Add documentation to more `From::from` implementations.

[rust.git] / library / alloc / src / collections / vec_deque / mod.rs

//! A double-ended queue implemented with a growable ring buffer.
//!
//! This queue has *O*(1) amortized inserts and removals from both ends of the
//! container. It also has *O*(1) indexing like a vector. The contained elements
//! are not required to be copyable, and the queue will be sendable if the
//! contained type is sendable.

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

use core::cmp::{self, Ordering};
use core::fmt;
use core::hash::{Hash, Hasher};
use core::iter::{repeat_with, FromIterator};
use core::marker::PhantomData;
use core::mem::{self, ManuallyDrop};
use core::ops::{Index, IndexMut, Range, RangeBounds};
use core::ptr::{self, NonNull};
use core::slice;

use crate::alloc::{Allocator, Global};
use crate::collections::TryReserveError;
use crate::collections::TryReserveErrorKind;
use crate::raw_vec::RawVec;
use crate::vec::Vec;

#[macro_use]
mod macros;

#[stable(feature = "drain", since = "1.6.0")]
pub use self::drain::Drain;

mod drain;

#[stable(feature = "rust1", since = "1.0.0")]
pub use self::iter_mut::IterMut;

mod iter_mut;

#[stable(feature = "rust1", since = "1.0.0")]
pub use self::into_iter::IntoIter;

mod into_iter;

#[stable(feature = "rust1", since = "1.0.0")]
pub use self::iter::Iter;

mod iter;

use self::pair_slices::PairSlices;

mod pair_slices;

use self::ring_slices::RingSlices;

mod ring_slices;

#[cfg(test)]
mod tests;

const INITIAL_CAPACITY: usize = 7; // 2^3 - 1
const MINIMUM_CAPACITY: usize = 1; // 2 - 1

const MAXIMUM_ZST_CAPACITY: usize = 1 << (usize::BITS - 1); // Largest possible power of two

/// A double-ended queue implemented with a growable ring buffer.
///
/// The "default" usage of this type as a queue is to use [`push_back`] to add to
/// the queue, and [`pop_front`] to remove from the queue. [`extend`] and [`append`]
/// push onto the back in this manner, and iterating over `VecDeque` goes front
/// to back.
///
/// A `VecDeque` with a known list of items can be initialized from an array:
///
/// ```
/// use std::collections::VecDeque;
///
/// let deq = VecDeque::from([-1, 0, 1]);
/// ```
///
/// Since `VecDeque` is a ring buffer, its elements are not necessarily contiguous
/// in memory. If you want to access the elements as a single slice, such as for
/// efficient sorting, you can use [`make_contiguous`]. It rotates the `VecDeque`
/// so that its elements do not wrap, and returns a mutable slice to the
/// now-contiguous element sequence.
///
/// [`push_back`]: VecDeque::push_back
/// [`pop_front`]: VecDeque::pop_front
/// [`extend`]: VecDeque::extend
/// [`append`]: VecDeque::append
/// [`make_contiguous`]: VecDeque::make_contiguous
#[cfg_attr(not(test), rustc_diagnostic_item = "VecDeque")]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_insignificant_dtor]
pub struct VecDeque<
    T,
    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
> {
    // tail and head are pointers into the buffer. Tail always points
    // to the first element that could be read, Head always points
    // to where data should be written.
    // If tail == head the buffer is empty. The length of the ringbuffer
    // is defined as the distance between the two.
    tail: usize,
    head: usize,
    buf: RawVec<T, A>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone, A: Allocator + Clone> Clone for VecDeque<T, A> {
    fn clone(&self) -> Self {
        let mut deq = Self::with_capacity_in(self.len(), self.allocator().clone());
        deq.extend(self.iter().cloned());
        deq
    }

    fn clone_from(&mut self, other: &Self) {
        self.truncate(other.len());

        let mut iter = PairSlices::from(self, other);
        while let Some((dst, src)) = iter.next() {
            dst.clone_from_slice(&src);
        }

        if iter.has_remainder() {
            for remainder in iter.remainder() {
                self.extend(remainder.iter().cloned());
            }
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T, A: Allocator> Drop for VecDeque<T, A> {
    fn drop(&mut self) {
        /// Runs the destructor for all items in the slice when it gets dropped (normally or
        /// during unwinding).
        struct Dropper<'a, T>(&'a mut [T]);

        impl<'a, T> Drop for Dropper<'a, T> {
            fn drop(&mut self) {
                unsafe {
                    ptr::drop_in_place(self.0);
                }
            }
        }

        let (front, back) = self.as_mut_slices();
        unsafe {
            let _back_dropper = Dropper(back);
            // use drop for [T]
            ptr::drop_in_place(front);
        }
        // RawVec handles deallocation
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Default for VecDeque<T> {
    /// Creates an empty `VecDeque<T>`.
    #[inline]
    fn default() -> VecDeque<T> {
        VecDeque::new()
    }
}

impl<T, A: Allocator> VecDeque<T, A> {
    /// Marginally more convenient
    #[inline]
    fn ptr(&self) -> *mut T {
        self.buf.ptr()
    }

    /// Marginally more convenient
    #[inline]
    fn cap(&self) -> usize {
        if mem::size_of::<T>() == 0 {
            // For zero sized types, we are always at maximum capacity
            MAXIMUM_ZST_CAPACITY
        } else {
            self.buf.capacity()
        }
    }

    /// Turn ptr into a slice
    #[inline]
    unsafe fn buffer_as_slice(&self) -> &[T] {
        unsafe { slice::from_raw_parts(self.ptr(), self.cap()) }
    }

    /// Turn ptr into a mut slice
    #[inline]
    unsafe fn buffer_as_mut_slice(&mut self) -> &mut [T] {
        unsafe { slice::from_raw_parts_mut(self.ptr(), self.cap()) }
    }

    /// Moves an element out of the buffer
    #[inline]
    unsafe fn buffer_read(&mut self, off: usize) -> T {
        unsafe { ptr::read(self.ptr().add(off)) }
    }

    /// Writes an element into the buffer, moving it.
    #[inline]
    unsafe fn buffer_write(&mut self, off: usize, value: T) {
        unsafe {
            ptr::write(self.ptr().add(off), value);
        }
    }

    /// Returns `true` if the buffer is at full capacity.
    #[inline]
    fn is_full(&self) -> bool {
        self.cap() - self.len() == 1
    }

    /// Returns the index in the underlying buffer for a given logical element
    /// index.
    #[inline]
    fn wrap_index(&self, idx: usize) -> usize {
        wrap_index(idx, self.cap())
    }

    /// Returns the index in the underlying buffer for a given logical element
    /// index + addend.
    #[inline]
    fn wrap_add(&self, idx: usize, addend: usize) -> usize {
        wrap_index(idx.wrapping_add(addend), self.cap())
    }

    /// Returns the index in the underlying buffer for a given logical element
    /// index - subtrahend.
    #[inline]
    fn wrap_sub(&self, idx: usize, subtrahend: usize) -> usize {
        wrap_index(idx.wrapping_sub(subtrahend), self.cap())
    }

    /// Copies a contiguous block of memory len long from src to dst
    #[inline]
    unsafe fn copy(&self, dst: usize, src: usize, len: usize) {
        debug_assert!(
            dst + len <= self.cap(),
            "cpy dst={} src={} len={} cap={}",
            dst,
            src,
            len,
            self.cap()
        );
        debug_assert!(
            src + len <= self.cap(),
            "cpy dst={} src={} len={} cap={}",
            dst,
            src,
            len,
            self.cap()
        );
        unsafe {
            ptr::copy(self.ptr().add(src), self.ptr().add(dst), len);
        }
    }

    /// Copies a contiguous block of memory len long from src to dst
    #[inline]
    unsafe fn copy_nonoverlapping(&self, dst: usize, src: usize, len: usize) {
        debug_assert!(
            dst + len <= self.cap(),
            "cno dst={} src={} len={} cap={}",
            dst,
            src,
            len,
            self.cap()
        );
        debug_assert!(
            src + len <= self.cap(),
            "cno dst={} src={} len={} cap={}",
            dst,
            src,
            len,
            self.cap()
        );
        unsafe {
            ptr::copy_nonoverlapping(self.ptr().add(src), self.ptr().add(dst), len);
        }
    }

    /// Copies a potentially wrapping block of memory len long from src to dest.
    /// (abs(dst - src) + len) must be no larger than cap() (There must be at
    /// most one continuous overlapping region between src and dest).
    unsafe fn wrap_copy(&self, dst: usize, src: usize, len: usize) {
        #[allow(dead_code)]
        fn diff(a: usize, b: usize) -> usize {
            if a <= b { b - a } else { a - b }
        }
        debug_assert!(
            cmp::min(diff(dst, src), self.cap() - diff(dst, src)) + len <= self.cap(),
            "wrc dst={} src={} len={} cap={}",
            dst,
            src,
            len,
            self.cap()
        );

        if src == dst || len == 0 {
            return;
        }

        let dst_after_src = self.wrap_sub(dst, src) < len;

        let src_pre_wrap_len = self.cap() - src;
        let dst_pre_wrap_len = self.cap() - dst;
        let src_wraps = src_pre_wrap_len < len;
        let dst_wraps = dst_pre_wrap_len < len;

        match (dst_after_src, src_wraps, dst_wraps) {
            (_, false, false) => {
                // src doesn't wrap, dst doesn't wrap
                //
                //        S . . .
                // 1 [_ _ A A B B C C _]
                // 2 [_ _ A A A A B B _]
                //            D . . .
                //
                unsafe {
                    self.copy(dst, src, len);
                }
            }
            (false, false, true) => {
                // dst before src, src doesn't wrap, dst wraps
                //
                //    S . . .
                // 1 [A A B B _ _ _ C C]
                // 2 [A A B B _ _ _ A A]
                // 3 [B B B B _ _ _ A A]
                //    . .           D .
                //
                unsafe {
                    self.copy(dst, src, dst_pre_wrap_len);
                    self.copy(0, src + dst_pre_wrap_len, len - dst_pre_wrap_len);
                }
            }
            (true, false, true) => {
                // src before dst, src doesn't wrap, dst wraps
                //
                //              S . . .
                // 1 [C C _ _ _ A A B B]
                // 2 [B B _ _ _ A A B B]
                // 3 [B B _ _ _ A A A A]
                //    . .           D .
                //
                unsafe {
                    self.copy(0, src + dst_pre_wrap_len, len - dst_pre_wrap_len);
                    self.copy(dst, src, dst_pre_wrap_len);
                }
            }
            (false, true, false) => {
                // dst before src, src wraps, dst doesn't wrap
                //
                //    . .           S .
                // 1 [C C _ _ _ A A B B]
                // 2 [C C _ _ _ B B B B]
                // 3 [C C _ _ _ B B C C]
                //              D . . .
                //
                unsafe {
                    self.copy(dst, src, src_pre_wrap_len);
                    self.copy(dst + src_pre_wrap_len, 0, len - src_pre_wrap_len);
                }
            }
            (true, true, false) => {
                // src before dst, src wraps, dst doesn't wrap
                //
                //    . .           S .
                // 1 [A A B B _ _ _ C C]
                // 2 [A A A A _ _ _ C C]
                // 3 [C C A A _ _ _ C C]
                //    D . . .
                //
                unsafe {
                    self.copy(dst + src_pre_wrap_len, 0, len - src_pre_wrap_len);
                    self.copy(dst, src, src_pre_wrap_len);
                }
            }
            (false, true, true) => {
                // dst before src, src wraps, dst wraps
                //
                //    . . .         S .
                // 1 [A B C D _ E F G H]
                // 2 [A B C D _ E G H H]
                // 3 [A B C D _ E G H A]
                // 4 [B C C D _ E G H A]
                //    . .         D . .
                //
                debug_assert!(dst_pre_wrap_len > src_pre_wrap_len);
                let delta = dst_pre_wrap_len - src_pre_wrap_len;
                unsafe {
                    self.copy(dst, src, src_pre_wrap_len);
                    self.copy(dst + src_pre_wrap_len, 0, delta);
                    self.copy(0, delta, len - dst_pre_wrap_len);
                }
            }
            (true, true, true) => {
                // src before dst, src wraps, dst wraps
                //
                //    . .         S . .
                // 1 [A B C D _ E F G H]
                // 2 [A A B D _ E F G H]
                // 3 [H A B D _ E F G H]
                // 4 [H A B D _ E F F G]
                //    . . .         D .
                //
                debug_assert!(src_pre_wrap_len > dst_pre_wrap_len);
                let delta = src_pre_wrap_len - dst_pre_wrap_len;
                unsafe {
                    self.copy(delta, 0, len - src_pre_wrap_len);
                    self.copy(0, self.cap() - delta, delta);
                    self.copy(dst, src, dst_pre_wrap_len);
                }
            }
        }
    }

