mod.rs

// ignore-tidy-filelength

//! Slice management and manipulation.
//!
//! For more details see [`std::slice`].
//!
//! [`std::slice`]: ../../std/slice/index.html

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

// How this module is organized.
//
// The library infrastructure for slices is fairly messy. There's
// a lot of stuff defined here. Let's keep it clean.
//
// The layout of this file is thus:
//
// * Inherent methods. This is where most of the slice API resides.
// * Implementations of a few common traits with important slice ops.
// * Definitions of a bunch of iterators.
// * Free functions.
// * The `raw` and `bytes` submodules.
// * Boilerplate trait implementations.

use crate::cmp;
use crate::cmp::Ordering::{self, Equal, Greater, Less};
use crate::fmt;
use crate::intrinsics::{assume, exact_div, is_aligned_and_not_null, unchecked_sub};
use crate::iter::*;
use crate::marker::{self, Copy, Send, Sized, Sync};
use crate::mem;
use crate::ops::{self, Bound, FnMut, Range, RangeBounds};
use crate::option::Option;
use crate::option::Option::{None, Some};
use crate::ptr::{self, NonNull};
use crate::result::Result;
use crate::result::Result::{Err, Ok};

#[unstable(
    feature = "slice_internals",
    issue = "none",
    reason = "exposed from core to be reused in std; use the memchr crate"
)]
/// Pure rust memchr implementation, taken from rust-memchr
pub mod memchr;

mod rotate;
mod sort;

//
// Extension traits
//

#[lang = "slice"]
#[cfg(not(test))]
impl<T> [T] {
    /// Returns the number of elements in the slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert_eq!(a.len(), 3);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_slice_len", since = "1.32.0")]
    #[inline]
    // SAFETY: const sound because we transmute out the length field as a usize (which it must be)
    #[allow(unused_attributes)]
    #[allow_internal_unstable(const_fn_union)]
    pub const fn len(&self) -> usize {
        // SAFETY: this is safe because `&[T]` and `FatPtr<T>` have the same layout.
        // Only `std` can make this guarantee.
        unsafe { crate::ptr::Repr { rust: self }.raw.len }
    }

    /// Returns `true` if the slice has a length of 0.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert!(!a.is_empty());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_slice_is_empty", since = "1.32.0")]
    #[inline]
    pub const fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Returns the first element of the slice, or `None` if it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = [10, 40, 30];
    /// assert_eq!(Some(&10), v.first());
    ///
    /// let w: &[i32] = &[];
    /// assert_eq!(None, w.first());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn first(&self) -> Option<&T> {
        if let [first, ..] = self { Some(first) } else { None }
    }

    /// Returns a mutable pointer to the first element of the slice, or `None` if it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &mut [0, 1, 2];
    ///
    /// if let Some(first) = x.first_mut() {
    ///     *first = 5;
    /// }
    /// assert_eq!(x, &[5, 1, 2]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn first_mut(&mut self) -> Option<&mut T> {
        if let [first, ..] = self { Some(first) } else { None }
    }

    /// Returns the first and all the rest of the elements of the slice, or `None` if it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &[0, 1, 2];
    ///
    /// if let Some((first, elements)) = x.split_first() {
    ///     assert_eq!(first, &0);
    ///     assert_eq!(elements, &[1, 2]);
    /// }
    /// ```
    #[stable(feature = "slice_splits", since = "1.5.0")]
    #[inline]
    pub fn split_first(&self) -> Option<(&T, &[T])> {
        if let [first, tail @ ..] = self { Some((first, tail)) } else { None }
    }

    /// Returns the first and all the rest of the elements of the slice, or `None` if it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &mut [0, 1, 2];
    ///
    /// if let Some((first, elements)) = x.split_first_mut() {
    ///     *first = 3;
    ///     elements[0] = 4;
    ///     elements[1] = 5;
    /// }
    /// assert_eq!(x, &[3, 4, 5]);
    /// ```
    #[stable(feature = "slice_splits", since = "1.5.0")]
    #[inline]
    pub fn split_first_mut(&mut self) -> Option<(&mut T, &mut [T])> {
        if let [first, tail @ ..] = self { Some((first, tail)) } else { None }
    }

    /// Returns the last and all the rest of the elements of the slice, or `None` if it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &[0, 1, 2];
    ///
    /// if let Some((last, elements)) = x.split_last() {
    ///     assert_eq!(last, &2);
    ///     assert_eq!(elements, &[0, 1]);
    /// }
    /// ```
    #[stable(feature = "slice_splits", since = "1.5.0")]
    #[inline]
    pub fn split_last(&self) -> Option<(&T, &[T])> {
        if let [init @ .., last] = self { Some((last, init)) } else { None }
    }

    /// Returns the last and all the rest of the elements of the slice, or `None` if it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &mut [0, 1, 2];
    ///
    /// if let Some((last, elements)) = x.split_last_mut() {
    ///     *last = 3;
    ///     elements[0] = 4;
    ///     elements[1] = 5;
    /// }
    /// assert_eq!(x, &[4, 5, 3]);
    /// ```
    #[stable(feature = "slice_splits", since = "1.5.0")]
    #[inline]
    pub fn split_last_mut(&mut self) -> Option<(&mut T, &mut [T])> {
        if let [init @ .., last] = self { Some((last, init)) } else { None }
    }

    /// Returns the last element of the slice, or `None` if it is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = [10, 40, 30];
    /// assert_eq!(Some(&30), v.last());
    ///
    /// let w: &[i32] = &[];
    /// assert_eq!(None, w.last());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn last(&self) -> Option<&T> {
        if let [.., last] = self { Some(last) } else { None }
    }

    /// Returns a mutable pointer to the last item in the slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &mut [0, 1, 2];
    ///
    /// if let Some(last) = x.last_mut() {
    ///     *last = 10;
    /// }
    /// assert_eq!(x, &[0, 1, 10]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn last_mut(&mut self) -> Option<&mut T> {
        if let [.., last] = self { Some(last) } else { None }
    }

    /// Returns a reference to an element or subslice depending on the type of
    /// index.
    ///
    /// - If given a position, returns a reference to the element at that
    ///   position or `None` if out of bounds.
    /// - If given a range, returns the subslice corresponding to that range,
    ///   or `None` if out of bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = [10, 40, 30];
    /// assert_eq!(Some(&40), v.get(1));
    /// assert_eq!(Some(&[10, 40][..]), v.get(0..2));
    /// assert_eq!(None, v.get(3));
    /// assert_eq!(None, v.get(0..4));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn get<I>(&self, index: I) -> Option<&I::Output>
    where
        I: SliceIndex<Self>,
    {
        index.get(self)
    }

    /// Returns a mutable reference to an element or subslice depending on the
    /// type of index (see [`get`]) or `None` if the index is out of bounds.
    ///
    /// [`get`]: #method.get
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &mut [0, 1, 2];
    ///
    /// if let Some(elem) = x.get_mut(1) {
    ///     *elem = 42;
    /// }
    /// assert_eq!(x, &[0, 42, 2]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn get_mut<I>(&mut self, index: I) -> Option<&mut I::Output>
    where
        I: SliceIndex<Self>,
    {
        index.get_mut(self)
    }

    /// Returns a reference to an element or subslice, without doing bounds
    /// checking.
    ///
    /// For a safe alternative see [`get`].
    ///
    /// # Safety
    ///
    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
    /// even if the resulting reference is not used.
    ///
    /// [`get`]: #method.get
    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &[1, 2, 4];
    ///
    /// unsafe {
    ///     assert_eq!(x.get_unchecked(1), &2);
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub unsafe fn get_unchecked<I>(&self, index: I) -> &I::Output
    where
        I: SliceIndex<Self>,
    {
        // SAFETY: the caller must uphold most of the safety requirements for `get_unchecked`;
        // the slice is dereferencable because `self` is a safe reference.
        // The returned pointer is safe because impls of `SliceIndex` have to guarantee that it is.
        unsafe { &*index.get_unchecked(self) }
    }

    /// Returns a mutable reference to an element or subslice, without doing
    /// bounds checking.
    ///
    /// For a safe alternative see [`get_mut`].
    ///
    /// # Safety
    ///
    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
    /// even if the resulting reference is not used.
    ///
    /// [`get_mut`]: #method.get_mut
    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &mut [1, 2, 4];
    ///
    /// unsafe {
    ///     let elem = x.get_unchecked_mut(1);
    ///     *elem = 13;
    /// }
    /// assert_eq!(x, &[1, 13, 4]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub unsafe fn get_unchecked_mut<I>(&mut self, index: I) -> &mut I::Output
    where
        I: SliceIndex<Self>,
    {
        // SAFETY: the caller must uphold the safety requirements for `get_unchecked_mut`;
        // the slice is dereferencable because `self` is a safe reference.
        // The returned pointer is safe because impls of `SliceIndex` have to guarantee that it is.
        unsafe { &mut *index.get_unchecked_mut(self) }
    }

    /// Converts a range over this slice to [`Range`].
    ///
    /// The returned range is safe to pass to [`get_unchecked`] and [`get_unchecked_mut`].
    ///
    /// [`get_unchecked`]: #method.get_unchecked
    /// [`get_unchecked_mut`]: #method.get_unchecked_mut
    ///
    /// # Panics
    ///
    /// Panics if the range is out of bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_check_range)]
    ///
    /// let v = [10, 40, 30];
    /// assert_eq!(1..2, v.check_range(1..2));
    /// assert_eq!(0..2, v.check_range(..2));
    /// assert_eq!(1..3, v.check_range(1..));
    /// ```
    ///
    /// Panics when [`Index::index`] would panic:
    ///
    /// ```should_panic
    /// #![feature(slice_check_range)]
    ///
    /// [10, 40, 30].check_range(2..1);
    /// ```
    ///
    /// ```should_panic
    /// #![feature(slice_check_range)]
    ///
    /// [10, 40, 30].check_range(1..4);
    /// ```
    ///
    /// ```should_panic
    /// #![feature(slice_check_range)]
    ///
    /// [10, 40, 30].check_range(1..=usize::MAX);
    /// ```
    ///
    /// [`Index::index`]: ops::Index::index
    #[track_caller]
    #[unstable(feature = "slice_check_range", issue = "76393")]
    pub fn check_range<R: RangeBounds<usize>>(&self, range: R) -> Range<usize> {
        let start = match range.start_bound() {
            Bound::Included(&start) => start,
            Bound::Excluded(start) => {
                start.checked_add(1).unwrap_or_else(|| slice_start_index_overflow_fail())
            }
            Bound::Unbounded => 0,
        };

        let len = self.len();
        let end = match range.end_bound() {
            Bound::Included(end) => {
                end.checked_add(1).unwrap_or_else(|| slice_end_index_overflow_fail())
            }
            Bound::Excluded(&end) => end,
            Bound::Unbounded => len,
        };

        if start > end {
            slice_index_order_fail(start, end);
        }
        if end > len {
            slice_end_index_len_fail(end, len);
        }

        Range { start, end }
    }

    /// Returns a raw pointer to the slice's buffer.
    ///
    /// The caller must ensure that the slice outlives the pointer this
    /// function returns, or else it will end up pointing to garbage.
    ///
    /// The caller must also ensure that the memory the pointer (non-transitively) points to
    /// is never written to (except inside an `UnsafeCell`) using this pointer or any pointer
    /// derived from it. If you need to mutate the contents of the slice, use [`as_mut_ptr`].
    ///
    /// Modifying the container referenced by this slice may cause its buffer
    /// to be reallocated, which would also make any pointers to it invalid.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &[1, 2, 4];
    /// let x_ptr = x.as_ptr();
    ///
    /// unsafe {
    ///     for i in 0..x.len() {
    ///         assert_eq!(x.get_unchecked(i), &*x_ptr.add(i));
    ///     }
    /// }
    /// ```
    ///
    /// [`as_mut_ptr`]: #method.as_mut_ptr
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_slice_as_ptr", since = "1.32.0")]
    #[inline]
    pub const fn as_ptr(&self) -> *const T {
        self as *const [T] as *const T
    }

    /// Returns an unsafe mutable pointer to the slice's buffer.
    ///
    /// The caller must ensure that the slice outlives the pointer this
    /// function returns, or else it will end up pointing to garbage.
    ///
    /// Modifying the container referenced by this slice may cause its buffer
    /// to be reallocated, which would also make any pointers to it invalid.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &mut [1, 2, 4];
    /// let x_ptr = x.as_mut_ptr();
    ///
    /// unsafe {
    ///     for i in 0..x.len() {
    ///         *x_ptr.add(i) += 2;
    ///     }
    /// }
    /// assert_eq!(x, &[3, 4, 6]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn as_mut_ptr(&mut self) -> *mut T {
        self as *mut [T] as *mut T
    }

    /// Returns the two raw pointers spanning the slice.
    ///
    /// The returned range is half-open, which means that the end pointer
    /// points *one past* the last element of the slice. This way, an empty
    /// slice is represented by two equal pointers, and the difference between
    /// the two pointers represents the size of the slice.
    ///
    /// See [`as_ptr`] for warnings on using these pointers. The end pointer
    /// requires extra caution, as it does not point to a valid element in the
    /// slice.
    ///
    /// This function is useful for interacting with foreign interfaces which
    /// use two pointers to refer to a range of elements in memory, as is
    /// common in C++.
    ///
    /// It can also be useful to check if a pointer to an element refers to an
    /// element of this slice:
    ///
    /// ```
    /// #![feature(slice_ptr_range)]
    ///
    /// let a = [1, 2, 3];
    /// let x = &a[1] as *const _;
    /// let y = &5 as *const _;
    ///
    /// assert!(a.as_ptr_range().contains(&x));
    /// assert!(!a.as_ptr_range().contains(&y));
    /// ```
    ///
    /// [`as_ptr`]: #method.as_ptr
    #[unstable(feature = "slice_ptr_range", issue = "65807")]
    #[inline]
    pub fn as_ptr_range(&self) -> Range<*const T> {
        let start = self.as_ptr();
        // SAFETY: The `add` here is safe, because:
        //
        //   - Both pointers are part of the same object, as pointing directly
        //     past the object also counts.
        //
        //   - The size of the slice is never larger than isize::MAX bytes, as
        //     noted here:
        //       - https://github.com/rust-lang/unsafe-code-guidelines/issues/102#issuecomment-473340447
        //       - https://doc.rust-lang.org/reference/behavior-considered-undefined.html
        //       - https://doc.rust-lang.org/core/slice/fn.from_raw_parts.html#safety
        //     (This doesn't seem normative yet, but the very same assumption is
        //     made in many places, including the Index implementation of slices.)
        //
        //   - There is no wrapping around involved, as slices do not wrap past
        //     the end of the address space.
        //
        // See the documentation of pointer::add.
        let end = unsafe { start.add(self.len()) };
        start..end
    }

    /// Returns the two unsafe mutable pointers spanning the slice.
    ///
    /// The returned range is half-open, which means that the end pointer
    /// points *one past* the last element of the slice. This way, an empty
    /// slice is represented by two equal pointers, and the difference between
    /// the two pointers represents the size of the slice.
    ///
    /// See [`as_mut_ptr`] for warnings on using these pointers. The end
    /// pointer requires extra caution, as it does not point to a valid element
    /// in the slice.
    ///
    /// This function is useful for interacting with foreign interfaces which
    /// use two pointers to refer to a range of elements in memory, as is
    /// common in C++.
    ///
    /// [`as_mut_ptr`]: #method.as_mut_ptr
    #[unstable(feature = "slice_ptr_range", issue = "65807")]
    #[inline]
    pub fn as_mut_ptr_range(&mut self) -> Range<*mut T> {
        let start = self.as_mut_ptr();
        // SAFETY: See as_ptr_range() above for why `add` here is safe.
        let end = unsafe { start.add(self.len()) };
        start..end
    }

    /// Swaps two elements in the slice.
    ///
    /// # Arguments
    ///
    /// * a - The index of the first element
    /// * b - The index of the second element
    ///
    /// # Panics
    ///
    /// Panics if `a` or `b` are out of bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = ["a", "b", "c", "d"];
    /// v.swap(1, 3);
    /// assert!(v == ["a", "d", "c", "b"]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn swap(&mut self, a: usize, b: usize) {
        // Can't take two mutable loans from one vector, so instead just cast
        // them to their raw pointers to do the swap.
        let pa: *mut T = &mut self[a];
        let pb: *mut T = &mut self[b];
        // SAFETY: `pa` and `pb` have been created from safe mutable references and refer
        // to elements in the slice and therefore are guaranteed to be valid and aligned.
        // Note that accessing the elements behind `a` and `b` is checked and will
        // panic when out of bounds.
        unsafe {
            ptr::swap(pa, pb);
        }
    }

    /// Reverses the order of elements in the slice, in place.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = [1, 2, 3];
    /// v.reverse();
    /// assert!(v == [3, 2, 1]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn reverse(&mut self) {
        let mut i: usize = 0;
        let ln = self.len();

        // For very small types, all the individual reads in the normal
        // path perform poorly.  We can do better, given efficient unaligned
        // load/store, by loading a larger chunk and reversing a register.

