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    // Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! 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
    //!
    //! Creating a box:
    //!
    //! ```
    //! let x = Box::new(5);
    //! ```
    //!
    //! 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.

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

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

    use raw_vec::RawVec;
    use 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
        }
    }

    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. Since the
        /// way `Box` allocates and releases memory is unspecified, the
        /// only valid pointer to pass to this function is the one taken
        /// from another `Box` via the [`Box::into_raw`] function.
        ///
        /// 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.
        ///
        /// [`Box::into_raw`]: struct.Box.html#method.into_raw
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let ptr = Box::into_raw(x);
        /// let x = unsafe { Box::from_raw(ptr) };
        /// ```
        #[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 the wrapped raw pointer.
        ///
        /// 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
        /// proper way to do so is to convert the raw pointer 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(b)` instead of `b.into_raw()`. 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
        ///
        /// ```
        /// let x = Box::new(5);
        /// let ptr = Box::into_raw(x);
        /// ```
        #[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
        /// proper 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);
        /// }
        /// ```
        #[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 unique = b.0;
            mem::forget(b);
            unique
        }

        /// Consumes and leaks the `Box`, returning a mutable reference,
        /// `&'a mut T`. Here, the lifetime `'a` 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) }
        }
    }

    #[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 {
            let len = self.len();
            let buf = RawVec::with_capacity(len);
            unsafe {
                ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
                from_boxed_utf8_unchecked(buf.into_box())
            }
        }
    }

    #[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> {
        fn from(t: T) -> Self {
            Box::new(t)
        }
    }

    #[stable(feature = "box_from_slice", since = "1.17.0")]
    impl<'a, T: Copy> From<&'a [T]> for Box<[T]> {
        fn from(slice: &'a [T]) -> Box<[T]> {
            let mut boxed = unsafe { RawVec::with_capacity(slice.len()).into_box() };
            boxed.copy_from_slice(slice);
            boxed
        }
    }

    #[stable(feature = "box_from_slice", since = "1.17.0")]
    impl<'a> From<&'a str> for Box<str> {
        #[inline]
        fn from(s: &'a 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]> {
        #[inline]
        fn from(s: Box<str>) -> Self {
            unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
        }
    }

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

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

    #[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()
        }
    }
    #[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> {}

    /// `FnBox` is a version of the `FnOnce` intended for use with boxed
    /// closure objects. The idea is that where one would normally store a
    /// `Box<FnOnce()>` in a data structure, you should use
    /// `Box<FnBox()>`. The two traits behave essentially the same, except
    /// that a `FnBox` closure can only be called if it is boxed. (Note
    /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
    /// closures become directly usable.)
    ///
    /// # Examples
    ///
    /// Here is a snippet of code which creates a hashmap full of boxed
    /// once closures and then removes them one by one, calling each
    /// closure as it is removed. Note that the type of the closures
    /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
    /// -> i32>`.
    ///
    /// ```
    /// #![feature(fnbox)]
    ///
    /// use std::boxed::FnBox;
    /// use std::collections::HashMap;
    ///
    /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
    ///     let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
    ///     map.insert(1, Box::new(|| 22));
    ///     map.insert(2, Box::new(|| 44));
    ///     map
    /// }
    ///
    /// fn main() {
    ///     let mut map = make_map();
    ///     for i in &[1, 2] {
    ///         let f = map.remove(&i).unwrap();
    ///         assert_eq!(f(), i * 22);
    ///     }
    /// }
    /// ```
    #[rustc_paren_sugar]
    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    pub trait FnBox<A> {
        type Output;

        fn call_box(self: Box<Self>, args: A) -> Self::Output;
    }

    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    impl<A, F> FnBox<A> for F
        where F: FnOnce<A>
    {
        type Output = F::Output;

        fn call_box(self: Box<F>, args: A) -> F::Output {
            self.call_once(args)
        }
    }

    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
        type Output = R;

        extern "rust-call" fn call_once(self, args: A) -> R {
            self.call_box(args)
        }
    }

    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a> {
        type Output = R;

        extern "rust-call" fn call_once(self, args: A) -> R {
            self.call_box(args)
        }
    }

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

    #[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.offset(self.len as isize) };

                    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
        }
    }

    #[unstable(feature = "generator_trait", issue = "43122")]
    impl<T> Generator for Box<T>
        where T: Generator + ?Sized
    {
        type Yield = T::Yield;
        type Return = T::Return;
        unsafe fn resume(&mut self) -> GeneratorState<Self::Yield, Self::Return> {
            (**self).resume()
        }
    }

    /// A pinned, heap allocated reference.
    #[unstable(feature = "pin", issue = "49150")]
    #[fundamental]
    #[repr(transparent)]
    pub struct PinBox<T: ?Sized> {
        inner: Box<T>,
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<T> PinBox<T> {
        /// Allocate memory on the heap, move the data into it and pin it.
        #[unstable(feature = "pin", issue = "49150")]
        pub fn new(data: T) -> PinBox<T> {
            PinBox { inner: Box::new(data) }
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<T: ?Sized> PinBox<T> {
        /// Get a pinned reference to the data in this PinBox.
        #[inline]
        pub fn as_pin_mut<'a>(&'a mut self) -> PinMut<'a, T> {
            unsafe { PinMut::new_unchecked(&mut *self.inner) }
        }