    /// Copies all values from `src` to `dst`, wrapping around if needed.
    /// Assumes capacity is sufficient.
    #[inline]
    unsafe fn copy_slice(&mut self, dst: usize, src: &[T]) {
        debug_assert!(src.len() <= self.cap());
        let head_room = self.cap() - dst;
        if src.len() <= head_room {
            unsafe {
                ptr::copy_nonoverlapping(src.as_ptr(), self.ptr().add(dst), src.len());
            }
        } else {
            let (left, right) = src.split_at(head_room);
            unsafe {
                ptr::copy_nonoverlapping(left.as_ptr(), self.ptr().add(dst), left.len());
                ptr::copy_nonoverlapping(right.as_ptr(), self.ptr(), right.len());
            }
        }
    }

    /// Frobs the head and tail sections around to handle the fact that we
    /// just reallocated. Unsafe because it trusts old_capacity.
    #[inline]
    unsafe fn handle_capacity_increase(&mut self, old_capacity: usize) {
        let new_capacity = self.cap();

        // Move the shortest contiguous section of the ring buffer
        //    T             H
        //   [o o o o o o o . ]
        //    T             H
        // A [o o o o o o o . . . . . . . . . ]
        //        H T
        //   [o o . o o o o o ]
        //          T             H
        // B [. . . o o o o o o o . . . . . . ]
        //              H T
        //   [o o o o o . o o ]
        //              H                 T
        // C [o o o o o . . . . . . . . . o o ]

        if self.tail <= self.head {
            // A
            // Nop
        } else if self.head < old_capacity - self.tail {
            // B
            unsafe {
                self.copy_nonoverlapping(old_capacity, 0, self.head);
            }
            self.head += old_capacity;
            debug_assert!(self.head > self.tail);
        } else {
            // C
            let new_tail = new_capacity - (old_capacity - self.tail);
            unsafe {
                self.copy_nonoverlapping(new_tail, self.tail, old_capacity - self.tail);
            }
            self.tail = new_tail;
            debug_assert!(self.head < self.tail);
        }
        debug_assert!(self.head < self.cap());
        debug_assert!(self.tail < self.cap());
        debug_assert!(self.cap().count_ones() == 1);
    }
}

impl<T> VecDeque<T> {
    /// Creates an empty `VecDeque`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let vector: VecDeque<u32> = VecDeque::new();
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    pub fn new() -> VecDeque<T> {
        VecDeque::new_in(Global)
    }

    /// Creates an empty `VecDeque` with space for at least `capacity` elements.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let vector: VecDeque<u32> = VecDeque::with_capacity(10);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    pub fn with_capacity(capacity: usize) -> VecDeque<T> {
        Self::with_capacity_in(capacity, Global)
    }
}

impl<T, A: Allocator> VecDeque<T, A> {
    /// Creates an empty `VecDeque`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let vector: VecDeque<u32> = VecDeque::new();
    /// ```
    #[inline]
    #[unstable(feature = "allocator_api", issue = "32838")]
    pub fn new_in(alloc: A) -> VecDeque<T, A> {
        VecDeque::with_capacity_in(INITIAL_CAPACITY, alloc)
    }

    /// Creates an empty `VecDeque` with space for at least `capacity` elements.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let vector: VecDeque<u32> = VecDeque::with_capacity(10);
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    pub fn with_capacity_in(capacity: usize, alloc: A) -> VecDeque<T, A> {
        assert!(capacity < 1_usize << usize::BITS - 1, "capacity overflow");
        // +1 since the ringbuffer always leaves one space empty
        let cap = cmp::max(capacity + 1, MINIMUM_CAPACITY + 1).next_power_of_two();

        VecDeque { tail: 0, head: 0, buf: RawVec::with_capacity_in(cap, alloc) }
    }

    /// Provides a reference to the element at the given index.
    ///
    /// Element at index 0 is the front of the queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(3);
    /// buf.push_back(4);
    /// buf.push_back(5);
    /// assert_eq!(buf.get(1), Some(&4));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn get(&self, index: usize) -> Option<&T> {
        if index < self.len() {
            let idx = self.wrap_add(self.tail, index);
            unsafe { Some(&*self.ptr().add(idx)) }
        } else {
            None
        }
    }

    /// Provides a mutable reference to the element at the given index.
    ///
    /// Element at index 0 is the front of the queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(3);
    /// buf.push_back(4);
    /// buf.push_back(5);
    /// if let Some(elem) = buf.get_mut(1) {
    ///     *elem = 7;
    /// }
    ///
    /// assert_eq!(buf[1], 7);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn get_mut(&mut self, index: usize) -> Option<&mut T> {
        if index < self.len() {
            let idx = self.wrap_add(self.tail, index);
            unsafe { Some(&mut *self.ptr().add(idx)) }
        } else {
            None
        }
    }

    /// Swaps elements at indices `i` and `j`.
    ///
    /// `i` and `j` may be equal.
    ///
    /// Element at index 0 is the front of the queue.
    ///
    /// # Panics
    ///
    /// Panics if either index is out of bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(3);
    /// buf.push_back(4);
    /// buf.push_back(5);
    /// assert_eq!(buf, [3, 4, 5]);
    /// buf.swap(0, 2);
    /// assert_eq!(buf, [5, 4, 3]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn swap(&mut self, i: usize, j: usize) {
        assert!(i < self.len());
        assert!(j < self.len());
        let ri = self.wrap_add(self.tail, i);
        let rj = self.wrap_add(self.tail, j);
        unsafe { ptr::swap(self.ptr().add(ri), self.ptr().add(rj)) }
    }

    /// Returns the number of elements the `VecDeque` can hold without
    /// reallocating.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let buf: VecDeque<i32> = VecDeque::with_capacity(10);
    /// assert!(buf.capacity() >= 10);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn capacity(&self) -> usize {
        self.cap() - 1
    }

    /// Reserves the minimum capacity for exactly `additional` more elements to be inserted in the
    /// given `VecDeque`. Does nothing if the capacity is already sufficient.
    ///
    /// Note that the allocator may give the collection more space than it requests. Therefore
    /// capacity can not be relied upon to be precisely minimal. Prefer [`reserve`] if future
    /// insertions are expected.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity overflows `usize`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf: VecDeque<i32> = vec![1].into_iter().collect();
    /// buf.reserve_exact(10);
    /// assert!(buf.capacity() >= 11);
    /// ```
    ///
    /// [`reserve`]: VecDeque::reserve
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn reserve_exact(&mut self, additional: usize) {
        self.reserve(additional);
    }

    /// Reserves capacity for at least `additional` more elements to be inserted in the given
    /// `VecDeque`. The collection may reserve more space to avoid frequent reallocations.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity overflows `usize`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf: VecDeque<i32> = vec![1].into_iter().collect();
    /// buf.reserve(10);
    /// assert!(buf.capacity() >= 11);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn reserve(&mut self, additional: usize) {
        let old_cap = self.cap();
        let used_cap = self.len() + 1;
        let new_cap = used_cap
            .checked_add(additional)
            .and_then(|needed_cap| needed_cap.checked_next_power_of_two())
            .expect("capacity overflow");

        if new_cap > old_cap {
            self.buf.reserve_exact(used_cap, new_cap - used_cap);
            unsafe {
                self.handle_capacity_increase(old_cap);
            }
        }
    }

    /// Tries to reserve the minimum capacity for exactly `additional` more elements to
    /// be inserted in the given `VecDeque<T>`. After calling `try_reserve_exact`,
    /// capacity will be greater than or equal to `self.len() + additional`.
    /// Does nothing if the capacity is already sufficient.
    ///
    /// Note that the allocator may give the collection more space than it
    /// requests. Therefore, capacity can not be relied upon to be precisely
    /// minimal. Prefer [`reserve`] if future insertions are expected.
    ///
    /// [`reserve`]: VecDeque::reserve
    ///
    /// # Errors
    ///
    /// If the capacity overflows `usize`, or the allocator reports a failure, then an error
    /// is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::TryReserveError;
    /// use std::collections::VecDeque;
    ///
    /// fn process_data(data: &[u32]) -> Result<VecDeque<u32>, TryReserveError> {
    ///     let mut output = VecDeque::new();
    ///
    ///     // Pre-reserve the memory, exiting if we can't
    ///     output.try_reserve_exact(data.len())?;
    ///
    ///     // Now we know this can't OOM(Out-Of-Memory) in the middle of our complex work
    ///     output.extend(data.iter().map(|&val| {
    ///         val * 2 + 5 // very complicated
    ///     }));
    ///
    ///     Ok(output)
    /// }
    /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
    /// ```
    #[stable(feature = "try_reserve", since = "1.57.0")]
    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.try_reserve(additional)
    }