        // Ideally LLVM would do this for us, as it knows better than we do
        // whether unaligned reads are efficient (since that changes between
        // different ARM versions, for example) and what the best chunk size
        // would be.  Unfortunately, as of LLVM 4.0 (2017-05) it only unrolls
        // the loop, so we need to do this ourselves.  (Hypothesis: reverse
        // is troublesome because the sides can be aligned differently --
        // will be, when the length is odd -- so there's no way of emitting
        // pre- and postludes to use fully-aligned SIMD in the middle.)

        let fast_unaligned = cfg!(any(target_arch = "x86", target_arch = "x86_64"));

        if fast_unaligned && mem::size_of::<T>() == 1 {
            // Use the llvm.bswap intrinsic to reverse u8s in a usize
            let chunk = mem::size_of::<usize>();
            while i + chunk - 1 < ln / 2 {
                // SAFETY: There are several things to check here:
                //
                // - Note that `chunk` is either 4 or 8 due to the cfg check
                //   above. So `chunk - 1` is positive.
                // - Indexing with index `i` is fine as the loop check guarantees
                //   `i + chunk - 1 < ln / 2`
                //   <=> `i < ln / 2 - (chunk - 1) < ln / 2 < ln`.
                // - Indexing with index `ln - i - chunk = ln - (i + chunk)` is fine:
                //   - `i + chunk > 0` is trivially true.
                //   - The loop check guarantees:
                //     `i + chunk - 1 < ln / 2`
                //     <=> `i + chunk ≤ ln / 2 ≤ ln`, thus subtraction does not underflow.
                // - The `read_unaligned` and `write_unaligned` calls are fine:
                //   - `pa` points to index `i` where `i < ln / 2 - (chunk - 1)`
                //     (see above) and `pb` points to index `ln - i - chunk`, so
                //     both are at least `chunk`
                //     many bytes away from the end of `self`.
                //   - Any initialized memory is valid `usize`.
                unsafe {
                    let pa: *mut T = self.get_unchecked_mut(i);
                    let pb: *mut T = self.get_unchecked_mut(ln - i - chunk);
                    let va = ptr::read_unaligned(pa as *mut usize);
                    let vb = ptr::read_unaligned(pb as *mut usize);
                    ptr::write_unaligned(pa as *mut usize, vb.swap_bytes());
                    ptr::write_unaligned(pb as *mut usize, va.swap_bytes());
                }
                i += chunk;
            }
        }

        if fast_unaligned && mem::size_of::<T>() == 2 {
            // Use rotate-by-16 to reverse u16s in a u32
            let chunk = mem::size_of::<u32>() / 2;
            while i + chunk - 1 < ln / 2 {
                // SAFETY: An unaligned u32 can be read from `i` if `i + 1 < ln`
                // (and obviously `i < ln`), because each element is 2 bytes and
                // we're reading 4.
                //
                // `i + chunk - 1 < ln / 2` # while condition
                // `i + 2 - 1 < ln / 2`
                // `i + 1 < ln / 2`
                //
                // Since it's less than the length divided by 2, then it must be
                // in bounds.
                //
                // This also means that the condition `0 < i + chunk <= ln` is
                // always respected, ensuring the `pb` pointer can be used
                // safely.
                unsafe {
                    let pa: *mut T = self.get_unchecked_mut(i);
                    let pb: *mut T = self.get_unchecked_mut(ln - i - chunk);
                    let va = ptr::read_unaligned(pa as *mut u32);
                    let vb = ptr::read_unaligned(pb as *mut u32);
                    ptr::write_unaligned(pa as *mut u32, vb.rotate_left(16));
                    ptr::write_unaligned(pb as *mut u32, va.rotate_left(16));
                }
                i += chunk;
            }
        }

        while i < ln / 2 {
            // SAFETY: `i` is inferior to half the length of the slice so
            // accessing `i` and `ln - i - 1` is safe (`i` starts at 0 and
            // will not go further than `ln / 2 - 1`).
            // The resulting pointers `pa` and `pb` are therefore valid and
            // aligned, and can be read from and written to.
            unsafe {
                // Unsafe swap to avoid the bounds check in safe swap.
                let pa: *mut T = self.get_unchecked_mut(i);
                let pb: *mut T = self.get_unchecked_mut(ln - i - 1);
                ptr::swap(pa, pb);
            }
            i += 1;
        }
    }

    /// Returns an iterator over the slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &[1, 2, 4];
    /// let mut iterator = x.iter();
    ///
    /// assert_eq!(iterator.next(), Some(&1));
    /// assert_eq!(iterator.next(), Some(&2));
    /// assert_eq!(iterator.next(), Some(&4));
    /// assert_eq!(iterator.next(), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn iter(&self) -> Iter<'_, T> {
        let ptr = self.as_ptr();
        // SAFETY: There are several things here:
        //
        // `ptr` has been obtained by `self.as_ptr()` where `self` is a valid
        // reference thus it is non-NUL and safe to use and pass to
        // `NonNull::new_unchecked` .
        //
        // Adding `self.len()` to the starting pointer gives a pointer
        // at the end of `self`. `end` will never be dereferenced, only checked
        // for direct pointer equality with `ptr` to check if the iterator is
        // done.
        //
        // In the case of a ZST, the end pointer is just the start pointer plus
        // the length, to also allows for the fast `ptr == end` check.
        //
        // See the `next_unchecked!` and `is_empty!` macros as well as the
        // `post_inc_start` method for more informations.
        unsafe {
            assume(!ptr.is_null());

            let end = if mem::size_of::<T>() == 0 {
                (ptr as *const u8).wrapping_add(self.len()) as *const T
            } else {
                ptr.add(self.len())
            };

            Iter { ptr: NonNull::new_unchecked(ptr as *mut T), end, _marker: marker::PhantomData }
        }
    }

    /// Returns an iterator that allows modifying each value.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = &mut [1, 2, 4];
    /// for elem in x.iter_mut() {
    ///     *elem += 2;
    /// }
    /// assert_eq!(x, &[3, 4, 6]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn iter_mut(&mut self) -> IterMut<'_, T> {
        let ptr = self.as_mut_ptr();
        // SAFETY: There are several things here:
        //
        // `ptr` has been obtained by `self.as_ptr()` where `self` is a valid
        // reference thus it is non-NUL and safe to use and pass to
        // `NonNull::new_unchecked` .
        //
        // Adding `self.len()` to the starting pointer gives a pointer
        // at the end of `self`. `end` will never be dereferenced, only checked
        // for direct pointer equality with `ptr` to check if the iterator is
        // done.
        //
        // In the case of a ZST, the end pointer is just the start pointer plus
        // the length, to also allows for the fast `ptr == end` check.
        //
        // See the `next_unchecked!` and `is_empty!` macros as well as the
        // `post_inc_start` method for more informations.
        unsafe {
            assume(!ptr.is_null());

            let end = if mem::size_of::<T>() == 0 {
                (ptr as *mut u8).wrapping_add(self.len()) as *mut T
            } else {
                ptr.add(self.len())
            };

            IterMut { ptr: NonNull::new_unchecked(ptr), end, _marker: marker::PhantomData }
        }
    }

    /// Returns an iterator over all contiguous windows of length
    /// `size`. The windows overlap. If the slice is shorter than
    /// `size`, the iterator returns no values.
    ///
    /// # Panics
    ///
    /// Panics if `size` is 0.
    ///
    /// # Examples
    ///
    /// ```
    /// let slice = ['r', 'u', 's', 't'];
    /// let mut iter = slice.windows(2);
    /// assert_eq!(iter.next().unwrap(), &['r', 'u']);
    /// assert_eq!(iter.next().unwrap(), &['u', 's']);
    /// assert_eq!(iter.next().unwrap(), &['s', 't']);
    /// assert!(iter.next().is_none());
    /// ```
    ///
    /// If the slice is shorter than `size`:
    ///
    /// ```
    /// let slice = ['f', 'o', 'o'];
    /// let mut iter = slice.windows(4);
    /// assert!(iter.next().is_none());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn windows(&self, size: usize) -> Windows<'_, T> {
        assert_ne!(size, 0);
        Windows { v: self, size }
    }

    /// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the
    /// beginning of the slice.
    ///
    /// The chunks are slices and do not overlap. If `chunk_size` does not divide the length of the
    /// slice, then the last chunk will not have length `chunk_size`.
    ///
    /// See [`chunks_exact`] for a variant of this iterator that returns chunks of always exactly
    /// `chunk_size` elements, and [`rchunks`] for the same iterator but starting at the end of the
    /// slice.
    ///
    /// # Panics
    ///
    /// Panics if `chunk_size` is 0.
    ///
    /// # Examples
    ///
    /// ```
    /// let slice = ['l', 'o', 'r', 'e', 'm'];
    /// let mut iter = slice.chunks(2);
    /// assert_eq!(iter.next().unwrap(), &['l', 'o']);
    /// assert_eq!(iter.next().unwrap(), &['r', 'e']);
    /// assert_eq!(iter.next().unwrap(), &['m']);
    /// assert!(iter.next().is_none());
    /// ```
    ///
    /// [`chunks_exact`]: #method.chunks_exact
    /// [`rchunks`]: #method.rchunks
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn chunks(&self, chunk_size: usize) -> Chunks<'_, T> {
        assert_ne!(chunk_size, 0);
        Chunks { v: self, chunk_size }
    }

    /// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the
    /// beginning of the slice.
    ///
    /// The chunks are mutable slices, and do not overlap. If `chunk_size` does not divide the
    /// length of the slice, then the last chunk will not have length `chunk_size`.
    ///
    /// See [`chunks_exact_mut`] for a variant of this iterator that returns chunks of always
    /// exactly `chunk_size` elements, and [`rchunks_mut`] for the same iterator but starting at
    /// the end of the slice.
    ///
    /// # Panics
    ///
    /// Panics if `chunk_size` is 0.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = &mut [0, 0, 0, 0, 0];
    /// let mut count = 1;
    ///
    /// for chunk in v.chunks_mut(2) {
    ///     for elem in chunk.iter_mut() {
    ///         *elem += count;
    ///     }
    ///     count += 1;
    /// }
    /// assert_eq!(v, &[1, 1, 2, 2, 3]);
    /// ```
    ///
    /// [`chunks_exact_mut`]: #method.chunks_exact_mut
    /// [`rchunks_mut`]: #method.rchunks_mut
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn chunks_mut(&mut self, chunk_size: usize) -> ChunksMut<'_, T> {
        assert_ne!(chunk_size, 0);
        ChunksMut { v: self, chunk_size }
    }

    /// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the
    /// beginning of the slice.
    ///
    /// The chunks are slices and do not overlap. If `chunk_size` does not divide the length of the
    /// slice, then the last up to `chunk_size-1` elements will be omitted and can be retrieved
    /// from the `remainder` function of the iterator.
    ///
    /// Due to each chunk having exactly `chunk_size` elements, the compiler can often optimize the
    /// resulting code better than in the case of [`chunks`].
    ///
    /// See [`chunks`] for a variant of this iterator that also returns the remainder as a smaller
    /// chunk, and [`rchunks_exact`] for the same iterator but starting at the end of the slice.
    ///
    /// # Panics
    ///
    /// Panics if `chunk_size` is 0.
    ///
    /// # Examples
    ///
    /// ```
    /// let slice = ['l', 'o', 'r', 'e', 'm'];
    /// let mut iter = slice.chunks_exact(2);
    /// assert_eq!(iter.next().unwrap(), &['l', 'o']);
    /// assert_eq!(iter.next().unwrap(), &['r', 'e']);
    /// assert!(iter.next().is_none());
    /// assert_eq!(iter.remainder(), &['m']);
    /// ```
    ///
    /// [`chunks`]: #method.chunks
    /// [`rchunks_exact`]: #method.rchunks_exact
    #[stable(feature = "chunks_exact", since = "1.31.0")]
    #[inline]
    pub fn chunks_exact(&self, chunk_size: usize) -> ChunksExact<'_, T> {
        assert_ne!(chunk_size, 0);
        let rem = self.len() % chunk_size;
        let fst_len = self.len() - rem;
        // SAFETY: 0 <= fst_len <= self.len() by construction above
        let (fst, snd) = unsafe { self.split_at_unchecked(fst_len) };
        ChunksExact { v: fst, rem: snd, chunk_size }
    }

    /// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the
    /// beginning of the slice.
    ///
    /// The chunks are mutable slices, and do not overlap. If `chunk_size` does not divide the
    /// length of the slice, then the last up to `chunk_size-1` elements will be omitted and can be
    /// retrieved from the `into_remainder` function of the iterator.
    ///
    /// Due to each chunk having exactly `chunk_size` elements, the compiler can often optimize the
    /// resulting code better than in the case of [`chunks_mut`].
    ///
    /// See [`chunks_mut`] for a variant of this iterator that also returns the remainder as a
    /// smaller chunk, and [`rchunks_exact_mut`] for the same iterator but starting at the end of
    /// the slice.
    ///
    /// # Panics
    ///
    /// Panics if `chunk_size` is 0.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = &mut [0, 0, 0, 0, 0];
    /// let mut count = 1;
    ///
    /// for chunk in v.chunks_exact_mut(2) {
    ///     for elem in chunk.iter_mut() {
    ///         *elem += count;
    ///     }
    ///     count += 1;
    /// }
    /// assert_eq!(v, &[1, 1, 2, 2, 0]);
    /// ```
    ///
    /// [`chunks_mut`]: #method.chunks_mut
    /// [`rchunks_exact_mut`]: #method.rchunks_exact_mut
    #[stable(feature = "chunks_exact", since = "1.31.0")]
    #[inline]
    pub fn chunks_exact_mut(&mut self, chunk_size: usize) -> ChunksExactMut<'_, T> {
        assert_ne!(chunk_size, 0);
        let rem = self.len() % chunk_size;
        let fst_len = self.len() - rem;
        // SAFETY: 0 <= fst_len <= self.len() by construction above
        let (fst, snd) = unsafe { self.split_at_mut_unchecked(fst_len) };
        ChunksExactMut { v: fst, rem: snd, chunk_size }
    }

    /// Returns an iterator over `N` elements of the slice at a time, starting at the
    /// beginning of the slice.
    ///
    /// The chunks are slices and do not overlap. If `N` does not divide the length of the
    /// slice, then the last up to `N-1` elements will be omitted and can be retrieved
    /// from the `remainder` function of the iterator.
    ///
    /// This method is the const generic equivalent of [`chunks_exact`].
    ///
    /// # Panics
    ///
    /// Panics if `N` is 0. This check will most probably get changed to a compile time
    /// error before this method gets stabilized.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(array_chunks)]
    /// let slice = ['l', 'o', 'r', 'e', 'm'];
    /// let mut iter = slice.array_chunks();
    /// assert_eq!(iter.next().unwrap(), &['l', 'o']);
    /// assert_eq!(iter.next().unwrap(), &['r', 'e']);
    /// assert!(iter.next().is_none());
    /// assert_eq!(iter.remainder(), &['m']);
    /// ```
    ///
    /// [`chunks_exact`]: #method.chunks_exact
    #[unstable(feature = "array_chunks", issue = "74985")]
    #[inline]
    pub fn array_chunks<const N: usize>(&self) -> ArrayChunks<'_, T, N> {
        assert_ne!(N, 0);
        let len = self.len() / N;
        let (fst, snd) = self.split_at(len * N);
        // SAFETY: We cast a slice of `len * N` elements into
        // a slice of `len` many `N` elements chunks.
        let array_slice: &[[T; N]] = unsafe { from_raw_parts(fst.as_ptr().cast(), len) };
        ArrayChunks { iter: array_slice.iter(), rem: snd }
    }

    /// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the end
    /// of the slice.
    ///
    /// The chunks are slices and do not overlap. If `chunk_size` does not divide the length of the
    /// slice, then the last chunk will not have length `chunk_size`.
    ///
    /// See [`rchunks_exact`] for a variant of this iterator that returns chunks of always exactly
    /// `chunk_size` elements, and [`chunks`] for the same iterator but starting at the beginning
    /// of the slice.
    ///
    /// # Panics
    ///
    /// Panics if `chunk_size` is 0.
    ///
    /// # Examples
    ///
    /// ```
    /// let slice = ['l', 'o', 'r', 'e', 'm'];
    /// let mut iter = slice.rchunks(2);
    /// assert_eq!(iter.next().unwrap(), &['e', 'm']);
    /// assert_eq!(iter.next().unwrap(), &['o', 'r']);
    /// assert_eq!(iter.next().unwrap(), &['l']);
    /// assert!(iter.next().is_none());
    /// ```
    ///
    /// [`rchunks_exact`]: #method.rchunks_exact
    /// [`chunks`]: #method.chunks
    #[stable(feature = "rchunks", since = "1.31.0")]
    #[inline]
    pub fn rchunks(&self, chunk_size: usize) -> RChunks<'_, T> {
        assert!(chunk_size != 0);
        RChunks { v: self, chunk_size }
    }