        /// Constructs a `PinBox` from a raw pointer.
        ///
        /// After calling this function, the raw pointer is owned by the
        /// resulting `PinBox`. Specifically, the `PinBox` destructor will call
        /// the destructor of `T` and free the allocated memory. Since the
        /// way `PinBox` allocates and releases memory is unspecified, the
        /// only valid pointer to pass to this function is the one taken
        /// from another `PinBox` via the [`PinBox::into_raw`] function.
        ///
        /// 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.
        ///
        /// [`PinBox::into_raw`]: struct.PinBox.html#method.into_raw
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(pin)]
        /// use std::boxed::PinBox;
        /// let x = PinBox::new(5);
        /// let ptr = PinBox::into_raw(x);
        /// let x = unsafe { PinBox::from_raw(ptr) };
        /// ```
        #[inline]
        pub unsafe fn from_raw(raw: *mut T) -> Self {
            PinBox { inner: Box::from_raw(raw) }
        }

        /// Consumes the `PinBox`, returning the wrapped raw pointer.
        ///
        /// After calling this function, the caller is responsible for the
        /// memory previously managed by the `PinBox`. In particular, the
        /// caller should properly destroy `T` and release the memory. The
        /// proper way to do so is to convert the raw pointer back into a
        /// `PinBox` with the [`PinBox::from_raw`] function.
        ///
        /// Note: this is an associated function, which means that you have
        /// to call it as `PinBox::into_raw(b)` instead of `b.into_raw()`. This
        /// is so that there is no conflict with a method on the inner type.
        ///
        /// [`PinBox::from_raw`]: struct.PinBox.html#method.from_raw
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(pin)]
        /// use std::boxed::PinBox;
        /// let x = PinBox::new(5);
        /// let ptr = PinBox::into_raw(x);
        /// ```
        #[inline]
        pub fn into_raw(b: PinBox<T>) -> *mut T {
            Box::into_raw(b.inner)
        }

        /// Get a mutable reference to the data inside this PinBox.
        ///
        /// This function is unsafe. Users must guarantee that the data is never
        /// moved out of this reference.
        #[inline]
        pub unsafe fn get_mut<'a>(this: &'a mut PinBox<T>) -> &'a mut T {
            &mut *this.inner
        }

        /// Convert this PinBox into an unpinned Box.
        ///
        /// This function is unsafe. Users must guarantee that the data is never
        /// moved out of the box.
        #[inline]
        pub unsafe fn unpin(this: PinBox<T>) -> Box<T> {
            this.inner
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<T: ?Sized> From<Box<T>> for PinBox<T> {
        fn from(boxed: Box<T>) -> PinBox<T> {
            PinBox { inner: boxed }
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<T: Unpin + ?Sized> From<PinBox<T>> for Box<T> {
        fn from(pinned: PinBox<T>) -> Box<T> {
            pinned.inner
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<T: ?Sized> Deref for PinBox<T> {
        type Target = T;

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

    #[unstable(feature = "pin", issue = "49150")]
    impl<T: Unpin + ?Sized> DerefMut for PinBox<T> {
        fn deref_mut(&mut self) -> &mut T {
            &mut *self.inner
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<T: fmt::Display + ?Sized> fmt::Display for PinBox<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Display::fmt(&*self.inner, f)
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<T: fmt::Debug + ?Sized> fmt::Debug for PinBox<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Debug::fmt(&*self.inner, f)
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<T: ?Sized> fmt::Pointer for PinBox<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.inner;
            fmt::Pointer::fmt(&ptr, f)
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<PinBox<U>> for PinBox<T> {}

    #[unstable(feature = "pin", issue = "49150")]
    impl<T: ?Sized> Unpin for PinBox<T> {}

    #[unstable(feature = "futures_api", issue = "50547")]
    impl<'a, F: ?Sized + Future + Unpin> Future for Box<F> {
        type Output = F::Output;

        fn poll(mut self: PinMut<Self>, cx: &mut Context) -> Poll<Self::Output> {
            PinMut::new(&mut **self).poll(cx)
        }
    }

    #[unstable(feature = "futures_api", issue = "50547")]
    impl<'a, F: ?Sized + Future> Future for PinBox<F> {
        type Output = F::Output;

        fn poll(mut self: PinMut<Self>, cx: &mut Context) -> Poll<Self::Output> {
            self.as_pin_mut().poll(cx)
        }
    }

    #[unstable(feature = "futures_api", issue = "50547")]
    unsafe impl<F: Future<Output = ()> + Send + 'static> UnsafeTask for PinBox<F> {
        fn into_raw(self) -> *mut () {
            PinBox::into_raw(self) as *mut ()
        }

        unsafe fn poll(task: *mut (), cx: &mut Context) -> Poll<()> {
            let ptr = task as *mut F;
            let pin: PinMut<F> = PinMut::new_unchecked(&mut *ptr);
            pin.poll(cx)
        }

        unsafe fn drop(task: *mut ()) {
            drop(PinBox::from_raw(task as *mut F))
        }
    }

    #[unstable(feature = "futures_api", issue = "50547")]
    impl<F: Future<Output = ()> + Send + 'static> From<PinBox<F>> for TaskObj {
        fn from(boxed: PinBox<F>) -> Self {
            TaskObj::new(boxed)
        }
    }

    #[unstable(feature = "futures_api", issue = "50547")]
    impl<F: Future<Output = ()> + Send + 'static> From<Box<F>> for TaskObj {
        fn from(boxed: Box<F>) -> Self {
            TaskObj::new(PinBox::from(boxed))
        }
    }

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