    /// Tries to reserve capacity for at least `additional` more elements to be inserted
    /// in the given `VecDeque<T>`. The collection may reserve more space to avoid
    /// frequent reallocations. After calling `try_reserve`, capacity will be
    /// greater than or equal to `self.len() + additional`. Does nothing if
    /// capacity is already sufficient.
    ///
    /// # Errors
    ///
    /// If the capacity overflows `usize`, or the allocator reports a failure, then an error
    /// is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::TryReserveError;
    /// use std::collections::VecDeque;
    ///
    /// fn process_data(data: &[u32]) -> Result<VecDeque<u32>, TryReserveError> {
    ///     let mut output = VecDeque::new();
    ///
    ///     // Pre-reserve the memory, exiting if we can't
    ///     output.try_reserve(data.len())?;
    ///
    ///     // Now we know this can't OOM in the middle of our complex work
    ///     output.extend(data.iter().map(|&val| {
    ///         val * 2 + 5 // very complicated
    ///     }));
    ///
    ///     Ok(output)
    /// }
    /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
    /// ```
    #[stable(feature = "try_reserve", since = "1.57.0")]
    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
        let old_cap = self.cap();
        let used_cap = self.len() + 1;
        let new_cap = used_cap
            .checked_add(additional)
            .and_then(|needed_cap| needed_cap.checked_next_power_of_two())
            .ok_or(TryReserveErrorKind::CapacityOverflow)?;

        if new_cap > old_cap {
            self.buf.try_reserve_exact(used_cap, new_cap - used_cap)?;
            unsafe {
                self.handle_capacity_increase(old_cap);
            }
        }
        Ok(())
    }

    /// Shrinks the capacity of the `VecDeque` as much as possible.
    ///
    /// It will drop down as close as possible to the length but the allocator may still inform the
    /// `VecDeque` that there is space for a few more elements.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::with_capacity(15);
    /// buf.extend(0..4);
    /// assert_eq!(buf.capacity(), 15);
    /// buf.shrink_to_fit();
    /// assert!(buf.capacity() >= 4);
    /// ```
    #[stable(feature = "deque_extras_15", since = "1.5.0")]
    pub fn shrink_to_fit(&mut self) {
        self.shrink_to(0);
    }

    /// Shrinks the capacity of the `VecDeque` with a lower bound.
    ///
    /// The capacity will remain at least as large as both the length
    /// and the supplied value.
    ///
    /// If the current capacity is less than the lower limit, this is a no-op.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::with_capacity(15);
    /// buf.extend(0..4);
    /// assert_eq!(buf.capacity(), 15);
    /// buf.shrink_to(6);
    /// assert!(buf.capacity() >= 6);
    /// buf.shrink_to(0);
    /// assert!(buf.capacity() >= 4);
    /// ```
    #[stable(feature = "shrink_to", since = "1.56.0")]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        let min_capacity = cmp::min(min_capacity, self.capacity());
        // We don't have to worry about an overflow as neither `self.len()` nor `self.capacity()`
        // can ever be `usize::MAX`. +1 as the ringbuffer always leaves one space empty.
        let target_cap = cmp::max(cmp::max(min_capacity, self.len()) + 1, MINIMUM_CAPACITY + 1)
            .next_power_of_two();

        if target_cap < self.cap() {
            // There are three cases of interest:
            //   All elements are out of desired bounds
            //   Elements are contiguous, and head is out of desired bounds
            //   Elements are discontiguous, and tail is out of desired bounds
            //
            // At all other times, element positions are unaffected.
            //
            // Indicates that elements at the head should be moved.
            let head_outside = self.head == 0 || self.head >= target_cap;
            // Move elements from out of desired bounds (positions after target_cap)
            if self.tail >= target_cap && head_outside {
                //                    T             H
                //   [. . . . . . . . o o o o o o o . ]
                //    T             H
                //   [o o o o o o o . ]
                unsafe {
                    self.copy_nonoverlapping(0, self.tail, self.len());
                }
                self.head = self.len();
                self.tail = 0;
            } else if self.tail != 0 && self.tail < target_cap && head_outside {
                //          T             H
                //   [. . . o o o o o o o . . . . . . ]
                //        H T
                //   [o o . o o o o o ]
                let len = self.wrap_sub(self.head, target_cap);
                unsafe {
                    self.copy_nonoverlapping(0, target_cap, len);
                }
                self.head = len;
                debug_assert!(self.head < self.tail);
            } else if self.tail >= target_cap {
                //              H                 T
                //   [o o o o o . . . . . . . . . o o ]
                //              H T
                //   [o o o o o . o o ]
                debug_assert!(self.wrap_sub(self.head, 1) < target_cap);
                let len = self.cap() - self.tail;
                let new_tail = target_cap - len;
                unsafe {
                    self.copy_nonoverlapping(new_tail, self.tail, len);
                }
                self.tail = new_tail;
                debug_assert!(self.head < self.tail);
            }

            self.buf.shrink_to_fit(target_cap);

            debug_assert!(self.head < self.cap());
            debug_assert!(self.tail < self.cap());
            debug_assert!(self.cap().count_ones() == 1);
        }
    }

    /// Shortens the `VecDeque`, keeping the first `len` elements and dropping
    /// the rest.
    ///
    /// If `len` is greater than the `VecDeque`'s current length, this has no
    /// effect.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(5);
    /// buf.push_back(10);
    /// buf.push_back(15);
    /// assert_eq!(buf, [5, 10, 15]);
    /// buf.truncate(1);
    /// assert_eq!(buf, [5]);
    /// ```
    #[stable(feature = "deque_extras", since = "1.16.0")]
    pub fn truncate(&mut self, len: usize) {
        /// Runs the destructor for all items in the slice when it gets dropped (normally or
        /// during unwinding).
        struct Dropper<'a, T>(&'a mut [T]);

        impl<'a, T> Drop for Dropper<'a, T> {
            fn drop(&mut self) {
                unsafe {
                    ptr::drop_in_place(self.0);
                }
            }
        }

        // Safe because:
        //
        // * Any slice passed to `drop_in_place` is valid; the second case has
        //   `len <= front.len()` and returning on `len > self.len()` ensures
        //   `begin <= back.len()` in the first case
        // * The head of the VecDeque is moved before calling `drop_in_place`,
        //   so no value is dropped twice if `drop_in_place` panics
        unsafe {
            if len > self.len() {
                return;
            }
            let num_dropped = self.len() - len;
            let (front, back) = self.as_mut_slices();
            if len > front.len() {
                let begin = len - front.len();
                let drop_back = back.get_unchecked_mut(begin..) as *mut _;
                self.head = self.wrap_sub(self.head, num_dropped);
                ptr::drop_in_place(drop_back);
            } else {
                let drop_back = back as *mut _;
                let drop_front = front.get_unchecked_mut(len..) as *mut _;
                self.head = self.wrap_sub(self.head, num_dropped);

                // Make sure the second half is dropped even when a destructor
                // in the first one panics.
                let _back_dropper = Dropper(&mut *drop_back);
                ptr::drop_in_place(drop_front);
            }
        }
    }

    /// Returns a reference to the underlying allocator.
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn allocator(&self) -> &A {
        self.buf.allocator()
    }

    /// Returns a front-to-back iterator.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(5);
    /// buf.push_back(3);
    /// buf.push_back(4);
    /// let b: &[_] = &[&5, &3, &4];
    /// let c: Vec<&i32> = buf.iter().collect();
    /// assert_eq!(&c[..], b);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter(&self) -> Iter<'_, T> {
        Iter { tail: self.tail, head: self.head, ring: unsafe { self.buffer_as_slice() } }
    }

    /// Returns a front-to-back iterator that returns mutable references.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(5);
    /// buf.push_back(3);
    /// buf.push_back(4);
    /// for num in buf.iter_mut() {
    ///     *num = *num - 2;
    /// }
    /// let b: &[_] = &[&mut 3, &mut 1, &mut 2];
    /// assert_eq!(&buf.iter_mut().collect::<Vec<&mut i32>>()[..], b);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn iter_mut(&mut self) -> IterMut<'_, T> {
        // SAFETY: The internal `IterMut` safety invariant is established because the
        // `ring` we create is a dereferencable slice for lifetime '_.
        let ring = ptr::slice_from_raw_parts_mut(self.ptr(), self.cap());

        unsafe { IterMut::new(ring, self.tail, self.head, PhantomData) }
    }

    /// Returns a pair of slices which contain, in order, the contents of the
    /// `VecDeque`.
    ///
    /// If [`make_contiguous`] was previously called, all elements of the
    /// `VecDeque` will be in the first slice and the second slice will be empty.
    ///
    /// [`make_contiguous`]: VecDeque::make_contiguous
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut vector = VecDeque::new();
    ///
    /// vector.push_back(0);
    /// vector.push_back(1);
    /// vector.push_back(2);
    ///
    /// assert_eq!(vector.as_slices(), (&[0, 1, 2][..], &[][..]));
    ///
    /// vector.push_front(10);
    /// vector.push_front(9);
    ///
    /// assert_eq!(vector.as_slices(), (&[9, 10][..], &[0, 1, 2][..]));
    /// ```
    #[inline]
    #[stable(feature = "deque_extras_15", since = "1.5.0")]
    pub fn as_slices(&self) -> (&[T], &[T]) {
        unsafe {
            let buf = self.buffer_as_slice();
            RingSlices::ring_slices(buf, self.head, self.tail)
        }
    }

    /// Returns a pair of slices which contain, in order, the contents of the
    /// `VecDeque`.
    ///
    /// If [`make_contiguous`] was previously called, all elements of the
    /// `VecDeque` will be in the first slice and the second slice will be empty.
    ///
    /// [`make_contiguous`]: VecDeque::make_contiguous
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut vector = VecDeque::new();
    ///
    /// vector.push_back(0);
    /// vector.push_back(1);
    ///
    /// vector.push_front(10);
    /// vector.push_front(9);
    ///
    /// vector.as_mut_slices().0[0] = 42;
    /// vector.as_mut_slices().1[0] = 24;
    /// assert_eq!(vector.as_slices(), (&[42, 10][..], &[24, 1][..]));
    /// ```
    #[inline]
    #[stable(feature = "deque_extras_15", since = "1.5.0")]
    pub fn as_mut_slices(&mut self) -> (&mut [T], &mut [T]) {
        unsafe {
            let head = self.head;
            let tail = self.tail;
            let buf = self.buffer_as_mut_slice();
            RingSlices::ring_slices(buf, head, tail)
        }
    }

    /// Returns the number of elements in the `VecDeque`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut v = VecDeque::new();
    /// assert_eq!(v.len(), 0);
    /// v.push_back(1);
    /// assert_eq!(v.len(), 1);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn len(&self) -> usize {
        count(self.tail, self.head, self.cap())
    }

    /// Returns `true` if the `VecDeque` is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut v = VecDeque::new();
    /// assert!(v.is_empty());
    /// v.push_front(1);
    /// assert!(!v.is_empty());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_empty(&self) -> bool {
        self.tail == self.head
    }

    fn range_tail_head<R>(&self, range: R) -> (usize, usize)
    where
        R: RangeBounds<usize>,
    {
        let Range { start, end } = slice::range(range, ..self.len());
        let tail = self.wrap_add(self.tail, start);
        let head = self.wrap_add(self.tail, end);
        (tail, head)
    }