    /// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the end
    /// of the slice.
    ///
    /// The chunks are mutable slices, and do not overlap. If `chunk_size` does not divide the
    /// length of the slice, then the last chunk will not have length `chunk_size`.
    ///
    /// See [`rchunks_exact_mut`] for a variant of this iterator that returns chunks of always
    /// exactly `chunk_size` elements, and [`chunks_mut`] for the same iterator but starting at the
    /// beginning of the slice.
    ///
    /// # Panics
    ///
    /// Panics if `chunk_size` is 0.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = &mut [0, 0, 0, 0, 0];
    /// let mut count = 1;
    ///
    /// for chunk in v.rchunks_mut(2) {
    ///     for elem in chunk.iter_mut() {
    ///         *elem += count;
    ///     }
    ///     count += 1;
    /// }
    /// assert_eq!(v, &[3, 2, 2, 1, 1]);
    /// ```
    ///
    /// [`rchunks_exact_mut`]: #method.rchunks_exact_mut
    /// [`chunks_mut`]: #method.chunks_mut
    #[stable(feature = "rchunks", since = "1.31.0")]
    #[inline]
    pub fn rchunks_mut(&mut self, chunk_size: usize) -> RChunksMut<'_, T> {
        assert!(chunk_size != 0);
        RChunksMut { v: self, chunk_size }
    }

    /// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the
    /// end of the slice.
    ///
    /// The chunks are slices and do not overlap. If `chunk_size` does not divide the length of the
    /// slice, then the last up to `chunk_size-1` elements will be omitted and can be retrieved
    /// from the `remainder` function of the iterator.
    ///
    /// Due to each chunk having exactly `chunk_size` elements, the compiler can often optimize the
    /// resulting code better than in the case of [`chunks`].
    ///
    /// See [`rchunks`] for a variant of this iterator that also returns the remainder as a smaller
    /// chunk, and [`chunks_exact`] for the same iterator but starting at the beginning of the
    /// slice.
    ///
    /// # Panics
    ///
    /// Panics if `chunk_size` is 0.
    ///
    /// # Examples
    ///
    /// ```
    /// let slice = ['l', 'o', 'r', 'e', 'm'];
    /// let mut iter = slice.rchunks_exact(2);
    /// assert_eq!(iter.next().unwrap(), &['e', 'm']);
    /// assert_eq!(iter.next().unwrap(), &['o', 'r']);
    /// assert!(iter.next().is_none());
    /// assert_eq!(iter.remainder(), &['l']);
    /// ```
    ///
    /// [`chunks`]: #method.chunks
    /// [`rchunks`]: #method.rchunks
    /// [`chunks_exact`]: #method.chunks_exact
    #[stable(feature = "rchunks", since = "1.31.0")]
    #[inline]
    pub fn rchunks_exact(&self, chunk_size: usize) -> RChunksExact<'_, T> {
        assert!(chunk_size != 0);
        let rem = self.len() % chunk_size;
        // SAFETY: 0 <= rem <= self.len() by construction above
        let (fst, snd) = unsafe { self.split_at_unchecked(rem) };
        RChunksExact { v: snd, rem: fst, chunk_size }
    }

    /// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the end
    /// of the slice.
    ///
    /// The chunks are mutable slices, and do not overlap. If `chunk_size` does not divide the
    /// length of the slice, then the last up to `chunk_size-1` elements will be omitted and can be
    /// retrieved from the `into_remainder` function of the iterator.
    ///
    /// Due to each chunk having exactly `chunk_size` elements, the compiler can often optimize the
    /// resulting code better than in the case of [`chunks_mut`].
    ///
    /// See [`rchunks_mut`] for a variant of this iterator that also returns the remainder as a
    /// smaller chunk, and [`chunks_exact_mut`] for the same iterator but starting at the beginning
    /// of the slice.
    ///
    /// # Panics
    ///
    /// Panics if `chunk_size` is 0.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = &mut [0, 0, 0, 0, 0];
    /// let mut count = 1;
    ///
    /// for chunk in v.rchunks_exact_mut(2) {
    ///     for elem in chunk.iter_mut() {
    ///         *elem += count;
    ///     }
    ///     count += 1;
    /// }
    /// assert_eq!(v, &[0, 2, 2, 1, 1]);
    /// ```
    ///
    /// [`chunks_mut`]: #method.chunks_mut
    /// [`rchunks_mut`]: #method.rchunks_mut
    /// [`chunks_exact_mut`]: #method.chunks_exact_mut
    #[stable(feature = "rchunks", since = "1.31.0")]
    #[inline]
    pub fn rchunks_exact_mut(&mut self, chunk_size: usize) -> RChunksExactMut<'_, T> {
        assert!(chunk_size != 0);
        let rem = self.len() % chunk_size;
        // SAFETY: 0 <= rem <= self.len() by construction above
        let (fst, snd) = unsafe { self.split_at_mut_unchecked(rem) };
        RChunksExactMut { v: snd, rem: fst, chunk_size }
    }

    /// Divides one slice into two at an index.
    ///
    /// The first will contain all indices from `[0, mid)` (excluding
    /// the index `mid` itself) and the second will contain all
    /// indices from `[mid, len)` (excluding the index `len` itself).
    ///
    /// # Panics
    ///
    /// Panics if `mid > len`.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = [1, 2, 3, 4, 5, 6];
    ///
    /// {
    ///    let (left, right) = v.split_at(0);
    ///    assert_eq!(left, []);
    ///    assert_eq!(right, [1, 2, 3, 4, 5, 6]);
    /// }
    ///
    /// {
    ///     let (left, right) = v.split_at(2);
    ///     assert_eq!(left, [1, 2]);
    ///     assert_eq!(right, [3, 4, 5, 6]);
    /// }
    ///
    /// {
    ///     let (left, right) = v.split_at(6);
    ///     assert_eq!(left, [1, 2, 3, 4, 5, 6]);
    ///     assert_eq!(right, []);
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn split_at(&self, mid: usize) -> (&[T], &[T]) {
        assert!(mid <= self.len());
        // SAFETY: `[ptr; mid]` and `[mid; len]` are inside `self`, which
        // fulfills the requirements of `from_raw_parts_mut`.
        unsafe { self.split_at_unchecked(mid) }
    }

    /// Divides one mutable slice into two at an index.
    ///
    /// The first will contain all indices from `[0, mid)` (excluding
    /// the index `mid` itself) and the second will contain all
    /// indices from `[mid, len)` (excluding the index `len` itself).
    ///
    /// # Panics
    ///
    /// Panics if `mid > len`.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = [1, 0, 3, 0, 5, 6];
    /// // scoped to restrict the lifetime of the borrows
    /// {
    ///     let (left, right) = v.split_at_mut(2);
    ///     assert_eq!(left, [1, 0]);
    ///     assert_eq!(right, [3, 0, 5, 6]);
    ///     left[1] = 2;
    ///     right[1] = 4;
    /// }
    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
        assert!(mid <= self.len());
        // SAFETY: `[ptr; mid]` and `[mid; len]` are inside `self`, which
        // fulfills the requirements of `from_raw_parts_mut`.
        unsafe { self.split_at_mut_unchecked(mid) }
    }

    /// Divides one slice into two at an index, without doing bounds checking.
    ///
    /// The first will contain all indices from `[0, mid)` (excluding
    /// the index `mid` itself) and the second will contain all
    /// indices from `[mid, len)` (excluding the index `len` itself).
    ///
    /// For a safe alternative see [`split_at`].
    ///
    /// # Safety
    ///
    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
    /// even if the resulting reference is not used. The caller has to ensure that
    /// `0 <= mid <= self.len()`.
    ///
    /// [`split_at`]: #method.split_at
    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    ///
    /// # Examples
    ///
    /// ```compile_fail
    /// #![feature(slice_split_at_unchecked)]
    ///
    /// let v = [1, 2, 3, 4, 5, 6];
    ///
    /// unsafe {
    ///    let (left, right) = v.split_at_unchecked(0);
    ///    assert_eq!(left, []);
    ///    assert_eq!(right, [1, 2, 3, 4, 5, 6]);
    /// }
    ///
    /// unsafe {
    ///     let (left, right) = v.split_at_unchecked(2);
    ///     assert_eq!(left, [1, 2]);
    ///     assert_eq!(right, [3, 4, 5, 6]);
    /// }
    ///
    /// unsafe {
    ///     let (left, right) = v.split_at_unchecked(6);
    ///     assert_eq!(left, [1, 2, 3, 4, 5, 6]);
    ///     assert_eq!(right, []);
    /// }
    /// ```
    #[unstable(feature = "slice_split_at_unchecked", reason = "new API", issue = "76014")]
    #[inline]
    unsafe fn split_at_unchecked(&self, mid: usize) -> (&[T], &[T]) {
        // SAFETY: Caller has to check that `0 <= mid <= self.len()`
        unsafe { (self.get_unchecked(..mid), self.get_unchecked(mid..)) }
    }

    /// Divides one mutable slice into two at an index, without doing bounds checking.
    ///
    /// The first will contain all indices from `[0, mid)` (excluding
    /// the index `mid` itself) and the second will contain all
    /// indices from `[mid, len)` (excluding the index `len` itself).
    ///
    /// For a safe alternative see [`split_at_mut`].
    ///
    /// # Safety
    ///
    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
    /// even if the resulting reference is not used. The caller has to ensure that
    /// `0 <= mid <= self.len()`.
    ///
    /// [`split_at_mut`]: #method.split_at_mut
    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    ///
    /// # Examples
    ///
    /// ```compile_fail
    /// #![feature(slice_split_at_unchecked)]
    ///
    /// let mut v = [1, 0, 3, 0, 5, 6];
    /// // scoped to restrict the lifetime of the borrows
    /// unsafe {
    ///     let (left, right) = v.split_at_mut_unchecked(2);
    ///     assert_eq!(left, [1, 0]);
    ///     assert_eq!(right, [3, 0, 5, 6]);
    ///     left[1] = 2;
    ///     right[1] = 4;
    /// }
    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
    /// ```
    #[unstable(feature = "slice_split_at_unchecked", reason = "new API", issue = "76014")]
    #[inline]
    unsafe fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut [T], &mut [T]) {
        let len = self.len();
        let ptr = self.as_mut_ptr();

        // SAFETY: Caller has to check that `0 <= mid <= self.len()`.
        //
        // `[ptr; mid]` and `[mid; len]` are not overlapping, so returning a mutable reference
        // is fine.
        unsafe { (from_raw_parts_mut(ptr, mid), from_raw_parts_mut(ptr.add(mid), len - mid)) }
    }

    /// Returns an iterator over subslices separated by elements that match
    /// `pred`. The matched element is not contained in the subslices.
    ///
    /// # Examples
    ///
    /// ```
    /// let slice = [10, 40, 33, 20];
    /// let mut iter = slice.split(|num| num % 3 == 0);
    ///
    /// assert_eq!(iter.next().unwrap(), &[10, 40]);
    /// assert_eq!(iter.next().unwrap(), &[20]);
    /// assert!(iter.next().is_none());
    /// ```
    ///
    /// If the first element is matched, an empty slice will be the first item
    /// returned by the iterator. Similarly, if the last element in the slice
    /// is matched, an empty slice will be the last item returned by the
    /// iterator:
    ///
    /// ```
    /// let slice = [10, 40, 33];
    /// let mut iter = slice.split(|num| num % 3 == 0);
    ///
    /// assert_eq!(iter.next().unwrap(), &[10, 40]);
    /// assert_eq!(iter.next().unwrap(), &[]);
    /// assert!(iter.next().is_none());
    /// ```
    ///
    /// If two matched elements are directly adjacent, an empty slice will be
    /// present between them:
    ///
    /// ```
    /// let slice = [10, 6, 33, 20];
    /// let mut iter = slice.split(|num| num % 3 == 0);
    ///
    /// assert_eq!(iter.next().unwrap(), &[10]);
    /// assert_eq!(iter.next().unwrap(), &[]);
    /// assert_eq!(iter.next().unwrap(), &[20]);
    /// assert!(iter.next().is_none());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn split<F>(&self, pred: F) -> Split<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        Split { v: self, pred, finished: false }
    }

    /// Returns an iterator over mutable subslices separated by elements that
    /// match `pred`. The matched element is not contained in the subslices.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = [10, 40, 30, 20, 60, 50];
    ///
    /// for group in v.split_mut(|num| *num % 3 == 0) {
    ///     group[0] = 1;
    /// }
    /// assert_eq!(v, [1, 40, 30, 1, 60, 1]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn split_mut<F>(&mut self, pred: F) -> SplitMut<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        SplitMut { v: self, pred, finished: false }
    }

    /// Returns an iterator over subslices separated by elements that match
    /// `pred`. The matched element is contained in the end of the previous
    /// subslice as a terminator.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_inclusive)]
    /// let slice = [10, 40, 33, 20];
    /// let mut iter = slice.split_inclusive(|num| num % 3 == 0);
    ///
    /// assert_eq!(iter.next().unwrap(), &[10, 40, 33]);
    /// assert_eq!(iter.next().unwrap(), &[20]);
    /// assert!(iter.next().is_none());
    /// ```
    ///
    /// If the last element of the slice is matched,
    /// that element will be considered the terminator of the preceding slice.
    /// That slice will be the last item returned by the iterator.
    ///
    /// ```
    /// #![feature(split_inclusive)]
    /// let slice = [3, 10, 40, 33];
    /// let mut iter = slice.split_inclusive(|num| num % 3 == 0);
    ///
    /// assert_eq!(iter.next().unwrap(), &[3]);
    /// assert_eq!(iter.next().unwrap(), &[10, 40, 33]);
    /// assert!(iter.next().is_none());
    /// ```
    #[unstable(feature = "split_inclusive", issue = "72360")]
    #[inline]
    pub fn split_inclusive<F>(&self, pred: F) -> SplitInclusive<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        SplitInclusive { v: self, pred, finished: false }
    }

    /// Returns an iterator over mutable subslices separated by elements that
    /// match `pred`. The matched element is contained in the previous
    /// subslice as a terminator.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(split_inclusive)]
    /// let mut v = [10, 40, 30, 20, 60, 50];
    ///
    /// for group in v.split_inclusive_mut(|num| *num % 3 == 0) {
    ///     let terminator_idx = group.len()-1;
    ///     group[terminator_idx] = 1;
    /// }
    /// assert_eq!(v, [10, 40, 1, 20, 1, 1]);
    /// ```
    #[unstable(feature = "split_inclusive", issue = "72360")]
    #[inline]
    pub fn split_inclusive_mut<F>(&mut self, pred: F) -> SplitInclusiveMut<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        SplitInclusiveMut { v: self, pred, finished: false }
    }

    /// Returns an iterator over subslices separated by elements that match
    /// `pred`, starting at the end of the slice and working backwards.
    /// The matched element is not contained in the subslices.
    ///
    /// # Examples
    ///
    /// ```
    /// let slice = [11, 22, 33, 0, 44, 55];
    /// let mut iter = slice.rsplit(|num| *num == 0);
    ///
    /// assert_eq!(iter.next().unwrap(), &[44, 55]);
    /// assert_eq!(iter.next().unwrap(), &[11, 22, 33]);
    /// assert_eq!(iter.next(), None);
    /// ```
    ///
    /// As with `split()`, if the first or last element is matched, an empty
    /// slice will be the first (or last) item returned by the iterator.
    ///
    /// ```
    /// let v = &[0, 1, 1, 2, 3, 5, 8];
    /// let mut it = v.rsplit(|n| *n % 2 == 0);
    /// assert_eq!(it.next().unwrap(), &[]);
    /// assert_eq!(it.next().unwrap(), &[3, 5]);
    /// assert_eq!(it.next().unwrap(), &[1, 1]);
    /// assert_eq!(it.next().unwrap(), &[]);
    /// assert_eq!(it.next(), None);
    /// ```
    #[stable(feature = "slice_rsplit", since = "1.27.0")]
    #[inline]
    pub fn rsplit<F>(&self, pred: F) -> RSplit<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        RSplit { inner: self.split(pred) }
    }

