a mut T
        where
            T: 'a // Technically not needed, but kept to be explicit.
        {
            unsafe { &mut *Box::into_raw(b) }
        }

        /// 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 = "0")]
        pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
            // 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> Drop for Box<T> {
        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() -> Box<T> {
            box Default::default()
        }
    }

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

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Clone> Clone for Box<T> {
        /// Returns a new box with a `clone()` of this box's contents.
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let y = x.clone();
        /// ```
        #[rustfmt::skip]
        #[inline]
        fn clone(&self) -> Box<T> {
            box { (**self).clone() }
        }
        /// Copies `source`'s contents into `self` without creating a new allocation.
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let mut y = Box::new(10);
        ///
        /// y.clone_from(&x);
        ///
        /// assert_eq!(*y, 5);
        /// ```
        #[inline]
        fn clone_from(&mut self, source: &Box<T>) {
            (**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> PartialEq for Box<T> {
        #[inline]
        fn eq(&self, other: &Box<T>) -> bool {
            PartialEq::eq(&**self, &**other)
        }
        #[inline]
        fn ne(&self, other: &Box<T>) -> bool {
            PartialEq::ne(&**self, &**other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
        #[inline]
        fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
            PartialOrd::partial_cmp(&**self, &**other)
        }
        #[inline]
        fn lt(&self, other: &Box<T>) -> bool {
            PartialOrd::lt(&**self, &**other)
        }
        #[inline]
        fn le(&self, other: &Box<T>) -> bool {
            PartialOrd::le(&**self, &**other)
        }
        #[inline]
        fn ge(&self, other: &Box<T>) -> bool {
            PartialOrd::ge(&**self, &**other)
        }
        #[inline]
        fn gt(&self, other: &Box<T>) -> bool {
            PartialOrd::gt(&**self, &**other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Ord> Ord for Box<T> {
        #[inline]
        fn cmp(&self, other: &Box<T>) -> Ordering {
            Ord::cmp(&**self, &**other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Eq> Eq for Box<T> {}

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

    #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
    impl<T: ?Sized + Hasher> Hasher for Box<T> {
        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> From<Box<T>> for Pin<Box<T>> {
        /// 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>) -> 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()
            }
        }
    }

    #[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 = "boxed_str_conv", since = "1.19.0")]
    impl From<Box<str>> for Box<[u8]> {
        /// 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>) -> Self {
            unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
        }
    }

    impl Box<dyn Any> {
        #[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);
        ///     }
        /// }
        ///
        /// fn main() {
        ///     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>, Box<dyn Any>> {
            if self.is::<T>() {
                unsafe {
                    let raw: *mut dyn Any = Box::into_raw(self);
                    Ok(Box::from_raw(raw as *mut T))
                }
            } else {
                Err(self)
            }
        }
    }

    impl Box<dyn Any + Send> {
        #[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);
        ///     }
        /// }
        ///
        /// fn main() {
        ///     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>, Box<dyn Any + Send>> {
            <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
                // reapply the Send marker
                Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
            })
        }
    }

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

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

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

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

    #[unstable(feature = "receiver_trait", issue = "0")]
    impl<T: ?Sized> Receiver for Box<T> {}

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

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

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

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

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

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

    #[unstable(feature = "dispatch_from_dyn", issue = "0")]
    impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}

    #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
    impl<A> FromIterator<A> for Box<[A]> {
        fn from_iter<T: IntoIterator<Item = A>>(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 {
            let mut new = BoxBuilder {
                data: RawVec::with_capacity(self.len()),
                len: 0,
            };

            let mut target = new.data.ptr();

            for item in self.iter() {
                unsafe {
                    ptr::write(target, item.clone());
                    target = target.offset(1);
                };

                new.len += 1;
            }

            return unsafe { new.into_box() };

            // Helper type for responding to panics correctly.
            struct BoxBuilder<T> {
                data: RawVec<T>,
                len: usize,
            }

            impl<T> BoxBuilder<T> {
                unsafe fn into_box(self) -> Box<[T]> {
                    let raw = ptr::read(&self.data);
                    mem::forget(self);
                    raw.into_box()
                }
            }

            impl<T> Drop for BoxBuilder<T> {
                fn drop(&mut self) {
                    let mut data = self.data.ptr();
                    let max = unsafe { data.add(self.len) };

                    while data != max {
                        unsafe {
                            ptr::read(data);
                            data = data.offset(1);
                        }
                    }
                }
            }
        }
    }

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

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

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

    #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
    impl<T: ?Sized> AsMut<T> for Box<T> {
        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> Unpin for Box<T> { }

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

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

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

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

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

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

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