    /// Creates an iterator that covers the specified range in the `VecDeque`.
    ///
    /// # Panics
    ///
    /// Panics if the starting point is greater than the end point or if
    /// the end point is greater than the length of the vector.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let v: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
    /// let range = v.range(2..).copied().collect::<VecDeque<_>>();
    /// assert_eq!(range, [3]);
    ///
    /// // A full range covers all contents
    /// let all = v.range(..);
    /// assert_eq!(all.len(), 3);
    /// ```
    #[inline]
    #[stable(feature = "deque_range", since = "1.51.0")]
    pub fn range<R>(&self, range: R) -> Iter<'_, T>
    where
        R: RangeBounds<usize>,
    {
        let (tail, head) = self.range_tail_head(range);
        Iter {
            tail,
            head,
            // The shared reference we have in &self is maintained in the '_ of Iter.
            ring: unsafe { self.buffer_as_slice() },
        }
    }

    /// Creates an iterator that covers the specified mutable range in the `VecDeque`.
    ///
    /// # Panics
    ///
    /// Panics if the starting point is greater than the end point or if
    /// the end point is greater than the length of the vector.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut v: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
    /// for v in v.range_mut(2..) {
    ///   *v *= 2;
    /// }
    /// assert_eq!(v, vec![1, 2, 6]);
    ///
    /// // A full range covers all contents
    /// for v in v.range_mut(..) {
    ///   *v *= 2;
    /// }
    /// assert_eq!(v, vec![2, 4, 12]);
    /// ```
    #[inline]
    #[stable(feature = "deque_range", since = "1.51.0")]
    pub fn range_mut<R>(&mut self, range: R) -> IterMut<'_, T>
    where
        R: RangeBounds<usize>,
    {
        let (tail, head) = self.range_tail_head(range);

        // SAFETY: The internal `IterMut` safety invariant is established because the
        // `ring` we create is a dereferencable slice for lifetime '_.
        let ring = ptr::slice_from_raw_parts_mut(self.ptr(), self.cap());

        unsafe { IterMut::new(ring, tail, head, PhantomData) }
    }

    /// Creates a draining iterator that removes the specified range in the
    /// `VecDeque` and yields the removed items.
    ///
    /// Note 1: The element range is removed even if the iterator is not
    /// consumed until the end.
    ///
    /// Note 2: It is unspecified how many elements are removed from the deque,
    /// if the `Drain` value is not dropped, but the borrow it holds expires
    /// (e.g., due to `mem::forget`).
    ///
    /// # Panics
    ///
    /// Panics if the starting point is greater than the end point or if
    /// the end point is greater than the length of the vector.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut v: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
    /// let drained = v.drain(2..).collect::<VecDeque<_>>();
    /// assert_eq!(drained, [3]);
    /// assert_eq!(v, [1, 2]);
    ///
    /// // A full range clears all contents
    /// v.drain(..);
    /// assert!(v.is_empty());
    /// ```
    #[inline]
    #[stable(feature = "drain", since = "1.6.0")]
    pub fn drain<R>(&mut self, range: R) -> Drain<'_, T, A>
    where
        R: RangeBounds<usize>,
    {
        // Memory safety
        //
        // When the Drain is first created, the source deque is shortened to
        // make sure no uninitialized or moved-from elements are accessible at
        // all if the Drain's destructor never gets to run.
        //
        // Drain will ptr::read out the values to remove.
        // When finished, the remaining data will be copied back to cover the hole,
        // and the head/tail values will be restored correctly.
        //
        let (drain_tail, drain_head) = self.range_tail_head(range);

        // The deque's elements are parted into three segments:
        // * self.tail  -> drain_tail
        // * drain_tail -> drain_head
        // * drain_head -> self.head
        //
        // T = self.tail; H = self.head; t = drain_tail; h = drain_head
        //
        // We store drain_tail as self.head, and drain_head and self.head as
        // after_tail and after_head respectively on the Drain. This also
        // truncates the effective array such that if the Drain is leaked, we
        // have forgotten about the potentially moved values after the start of
        // the drain.
        //
        //        T   t   h   H
        // [. . . o o x x o o . . .]
        //
        let head = self.head;

        // "forget" about the values after the start of the drain until after
        // the drain is complete and the Drain destructor is run.
        self.head = drain_tail;

        let deque = NonNull::from(&mut *self);
        let iter = Iter {
            tail: drain_tail,
            head: drain_head,
            // Crucially, we only create shared references from `self` here and read from
            // it.  We do not write to `self` nor reborrow to a mutable reference.
            // Hence the raw pointer we created above, for `deque`, remains valid.
            ring: unsafe { self.buffer_as_slice() },
        };

        unsafe { Drain::new(drain_head, head, iter, deque) }
    }

    /// Clears the `VecDeque`, removing all values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut v = VecDeque::new();
    /// v.push_back(1);
    /// v.clear();
    /// assert!(v.is_empty());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn clear(&mut self) {
        self.truncate(0);
    }

    /// Returns `true` if the `VecDeque` contains an element equal to the
    /// given value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut vector: VecDeque<u32> = VecDeque::new();
    ///
    /// vector.push_back(0);
    /// vector.push_back(1);
    ///
    /// assert_eq!(vector.contains(&1), true);
    /// assert_eq!(vector.contains(&10), false);
    /// ```
    #[stable(feature = "vec_deque_contains", since = "1.12.0")]
    pub fn contains(&self, x: &T) -> bool
    where
        T: PartialEq<T>,
    {
        let (a, b) = self.as_slices();
        a.contains(x) || b.contains(x)
    }

    /// Provides a reference to the front element, or `None` if the `VecDeque` is
    /// empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// assert_eq!(d.front(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// assert_eq!(d.front(), Some(&1));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn front(&self) -> Option<&T> {
        self.get(0)
    }

    /// Provides a mutable reference to the front element, or `None` if the
    /// `VecDeque` is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// assert_eq!(d.front_mut(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// match d.front_mut() {
    ///     Some(x) => *x = 9,
    ///     None => (),
    /// }
    /// assert_eq!(d.front(), Some(&9));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn front_mut(&mut self) -> Option<&mut T> {
        self.get_mut(0)
    }

    /// Provides a reference to the back element, or `None` if the `VecDeque` is
    /// empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// assert_eq!(d.back(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// assert_eq!(d.back(), Some(&2));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn back(&self) -> Option<&T> {
        self.get(self.len().wrapping_sub(1))
    }

    /// Provides a mutable reference to the back element, or `None` if the
    /// `VecDeque` is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// assert_eq!(d.back(), None);
    ///
    /// d.push_back(1);
    /// d.push_back(2);
    /// match d.back_mut() {
    ///     Some(x) => *x = 9,
    ///     None => (),
    /// }
    /// assert_eq!(d.back(), Some(&9));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn back_mut(&mut self) -> Option<&mut T> {
        self.get_mut(self.len().wrapping_sub(1))
    }

    /// Removes the first element and returns it, or `None` if the `VecDeque` is
    /// empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// d.push_back(1);
    /// d.push_back(2);
    ///
    /// assert_eq!(d.pop_front(), Some(1));
    /// assert_eq!(d.pop_front(), Some(2));
    /// assert_eq!(d.pop_front(), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn pop_front(&mut self) -> Option<T> {
        if self.is_empty() {
            None
        } else {
            let tail = self.tail;
            self.tail = self.wrap_add(self.tail, 1);
            unsafe { Some(self.buffer_read(tail)) }
        }
    }

    /// Removes the last element from the `VecDeque` and returns it, or `None` if
    /// it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// assert_eq!(buf.pop_back(), None);
    /// buf.push_back(1);
    /// buf.push_back(3);
    /// assert_eq!(buf.pop_back(), Some(3));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn pop_back(&mut self) -> Option<T> {
        if self.is_empty() {
            None
        } else {
            self.head = self.wrap_sub(self.head, 1);
            let head = self.head;
            unsafe { Some(self.buffer_read(head)) }
        }
    }

    /// Prepends an element to the `VecDeque`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut d = VecDeque::new();
    /// d.push_front(1);
    /// d.push_front(2);
    /// assert_eq!(d.front(), Some(&2));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn push_front(&mut self, value: T) {
        if self.is_full() {
            self.grow();
        }

        self.tail = self.wrap_sub(self.tail, 1);
        let tail = self.tail;
        unsafe {
            self.buffer_write(tail, value);
        }
    }

    /// Appends an element to the back of the `VecDeque`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(1);
    /// buf.push_back(3);
    /// assert_eq!(3, *buf.back().unwrap());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn push_back(&mut self, value: T) {
        if self.is_full() {
            self.grow();
        }

        let head = self.head;
        self.head = self.wrap_add(self.head, 1);
        unsafe { self.buffer_write(head, value) }
    }

    #[inline]
    fn is_contiguous(&self) -> bool {
        // FIXME: Should we consider `head == 0` to mean
        // that `self` is contiguous?
        self.tail <= self.head
    }

    /// Removes an element from anywhere in the `VecDeque` and returns it,
    /// replacing it with the first element.
    ///
    /// This does not preserve ordering, but is *O*(1).
    ///
    /// Returns `None` if `index` is out of bounds.
    ///
    /// Element at index 0 is the front of the queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// assert_eq!(buf.swap_remove_front(0), None);
    /// buf.push_back(1);
    /// buf.push_back(2);
    /// buf.push_back(3);
    /// assert_eq!(buf, [1, 2, 3]);
    ///
    /// assert_eq!(buf.swap_remove_front(2), Some(3));
    /// assert_eq!(buf, [2, 1]);
    /// ```
    #[stable(feature = "deque_extras_15", since = "1.5.0")]
    pub fn swap_remove_front(&mut self, index: usize) -> Option<T> {
        let length = self.len();
        if length > 0 && index < length && index != 0 {
            self.swap(index, 0);
        } else if index >= length {
            return None;
        }
        self.pop_front()
    }

    /// Removes an element from anywhere in the `VecDeque` and returns it, replacing it with the
    /// last element.
    ///
    /// This does not preserve ordering, but is *O*(1).
    ///
    /// Returns `None` if `index` is out of bounds.
    ///
    /// Element at index 0 is the front of the queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// assert_eq!(buf.swap_remove_back(0), None);
    /// buf.push_back(1);
    /// buf.push_back(2);
    /// buf.push_back(3);
    /// assert_eq!(buf, [1, 2, 3]);
    ///
    /// assert_eq!(buf.swap_remove_back(0), Some(1));
    /// assert_eq!(buf, [3, 2]);
    /// ```
    #[stable(feature = "deque_extras_15", since = "1.5.0")]
    pub fn swap_remove_back(&mut self, index: usize) -> Option<T> {
        let length = self.len();
        if length > 0 && index < length - 1 {
            self.swap(index, length - 1);
        } else if index >= length {
            return None;
        }
        self.pop_back()
    }

    /// Inserts an element at `index` within the `VecDeque`, shifting all elements with indices
    /// greater than or equal to `index` towards the back.
    ///
    /// Element at index 0 is the front of the queue.
    ///
    /// # Panics
    ///
    /// Panics if `index` is greater than `VecDeque`'s length
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut vec_deque = VecDeque::new();
    /// vec_deque.push_back('a');
    /// vec_deque.push_back('b');
    /// vec_deque.push_back('c');
    /// assert_eq!(vec_deque, &['a', 'b', 'c']);
    ///
    /// vec_deque.insert(1, 'd');
    /// assert_eq!(vec_deque, &['a', 'd', 'b', 'c']);
    /// ```
    #[stable(feature = "deque_extras_15", since = "1.5.0")]
    pub fn insert(&mut self, index: usize, value: T) {
        assert!(index <= self.len(), "index out of bounds");
        if self.is_full() {
            self.grow();
        }