    /// Returns an iterator over mutable subslices separated by elements that
    /// match `pred`, starting at the end of the slice and working
    /// backwards. The matched element is not contained in the subslices.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = [100, 400, 300, 200, 600, 500];
    ///
    /// let mut count = 0;
    /// for group in v.rsplit_mut(|num| *num % 3 == 0) {
    ///     count += 1;
    ///     group[0] = count;
    /// }
    /// assert_eq!(v, [3, 400, 300, 2, 600, 1]);
    /// ```
    ///
    #[stable(feature = "slice_rsplit", since = "1.27.0")]
    #[inline]
    pub fn rsplit_mut<F>(&mut self, pred: F) -> RSplitMut<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        RSplitMut { inner: self.split_mut(pred) }
    }

    /// Returns an iterator over subslices separated by elements that match
    /// `pred`, limited to returning at most `n` items. The matched element is
    /// not contained in the subslices.
    ///
    /// The last element returned, if any, will contain the remainder of the
    /// slice.
    ///
    /// # Examples
    ///
    /// Print the slice split once by numbers divisible by 3 (i.e., `[10, 40]`,
    /// `[20, 60, 50]`):
    ///
    /// ```
    /// let v = [10, 40, 30, 20, 60, 50];
    ///
    /// for group in v.splitn(2, |num| *num % 3 == 0) {
    ///     println!("{:?}", group);
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn splitn<F>(&self, n: usize, pred: F) -> SplitN<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        SplitN { inner: GenericSplitN { iter: self.split(pred), count: n } }
    }

    /// Returns an iterator over subslices separated by elements that match
    /// `pred`, limited to returning at most `n` items. The matched element is
    /// not contained in the subslices.
    ///
    /// The last element returned, if any, will contain the remainder of the
    /// slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = [10, 40, 30, 20, 60, 50];
    ///
    /// for group in v.splitn_mut(2, |num| *num % 3 == 0) {
    ///     group[0] = 1;
    /// }
    /// assert_eq!(v, [1, 40, 30, 1, 60, 50]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn splitn_mut<F>(&mut self, n: usize, pred: F) -> SplitNMut<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        SplitNMut { inner: GenericSplitN { iter: self.split_mut(pred), count: n } }
    }

    /// Returns an iterator over subslices separated by elements that match
    /// `pred` limited to returning at most `n` items. This starts at the end of
    /// the slice and works backwards. The matched element is not contained in
    /// the subslices.
    ///
    /// The last element returned, if any, will contain the remainder of the
    /// slice.
    ///
    /// # Examples
    ///
    /// Print the slice split once, starting from the end, by numbers divisible
    /// by 3 (i.e., `[50]`, `[10, 40, 30, 20]`):
    ///
    /// ```
    /// let v = [10, 40, 30, 20, 60, 50];
    ///
    /// for group in v.rsplitn(2, |num| *num % 3 == 0) {
    ///     println!("{:?}", group);
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn rsplitn<F>(&self, n: usize, pred: F) -> RSplitN<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        RSplitN { inner: GenericSplitN { iter: self.rsplit(pred), count: n } }
    }

    /// Returns an iterator over subslices separated by elements that match
    /// `pred` limited to returning at most `n` items. This starts at the end of
    /// the slice and works backwards. The matched element is not contained in
    /// the subslices.
    ///
    /// The last element returned, if any, will contain the remainder of the
    /// slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = [10, 40, 30, 20, 60, 50];
    ///
    /// for group in s.rsplitn_mut(2, |num| *num % 3 == 0) {
    ///     group[0] = 1;
    /// }
    /// assert_eq!(s, [1, 40, 30, 20, 60, 1]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn rsplitn_mut<F>(&mut self, n: usize, pred: F) -> RSplitNMut<'_, T, F>
    where
        F: FnMut(&T) -> bool,
    {
        RSplitNMut { inner: GenericSplitN { iter: self.rsplit_mut(pred), count: n } }
    }

    /// Returns `true` if the slice contains an element with the given value.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = [10, 40, 30];
    /// assert!(v.contains(&30));
    /// assert!(!v.contains(&50));
    /// ```
    ///
    /// If you do not have an `&T`, but just an `&U` such that `T: Borrow<U>`
    /// (e.g. `String: Borrow<str>`), you can use `iter().any`:
    ///
    /// ```
    /// let v = [String::from("hello"), String::from("world")]; // slice of `String`
    /// assert!(v.iter().any(|e| e == "hello")); // search with `&str`
    /// assert!(!v.iter().any(|e| e == "hi"));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn contains(&self, x: &T) -> bool
    where
        T: PartialEq,
    {
        x.slice_contains(self)
    }

    /// Returns `true` if `needle` is a prefix of the slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = [10, 40, 30];
    /// assert!(v.starts_with(&[10]));
    /// assert!(v.starts_with(&[10, 40]));
    /// assert!(!v.starts_with(&[50]));
    /// assert!(!v.starts_with(&[10, 50]));
    /// ```
    ///
    /// Always returns `true` if `needle` is an empty slice:
    ///
    /// ```
    /// let v = &[10, 40, 30];
    /// assert!(v.starts_with(&[]));
    /// let v: &[u8] = &[];
    /// assert!(v.starts_with(&[]));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn starts_with(&self, needle: &[T]) -> bool
    where
        T: PartialEq,
    {
        let n = needle.len();
        self.len() >= n && needle == &self[..n]
    }

    /// Returns `true` if `needle` is a suffix of the slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let v = [10, 40, 30];
    /// assert!(v.ends_with(&[30]));
    /// assert!(v.ends_with(&[40, 30]));
    /// assert!(!v.ends_with(&[50]));
    /// assert!(!v.ends_with(&[50, 30]));
    /// ```
    ///
    /// Always returns `true` if `needle` is an empty slice:
    ///
    /// ```
    /// let v = &[10, 40, 30];
    /// assert!(v.ends_with(&[]));
    /// let v: &[u8] = &[];
    /// assert!(v.ends_with(&[]));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn ends_with(&self, needle: &[T]) -> bool
    where
        T: PartialEq,
    {
        let (m, n) = (self.len(), needle.len());
        m >= n && needle == &self[m - n..]
    }

    /// Returns a subslice with the prefix removed.
    ///
    /// This method returns [`None`] if slice does not start with `prefix`.
    /// Also it returns the original slice if `prefix` is an empty slice.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_strip)]
    /// let v = &[10, 40, 30];
    /// assert_eq!(v.strip_prefix(&[10]), Some(&[40, 30][..]));
    /// assert_eq!(v.strip_prefix(&[10, 40]), Some(&[30][..]));
    /// assert_eq!(v.strip_prefix(&[50]), None);
    /// assert_eq!(v.strip_prefix(&[10, 50]), None);
    /// ```
    #[must_use = "returns the subslice without modifying the original"]
    #[unstable(feature = "slice_strip", issue = "73413")]
    pub fn strip_prefix(&self, prefix: &[T]) -> Option<&[T]>
    where
        T: PartialEq,
    {
        let n = prefix.len();
        if n <= self.len() {
            let (head, tail) = self.split_at(n);
            if head == prefix {
                return Some(tail);
            }
        }
        None
    }

    /// Returns a subslice with the suffix removed.
    ///
    /// This method returns [`None`] if slice does not end with `suffix`.
    /// Also it returns the original slice if `suffix` is an empty slice
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_strip)]
    /// let v = &[10, 40, 30];
    /// assert_eq!(v.strip_suffix(&[30]), Some(&[10, 40][..]));
    /// assert_eq!(v.strip_suffix(&[40, 30]), Some(&[10][..]));
    /// assert_eq!(v.strip_suffix(&[50]), None);
    /// assert_eq!(v.strip_suffix(&[50, 30]), None);
    /// ```
    #[must_use = "returns the subslice without modifying the original"]
    #[unstable(feature = "slice_strip", issue = "73413")]
    pub fn strip_suffix(&self, suffix: &[T]) -> Option<&[T]>
    where
        T: PartialEq,
    {
        let (len, n) = (self.len(), suffix.len());
        if n <= len {
            let (head, tail) = self.split_at(len - n);
            if tail == suffix {
                return Some(head);
            }
        }
        None
    }

    /// Binary searches this sorted slice 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.
    ///
    /// # 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]`.
    ///
    /// ```
    /// let s = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55];
    ///
    /// assert_eq!(s.binary_search(&13),  Ok(9));
    /// assert_eq!(s.binary_search(&4),   Err(7));
    /// assert_eq!(s.binary_search(&100), Err(13));
    /// let r = s.binary_search(&1);
    /// assert!(match r { Ok(1..=4) => true, _ => false, });
    /// ```
    ///
    /// If you want to insert an item to a sorted vector, while maintaining
    /// sort order:
    ///
    /// ```
    /// let mut s = vec![0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55];
    /// let num = 42;
    /// let idx = s.binary_search(&num).unwrap_or_else(|x| x);
    /// s.insert(idx, num);
    /// assert_eq!(s, [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 42, 55]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn binary_search(&self, x: &T) -> Result<usize, usize>
    where
        T: Ord,
    {
        self.binary_search_by(|p| p.cmp(x))
    }

    /// Binary searches this sorted slice with a comparator function.
    ///
    /// The comparator function should implement an order consistent
    /// with the sort order of the underlying slice, returning an
    /// order code that indicates whether its argument is `Less`,
    /// `Equal` or `Greater` 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.
    ///
    /// # 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]`.
    ///
    /// ```
    /// let s = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55];
    ///
    /// let seek = 13;
    /// assert_eq!(s.binary_search_by(|probe| probe.cmp(&seek)), Ok(9));
    /// let seek = 4;
    /// assert_eq!(s.binary_search_by(|probe| probe.cmp(&seek)), Err(7));
    /// let seek = 100;
    /// assert_eq!(s.binary_search_by(|probe| probe.cmp(&seek)), Err(13));
    /// let seek = 1;
    /// let r = s.binary_search_by(|probe| probe.cmp(&seek));
    /// assert!(match r { Ok(1..=4) => true, _ => false, });
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    pub fn binary_search_by<'a, F>(&'a self, mut f: F) -> Result<usize, usize>
    where
        F: FnMut(&'a T) -> Ordering,
    {
        let s = self;
        let mut size = s.len();
        if size == 0 {
            return Err(0);
        }
        let mut base = 0usize;
        while size > 1 {
            let half = size / 2;
            let mid = base + half;
            // SAFETY: the call is made safe by the following inconstants:
            // - `mid >= 0`: by definition
            // - `mid < size`: `mid = size / 2 + size / 4 + size / 8 ...`
            let cmp = f(unsafe { s.get_unchecked(mid) });
            base = if cmp == Greater { base } else { mid };
            size -= half;
        }
        // SAFETY: base is always in [0, size) because base <= mid.
        let cmp = f(unsafe { s.get_unchecked(base) });
        if cmp == Equal { Ok(base) } else { Err(base + (cmp == Less) as usize) }
    }

    /// Binary searches this sorted slice with a key extraction function.
    ///
    /// Assumes that the slice is sorted by the key, for instance with
    /// [`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.
    ///
    /// [`sort_by_key`]: #method.sort_by_key
    ///
    /// # 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]`.
    ///
    /// ```
    /// let s = [(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)];
    ///
    /// assert_eq!(s.binary_search_by_key(&13, |&(a,b)| b),  Ok(9));
    /// assert_eq!(s.binary_search_by_key(&4, |&(a,b)| b),   Err(7));
    /// assert_eq!(s.binary_search_by_key(&100, |&(a,b)| b), Err(13));
    /// let r = s.binary_search_by_key(&1, |&(a,b)| b);
    /// assert!(match r { Ok(1..=4) => true, _ => false, });
    /// ```
    #[stable(feature = "slice_binary_search_by_key", since = "1.10.0")]
    #[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))
    }

    /// Sorts the slice, but may not preserve the order of equal elements.
    ///
    /// This sort is unstable (i.e., may reorder equal elements), in-place
    /// (i.e., does not allocate), and *O*(*n* \* log(*n*)) worst-case.
    ///
    /// # Current implementation
    ///
    /// The current algorithm is based on [pattern-defeating quicksort][pdqsort] by Orson Peters,
    /// which combines the fast average case of randomized quicksort with the fast worst case of
    /// heapsort, while achieving linear time on slices with certain patterns. It uses some
    /// randomization to avoid degenerate cases, but with a fixed seed to always provide
    /// deterministic behavior.
    ///
    /// It is typically faster than stable sorting, except in a few special cases, e.g., when the
    /// slice consists of several concatenated sorted sequences.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = [-5, 4, 1, -3, 2];
    ///
    /// v.sort_unstable();
    /// assert!(v == [-5, -3, 1, 2, 4]);
    /// ```
    ///
    /// [pdqsort]: https://github.com/orlp/pdqsort
    #[stable(feature = "sort_unstable", since = "1.20.0")]
    #[inline]
    pub fn sort_unstable(&mut self)
    where
        T: Ord,
    {
        sort::quicksort(self, |a, b| a.lt(b));
    }

    /// Sorts the slice with a comparator function, but may not preserve the order of equal
    /// elements.
    ///
    /// This sort is unstable (i.e., may reorder equal elements), in-place
    /// (i.e., does not allocate), and *O*(*n* \* log(*n*)) worst-case.
    ///
    /// The comparator function must define a total ordering for the elements in the slice. If
    /// the ordering is not total, the order of the elements is unspecified. An order is a
    /// total order if it is (for all a, b and c):
    ///
    /// * total and antisymmetric: exactly one of a < b, a == b or a > b is true; and
    /// * transitive, a < b and b < c implies a < c. The same must hold for both == and >.
    ///
    /// For example, while [`f64`] doesn't implement [`Ord`] because `NaN != NaN`, we can use
    /// `partial_cmp` as our sort function when we know the slice doesn't contain a `NaN`.
    ///
    /// ```
    /// let mut floats = [5f64, 4.0, 1.0, 3.0, 2.0];
    /// floats.sort_unstable_by(|a, b| a.partial_cmp(b).unwrap());
    /// assert_eq!(floats, [1.0, 2.0, 3.0, 4.0, 5.0]);
    /// ```
    ///
    /// # Current implementation
    ///
    /// The current algorithm is based on [pattern-defeating quicksort][pdqsort] by Orson Peters,
    /// which combines the fast average case of randomized quicksort with the fast worst case of
    /// heapsort, while achieving linear time on slices with certain patterns. It uses some
    /// randomization to avoid degenerate cases, but with a fixed seed to always provide
    /// deterministic behavior.
    ///
    /// It is typically faster than stable sorting, except in a few special cases, e.g., when the
    /// slice consists of several concatenated sorted sequences.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = [5, 4, 1, 3, 2];
    /// v.sort_unstable_by(|a, b| a.cmp(b));
    /// assert!(v == [1, 2, 3, 4, 5]);
    ///
    /// // reverse sorting
    /// v.sort_unstable_by(|a, b| b.cmp(a));
    /// assert!(v == [5, 4, 3, 2, 1]);
    /// ```
    ///
    /// [pdqsort]: https://github.com/orlp/pdqsort
    #[stable(feature = "sort_unstable", since = "1.20.0")]
    #[inline]
    pub fn sort_unstable_by<F>(&mut self, mut compare: F)
    where
        F: FnMut(&T, &T) -> Ordering,
    {
        sort::quicksort(self, |a, b| compare(a, b) == Ordering::Less);
    }

    /// Sorts the slice with a key extraction function, but may not preserve the order of equal
    /// elements.
    ///
    /// This sort is unstable (i.e., may reorder equal elements), in-place
    /// (i.e., does not allocate), and *O*(m \* *n* \* log(*n*)) worst-case, where the key function is
    /// *O*(*m*).
    ///
    /// # Current implementation
    ///
    /// The current algorithm is based on [pattern-defeating quicksort][pdqsort] by Orson Peters,
    /// which combines the fast average case of randomized quicksort with the fast worst case of
    /// heapsort, while achieving linear time on slices with certain patterns. It uses some
    /// randomization to avoid degenerate cases, but with a fixed seed to always provide
    /// deterministic behavior.
    ///
    /// Due to its key calling strategy, [`sort_unstable_by_key`](#method.sort_unstable_by_key)
    /// is likely to be slower than [`sort_by_cached_key`](#method.sort_by_cached_key) in
    /// cases where the key function is expensive.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = [-5i32, 4, 1, -3, 2];
    ///
    /// v.sort_unstable_by_key(|k| k.abs());
    /// assert!(v == [1, 2, -3, 4, -5]);
    /// ```
    ///
    /// [pdqsort]: https://github.com/orlp/pdqsort
    #[stable(feature = "sort_unstable", since = "1.20.0")]
    #[inline]
    pub fn sort_unstable_by_key<K, F>(&mut self, mut f: F)
    where
        F: FnMut(&T) -> K,
        K: Ord,
    {
        sort::quicksort(self, |a, b| f(a).lt(&f(b)));
    }