        // Move the least number of elements in the ring buffer and insert
        // the given object
        //
        // At most len/2 - 1 elements will be moved. O(min(n, n-i))
        //
        // There are three main cases:
        //  Elements are contiguous
        //      - special case when tail is 0
        //  Elements are discontiguous and the insert is in the tail section
        //  Elements are discontiguous and the insert is in the head section
        //
        // For each of those there are two more cases:
        //  Insert is closer to tail
        //  Insert is closer to head
        //
        // Key: H - self.head
        //      T - self.tail
        //      o - Valid element
        //      I - Insertion element
        //      A - The element that should be after the insertion point
        //      M - Indicates element was moved

        let idx = self.wrap_add(self.tail, index);

        let distance_to_tail = index;
        let distance_to_head = self.len() - index;

        let contiguous = self.is_contiguous();

        match (contiguous, distance_to_tail <= distance_to_head, idx >= self.tail) {
            (true, true, _) if index == 0 => {
                // push_front
                //
                //       T
                //       I             H
                //      [A o o o o o o . . . . . . . . .]
                //
                //                       H         T
                //      [A o o o o o o o . . . . . I]
                //

                self.tail = self.wrap_sub(self.tail, 1);
            }
            (true, true, _) => {
                unsafe {
                    // contiguous, insert closer to tail:
                    //
                    //             T   I         H
                    //      [. . . o o A o o o o . . . . . .]
                    //
                    //           T               H
                    //      [. . o o I A o o o o . . . . . .]
                    //           M M
                    //
                    // contiguous, insert closer to tail and tail is 0:
                    //
                    //
                    //       T   I         H
                    //      [o o A o o o o . . . . . . . . .]
                    //
                    //                       H             T
                    //      [o I A o o o o o . . . . . . . o]
                    //       M                             M

                    let new_tail = self.wrap_sub(self.tail, 1);

                    self.copy(new_tail, self.tail, 1);
                    // Already moved the tail, so we only copy `index - 1` elements.
                    self.copy(self.tail, self.tail + 1, index - 1);

                    self.tail = new_tail;
                }
            }
            (true, false, _) => {
                unsafe {
                    //  contiguous, insert closer to head:
                    //
                    //             T       I     H
                    //      [. . . o o o o A o o . . . . . .]
                    //
                    //             T               H
                    //      [. . . o o o o I A o o . . . . .]
                    //                       M M M

                    self.copy(idx + 1, idx, self.head - idx);
                    self.head = self.wrap_add(self.head, 1);
                }
            }
            (false, true, true) => {
                unsafe {
                    // discontiguous, insert closer to tail, tail section:
                    //
                    //                   H         T   I
                    //      [o o o o o o . . . . . o o A o o]
                    //
                    //                   H       T
                    //      [o o o o o o . . . . o o I A o o]
                    //                           M M

                    self.copy(self.tail - 1, self.tail, index);
                    self.tail -= 1;
                }
            }
            (false, false, true) => {
                unsafe {
                    // discontiguous, insert closer to head, tail section:
                    //
                    //           H             T         I
                    //      [o o . . . . . . . o o o o o A o]
                    //
                    //             H           T
                    //      [o o o . . . . . . o o o o o I A]
                    //       M M M                         M

                    // copy elements up to new head
                    self.copy(1, 0, self.head);

                    // copy last element into empty spot at bottom of buffer
                    self.copy(0, self.cap() - 1, 1);

                    // move elements from idx to end forward not including ^ element
                    self.copy(idx + 1, idx, self.cap() - 1 - idx);

                    self.head += 1;
                }
            }
            (false, true, false) if idx == 0 => {
                unsafe {
                    // discontiguous, insert is closer to tail, head section,
                    // and is at index zero in the internal buffer:
                    //
                    //       I                   H     T
                    //      [A o o o o o o o o o . . . o o o]
                    //
                    //                           H   T
                    //      [A o o o o o o o o o . . o o o I]
                    //                               M M M

                    // copy elements up to new tail
                    self.copy(self.tail - 1, self.tail, self.cap() - self.tail);

                    // copy last element into empty spot at bottom of buffer
                    self.copy(self.cap() - 1, 0, 1);

                    self.tail -= 1;
                }
            }
            (false, true, false) => {
                unsafe {
                    // discontiguous, insert closer to tail, head section:
                    //
                    //             I             H     T
                    //      [o o o A o o o o o o . . . o o o]
                    //
                    //                           H   T
                    //      [o o I A o o o o o o . . o o o o]
                    //       M M                     M M M M

                    // copy elements up to new tail
                    self.copy(self.tail - 1, self.tail, self.cap() - self.tail);

                    // copy last element into empty spot at bottom of buffer
                    self.copy(self.cap() - 1, 0, 1);

                    // move elements from idx-1 to end forward not including ^ element
                    self.copy(0, 1, idx - 1);

                    self.tail -= 1;
                }
            }
            (false, false, false) => {
                unsafe {
                    // discontiguous, insert closer to head, head section:
                    //
                    //               I     H           T
                    //      [o o o o A o o . . . . . . o o o]
                    //
                    //                     H           T
                    //      [o o o o I A o o . . . . . o o o]
                    //                 M M M

                    self.copy(idx + 1, idx, self.head - idx);
                    self.head += 1;
                }
            }
        }

        // tail might've been changed so we need to recalculate
        let new_idx = self.wrap_add(self.tail, index);
        unsafe {
            self.buffer_write(new_idx, value);
        }
    }

    /// Removes and returns the element at `index` from the `VecDeque`.
    /// Whichever end is closer to the removal point will be moved to make
    /// room, and all the affected elements will be moved to new positions.
    /// Returns `None` if `index` is out of bounds.
    ///
    /// Element at index 0 is the front of the queue.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(1);
    /// buf.push_back(2);
    /// buf.push_back(3);
    /// assert_eq!(buf, [1, 2, 3]);
    ///
    /// assert_eq!(buf.remove(1), Some(2));
    /// assert_eq!(buf, [1, 3]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn remove(&mut self, index: usize) -> Option<T> {
        if self.is_empty() || self.len() <= index {
            return None;
        }

        // There are three main cases:
        //  Elements are contiguous
        //  Elements are discontiguous and the removal is in the tail section
        //  Elements are discontiguous and the removal is in the head section
        //      - special case when elements are technically contiguous,
        //        but self.head = 0
        //
        // For each of those there are two more cases:
        //  Insert is closer to tail
        //  Insert is closer to head
        //
        // Key: H - self.head
        //      T - self.tail
        //      o - Valid element
        //      x - Element marked for removal
        //      R - Indicates element that is being removed
        //      M - Indicates element was moved

        let idx = self.wrap_add(self.tail, index);

        let elem = unsafe { Some(self.buffer_read(idx)) };

        let distance_to_tail = index;
        let distance_to_head = self.len() - index;

        let contiguous = self.is_contiguous();

        match (contiguous, distance_to_tail <= distance_to_head, idx >= self.tail) {
            (true, true, _) => {
                unsafe {
                    // contiguous, remove closer to tail:
                    //
                    //             T   R         H
                    //      [. . . o o x o o o o . . . . . .]
                    //
                    //               T           H
                    //      [. . . . o o o o o o . . . . . .]
                    //               M M

                    self.copy(self.tail + 1, self.tail, index);
                    self.tail += 1;
                }
            }
            (true, false, _) => {
                unsafe {
                    // contiguous, remove closer to head:
                    //
                    //             T       R     H
                    //      [. . . o o o o x o o . . . . . .]
                    //
                    //             T           H
                    //      [. . . o o o o o o . . . . . . .]
                    //                     M M

                    self.copy(idx, idx + 1, self.head - idx - 1);
                    self.head -= 1;
                }
            }
            (false, true, true) => {
                unsafe {
                    // discontiguous, remove closer to tail, tail section:
                    //
                    //                   H         T   R
                    //      [o o o o o o . . . . . o o x o o]
                    //
                    //                   H           T
                    //      [o o o o o o . . . . . . o o o o]
                    //                               M M

                    self.copy(self.tail + 1, self.tail, index);
                    self.tail = self.wrap_add(self.tail, 1);
                }
            }
            (false, false, false) => {
                unsafe {
                    // discontiguous, remove closer to head, head section:
                    //
                    //               R     H           T
                    //      [o o o o x o o . . . . . . o o o]
                    //
                    //                   H             T
                    //      [o o o o o o . . . . . . . o o o]
                    //               M M

                    self.copy(idx, idx + 1, self.head - idx - 1);
                    self.head -= 1;
                }
            }
            (false, false, true) => {
                unsafe {
                    // discontiguous, remove closer to head, tail section:
                    //
                    //             H           T         R
                    //      [o o o . . . . . . o o o o o x o]
                    //
                    //           H             T
                    //      [o o . . . . . . . o o o o o o o]
                    //       M M                         M M
                    //
                    // or quasi-discontiguous, remove next to head, tail section:
                    //
                    //       H                 T         R
                    //      [. . . . . . . . . o o o o o x o]
                    //
                    //                         T           H
                    //      [. . . . . . . . . o o o o o o .]
                    //                                   M

                    // draw in elements in the tail section
                    self.copy(idx, idx + 1, self.cap() - idx - 1);

                    // Prevents underflow.
                    if self.head != 0 {
                        // copy first element into empty spot
                        self.copy(self.cap() - 1, 0, 1);

                        // move elements in the head section backwards
                        self.copy(0, 1, self.head - 1);
                    }

                    self.head = self.wrap_sub(self.head, 1);
                }
            }
            (false, true, false) => {
                unsafe {
                    // discontiguous, remove closer to tail, head section:
                    //
                    //           R               H     T
                    //      [o o x o o o o o o o . . . o o o]
                    //
                    //                           H       T
                    //      [o o o o o o o o o o . . . . o o]
                    //       M M M                       M M

                    // draw in elements up to idx
                    self.copy(1, 0, idx);

                    // copy last element into empty spot
                    self.copy(0, self.cap() - 1, 1);

                    // move elements from tail to end forward, excluding the last one
                    self.copy(self.tail + 1, self.tail, self.cap() - self.tail - 1);

                    self.tail = self.wrap_add(self.tail, 1);
                }
            }
        }

        elem
    }

    /// Splits the `VecDeque` into two at the given index.
    ///
    /// Returns a newly allocated `VecDeque`. `self` contains elements `[0, at)`,
    /// and the returned `VecDeque` contains elements `[at, len)`.
    ///
    /// Note that the capacity of `self` does not change.
    ///
    /// Element at index 0 is the front of the queue.
    ///
    /// # Panics
    ///
    /// Panics if `at > len`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
    /// let buf2 = buf.split_off(1);
    /// assert_eq!(buf, [1]);
    /// assert_eq!(buf2, [2, 3]);
    /// ```
    #[inline]
    #[must_use = "use `.truncate()` if you don't need the other half"]
    #[stable(feature = "split_off", since = "1.4.0")]
    pub fn split_off(&mut self, at: usize) -> Self
    where
        A: Clone,
    {
        let len = self.len();
        assert!(at <= len, "`at` out of bounds");

        let other_len = len - at;
        let mut other = VecDeque::with_capacity_in(other_len, self.allocator().clone());

        unsafe {
            let (first_half, second_half) = self.as_slices();

            let first_len = first_half.len();
            let second_len = second_half.len();
            if at < first_len {
                // `at` lies in the first half.
                let amount_in_first = first_len - at;

                ptr::copy_nonoverlapping(first_half.as_ptr().add(at), other.ptr(), amount_in_first);

                // just take all of the second half.
                ptr::copy_nonoverlapping(
                    second_half.as_ptr(),
                    other.ptr().add(amount_in_first),
                    second_len,
                );
            } else {
                // `at` lies in the second half, need to factor in the elements we skipped
                // in the first half.
                let offset = at - first_len;
                let amount_in_second = second_len - offset;
                ptr::copy_nonoverlapping(
                    second_half.as_ptr().add(offset),
                    other.ptr(),
                    amount_in_second,
                );
            }
        }

        // Cleanup where the ends of the buffers are
        self.head = self.wrap_sub(self.head, other_len);
        other.head = other.wrap_index(other_len);

        other
    }

    /// Moves all the elements of `other` into `self`, leaving `other` empty.
    ///
    /// # Panics
    ///
    /// Panics if the new number of elements in self overflows a `usize`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf: VecDeque<_> = vec![1, 2].into_iter().collect();
    /// let mut buf2: VecDeque<_> = vec![3, 4].into_iter().collect();
    /// buf.append(&mut buf2);
    /// assert_eq!(buf, [1, 2, 3, 4]);
    /// assert_eq!(buf2, []);
    /// ```
    #[inline]
    #[stable(feature = "append", since = "1.4.0")]
    pub fn append(&mut self, other: &mut Self) {
        self.reserve(other.len());
        unsafe {
            let (left, right) = other.as_slices();
            self.copy_slice(self.head, left);
            self.copy_slice(self.wrap_add(self.head, left.len()), right);
        }
        // SAFETY: Update pointers after copying to avoid leaving doppelganger
        // in case of panics.
        self.head = self.wrap_add(self.head, other.len());
        // Silently drop values in `other`.
        other.tail = other.head;
    }

    /// Retains only the elements specified by the predicate.
    ///
    /// In other words, remove all elements `e` such that `f(&e)` returns false.
    /// This method operates in place, visiting each element exactly once in the
    /// original order, and preserves the order of the retained elements.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.extend(1..5);
    /// buf.retain(|&x| x % 2 == 0);
    /// assert_eq!(buf, [2, 4]);
    /// ```
    ///
    /// Because the elements are visited exactly once in the original order,
    /// external state may be used to decide which elements to keep.
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.extend(1..6);
    ///
    /// let keep = [false, true, true, false, true];
    /// let mut iter = keep.iter();
    /// buf.retain(|_| *iter.next().unwrap());
    /// assert_eq!(buf, [2, 3, 5]);
    /// ```
    #[stable(feature = "vec_deque_retain", since = "1.4.0")]
    pub fn retain<F>(&mut self, mut f: F)
    where
        F: FnMut(&T) -> bool,
    {
        let len = self.len();
        let mut idx = 0;
        let mut cur = 0;

        // Stage 1: All values are retained.
        while cur < len {
            if !f(&self[cur]) {
                cur += 1;
                break;
            }
            cur += 1;
            idx += 1;
        }
        // Stage 2: Swap retained value into current idx.
        while cur < len {
            if !f(&self[cur]) {
                cur += 1;
                continue;
            }

            self.swap(idx, cur);
            cur += 1;
            idx += 1;
        }
        // Stage 3: Trancate all values after idx.
        if cur != idx {
            self.truncate(idx);
        }
    }

    // Double the buffer size. This method is inline(never), so we expect it to only
    // be called in cold paths.
    // This may panic or abort
    #[inline(never)]
    fn grow(&mut self) {
        // Extend or possibly remove this assertion when valid use-cases for growing the
        // buffer without it being full emerge
        debug_assert!(self.is_full());
        let old_cap = self.cap();
        self.buf.reserve_exact(old_cap, old_cap);
        assert!(self.cap() == old_cap * 2);
        unsafe {
            self.handle_capacity_increase(old_cap);
        }
        debug_assert!(!self.is_full());
    }

    /// Modifies the `VecDeque` in-place so that `len()` is equal to `new_len`,
    /// either by removing excess elements from the back or by appending
    /// elements generated by calling `generator` to the back.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(5);
    /// buf.push_back(10);
    /// buf.push_back(15);
    /// assert_eq!(buf, [5, 10, 15]);
    ///
    /// buf.resize_with(5, Default::default);
    /// assert_eq!(buf, [5, 10, 15, 0, 0]);
    ///
    /// buf.resize_with(2, || unreachable!());
    /// assert_eq!(buf, [5, 10]);
    ///
    /// let mut state = 100;
    /// buf.resize_with(5, || { state += 1; state });
    /// assert_eq!(buf, [5, 10, 101, 102, 103]);
    /// ```
    #[stable(feature = "vec_resize_with", since = "1.33.0")]
    pub fn resize_with(&mut self, new_len: usize, generator: impl FnMut() -> T) {
        let len = self.len();

        if new_len > len {
            self.extend(repeat_with(generator).take(new_len - len))
        } else {
            self.truncate(new_len);
        }
    }

    /// Rearranges the internal storage of this deque so it is one contiguous
    /// slice, which is then returned.
    ///
    /// This method does not allocate and does not change the order of the
    /// inserted elements. As it returns a mutable slice, this can be used to
    /// sort a deque.
    ///
    /// Once the internal storage is contiguous, the [`as_slices`] and
    /// [`as_mut_slices`] methods will return the entire contents of the
    /// `VecDeque` in a single slice.
    ///
    /// [`as_slices`]: VecDeque::as_slices
    /// [`as_mut_slices`]: VecDeque::as_mut_slices
    ///
    /// # Examples
    ///
    /// Sorting the content of a deque.
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::with_capacity(15);
    ///
    /// buf.push_back(2);
    /// buf.push_back(1);
    /// buf.push_front(3);
    ///
    /// // sorting the deque
    /// buf.make_contiguous().sort();
    /// assert_eq!(buf.as_slices(), (&[1, 2, 3] as &[_], &[] as &[_]));
    ///
    /// // sorting it in reverse order
    /// buf.make_contiguous().sort_by(|a, b| b.cmp(a));
    /// assert_eq!(buf.as_slices(), (&[3, 2, 1] as &[_], &[] as &[_]));
    /// ```
    ///
    /// Getting immutable access to the contiguous slice.
    ///
    /// ```rust
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    ///
    /// buf.push_back(2);
    /// buf.push_back(1);
    /// buf.push_front(3);
    ///
    /// buf.make_contiguous();
    /// if let (slice, &[]) = buf.as_slices() {
    ///     // we can now be sure that `slice` contains all elements of the deque,
    ///     // while still having immutable access to `buf`.
    ///     assert_eq!(buf.len(), slice.len());
    ///     assert_eq!(slice, &[3, 2, 1] as &[_]);
    /// }
    /// ```
    #[stable(feature = "deque_make_contiguous", since = "1.48.0")]
    pub fn make_contiguous(&mut self) -> &mut [T] {
        if self.is_contiguous() {
            let tail = self.tail;
            let head = self.head;
            return unsafe { RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0 };
        }

        let buf = self.buf.ptr();
        let cap = self.cap();
        let len = self.len();

        let free = self.tail - self.head;
        let tail_len = cap - self.tail;

        if free >= tail_len {
            // there is enough free space to copy the tail in one go,
            // this means that we first shift the head backwards, and then
            // copy the tail to the correct position.
            //
            // from: DEFGH....ABC
            // to:   ABCDEFGH....
            unsafe {
                ptr::copy(buf, buf.add(tail_len), self.head);
                // ...DEFGH.ABC
                ptr::copy_nonoverlapping(buf.add(self.tail), buf, tail_len);
                // ABCDEFGH....

                self.tail = 0;
                self.head = len;
            }
        } else if free > self.head {
            // FIXME: We currently do not consider ....ABCDEFGH
            // to be contiguous because `head` would be `0` in this
            // case. While we probably want to change this it
            // isn't trivial as a few places expect `is_contiguous`
            // to mean that we can just slice using `buf[tail..head]`.

            // there is enough free space to copy the head in one go,
            // this means that we first shift the tail forwards, and then
            // copy the head to the correct position.
            //
            // from: FGH....ABCDE
            // to:   ...ABCDEFGH.
            unsafe {
                ptr::copy(buf.add(self.tail), buf.add(self.head), tail_len);
                // FGHABCDE....
                ptr::copy_nonoverlapping(buf, buf.add(self.head + tail_len), self.head);
                // ...ABCDEFGH.

                self.tail = self.head;
                self.head = self.wrap_add(self.tail, len);
            }
        } else {
            // free is smaller than both head and tail,
            // this means we have to slowly "swap" the tail and the head.
            //
            // from: EFGHI...ABCD or HIJK.ABCDEFG
            // to:   ABCDEFGHI... or ABCDEFGHIJK.
            let mut left_edge: usize = 0;
            let mut right_edge: usize = self.tail;
            unsafe {
                // The general problem looks like this
                // GHIJKLM...ABCDEF - before any swaps
                // ABCDEFM...GHIJKL - after 1 pass of swaps
                // ABCDEFGHIJM...KL - swap until the left edge reaches the temp store
                //                  - then restart the algorithm with a new (smaller) store
                // Sometimes the temp store is reached when the right edge is at the end
                // of the buffer - this means we've hit the right order with fewer swaps!
                // E.g
                // EF..ABCD
                // ABCDEF.. - after four only swaps we've finished
                while left_edge < len && right_edge != cap {
                    let mut right_offset = 0;
                    for i in left_edge..right_edge {
                        right_offset = (i - left_edge) % (cap - right_edge);
                        let src: isize = (right_edge + right_offset) as isize;
                        ptr::swap(buf.add(i), buf.offset(src));
                    }
                    let n_ops = right_edge - left_edge;
                    left_edge += n_ops;
                    right_edge += right_offset + 1;
                }

                self.tail = 0;
                self.head = len;
            }
        }

        let tail = self.tail;
        let head = self.head;
        unsafe { RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0 }
    }