    /// Reorder the slice such that the element at `index` is at its final sorted position.
    ///
    /// This reordering has the additional property that any value at position `i < index` will be
    /// less than or equal to any value at a position `j > index`. Additionally, this reordering is
    /// unstable (i.e. any number of equal elements may end up at position `index`), in-place
    /// (i.e. does not allocate), and *O*(*n*) worst-case. This function is also/ known as "kth
    /// element" in other libraries. It returns a triplet of the following values: all elements less
    /// than the one at the given index, the value at the given index, and all elements greater than
    /// the one at the given index.
    ///
    /// # Current implementation
    ///
    /// The current algorithm is based on the quickselect portion of the same quicksort algorithm
    /// used for [`sort_unstable`].
    ///
    /// [`sort_unstable`]: #method.sort_unstable
    ///
    /// # Panics
    ///
    /// Panics when `index >= len()`, meaning it always panics on empty slices.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_partition_at_index)]
    ///
    /// let mut v = [-5i32, 4, 1, -3, 2];
    ///
    /// // Find the median
    /// v.partition_at_index(2);
    ///
    /// // We are only guaranteed the slice will be one of the following, based on the way we sort
    /// // about the specified index.
    /// assert!(v == [-3, -5, 1, 2, 4] ||
    ///         v == [-5, -3, 1, 2, 4] ||
    ///         v == [-3, -5, 1, 4, 2] ||
    ///         v == [-5, -3, 1, 4, 2]);
    /// ```
    #[unstable(feature = "slice_partition_at_index", issue = "55300")]
    #[inline]
    pub fn partition_at_index(&mut self, index: usize) -> (&mut [T], &mut T, &mut [T])
    where
        T: Ord,
    {
        let mut f = |a: &T, b: &T| a.lt(b);
        sort::partition_at_index(self, index, &mut f)
    }

    /// Reorder the slice with a comparator function such that the element at `index` is at its
    /// final sorted position.
    ///
    /// This reordering has the additional property that any value at position `i < index` will be
    /// less than or equal to any value at a position `j > index` using the comparator function.
    /// Additionally, this reordering is unstable (i.e. any number of equal elements may end up at
    /// position `index`), in-place (i.e. does not allocate), and *O*(*n*) worst-case. This function
    /// is also known as "kth element" in other libraries. It returns a triplet of the following
    /// values: all elements less than the one at the given index, the value at the given index,
    /// and all elements greater than the one at the given index, using the provided comparator
    /// function.
    ///
    /// # Current implementation
    ///
    /// The current algorithm is based on the quickselect portion of the same quicksort algorithm
    /// used for [`sort_unstable`].
    ///
    /// [`sort_unstable`]: #method.sort_unstable
    ///
    /// # Panics
    ///
    /// Panics when `index >= len()`, meaning it always panics on empty slices.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_partition_at_index)]
    ///
    /// let mut v = [-5i32, 4, 1, -3, 2];
    ///
    /// // Find the median as if the slice were sorted in descending order.
    /// v.partition_at_index_by(2, |a, b| b.cmp(a));
    ///
    /// // We are only guaranteed the slice will be one of the following, based on the way we sort
    /// // about the specified index.
    /// assert!(v == [2, 4, 1, -5, -3] ||
    ///         v == [2, 4, 1, -3, -5] ||
    ///         v == [4, 2, 1, -5, -3] ||
    ///         v == [4, 2, 1, -3, -5]);
    /// ```
    #[unstable(feature = "slice_partition_at_index", issue = "55300")]
    #[inline]
    pub fn partition_at_index_by<F>(
        &mut self,
        index: usize,
        mut compare: F,
    ) -> (&mut [T], &mut T, &mut [T])
    where
        F: FnMut(&T, &T) -> Ordering,
    {
        let mut f = |a: &T, b: &T| compare(a, b) == Less;
        sort::partition_at_index(self, index, &mut f)
    }

    /// Reorder the slice with a key extraction function such that the element at `index` is at its
    /// final sorted position.
    ///
    /// This reordering has the additional property that any value at position `i < index` will be
    /// less than or equal to any value at a position `j > index` using the key extraction function.
    /// Additionally, this reordering is unstable (i.e. any number of equal elements may end up at
    /// position `index`), in-place (i.e. does not allocate), and *O*(*n*) worst-case. This function
    /// is also known as "kth element" in other libraries. It returns a triplet of the following
    /// values: all elements less than the one at the given index, the value at the given index, and
    /// all elements greater than the one at the given index, using the provided key extraction
    /// function.
    ///
    /// # Current implementation
    ///
    /// The current algorithm is based on the quickselect portion of the same quicksort algorithm
    /// used for [`sort_unstable`].
    ///
    /// [`sort_unstable`]: #method.sort_unstable
    ///
    /// # Panics
    ///
    /// Panics when `index >= len()`, meaning it always panics on empty slices.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_partition_at_index)]
    ///
    /// let mut v = [-5i32, 4, 1, -3, 2];
    ///
    /// // Return the median as if the array were sorted according to absolute value.
    /// v.partition_at_index_by_key(2, |a| a.abs());
    ///
    /// // We are only guaranteed the slice will be one of the following, based on the way we sort
    /// // about the specified index.
    /// assert!(v == [1, 2, -3, 4, -5] ||
    ///         v == [1, 2, -3, -5, 4] ||
    ///         v == [2, 1, -3, 4, -5] ||
    ///         v == [2, 1, -3, -5, 4]);
    /// ```
    #[unstable(feature = "slice_partition_at_index", issue = "55300")]
    #[inline]
    pub fn partition_at_index_by_key<K, F>(
        &mut self,
        index: usize,
        mut f: F,
    ) -> (&mut [T], &mut T, &mut [T])
    where
        F: FnMut(&T) -> K,
        K: Ord,
    {
        let mut g = |a: &T, b: &T| f(a).lt(&f(b));
        sort::partition_at_index(self, index, &mut g)
    }

    /// Moves all consecutive repeated elements to the end of the slice according to the
    /// [`PartialEq`] trait implementation.
    ///
    /// Returns two slices. The first contains no consecutive repeated elements.
    /// The second contains all the duplicates in no specified order.
    ///
    /// If the slice is sorted, the first returned slice contains no duplicates.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_partition_dedup)]
    ///
    /// let mut slice = [1, 2, 2, 3, 3, 2, 1, 1];
    ///
    /// let (dedup, duplicates) = slice.partition_dedup();
    ///
    /// assert_eq!(dedup, [1, 2, 3, 2, 1]);
    /// assert_eq!(duplicates, [2, 3, 1]);
    /// ```
    #[unstable(feature = "slice_partition_dedup", issue = "54279")]
    #[inline]
    pub fn partition_dedup(&mut self) -> (&mut [T], &mut [T])
    where
        T: PartialEq,
    {
        self.partition_dedup_by(|a, b| a == b)
    }

    /// Moves all but the first of consecutive elements to the end of the slice satisfying
    /// a given equality relation.
    ///
    /// Returns two slices. The first contains no consecutive repeated elements.
    /// The second contains all the duplicates in no specified order.
    ///
    /// The `same_bucket` function is passed references to two elements from the slice and
    /// must determine if the elements compare equal. The elements are passed in opposite order
    /// from their order in the slice, so if `same_bucket(a, b)` returns `true`, `a` is moved
    /// at the end of the slice.
    ///
    /// If the slice is sorted, the first returned slice contains no duplicates.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_partition_dedup)]
    ///
    /// let mut slice = ["foo", "Foo", "BAZ", "Bar", "bar", "baz", "BAZ"];
    ///
    /// let (dedup, duplicates) = slice.partition_dedup_by(|a, b| a.eq_ignore_ascii_case(b));
    ///
    /// assert_eq!(dedup, ["foo", "BAZ", "Bar", "baz"]);
    /// assert_eq!(duplicates, ["bar", "Foo", "BAZ"]);
    /// ```
    #[unstable(feature = "slice_partition_dedup", issue = "54279")]
    #[inline]
    pub fn partition_dedup_by<F>(&mut self, mut same_bucket: F) -> (&mut [T], &mut [T])
    where
        F: FnMut(&mut T, &mut T) -> bool,
    {
        // Although we have a mutable reference to `self`, we cannot make
        // *arbitrary* changes. The `same_bucket` calls could panic, so we
        // must ensure that the slice is in a valid state at all times.
        //
        // The way that we handle this is by using swaps; we iterate
        // over all the elements, swapping as we go so that at the end
        // the elements we wish to keep are in the front, and those we
        // wish to reject are at the back. We can then split the slice.
        // This operation is still `O(n)`.
        //
        // Example: We start in this state, where `r` represents "next
        // read" and `w` represents "next_write`.
        //
        //           r
        //     +---+---+---+---+---+---+
        //     | 0 | 1 | 1 | 2 | 3 | 3 |
        //     +---+---+---+---+---+---+
        //           w
        //
        // Comparing self[r] against self[w-1], this is not a duplicate, so
        // we swap self[r] and self[w] (no effect as r==w) and then increment both
        // r and w, leaving us with:
        //
        //               r
        //     +---+---+---+---+---+---+
        //     | 0 | 1 | 1 | 2 | 3 | 3 |
        //     +---+---+---+---+---+---+
        //               w
        //
        // Comparing self[r] against self[w-1], this value is a duplicate,
        // so we increment `r` but leave everything else unchanged:
        //
        //                   r
        //     +---+---+---+---+---+---+
        //     | 0 | 1 | 1 | 2 | 3 | 3 |
        //     +---+---+---+---+---+---+
        //               w
        //
        // Comparing self[r] against self[w-1], this is not a duplicate,
        // so swap self[r] and self[w] and advance r and w:
        //
        //                       r
        //     +---+---+---+---+---+---+
        //     | 0 | 1 | 2 | 1 | 3 | 3 |
        //     +---+---+---+---+---+---+
        //                   w
        //
        // Not a duplicate, repeat:
        //
        //                           r
        //     +---+---+---+---+---+---+
        //     | 0 | 1 | 2 | 3 | 1 | 3 |
        //     +---+---+---+---+---+---+
        //                       w
        //
        // Duplicate, advance r. End of slice. Split at w.

        let len = self.len();
        if len <= 1 {
            return (self, &mut []);
        }

        let ptr = self.as_mut_ptr();
        let mut next_read: usize = 1;
        let mut next_write: usize = 1;

        // SAFETY: the `while` condition guarantees `next_read` and `next_write`
        // are less than `len`, thus are inside `self`. `prev_ptr_write` points to
        // one element before `ptr_write`, but `next_write` starts at 1, so
        // `prev_ptr_write` is never less than 0 and is inside the slice.
        // This fulfils the requirements for dereferencing `ptr_read`, `prev_ptr_write`
        // and `ptr_write`, and for using `ptr.add(next_read)`, `ptr.add(next_write - 1)`
        // and `prev_ptr_write.offset(1)`.
        //
        // `next_write` is also incremented at most once per loop at most meaning
        // no element is skipped when it may need to be swapped.
        //
        // `ptr_read` and `prev_ptr_write` never point to the same element. This
        // is required for `&mut *ptr_read`, `&mut *prev_ptr_write` to be safe.
        // The explanation is simply that `next_read >= next_write` is always true,
        // thus `next_read > next_write - 1` is too.
        unsafe {
            // Avoid bounds checks by using raw pointers.
            while next_read < len {
                let ptr_read = ptr.add(next_read);
                let prev_ptr_write = ptr.add(next_write - 1);
                if !same_bucket(&mut *ptr_read, &mut *prev_ptr_write) {
                    if next_read != next_write {
                        let ptr_write = prev_ptr_write.offset(1);
                        mem::swap(&mut *ptr_read, &mut *ptr_write);
                    }
                    next_write += 1;
                }
                next_read += 1;
            }
        }

        self.split_at_mut(next_write)
    }

    /// Moves all but the first of consecutive elements to the end of the slice that resolve
    /// to the same key.
    ///
    /// Returns two slices. The first contains no consecutive repeated elements.
    /// The second contains all the duplicates in no specified order.
    ///
    /// If the slice is sorted, the first returned slice contains no duplicates.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_partition_dedup)]
    ///
    /// let mut slice = [10, 20, 21, 30, 30, 20, 11, 13];
    ///
    /// let (dedup, duplicates) = slice.partition_dedup_by_key(|i| *i / 10);
    ///
    /// assert_eq!(dedup, [10, 20, 30, 20, 11]);
    /// assert_eq!(duplicates, [21, 30, 13]);
    /// ```
    #[unstable(feature = "slice_partition_dedup", issue = "54279")]
    #[inline]
    pub fn partition_dedup_by_key<K, F>(&mut self, mut key: F) -> (&mut [T], &mut [T])
    where
        F: FnMut(&mut T) -> K,
        K: PartialEq,
    {
        self.partition_dedup_by(|a, b| key(a) == key(b))
    }

    /// Rotates the slice in-place such that the first `mid` elements of the
    /// slice move to the end while the last `self.len() - mid` elements move to
    /// the front. After calling `rotate_left`, the element previously at index
    /// `mid` will become the first element in the slice.
    ///
    /// # Panics
    ///
    /// This function will panic if `mid` is greater than the length of the
    /// slice. Note that `mid == self.len()` does _not_ panic and is a no-op
    /// rotation.
    ///
    /// # Complexity
    ///
    /// Takes linear (in `self.len()`) time.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut a = ['a', 'b', 'c', 'd', 'e', 'f'];
    /// a.rotate_left(2);
    /// assert_eq!(a, ['c', 'd', 'e', 'f', 'a', 'b']);
    /// ```
    ///
    /// Rotating a subslice:
    ///
    /// ```
    /// let mut a = ['a', 'b', 'c', 'd', 'e', 'f'];
    /// a[1..5].rotate_left(1);
    /// assert_eq!(a, ['a', 'c', 'd', 'e', 'b', 'f']);
    /// ```
    #[stable(feature = "slice_rotate", since = "1.26.0")]
    pub fn rotate_left(&mut self, mid: usize) {
        assert!(mid <= self.len());
        let k = self.len() - mid;
        let p = self.as_mut_ptr();

        // SAFETY: The range `[p.add(mid) - mid, p.add(mid) + k)` is trivially
        // valid for reading and writing, as required by `ptr_rotate`.
        unsafe {
            rotate::ptr_rotate(mid, p.add(mid), k);
        }
    }

    /// Rotates the slice in-place such that the first `self.len() - k`
    /// elements of the slice move to the end while the last `k` elements move
    /// to the front. After calling `rotate_right`, the element previously at
    /// index `self.len() - k` will become the first element in the slice.
    ///
    /// # Panics
    ///
    /// This function will panic if `k` is greater than the length of the
    /// slice. Note that `k == self.len()` does _not_ panic and is a no-op
    /// rotation.
    ///
    /// # Complexity
    ///
    /// Takes linear (in `self.len()`) time.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut a = ['a', 'b', 'c', 'd', 'e', 'f'];
    /// a.rotate_right(2);
    /// assert_eq!(a, ['e', 'f', 'a', 'b', 'c', 'd']);
    /// ```
    ///
    /// Rotate a subslice:
    ///
    /// ```
    /// let mut a = ['a', 'b', 'c', 'd', 'e', 'f'];
    /// a[1..5].rotate_right(1);
    /// assert_eq!(a, ['a', 'e', 'b', 'c', 'd', 'f']);
    /// ```
    #[stable(feature = "slice_rotate", since = "1.26.0")]
    pub fn rotate_right(&mut self, k: usize) {
        assert!(k <= self.len());
        let mid = self.len() - k;
        let p = self.as_mut_ptr();

        // SAFETY: The range `[p.add(mid) - mid, p.add(mid) + k)` is trivially
        // valid for reading and writing, as required by `ptr_rotate`.
        unsafe {
            rotate::ptr_rotate(mid, p.add(mid), k);
        }
    }

    /// Fills `self` with elements by cloning `value`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_fill)]
    ///
    /// let mut buf = vec![0; 10];
    /// buf.fill(1);
    /// assert_eq!(buf, vec![1; 10]);
    /// ```
    #[unstable(feature = "slice_fill", issue = "70758")]
    pub fn fill(&mut self, value: T)
    where
        T: Clone,
    {
        if let Some((last, elems)) = self.split_last_mut() {
            for el in elems {
                el.clone_from(&value);
            }

            *last = value
        }
    }

    /// Copies the elements from `src` into `self`.
    ///
    /// The length of `src` must be the same as `self`.
    ///
    /// If `T` implements `Copy`, it can be more performant to use
    /// [`copy_from_slice`].
    ///
    /// # Panics
    ///
    /// This function will panic if the two slices have different lengths.
    ///
    /// # Examples
    ///
    /// Cloning two elements from a slice into another:
    ///
    /// ```
    /// let src = [1, 2, 3, 4];
    /// let mut dst = [0, 0];
    ///
    /// // Because the slices have to be the same length,
    /// // we slice the source slice from four elements
    /// // to two. It will panic if we don't do this.
    /// dst.clone_from_slice(&src[2..]);
    ///
    /// assert_eq!(src, [1, 2, 3, 4]);
    /// assert_eq!(dst, [3, 4]);
    /// ```
    ///
    /// Rust enforces that there can only be one mutable reference with no
    /// immutable references to a particular piece of data in a particular
    /// scope. Because of this, attempting to use `clone_from_slice` on a
    /// single slice will result in a compile failure:
    ///
    /// ```compile_fail
    /// let mut slice = [1, 2, 3, 4, 5];
    ///
    /// slice[..2].clone_from_slice(&slice[3..]); // compile fail!
    /// ```
    ///
    /// To work around this, we can use [`split_at_mut`] to create two distinct
    /// sub-slices from a slice:
    ///
    /// ```
    /// let mut slice = [1, 2, 3, 4, 5];
    ///
    /// {
    ///     let (left, right) = slice.split_at_mut(2);
    ///     left.clone_from_slice(&right[1..]);
    /// }
    ///
    /// assert_eq!(slice, [4, 5, 3, 4, 5]);
    /// ```
    ///
    /// [`copy_from_slice`]: #method.copy_from_slice
    /// [`split_at_mut`]: #method.split_at_mut
    #[stable(feature = "clone_from_slice", since = "1.7.0")]
    pub fn clone_from_slice(&mut self, src: &[T])
    where
        T: Clone,
    {
        assert!(self.len() == src.len(), "destination and source slices have different lengths");
        // NOTE: We need to explicitly slice them to the same length
        // for bounds checking to be elided, and the optimizer will
        // generate memcpy for simple cases (for example T = u8).
        let len = self.len();
        let src = &src[..len];
        for i in 0..len {
            self[i].clone_from(&src[i]);
        }
    }

    /// Copies all elements from `src` into `self`, using a memcpy.
    ///
    /// The length of `src` must be the same as `self`.
    ///
    /// If `T` does not implement `Copy`, use [`clone_from_slice`].
    ///
    /// # Panics
    ///
    /// This function will panic if the two slices have different lengths.
    ///
    /// # Examples
    ///
    /// Copying two elements from a slice into another:
    ///
    /// ```
    /// let src = [1, 2, 3, 4];
    /// let mut dst = [0, 0];
    ///
    /// // Because the slices have to be the same length,
    /// // we slice the source slice from four elements
    /// // to two. It will panic if we don't do this.
    /// dst.copy_from_slice(&src[2..]);
    ///
    /// assert_eq!(src, [1, 2, 3, 4]);
    /// assert_eq!(dst, [3, 4]);
    /// ```
    ///
    /// Rust enforces that there can only be one mutable reference with no
    /// immutable references to a particular piece of data in a particular
    /// scope. Because of this, attempting to use `copy_from_slice` on a
    /// single slice will result in a compile failure:
    ///
    /// ```compile_fail
    /// let mut slice = [1, 2, 3, 4, 5];
    ///
    /// slice[..2].copy_from_slice(&slice[3..]); // compile fail!
    /// ```
    ///
    /// To work around this, we can use [`split_at_mut`] to create two distinct
    /// sub-slices from a slice:
    ///
    /// ```
    /// let mut slice = [1, 2, 3, 4, 5];
    ///
    /// {
    ///     let (left, right) = slice.split_at_mut(2);
    ///     left.copy_from_slice(&right[1..]);
    /// }
    ///
    /// assert_eq!(slice, [4, 5, 3, 4, 5]);
    /// ```
    ///
    /// [`clone_from_slice`]: #method.clone_from_slice
    /// [`split_at_mut`]: #method.split_at_mut
    #[stable(feature = "copy_from_slice", since = "1.9.0")]
    pub fn copy_from_slice(&mut self, src: &[T])
    where
        T: Copy,
    {
        // The panic code path was put into a cold function to not bloat the
        // call site.
        #[inline(never)]
        #[cold]
        #[track_caller]
        fn len_mismatch_fail(dst_len: usize, src_len: usize) -> ! {
            panic!(
                "source slice length ({}) does not match destination slice length ({})",
                src_len, dst_len,
            );
        }

        if self.len() != src.len() {
            len_mismatch_fail(self.len(), src.len());
        }

        // SAFETY: `self` is valid for `self.len()` elements by definition, and `src` was
        // checked to have the same length. The slices cannot overlap because
        // mutable references are exclusive.
        unsafe {
            ptr::copy_nonoverlapping(src.as_ptr(), self.as_mut_ptr(), self.len());
        }
    }

    /// Copies elements from one part of the slice to another part of itself,
    /// using a memmove.
    ///
    /// `src` is the range within `self` to copy from. `dest` is the starting
    /// index of the range within `self` to copy to, which will have the same
    /// length as `src`. The two ranges may overlap. The ends of the two ranges
    /// must be less than or equal to `self.len()`.
    ///
    /// # Panics
    ///
    /// This function will panic if either range exceeds the end of the slice,
    /// or if the end of `src` is before the start.
    ///
    /// # Examples
    ///
    /// Copying four bytes within a slice:
    ///
    /// ```
    /// let mut bytes = *b"Hello, World!";
    ///
    /// bytes.copy_within(1..5, 8);
    ///
    /// assert_eq!(&bytes, b"Hello, Wello!");
    /// ```
    #[stable(feature = "copy_within", since = "1.37.0")]
    #[track_caller]
    pub fn copy_within<R: RangeBounds<usize>>(&mut self, src: R, dest: usize)
    where
        T: Copy,
    {
        let Range { start: src_start, end: src_end } = self.check_range(src);
        let count = src_end - src_start;
        assert!(dest <= self.len() - count, "dest is out of bounds");
        // SAFETY: the conditions for `ptr::copy` have all been checked above,
        // as have those for `ptr::add`.
        unsafe {
            ptr::copy(self.as_ptr().add(src_start), self.as_mut_ptr().add(dest), count);
        }
    }

    /// Swaps all elements in `self` with those in `other`.
    ///
    /// The length of `other` must be the same as `self`.
    ///
    /// # Panics
    ///
    /// This function will panic if the two slices have different lengths.
    ///
    /// # Example
    ///
    /// Swapping two elements across slices:
    ///
    /// ```
    /// let mut slice1 = [0, 0];
    /// let mut slice2 = [1, 2, 3, 4];
    ///
    /// slice1.swap_with_slice(&mut slice2[2..]);
    ///
    /// assert_eq!(slice1, [3, 4]);
    /// assert_eq!(slice2, [1, 2, 0, 0]);
    /// ```
    ///
    /// Rust enforces that there can only be one mutable reference to a
    /// particular piece of data in a particular scope. Because of this,
    /// attempting to use `swap_with_slice` on a single slice will result in
    /// a compile failure:
    ///
    /// ```compile_fail
    /// let mut slice = [1, 2, 3, 4, 5];
    /// slice[..2].swap_with_slice(&mut slice[3..]); // compile fail!
    /// ```
    ///
    /// To work around this, we can use [`split_at_mut`] to create two distinct
    /// mutable sub-slices from a slice:
    ///
    /// ```
    /// let mut slice = [1, 2, 3, 4, 5];
    ///
    /// {
    ///     let (left, right) = slice.split_at_mut(2);
    ///     left.swap_with_slice(&mut right[1..]);
    /// }
    ///
    /// assert_eq!(slice, [4, 5, 3, 1, 2]);
    /// ```
    ///
    /// [`split_at_mut`]: #method.split_at_mut
    #[stable(feature = "swap_with_slice", since = "1.27.0")]
    pub fn swap_with_slice(&mut self, other: &mut [T]) {
        assert!(self.len() == other.len(), "destination and source slices have different lengths");
        // SAFETY: `self` is valid for `self.len()` elements by definition, and `src` was
        // checked to have the same length. The slices cannot overlap because
        // mutable references are exclusive.
        unsafe {
            ptr::swap_nonoverlapping(self.as_mut_ptr(), other.as_mut_ptr(), self.len());
        }
    }

    /// Function to calculate lengths of the middle and trailing slice for `align_to{,_mut}`.
    fn align_to_offsets<U>(&self) -> (usize, usize) {
        // What we gonna do about `rest` is figure out what multiple of `U`s we can put in a
        // lowest number of `T`s. And how many `T`s we need for each such "multiple".
        //
        // Consider for example T=u8 U=u16. Then we can put 1 U in 2 Ts. Simple. Now, consider
        // for example a case where size_of::<T> = 16, size_of::<U> = 24. We can put 2 Us in
        // place of every 3 Ts in the `rest` slice. A bit more complicated.
        //
        // Formula to calculate this is:
        //
        // Us = lcm(size_of::<T>, size_of::<U>) / size_of::<U>
        // Ts = lcm(size_of::<T>, size_of::<U>) / size_of::<T>
        //
        // Expanded and simplified:
        //
        // Us = size_of::<T> / gcd(size_of::<T>, size_of::<U>)
        // Ts = size_of::<U> / gcd(size_of::<T>, size_of::<U>)
        //
        // Luckily since all this is constant-evaluated... performance here matters not!
        #[inline]
        fn gcd(a: usize, b: usize) -> usize {
            use crate::intrinsics;
            // iterative stein’s algorithm
            // We should still make this `const fn` (and revert to recursive algorithm if we do)
            // because relying on llvm to consteval all this is… well, it makes me uncomfortable.

            // SAFETY: `a` and `b` are checked to be non-zero values.
            let (ctz_a, mut ctz_b) = unsafe {
                if a == 0 {
                    return b;
                }
                if b == 0 {
                    return a;
                }
                (intrinsics::cttz_nonzero(a), intrinsics::cttz_nonzero(b))
            };
            let k = ctz_a.min(ctz_b);
            let mut a = a >> ctz_a;
            let mut b = b;
            loop {
                // remove all factors of 2 from b
                b >>= ctz_b;
                if a > b {
                    mem::swap(&mut a, &mut b);
                }
                b = b - a;
                // SAFETY: `b` is checked to be non-zero.
                unsafe {
                    if b == 0 {
                        break;
                    }
                    ctz_b = intrinsics::cttz_nonzero(b);
                }
            }
            a << k
        }
        let gcd: usize = gcd(mem::size_of::<T>(), mem::size_of::<U>());
        let ts: usize = mem::size_of::<U>() / gcd;
        let us: usize = mem::size_of::<T>() / gcd;

        // Armed with this knowledge, we can find how many `U`s we can fit!
        let us_len = self.len() / ts * us;
        // And how many `T`s will be in the trailing slice!
        let ts_len = self.len() % ts;
        (us_len, ts_len)
    }

    /// Transmute the slice to a slice of another type, ensuring alignment of the types is
    /// maintained.
    ///
    /// This method splits the slice into three distinct slices: prefix, correctly aligned middle
    /// slice of a new type, and the suffix slice. The method may make the middle slice the greatest
    /// length possible for a given type and input slice, but only your algorithm's performance
    /// should depend on that, not its correctness. It is permissible for all of the input data to
    /// be returned as the prefix or suffix slice.
    ///
    /// This method has no purpose when either input element `T` or output element `U` are
    /// zero-sized and will return the original slice without splitting anything.
    ///
    /// # Safety
    ///
    /// This method is essentially a `transmute` with respect to the elements in the returned
    /// middle slice, so all the usual caveats pertaining to `transmute::<T, U>` also apply here.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// unsafe {
    ///     let bytes: [u8; 7] = [1, 2, 3, 4, 5, 6, 7];
    ///     let (prefix, shorts, suffix) = bytes.align_to::<u16>();
    ///     // less_efficient_algorithm_for_bytes(prefix);
    ///     // more_efficient_algorithm_for_aligned_shorts(shorts);
    ///     // less_efficient_algorithm_for_bytes(suffix);
    /// }
    /// ```
    #[stable(feature = "slice_align_to", since = "1.30.0")]
    pub unsafe fn align_to<U>(&self) -> (&[T], &[U], &[T]) {
        // Note that most of this function will be constant-evaluated,
        if mem::size_of::<U>() == 0 || mem::size_of::<T>() == 0 {
            // handle ZSTs specially, which is – don't handle them at all.
            return (self, &[], &[]);
        }

        // First, find at what point do we split between the first and 2nd slice. Easy with
        // ptr.align_offset.
        let ptr = self.as_ptr();
        // SAFETY: See the `align_to_mut` method for the detailed safety comment.
        let offset = unsafe { crate::ptr::align_offset(ptr, mem::align_of::<U>()) };
        if offset > self.len() {
            (self, &[], &[])
        } else {
            let (left, rest) = self.split_at(offset);
            let (us_len, ts_len) = rest.align_to_offsets::<U>();
            // SAFETY: now `rest` is definitely aligned, so `from_raw_parts` below is okay,
            // since the caller guarantees that we can transmute `T` to `U` safely.
            unsafe {
                (
                    left,
                    from_raw_parts(rest.as_ptr() as *const U, us_len),
                    from_raw_parts(rest.as_ptr().add(rest.len() - ts_len), ts_len),
                )
            }
        }
    }

    /// Transmute the slice to a slice of another type, ensuring alignment of the types is
    /// maintained.
    ///
    /// This method splits the slice into three distinct slices: prefix, correctly aligned middle
    /// slice of a new type, and the suffix slice. The method may make the middle slice the greatest
    /// length possible for a given type and input slice, but only your algorithm's performance
    /// should depend on that, not its correctness. It is permissible for all of the input data to
    /// be returned as the prefix or suffix slice.
    ///
    /// This method has no purpose when either input element `T` or output element `U` are
    /// zero-sized and will return the original slice without splitting anything.
    ///
    /// # Safety
    ///
    /// This method is essentially a `transmute` with respect to the elements in the returned
    /// middle slice, so all the usual caveats pertaining to `transmute::<T, U>` also apply here.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// unsafe {
    ///     let mut bytes: [u8; 7] = [1, 2, 3, 4, 5, 6, 7];
    ///     let (prefix, shorts, suffix) = bytes.align_to_mut::<u16>();
    ///     // less_efficient_algorithm_for_bytes(prefix);
    ///     // more_efficient_algorithm_for_aligned_shorts(shorts);
    ///     // less_efficient_algorithm_for_bytes(suffix);
    /// }
    /// ```
    #[stable(feature = "slice_align_to", since = "1.30.0")]
    pub unsafe fn align_to_mut<U>(&mut self) -> (&mut [T], &mut [U], &mut [T]) {
        // Note that most of this function will be constant-evaluated,
        if mem::size_of::<U>() == 0 || mem::size_of::<T>() == 0 {
            // handle ZSTs specially, which is – don't handle them at all.
            return (self, &mut [], &mut []);
        }

        // First, find at what point do we split between the first and 2nd slice. Easy with
        // ptr.align_offset.
        let ptr = self.as_ptr();
        // SAFETY: Here we are ensuring we will use aligned pointers for U for the
        // rest of the method. This is done by passing a pointer to &[T] with an
        // alignment targeted for U.
        // `crate::ptr::align_offset` is called with a correctly aligned and
        // valid pointer `ptr` (it comes from a reference to `self`) and with
        // a size that is a power of two (since it comes from the alignement for U),
        // satisfying its safety constraints.
        let offset = unsafe { crate::ptr::align_offset(ptr, mem::align_of::<U>()) };
        if offset > self.len() {
            (self, &mut [], &mut [])
        } else {
            let (left, rest) = self.split_at_mut(offset);
            let (us_len, ts_len) = rest.align_to_offsets::<U>();
            let rest_len = rest.len();
            let mut_ptr = rest.as_mut_ptr();
            // We can't use `rest` again after this, that would invalidate its alias `mut_ptr`!
            // SAFETY: see comments for `align_to`.
            unsafe {
                (
                    left,
                    from_raw_parts_mut(mut_ptr as *mut U, us_len),
                    from_raw_parts_mut(mut_ptr.add(rest_len - ts_len), ts_len),
                )
            }
        }
    }

    /// Checks if the elements of this slice are sorted.
    ///
    /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
    /// slice yields exactly zero or one element, `true` is returned.
    ///
    /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
    /// implies that this function returns `false` if any two consecutive items are not
    /// comparable.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(is_sorted)]
    /// let empty: [i32; 0] = [];
    ///
    /// assert!([1, 2, 2, 9].is_sorted());
    /// assert!(![1, 3, 2, 4].is_sorted());
    /// assert!([0].is_sorted());
    /// assert!(empty.is_sorted());
    /// assert!(![0.0, 1.0, f32::NAN].is_sorted());
    /// ```
    #[inline]
    #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
    pub fn is_sorted(&self) -> bool
    where
        T: PartialOrd,
    {
        self.is_sorted_by(|a, b| a.partial_cmp(b))
    }