    /// Rotates the double-ended queue `mid` places to the left.
    ///
    /// Equivalently,
    /// - Rotates item `mid` into the first position.
    /// - Pops the first `mid` items and pushes them to the end.
    /// - Rotates `len() - mid` places to the right.
    ///
    /// # Panics
    ///
    /// If `mid` is greater than `len()`. Note that `mid == len()`
    /// does _not_ panic and is a no-op rotation.
    ///
    /// # Complexity
    ///
    /// Takes `*O*(min(mid, len() - mid))` time and no extra space.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf: VecDeque<_> = (0..10).collect();
    ///
    /// buf.rotate_left(3);
    /// assert_eq!(buf, [3, 4, 5, 6, 7, 8, 9, 0, 1, 2]);
    ///
    /// for i in 1..10 {
    ///     assert_eq!(i * 3 % 10, buf[0]);
    ///     buf.rotate_left(3);
    /// }
    /// assert_eq!(buf, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
    /// ```
    #[stable(feature = "vecdeque_rotate", since = "1.36.0")]
    pub fn rotate_left(&mut self, mid: usize) {
        assert!(mid <= self.len());
        let k = self.len() - mid;
        if mid <= k {
            unsafe { self.rotate_left_inner(mid) }
        } else {
            unsafe { self.rotate_right_inner(k) }
        }
    }

    /// Rotates the double-ended queue `k` places to the right.
    ///
    /// Equivalently,
    /// - Rotates the first item into position `k`.
    /// - Pops the last `k` items and pushes them to the front.
    /// - Rotates `len() - k` places to the left.
    ///
    /// # Panics
    ///
    /// If `k` is greater than `len()`. Note that `k == len()`
    /// does _not_ panic and is a no-op rotation.
    ///
    /// # Complexity
    ///
    /// Takes `*O*(min(k, len() - k))` time and no extra space.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf: VecDeque<_> = (0..10).collect();
    ///
    /// buf.rotate_right(3);
    /// assert_eq!(buf, [7, 8, 9, 0, 1, 2, 3, 4, 5, 6]);
    ///
    /// for i in 1..10 {
    ///     assert_eq!(0, buf[i * 3 % 10]);
    ///     buf.rotate_right(3);
    /// }
    /// assert_eq!(buf, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
    /// ```
    #[stable(feature = "vecdeque_rotate", since = "1.36.0")]
    pub fn rotate_right(&mut self, k: usize) {
        assert!(k <= self.len());
        let mid = self.len() - k;
        if k <= mid {
            unsafe { self.rotate_right_inner(k) }
        } else {
            unsafe { self.rotate_left_inner(mid) }
        }
    }

    // SAFETY: the following two methods require that the rotation amount
    // be less than half the length of the deque.
    //
    // `wrap_copy` requires that `min(x, cap() - x) + copy_len <= cap()`,
    // but than `min` is never more than half the capacity, regardless of x,
    // so it's sound to call here because we're calling with something
    // less than half the length, which is never above half the capacity.

    unsafe fn rotate_left_inner(&mut self, mid: usize) {
        debug_assert!(mid * 2 <= self.len());
        unsafe {
            self.wrap_copy(self.head, self.tail, mid);
        }
        self.head = self.wrap_add(self.head, mid);
        self.tail = self.wrap_add(self.tail, mid);
    }

    unsafe fn rotate_right_inner(&mut self, k: usize) {
        debug_assert!(k * 2 <= self.len());
        self.head = self.wrap_sub(self.head, k);
        self.tail = self.wrap_sub(self.tail, k);
        unsafe {
            self.wrap_copy(self.tail, self.head, k);
        }
    }

    /// Binary searches this sorted `VecDeque` for a given element.
    ///
    /// If the value is found then [`Result::Ok`] is returned, containing the
    /// index of the matching element. If there are multiple matches, then any
    /// one of the matches could be returned. If the value is not found then
    /// [`Result::Err`] is returned, containing the index where a matching
    /// element could be inserted while maintaining sorted order.
    ///
    /// See also [`binary_search_by`], [`binary_search_by_key`], and [`partition_point`].
    ///
    /// [`binary_search_by`]: VecDeque::binary_search_by
    /// [`binary_search_by_key`]: VecDeque::binary_search_by_key
    /// [`partition_point`]: VecDeque::partition_point
    ///
    /// # Examples
    ///
    /// Looks up a series of four elements. The first is found, with a
    /// uniquely determined position; the second and third are not
    /// found; the fourth could match any position in `[1, 4]`.
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let deque: VecDeque<_> = vec![0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();
    ///
    /// assert_eq!(deque.binary_search(&13),  Ok(9));
    /// assert_eq!(deque.binary_search(&4),   Err(7));
    /// assert_eq!(deque.binary_search(&100), Err(13));
    /// let r = deque.binary_search(&1);
    /// assert!(matches!(r, Ok(1..=4)));
    /// ```
    ///
    /// If you want to insert an item to a sorted `VecDeque`, while maintaining
    /// sort order:
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut deque: VecDeque<_> = vec![0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();
    /// let num = 42;
    /// let idx = deque.binary_search(&num).unwrap_or_else(|x| x);
    /// deque.insert(idx, num);
    /// assert_eq!(deque, &[0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 42, 55]);
    /// ```
    #[stable(feature = "vecdeque_binary_search", since = "1.54.0")]
    #[inline]
    pub fn binary_search(&self, x: &T) -> Result<usize, usize>
    where
        T: Ord,
    {
        self.binary_search_by(|e| e.cmp(x))
    }

    /// Binary searches this sorted `VecDeque` with a comparator function.
    ///
    /// The comparator function should implement an order consistent
    /// with the sort order of the underlying `VecDeque`, returning an
    /// order code that indicates whether its argument is `Less`,
    /// `Equal` or `Greater` than the desired target.
    ///
    /// If the value is found then [`Result::Ok`] is returned, containing the
    /// index of the matching element. If there are multiple matches, then any
    /// one of the matches could be returned. If the value is not found then
    /// [`Result::Err`] is returned, containing the index where a matching
    /// element could be inserted while maintaining sorted order.
    ///
    /// See also [`binary_search`], [`binary_search_by_key`], and [`partition_point`].
    ///
    /// [`binary_search`]: VecDeque::binary_search
    /// [`binary_search_by_key`]: VecDeque::binary_search_by_key
    /// [`partition_point`]: VecDeque::partition_point
    ///
    /// # Examples
    ///
    /// Looks up a series of four elements. The first is found, with a
    /// uniquely determined position; the second and third are not
    /// found; the fourth could match any position in `[1, 4]`.
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let deque: VecDeque<_> = vec![0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();
    ///
    /// assert_eq!(deque.binary_search_by(|x| x.cmp(&13)),  Ok(9));
    /// assert_eq!(deque.binary_search_by(|x| x.cmp(&4)),   Err(7));
    /// assert_eq!(deque.binary_search_by(|x| x.cmp(&100)), Err(13));
    /// let r = deque.binary_search_by(|x| x.cmp(&1));
    /// assert!(matches!(r, Ok(1..=4)));
    /// ```
    #[stable(feature = "vecdeque_binary_search", since = "1.54.0")]
    pub fn binary_search_by<'a, F>(&'a self, mut f: F) -> Result<usize, usize>
    where
        F: FnMut(&'a T) -> Ordering,
    {
        let (front, back) = self.as_slices();
        let cmp_back = back.first().map(|elem| f(elem));

        if let Some(Ordering::Equal) = cmp_back {
            Ok(front.len())
        } else if let Some(Ordering::Less) = cmp_back {
            back.binary_search_by(f).map(|idx| idx + front.len()).map_err(|idx| idx + front.len())
        } else {
            front.binary_search_by(f)
        }
    }

    /// Binary searches this sorted `VecDeque` with a key extraction function.
    ///
    /// Assumes that the `VecDeque` is sorted by the key, for instance with
    /// [`make_contiguous().sort_by_key()`] using the same key extraction function.
    ///
    /// If the value is found then [`Result::Ok`] is returned, containing the
    /// index of the matching element. If there are multiple matches, then any
    /// one of the matches could be returned. If the value is not found then
    /// [`Result::Err`] is returned, containing the index where a matching
    /// element could be inserted while maintaining sorted order.
    ///
    /// See also [`binary_search`], [`binary_search_by`], and [`partition_point`].
    ///
    /// [`make_contiguous().sort_by_key()`]: VecDeque::make_contiguous
    /// [`binary_search`]: VecDeque::binary_search
    /// [`binary_search_by`]: VecDeque::binary_search_by
    /// [`partition_point`]: VecDeque::partition_point
    ///
    /// # Examples
    ///
    /// Looks up a series of four elements in a slice of pairs sorted by
    /// their second elements. The first is found, with a uniquely
    /// determined position; the second and third are not found; the
    /// fourth could match any position in `[1, 4]`.
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let deque: VecDeque<_> = vec![(0, 0), (2, 1), (4, 1), (5, 1),
    ///          (3, 1), (1, 2), (2, 3), (4, 5), (5, 8), (3, 13),
    ///          (1, 21), (2, 34), (4, 55)].into();
    ///
    /// assert_eq!(deque.binary_search_by_key(&13, |&(a, b)| b),  Ok(9));
    /// assert_eq!(deque.binary_search_by_key(&4, |&(a, b)| b),   Err(7));
    /// assert_eq!(deque.binary_search_by_key(&100, |&(a, b)| b), Err(13));
    /// let r = deque.binary_search_by_key(&1, |&(a, b)| b);
    /// assert!(matches!(r, Ok(1..=4)));
    /// ```
    #[stable(feature = "vecdeque_binary_search", since = "1.54.0")]
    #[inline]
    pub fn binary_search_by_key<'a, B, F>(&'a self, b: &B, mut f: F) -> Result<usize, usize>
    where
        F: FnMut(&'a T) -> B,
        B: Ord,
    {
        self.binary_search_by(|k| f(k).cmp(b))
    }

    /// Returns the index of the partition point according to the given predicate
    /// (the index of the first element of the second partition).
    ///
    /// The deque is assumed to be partitioned according to the given predicate.
    /// This means that all elements for which the predicate returns true are at the start of the deque
    /// and all elements for which the predicate returns false are at the end.
    /// For example, [7, 15, 3, 5, 4, 12, 6] is a partitioned under the predicate x % 2 != 0
    /// (all odd numbers are at the start, all even at the end).
    ///
    /// If this deque is not partitioned, the returned result is unspecified and meaningless,
    /// as this method performs a kind of binary search.
    ///
    /// See also [`binary_search`], [`binary_search_by`], and [`binary_search_by_key`].
    ///
    /// [`binary_search`]: VecDeque::binary_search
    /// [`binary_search_by`]: VecDeque::binary_search_by
    /// [`binary_search_by_key`]: VecDeque::binary_search_by_key
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let deque: VecDeque<_> = vec![1, 2, 3, 3, 5, 6, 7].into();
    /// let i = deque.partition_point(|&x| x < 5);
    ///
    /// assert_eq!(i, 4);
    /// assert!(deque.iter().take(i).all(|&x| x < 5));
    /// assert!(deque.iter().skip(i).all(|&x| !(x < 5)));
    /// ```
    #[stable(feature = "vecdeque_binary_search", since = "1.54.0")]
    pub fn partition_point<P>(&self, mut pred: P) -> usize
    where
        P: FnMut(&T) -> bool,
    {
        let (front, back) = self.as_slices();

        if let Some(true) = back.first().map(|v| pred(v)) {
            back.partition_point(pred) + front.len()
        } else {
            front.partition_point(pred)
        }
    }
}

impl<T: Clone, A: Allocator> VecDeque<T, A> {
    /// Modifies the `VecDeque` in-place so that `len()` is equal to new_len,
    /// either by removing excess elements from the back or by appending clones of `value`
    /// to the back.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let mut buf = VecDeque::new();
    /// buf.push_back(5);
    /// buf.push_back(10);
    /// buf.push_back(15);
    /// assert_eq!(buf, [5, 10, 15]);
    ///
    /// buf.resize(2, 0);
    /// assert_eq!(buf, [5, 10]);
    ///
    /// buf.resize(5, 20);
    /// assert_eq!(buf, [5, 10, 20, 20, 20]);
    /// ```
    #[stable(feature = "deque_extras", since = "1.16.0")]
    pub fn resize(&mut self, new_len: usize, value: T) {
        self.resize_with(new_len, || value.clone());
    }
}