    /// Checks if the elements of this slice are sorted using the given comparator function.
    ///
    /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
    /// function to determine the ordering of two elements. Apart from that, it's equivalent to
    /// [`is_sorted`]; see its documentation for more information.
    ///
    /// [`is_sorted`]: #method.is_sorted
    #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
    pub fn is_sorted_by<F>(&self, mut compare: F) -> bool
    where
        F: FnMut(&T, &T) -> Option<Ordering>,
    {
        self.iter().is_sorted_by(|a, b| compare(*a, *b))
    }

    /// Checks if the elements of this slice are sorted using the given key extraction function.
    ///
    /// Instead of comparing the slice's elements directly, this function compares the keys of the
    /// elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see its
    /// documentation for more information.
    ///
    /// [`is_sorted`]: #method.is_sorted
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(is_sorted)]
    ///
    /// assert!(["c", "bb", "aaa"].is_sorted_by_key(|s| s.len()));
    /// assert!(![-2i32, -1, 0, 3].is_sorted_by_key(|n| n.abs()));
    /// ```
    #[inline]
    #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
    pub fn is_sorted_by_key<F, K>(&self, f: F) -> bool
    where
        F: FnMut(&T) -> K,
        K: PartialOrd,
    {
        self.iter().is_sorted_by_key(f)
    }

    /// Returns the index of the partition point according to the given predicate
    /// (the index of the first element of the second partition).
    ///
    /// The slice 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 slice
    /// 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 slice is not partitioned, the returned result is unspecified and meaningless,
    /// as this method performs a kind of binary search.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(partition_point)]
    ///
    /// let v = [1, 2, 3, 3, 5, 6, 7];
    /// let i = v.partition_point(|&x| x < 5);
    ///
    /// assert_eq!(i, 4);
    /// assert!(v[..i].iter().all(|&x| x < 5));
    /// assert!(v[i..].iter().all(|&x| !(x < 5)));
    /// ```
    #[unstable(feature = "partition_point", reason = "new API", issue = "73831")]
    pub fn partition_point<P>(&self, mut pred: P) -> usize
    where
        P: FnMut(&T) -> bool,
    {
        let mut left = 0;
        let mut right = self.len();

        while left != right {
            let mid = left + (right - left) / 2;
            // SAFETY: When `left < right`, `left <= mid < right`.
            // Therefore `left` always increases and `right` always decreases,
            // and either of them is selected. In both cases `left <= right` is
            // satisfied. Therefore if `left < right` in a step, `left <= right`
            // is satisfied in the next step. Therefore as long as `left != right`,
            // `0 <= left < right <= len` is satisfied and if this case
            // `0 <= mid < len` is satisfied too.
            let value = unsafe { self.get_unchecked(mid) };
            if pred(value) {
                left = mid + 1;
            } else {
                right = mid;
            }
        }

        left
    }
}

#[lang = "slice_u8"]
#[cfg(not(test))]
impl [u8] {
    /// Checks if all bytes in this slice are within the ASCII range.
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[inline]
    pub fn is_ascii(&self) -> bool {
        is_ascii(self)
    }

    /// Checks that two slices are an ASCII case-insensitive match.
    ///
    /// Same as `to_ascii_lowercase(a) == to_ascii_lowercase(b)`,
    /// but without allocating and copying temporaries.
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[inline]
    pub fn eq_ignore_ascii_case(&self, other: &[u8]) -> bool {
        self.len() == other.len() && self.iter().zip(other).all(|(a, b)| a.eq_ignore_ascii_case(b))
    }

    /// Converts this slice to its ASCII upper case equivalent in-place.
    ///
    /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
    /// but non-ASCII letters are unchanged.
    ///
    /// To return a new uppercased value without modifying the existing one, use
    /// [`to_ascii_uppercase`].
    ///
    /// [`to_ascii_uppercase`]: #method.to_ascii_uppercase
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[inline]
    pub fn make_ascii_uppercase(&mut self) {
        for byte in self {
            byte.make_ascii_uppercase();
        }
    }

    /// Converts this slice to its ASCII lower case equivalent in-place.
    ///
    /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
    /// but non-ASCII letters are unchanged.
    ///
    /// To return a new lowercased value without modifying the existing one, use
    /// [`to_ascii_lowercase`].
    ///
    /// [`to_ascii_lowercase`]: #method.to_ascii_lowercase
    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    #[inline]
    pub fn make_ascii_lowercase(&mut self) {
        for byte in self {
            byte.make_ascii_lowercase();
        }
    }
}

/// Returns `true` if any byte in the word `v` is nonascii (>= 128). Snarfed
/// from `../str/mod.rs`, which does something similar for utf8 validation.
#[inline]
fn contains_nonascii(v: usize) -> bool {
    const NONASCII_MASK: usize = 0x80808080_80808080u64 as usize;
    (NONASCII_MASK & v) != 0
}

/// Optimized ASCII test that will use usize-at-a-time operations instead of
/// byte-at-a-time operations (when possible).
///
/// The algorithm we use here is pretty simple. If `s` is too short, we just
/// check each byte and be done with it. Otherwise:
///
/// - Read the first word with an unaligned load.
/// - Align the pointer, read subsequent words until end with aligned loads.
/// - Read the last `usize` from `s` with an unaligned load.
///
/// If any of these loads produces something for which `contains_nonascii`
/// (above) returns true, then we know the answer is false.
#[inline]
fn is_ascii(s: &[u8]) -> bool {
    const USIZE_SIZE: usize = mem::size_of::<usize>();

    let len = s.len();
    let align_offset = s.as_ptr().align_offset(USIZE_SIZE);

    // If we wouldn't gain anything from the word-at-a-time implementation, fall
    // back to a scalar loop.
    //
    // We also do this for architectures where `size_of::<usize>()` isn't
    // sufficient alignment for `usize`, because it's a weird edge case.
    if len < USIZE_SIZE || len < align_offset || USIZE_SIZE < mem::align_of::<usize>() {
        return s.iter().all(|b| b.is_ascii());
    }

    // We always read the first word unaligned, which means `align_offset` is
    // 0, we'd read the same value again for the aligned read.
    let offset_to_aligned = if align_offset == 0 { USIZE_SIZE } else { align_offset };

    let start = s.as_ptr();
    // SAFETY: We verify `len < USIZE_SIZE` above.
    let first_word = unsafe { (start as *const usize).read_unaligned() };

    if contains_nonascii(first_word) {
        return false;
    }
    // We checked this above, somewhat implicitly. Note that `offset_to_aligned`
    // is either `align_offset` or `USIZE_SIZE`, both of are explicitly checked
    // above.
    debug_assert!(offset_to_aligned <= len);

    // SAFETY: word_ptr is the (properly aligned) usize ptr we use to read the
    // middle chunk of the slice.
    let mut word_ptr = unsafe { start.add(offset_to_aligned) as *const usize };

    // `byte_pos` is the byte index of `word_ptr`, used for loop end checks.
    let mut byte_pos = offset_to_aligned;

    // Paranoia check about alignment, since we're about to do a bunch of
    // unaligned loads. In practice this should be impossible barring a bug in
    // `align_offset` though.
    debug_assert_eq!((word_ptr as usize) % mem::align_of::<usize>(), 0);

    // Read subsequent words until the last aligned word, excluding the last
    // aligned word by itself to be done in tail check later, to ensure that
    // tail is always one `usize` at most to extra branch `byte_pos == len`.
    while byte_pos < len - USIZE_SIZE {
        debug_assert!(
            // Sanity check that the read is in bounds
            (word_ptr as usize + USIZE_SIZE) <= (start.wrapping_add(len) as usize) &&
            // And that our assumptions about `byte_pos` hold.
            (word_ptr as usize) - (start as usize) == byte_pos
        );

        // Safety: We know `word_ptr` is properly aligned (because of
        // `align_offset`), and we know that we have enough bytes between `word_ptr` and the end
        let word = unsafe { word_ptr.read() };
        if contains_nonascii(word) {
            return false;
        }

        byte_pos += USIZE_SIZE;
        // SAFETY: We know that `byte_pos <= len - USIZE_SIZE`, which means that
        // after this `add`, `word_ptr` will be at most one-past-the-end.
        word_ptr = unsafe { word_ptr.add(1) };
    }

    // Sanity check to ensure there really is only one `usize` left. This should
    // be guaranteed by our loop condition.
    debug_assert!(byte_pos <= len && len - byte_pos <= USIZE_SIZE);

    // SAFETY: This relies on `len >= USIZE_SIZE`, which we check at the start.
    let last_word = unsafe { (start.add(len - USIZE_SIZE) as *const usize).read_unaligned() };

    !contains_nonascii(last_word)
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, I> ops::Index<I> for [T]
where
    I: SliceIndex<[T]>,
{
    type Output = I::Output;

    #[inline]
    fn index(&self, index: I) -> &I::Output {
        index.index(self)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, I> ops::IndexMut<I> for [T]
where
    I: SliceIndex<[T]>,
{
    #[inline]
    fn index_mut(&mut self, index: I) -> &mut I::Output {
        index.index_mut(self)
    }
}

#[inline(never)]
#[cold]
#[track_caller]
fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
    panic!("range start index {} out of range for slice of length {}", index, len);
}

#[inline(never)]
#[cold]
#[track_caller]
fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
    panic!("range end index {} out of range for slice of length {}", index, len);
}

#[inline(never)]
#[cold]
#[track_caller]
fn slice_index_order_fail(index: usize, end: usize) -> ! {
    panic!("slice index starts at {} but ends at {}", index, end);
}

#[inline(never)]
#[cold]
#[track_caller]
fn slice_start_index_overflow_fail() -> ! {
    panic!("attempted to index slice from after maximum usize");
}

#[inline(never)]
#[cold]
#[track_caller]
fn slice_end_index_overflow_fail() -> ! {
    panic!("attempted to index slice up to maximum usize");
}

mod private_slice_index {
    use super::ops;
    #[stable(feature = "slice_get_slice", since = "1.28.0")]
    pub trait Sealed {}

    #[stable(feature = "slice_get_slice", since = "1.28.0")]
    impl Sealed for usize {}
    #[stable(feature = "slice_get_slice", since = "1.28.0")]
    impl Sealed for ops::Range<usize> {}
    #[stable(feature = "slice_get_slice", since = "1.28.0")]
    impl Sealed for ops::RangeTo<usize> {}
    #[stable(feature = "slice_get_slice", since = "1.28.0")]
    impl Sealed for ops::RangeFrom<usize> {}
    #[stable(feature = "slice_get_slice", since = "1.28.0")]
    impl Sealed for ops::RangeFull {}
    #[stable(feature = "slice_get_slice", since = "1.28.0")]
    impl Sealed for ops::RangeInclusive<usize> {}
    #[stable(feature = "slice_get_slice", since = "1.28.0")]
    impl Sealed for ops::RangeToInclusive<usize> {}
}

/// A helper trait used for indexing operations.
///
/// Implementations of this trait have to promise that if the argument
/// to `get_(mut_)unchecked` is a safe reference, then so is the result.
#[stable(feature = "slice_get_slice", since = "1.28.0")]
#[rustc_on_unimplemented(
    on(T = "str", label = "string indices are ranges of `usize`",),
    on(
        all(any(T = "str", T = "&str", T = "std::string::String"), _Self = "{integer}"),
        note = "you can use `.chars().nth()` or `.bytes().nth()`
see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
    ),
    message = "the type `{T}` cannot be indexed by `{Self}`",
    label = "slice indices are of type `usize` or ranges of `usize`"
)]
pub unsafe trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
    /// The output type returned by methods.
    #[stable(feature = "slice_get_slice", since = "1.28.0")]
    type Output: ?Sized;

    /// Returns a shared reference to the output at this location, if in
    /// bounds.
    #[unstable(feature = "slice_index_methods", issue = "none")]
    fn get(self, slice: &T) -> Option<&Self::Output>;

    /// Returns a mutable reference to the output at this location, if in
    /// bounds.
    #[unstable(feature = "slice_index_methods", issue = "none")]
    fn get_mut(self, slice: &mut T) -> Option<&mut Self::Output>;

    /// Returns a shared reference to the output at this location, without
    /// performing any bounds checking.
    /// Calling this method with an out-of-bounds index or a dangling `slice` pointer
    /// is *[undefined behavior]* even if the resulting reference is not used.
    ///
    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    #[unstable(feature = "slice_index_methods", issue = "none")]
    unsafe fn get_unchecked(self, slice: *const T) -> *const Self::Output;

    /// Returns a mutable reference to the output at this location, without
    /// performing any bounds checking.
    /// Calling this method with an out-of-bounds index or a dangling `slice` pointer
    /// is *[undefined behavior]* even if the resulting reference is not used.
    ///
    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    #[unstable(feature = "slice_index_methods", issue = "none")]
    unsafe fn get_unchecked_mut(self, slice: *mut T) -> *mut Self::Output;

    /// Returns a shared reference to the output at this location, panicking
    /// if out of bounds.
    #[unstable(feature = "slice_index_methods", issue = "none")]
    #[track_caller]
    fn index(self, slice: &T) -> &Self::Output;

    /// Returns a mutable reference to the output at this location, panicking
    /// if out of bounds.
    #[unstable(feature = "slice_index_methods", issue = "none")]
    #[track_caller]
    fn index_mut(self, slice: &mut T) -> &mut Self::Output;
}

#[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
unsafe impl<T> SliceIndex<[T]> for usize {
    type Output = T;

    #[inline]
    fn get(self, slice: &[T]) -> Option<&T> {
        // SAFETY: `self` is checked to be in bounds.
        if self < slice.len() { unsafe { Some(&*self.get_unchecked(slice)) } } else { None }
    }

    #[inline]
    fn get_mut(self, slice: &mut [T]) -> Option<&mut T> {
        // SAFETY: `self` is checked to be in bounds.
        if self < slice.len() { unsafe { Some(&mut *self.get_unchecked_mut(slice)) } } else { None }
    }

    #[inline]
    unsafe fn get_unchecked(self, slice: *const [T]) -> *const T {
        // SAFETY: the caller guarantees that `slice` is not dangling, so it
        // cannot be longer than `isize::MAX`. They also guarantee that
        // `self` is in bounds of `slice` so `self` cannot overflow an `isize`,
        // so the call to `add` is safe.
        unsafe { slice.as_ptr().add(self) }
    }

    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut [T]) -> *mut T {
        // SAFETY: see comments for `get_unchecked` above.
        unsafe { slice.as_mut_ptr().add(self) }
    }

    #[inline]
    fn index(self, slice: &[T]) -> &T {
        // N.B., use intrinsic indexing
        &(*slice)[self]
    }

    #[inline]
    fn index_mut(self, slice: &mut [T]) -> &mut T {
        // N.B., use intrinsic indexing
        &mut (*slice)[self]
    }
}

#[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
unsafe impl<T> SliceIndex<[T]> for ops::Range<usize> {
    type Output = [T];

    #[inline]
    fn get(self, slice: &[T]) -> Option<&[T]> {
        if self.start > self.end || self.end > slice.len() {
            None
        } else {
            // SAFETY: `self` is checked to be valid and in bounds above.
            unsafe { Some(&*self.get_unchecked(slice)) }
        }
    }

    #[inline]
    fn get_mut(self, slice: &mut [T]) -> Option<&mut [T]> {
        if self.start > self.end || self.end > slice.len() {
            None
        } else {
            // SAFETY: `self` is checked to be valid and in bounds above.
            unsafe { Some(&mut *self.get_unchecked_mut(slice)) }
        }
    }

    #[inline]
    unsafe fn get_unchecked(self, slice: *const [T]) -> *const [T] {
        // SAFETY: the caller guarantees that `slice` is not dangling, so it
        // cannot be longer than `isize::MAX`. They also guarantee that
        // `self` is in bounds of `slice` so `self` cannot overflow an `isize`,
        // so the call to `add` is safe.
        unsafe { ptr::slice_from_raw_parts(slice.as_ptr().add(self.start), self.end - self.start) }
    }

    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut [T]) -> *mut [T] {
        // SAFETY: see comments for `get_unchecked` above.
        unsafe {
            ptr::slice_from_raw_parts_mut(slice.as_mut_ptr().add(self.start), self.end - self.start)
        }
    }

    #[inline]
    fn index(self, slice: &[T]) -> &[T] {
        if self.start > self.end {
            slice_index_order_fail(self.start, self.end);
        } else if self.end > slice.len() {
            slice_end_index_len_fail(self.end, slice.len());
        }
        // SAFETY: `self` is checked to be valid and in bounds above.
        unsafe { &*self.get_unchecked(slice) }
    }