/// Returns the index in the underlying buffer for a given logical element index.
#[inline]
fn wrap_index(index: usize, size: usize) -> usize {
    // size is always a power of 2
    debug_assert!(size.is_power_of_two());
    index & (size - 1)
}

/// Calculate the number of elements left to be read in the buffer
#[inline]
fn count(tail: usize, head: usize, size: usize) -> usize {
    // size is always a power of 2
    (head.wrapping_sub(tail)) & (size - 1)
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: PartialEq, A: Allocator> PartialEq for VecDeque<T, A> {
    fn eq(&self, other: &Self) -> bool {
        if self.len() != other.len() {
            return false;
        }
        let (sa, sb) = self.as_slices();
        let (oa, ob) = other.as_slices();
        if sa.len() == oa.len() {
            sa == oa && sb == ob
        } else if sa.len() < oa.len() {
            // Always divisible in three sections, for example:
            // self:  [a b c|d e f]
            // other: [0 1 2 3|4 5]
            // front = 3, mid = 1,
            // [a b c] == [0 1 2] && [d] == [3] && [e f] == [4 5]
            let front = sa.len();
            let mid = oa.len() - front;

            let (oa_front, oa_mid) = oa.split_at(front);
            let (sb_mid, sb_back) = sb.split_at(mid);
            debug_assert_eq!(sa.len(), oa_front.len());
            debug_assert_eq!(sb_mid.len(), oa_mid.len());
            debug_assert_eq!(sb_back.len(), ob.len());
            sa == oa_front && sb_mid == oa_mid && sb_back == ob
        } else {
            let front = oa.len();
            let mid = sa.len() - front;

            let (sa_front, sa_mid) = sa.split_at(front);
            let (ob_mid, ob_back) = ob.split_at(mid);
            debug_assert_eq!(sa_front.len(), oa.len());
            debug_assert_eq!(sa_mid.len(), ob_mid.len());
            debug_assert_eq!(sb.len(), ob_back.len());
            sa_front == oa && sa_mid == ob_mid && sb == ob_back
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Eq, A: Allocator> Eq for VecDeque<T, A> {}

__impl_slice_eq1! { [] VecDeque<T, A>, Vec<U, A>, }
__impl_slice_eq1! { [] VecDeque<T, A>, &[U], }
__impl_slice_eq1! { [] VecDeque<T, A>, &mut [U], }
__impl_slice_eq1! { [const N: usize] VecDeque<T, A>, [U; N], }
__impl_slice_eq1! { [const N: usize] VecDeque<T, A>, &[U; N], }
__impl_slice_eq1! { [const N: usize] VecDeque<T, A>, &mut [U; N], }

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: PartialOrd, A: Allocator> PartialOrd for VecDeque<T, A> {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        self.iter().partial_cmp(other.iter())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Ord, A: Allocator> Ord for VecDeque<T, A> {
    #[inline]
    fn cmp(&self, other: &Self) -> Ordering {
        self.iter().cmp(other.iter())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Hash, A: Allocator> Hash for VecDeque<T, A> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.len().hash(state);
        // It's not possible to use Hash::hash_slice on slices
        // returned by as_slices method as their length can vary
        // in otherwise identical deques.
        //
        // Hasher only guarantees equivalence for the exact same
        // set of calls to its methods.
        self.iter().for_each(|elem| elem.hash(state));
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> Index<usize> for VecDeque<T, A> {
    type Output = T;

    #[inline]
    fn index(&self, index: usize) -> &T {
        self.get(index).expect("Out of bounds access")
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> IndexMut<usize> for VecDeque<T, A> {
    #[inline]
    fn index_mut(&mut self, index: usize) -> &mut T {
        self.get_mut(index).expect("Out of bounds access")
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> FromIterator<T> for VecDeque<T> {
    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> VecDeque<T> {
        let iterator = iter.into_iter();
        let (lower, _) = iterator.size_hint();
        let mut deq = VecDeque::with_capacity(lower);
        deq.extend(iterator);
        deq
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> IntoIterator for VecDeque<T, A> {
    type Item = T;
    type IntoIter = IntoIter<T, A>;

    /// Consumes the `VecDeque` into a front-to-back iterator yielding elements by
    /// value.
    fn into_iter(self) -> IntoIter<T, A> {
        IntoIter::new(self)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, A: Allocator> IntoIterator for &'a VecDeque<T, A> {
    type Item = &'a T;
    type IntoIter = Iter<'a, T>;

    fn into_iter(self) -> Iter<'a, T> {
        self.iter()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, A: Allocator> IntoIterator for &'a mut VecDeque<T, A> {
    type Item = &'a mut T;
    type IntoIter = IterMut<'a, T>;

    fn into_iter(self) -> IterMut<'a, T> {
        self.iter_mut()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, A: Allocator> Extend<T> for VecDeque<T, A> {
    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
        // This function should be the moral equivalent of:
        //
        //      for item in iter.into_iter() {
        //          self.push_back(item);
        //      }
        let mut iter = iter.into_iter();
        while let Some(element) = iter.next() {
            if self.len() == self.capacity() {
                let (lower, _) = iter.size_hint();
                self.reserve(lower.saturating_add(1));
            }

            let head = self.head;
            self.head = self.wrap_add(self.head, 1);
            unsafe {
                self.buffer_write(head, element);
            }
        }
    }

    #[inline]
    fn extend_one(&mut self, elem: T) {
        self.push_back(elem);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        self.reserve(additional);
    }
}

#[stable(feature = "extend_ref", since = "1.2.0")]
impl<'a, T: 'a + Copy, A: Allocator> Extend<&'a T> for VecDeque<T, A> {
    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
        self.extend(iter.into_iter().cloned());
    }

    #[inline]
    fn extend_one(&mut self, &elem: &T) {
        self.push_back(elem);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        self.reserve(additional);
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Debug, A: Allocator> fmt::Debug for VecDeque<T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self).finish()
    }
}

#[stable(feature = "vecdeque_vec_conversions", since = "1.10.0")]
impl<T, A: Allocator> From<Vec<T, A>> for VecDeque<T, A> {
    /// Turn a [`Vec<T>`] into a [`VecDeque<T>`].
    ///
    /// [`Vec<T>`]: crate::vec::Vec
    /// [`VecDeque<T>`]: crate::collections::VecDeque
    ///
    /// This avoids reallocating where possible, but the conditions for that are
    /// strict, and subject to change, and so shouldn't be relied upon unless the
    /// `Vec<T>` came from `From<VecDeque<T>>` and hasn't been reallocated.
    fn from(mut other: Vec<T, A>) -> Self {
        let len = other.len();
        if mem::size_of::<T>() == 0 {
            // There's no actual allocation for ZSTs to worry about capacity,
            // but `VecDeque` can't handle as much length as `Vec`.
            assert!(len < MAXIMUM_ZST_CAPACITY, "capacity overflow");
        } else {
            // We need to resize if the capacity is not a power of two, too small or
            // doesn't have at least one free space. We do this while it's still in
            // the `Vec` so the items will drop on panic.
            let min_cap = cmp::max(MINIMUM_CAPACITY, len) + 1;
            let cap = cmp::max(min_cap, other.capacity()).next_power_of_two();
            if other.capacity() != cap {
                other.reserve_exact(cap - len);
            }
        }

        unsafe {
            let (other_buf, len, capacity, alloc) = other.into_raw_parts_with_alloc();
            let buf = RawVec::from_raw_parts_in(other_buf, capacity, alloc);
            VecDeque { tail: 0, head: len, buf }
        }
    }
}

#[stable(feature = "vecdeque_vec_conversions", since = "1.10.0")]
impl<T, A: Allocator> From<VecDeque<T, A>> for Vec<T, A> {
    /// Turn a [`VecDeque<T>`] into a [`Vec<T>`].
    ///
    /// [`Vec<T>`]: crate::vec::Vec
    /// [`VecDeque<T>`]: crate::collections::VecDeque
    ///
    /// This never needs to re-allocate, but does need to do *O*(*n*) data movement if
    /// the circular buffer doesn't happen to be at the beginning of the allocation.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// // This one is *O*(1).
    /// let deque: VecDeque<_> = (1..5).collect();
    /// let ptr = deque.as_slices().0.as_ptr();
    /// let vec = Vec::from(deque);
    /// assert_eq!(vec, [1, 2, 3, 4]);
    /// assert_eq!(vec.as_ptr(), ptr);
    ///
    /// // This one needs data rearranging.
    /// let mut deque: VecDeque<_> = (1..5).collect();
    /// deque.push_front(9);
    /// deque.push_front(8);
    /// let ptr = deque.as_slices().1.as_ptr();
    /// let vec = Vec::from(deque);
    /// assert_eq!(vec, [8, 9, 1, 2, 3, 4]);
    /// assert_eq!(vec.as_ptr(), ptr);
    /// ```
    fn from(mut other: VecDeque<T, A>) -> Self {
        other.make_contiguous();

        unsafe {
            let other = ManuallyDrop::new(other);
            let buf = other.buf.ptr();
            let len = other.len();
            let cap = other.cap();
            let alloc = ptr::read(other.allocator());

            if other.tail != 0 {
                ptr::copy(buf.add(other.tail), buf, len);
            }
            Vec::from_raw_parts_in(buf, len, cap, alloc)
        }
    }
}

#[stable(feature = "std_collections_from_array", since = "1.56.0")]
impl<T, const N: usize> From<[T; N]> for VecDeque<T> {
    /// Converts a `[T; N]` into a `VecDeque<T>`.
    ///
    /// ```
    /// use std::collections::VecDeque;
    ///
    /// let deq1 = VecDeque::from([1, 2, 3, 4]);
    /// let deq2: VecDeque<_> = [1, 2, 3, 4].into();
    /// assert_eq!(deq1, deq2);
    /// ```
    fn from(arr: [T; N]) -> Self {
        let mut deq = VecDeque::with_capacity(N);
        let arr = ManuallyDrop::new(arr);
        if mem::size_of::<T>() != 0 {
            // SAFETY: VecDeque::with_capacity ensures that there is enough capacity.
            unsafe {
                ptr::copy_nonoverlapping(arr.as_ptr(), deq.ptr(), N);
            }
        }
        deq.tail = 0;
        deq.head = N;
        deq
    }
}