    #[inline]
    fn index_mut(self, slice: &mut [T]) -> &mut [T] {
        if self.start > self.end {
            slice_index_order_fail(self.start, self.end);
        } else if self.end > slice.len() {
            slice_end_index_len_fail(self.end, slice.len());
        }
        // SAFETY: `self` is checked to be valid and in bounds above.
        unsafe { &mut *self.get_unchecked_mut(slice) }
    }
}

#[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
unsafe impl<T> SliceIndex<[T]> for ops::RangeTo<usize> {
    type Output = [T];

    #[inline]
    fn get(self, slice: &[T]) -> Option<&[T]> {
        (0..self.end).get(slice)
    }

    #[inline]
    fn get_mut(self, slice: &mut [T]) -> Option<&mut [T]> {
        (0..self.end).get_mut(slice)
    }

    #[inline]
    unsafe fn get_unchecked(self, slice: *const [T]) -> *const [T] {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { (0..self.end).get_unchecked(slice) }
    }

    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut [T]) -> *mut [T] {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { (0..self.end).get_unchecked_mut(slice) }
    }

    #[inline]
    fn index(self, slice: &[T]) -> &[T] {
        (0..self.end).index(slice)
    }

    #[inline]
    fn index_mut(self, slice: &mut [T]) -> &mut [T] {
        (0..self.end).index_mut(slice)
    }
}

#[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
unsafe impl<T> SliceIndex<[T]> for ops::RangeFrom<usize> {
    type Output = [T];

    #[inline]
    fn get(self, slice: &[T]) -> Option<&[T]> {
        (self.start..slice.len()).get(slice)
    }

    #[inline]
    fn get_mut(self, slice: &mut [T]) -> Option<&mut [T]> {
        (self.start..slice.len()).get_mut(slice)
    }

    #[inline]
    unsafe fn get_unchecked(self, slice: *const [T]) -> *const [T] {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { (self.start..slice.len()).get_unchecked(slice) }
    }

    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut [T]) -> *mut [T] {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { (self.start..slice.len()).get_unchecked_mut(slice) }
    }

    #[inline]
    fn index(self, slice: &[T]) -> &[T] {
        if self.start > slice.len() {
            slice_start_index_len_fail(self.start, slice.len());
        }
        // SAFETY: `self` is checked to be valid and in bounds above.
        unsafe { &*self.get_unchecked(slice) }
    }

    #[inline]
    fn index_mut(self, slice: &mut [T]) -> &mut [T] {
        if self.start > slice.len() {
            slice_start_index_len_fail(self.start, slice.len());
        }
        // SAFETY: `self` is checked to be valid and in bounds above.
        unsafe { &mut *self.get_unchecked_mut(slice) }
    }
}

#[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
unsafe impl<T> SliceIndex<[T]> for ops::RangeFull {
    type Output = [T];

    #[inline]
    fn get(self, slice: &[T]) -> Option<&[T]> {
        Some(slice)
    }

    #[inline]
    fn get_mut(self, slice: &mut [T]) -> Option<&mut [T]> {
        Some(slice)
    }

    #[inline]
    unsafe fn get_unchecked(self, slice: *const [T]) -> *const [T] {
        slice
    }

    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut [T]) -> *mut [T] {
        slice
    }

    #[inline]
    fn index(self, slice: &[T]) -> &[T] {
        slice
    }

    #[inline]
    fn index_mut(self, slice: &mut [T]) -> &mut [T] {
        slice
    }
}

#[stable(feature = "inclusive_range", since = "1.26.0")]
unsafe impl<T> SliceIndex<[T]> for ops::RangeInclusive<usize> {
    type Output = [T];

    #[inline]
    fn get(self, slice: &[T]) -> Option<&[T]> {
        if *self.end() == usize::MAX { None } else { (*self.start()..self.end() + 1).get(slice) }
    }

    #[inline]
    fn get_mut(self, slice: &mut [T]) -> Option<&mut [T]> {
        if *self.end() == usize::MAX {
            None
        } else {
            (*self.start()..self.end() + 1).get_mut(slice)
        }
    }

    #[inline]
    unsafe fn get_unchecked(self, slice: *const [T]) -> *const [T] {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { (*self.start()..self.end() + 1).get_unchecked(slice) }
    }

    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut [T]) -> *mut [T] {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { (*self.start()..self.end() + 1).get_unchecked_mut(slice) }
    }

    #[inline]
    fn index(self, slice: &[T]) -> &[T] {
        if *self.end() == usize::MAX {
            slice_end_index_overflow_fail();
        }
        (*self.start()..self.end() + 1).index(slice)
    }

    #[inline]
    fn index_mut(self, slice: &mut [T]) -> &mut [T] {
        if *self.end() == usize::MAX {
            slice_end_index_overflow_fail();
        }
        (*self.start()..self.end() + 1).index_mut(slice)
    }
}

#[stable(feature = "inclusive_range", since = "1.26.0")]
unsafe impl<T> SliceIndex<[T]> for ops::RangeToInclusive<usize> {
    type Output = [T];

    #[inline]
    fn get(self, slice: &[T]) -> Option<&[T]> {
        (0..=self.end).get(slice)
    }

    #[inline]
    fn get_mut(self, slice: &mut [T]) -> Option<&mut [T]> {
        (0..=self.end).get_mut(slice)
    }

    #[inline]
    unsafe fn get_unchecked(self, slice: *const [T]) -> *const [T] {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked`.
        unsafe { (0..=self.end).get_unchecked(slice) }
    }

    #[inline]
    unsafe fn get_unchecked_mut(self, slice: *mut [T]) -> *mut [T] {
        // SAFETY: the caller has to uphold the safety contract for `get_unchecked_mut`.
        unsafe { (0..=self.end).get_unchecked_mut(slice) }
    }

    #[inline]
    fn index(self, slice: &[T]) -> &[T] {
        (0..=self.end).index(slice)
    }

    #[inline]
    fn index_mut(self, slice: &mut [T]) -> &mut [T] {
        (0..=self.end).index_mut(slice)
    }
}

////////////////////////////////////////////////////////////////////////////////
// Common traits
////////////////////////////////////////////////////////////////////////////////

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Default for &[T] {
    /// Creates an empty slice.
    fn default() -> Self {
        &[]
    }
}

#[stable(feature = "mut_slice_default", since = "1.5.0")]
impl<T> Default for &mut [T] {
    /// Creates a mutable empty slice.
    fn default() -> Self {
        &mut []
    }
}

//
// Iterators
//

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

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

// Macro helper functions
#[inline(always)]
fn size_from_ptr<T>(_: *const T) -> usize {
    mem::size_of::<T>()
}

// Inlining is_empty and len makes a huge performance difference
macro_rules! is_empty {
    // The way we encode the length of a ZST iterator, this works both for ZST
    // and non-ZST.
    ($self: ident) => {
        $self.ptr.as_ptr() as *const T == $self.end
    };
}

// To get rid of some bounds checks (see `position`), we compute the length in a somewhat
// unexpected way. (Tested by `codegen/slice-position-bounds-check`.)
macro_rules! len {
    ($self: ident) => {{
        #![allow(unused_unsafe)] // we're sometimes used within an unsafe block

        let start = $self.ptr;
        let size = size_from_ptr(start.as_ptr());
        if size == 0 {
            // This _cannot_ use `unchecked_sub` because we depend on wrapping
            // to represent the length of long ZST slice iterators.
            ($self.end as usize).wrapping_sub(start.as_ptr() as usize)
        } else {
            // We know that `start <= end`, so can do better than `offset_from`,
            // which needs to deal in signed.  By setting appropriate flags here
            // we can tell LLVM this, which helps it remove bounds checks.
            // SAFETY: By the type invariant, `start <= end`
            let diff = unsafe { unchecked_sub($self.end as usize, start.as_ptr() as usize) };
            // By also telling LLVM that the pointers are apart by an exact
            // multiple of the type size, it can optimize `len() == 0` down to
            // `start == end` instead of `(end - start) < size`.
            // SAFETY: By the type invariant, the pointers are aligned so the
            //         distance between them must be a multiple of pointee size
            unsafe { exact_div(diff, size) }
        }
    }};
}

// The shared definition of the `Iter` and `IterMut` iterators
macro_rules! iterator {
    (
        struct $name:ident -> $ptr:ty,
        $elem:ty,
        $raw_mut:tt,
        {$( $mut_:tt )?},
        {$($extra:tt)*}
    ) => {
        // Returns the first element and moves the start of the iterator forwards by 1.
        // Greatly improves performance compared to an inlined function. The iterator
        // must not be empty.
        macro_rules! next_unchecked {
            ($self: ident) => {& $( $mut_ )? *$self.post_inc_start(1)}
        }

        // Returns the last element and moves the end of the iterator backwards by 1.
        // Greatly improves performance compared to an inlined function. The iterator
        // must not be empty.
        macro_rules! next_back_unchecked {
            ($self: ident) => {& $( $mut_ )? *$self.pre_dec_end(1)}
        }

        // Shrinks the iterator when T is a ZST, by moving the end of the iterator
        // backwards by `n`. `n` must not exceed `self.len()`.
        macro_rules! zst_shrink {
            ($self: ident, $n: ident) => {
                $self.end = ($self.end as * $raw_mut u8).wrapping_offset(-$n) as * $raw_mut T;
            }
        }

        impl<'a, T> $name<'a, T> {
            // Helper function for creating a slice from the iterator.
            #[inline(always)]
            fn make_slice(&self) -> &'a [T] {
                // SAFETY: the iterator was created from a slice with pointer
                // `self.ptr` and length `len!(self)`. This guarantees that all
                // the prerequisites for `from_raw_parts` are fulfilled.
                unsafe { from_raw_parts(self.ptr.as_ptr(), len!(self)) }
            }

            // Helper function for moving the start of the iterator forwards by `offset` elements,
            // returning the old start.
            // Unsafe because the offset must not exceed `self.len()`.
            #[inline(always)]
            unsafe fn post_inc_start(&mut self, offset: isize) -> * $raw_mut T {
                if mem::size_of::<T>() == 0 {
                    zst_shrink!(self, offset);
                    self.ptr.as_ptr()
                } else {
                    let old = self.ptr.as_ptr();
                    // SAFETY: the caller guarantees that `offset` doesn't exceed `self.len()`,
                    // so this new pointer is inside `self` and thus guaranteed to be non-null.
                    self.ptr = unsafe { NonNull::new_unchecked(self.ptr.as_ptr().offset(offset)) };
                    old
                }
            }

            // Helper function for moving the end of the iterator backwards by `offset` elements,
            // returning the new end.
            // Unsafe because the offset must not exceed `self.len()`.
            #[inline(always)]
            unsafe fn pre_dec_end(&mut self, offset: isize) -> * $raw_mut T {
                if mem::size_of::<T>() == 0 {
                    zst_shrink!(self, offset);
                    self.ptr.as_ptr()
                } else {
                    // SAFETY: the caller guarantees that `offset` doesn't exceed `self.len()`,
                    // which is guaranteed to not overflow an `isize`. Also, the resulting pointer
                    // is in bounds of `slice`, which fulfills the other requirements for `offset`.
                    self.end = unsafe { self.end.offset(-offset) };
                    self.end
                }
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<T> ExactSizeIterator for $name<'_, T> {
            #[inline(always)]
            fn len(&self) -> usize {
                len!(self)
            }

            #[inline(always)]
            fn is_empty(&self) -> bool {
                is_empty!(self)
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<'a, T> Iterator for $name<'a, T> {
            type Item = $elem;

            #[inline]
            fn next(&mut self) -> Option<$elem> {
                // could be implemented with slices, but this avoids bounds checks

                // SAFETY: `assume` calls are safe since a slice's start pointer
                // must be non-null, and slices over non-ZSTs must also have a
                // non-null end pointer. The call to `next_unchecked!` is safe
                // since we check if the iterator is empty first.
                unsafe {
                    assume(!self.ptr.as_ptr().is_null());
                    if mem::size_of::<T>() != 0 {
                        assume(!self.end.is_null());
                    }
                    if is_empty!(self) {
                        None
                    } else {
                        Some(next_unchecked!(self))
                    }
                }
            }

            #[inline]
            fn size_hint(&self) -> (usize, Option<usize>) {
                let exact = len!(self);
                (exact, Some(exact))
            }

            #[inline]
            fn count(self) -> usize {
                len!(self)
            }

            #[inline]
            fn nth(&mut self, n: usize) -> Option<$elem> {
                if n >= len!(self) {
                    // This iterator is now empty.
                    if mem::size_of::<T>() == 0 {
                        // We have to do it this way as `ptr` may never be 0, but `end`
                        // could be (due to wrapping).
                        self.end = self.ptr.as_ptr();
                    } else {
                        // SAFETY: end can't be 0 if T isn't ZST because ptr isn't 0 and end >= ptr
                        unsafe {
                            self.ptr = NonNull::new_unchecked(self.end as *mut T);
                        }
                    }
                    return None;
                }
                // SAFETY: We are in bounds. `post_inc_start` does the right thing even for ZSTs.
                unsafe {
                    self.post_inc_start(n as isize);
                    Some(next_unchecked!(self))
                }
            }

            #[inline]
            fn last(mut self) -> Option<$elem> {
                self.next_back()
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile.
            #[inline]
            fn for_each<F>(mut self, mut f: F)
            where
                Self: Sized,
                F: FnMut(Self::Item),
            {
                while let Some(x) = self.next() {
                    f(x);
                }
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile.
            #[inline]
            fn all<F>(&mut self, mut f: F) -> bool
            where
                Self: Sized,
                F: FnMut(Self::Item) -> bool,
            {
                while let Some(x) = self.next() {
                    if !f(x) {
                        return false;
                    }
                }
                true
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile.
            #[inline]
            fn any<F>(&mut self, mut f: F) -> bool
            where
                Self: Sized,
                F: FnMut(Self::Item) -> bool,
            {
                while let Some(x) = self.next() {
                    if f(x) {
                        return true;
                    }
                }
                false
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile.
            #[inline]
            fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item>
            where
                Self: Sized,
                P: FnMut(&Self::Item) -> bool,
            {
                while let Some(x) = self.next() {
                    if predicate(&x) {
                        return Some(x);
                    }
                }
                None
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile.
            #[inline]
            fn find_map<B, F>(&mut self, mut f: F) -> Option<B>
            where
                Self: Sized,
                F: FnMut(Self::Item) -> Option<B>,
            {
                while let Some(x) = self.next() {
                    if let Some(y) = f(x) {
                        return Some(y);
                    }
                }
                None
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile. Also, the `assume` avoids a bounds check.
            #[inline]
            #[rustc_inherit_overflow_checks]
            fn position<P>(&mut self, mut predicate: P) -> Option<usize> where
                Self: Sized,
                P: FnMut(Self::Item) -> bool,
            {
                let n = len!(self);
                let mut i = 0;
                while let Some(x) = self.next() {
                    if predicate(x) {
                        // SAFETY: we are guaranteed to be in bounds by the loop invariant:
                        // when `i >= n`, `self.next()` returns `None` and the loop breaks.
                        unsafe { assume(i < n) };
                        return Some(i);
                    }
                    i += 1;
                }
                None
            }

            // We override the default implementation, which uses `try_fold`,
            // because this simple implementation generates less LLVM IR and is
            // faster to compile. Also, the `assume` avoids a bounds check.
            #[inline]
            fn rposition<P>(&mut self, mut predicate: P) -> Option<usize> where
                P: FnMut(Self::Item) -> bool,
                Self: Sized + ExactSizeIterator + DoubleEndedIterator
            {
                let n = len!(self);
                let mut i = n;
                while let Some(x) = self.next_back() {
                    i -= 1;
                    if predicate(x) {
                        // SAFETY: `i` must be lower than `n` since it starts at `n`
                        // and is only decreasing.
                        unsafe { assume(i < n) };
                        return Some(i);
                    }
                }
                None
            }

            #[doc(hidden)]
            unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
                // SAFETY: the caller must guarantee that `i` is in bounds of
                // the underlying slice, so `i` cannot overflow an `isize`, and
                // the returned references is guaranteed to refer to an element
                // of the slice and thus guaranteed to be valid.
                //
                // Also note that the caller also guarantees that we're never
                // called with the same index again, and that no other methods
                // that will access this subslice are called, so it is valid
                // for the returned reference to be mutable in the case of
                // `IterMut`
                unsafe { & $( $mut_ )? * self.ptr.as_ptr().add(idx) }
            }

            $($extra)*
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<'a, T> DoubleEndedIterator for $name<'a, T> {
            #[inline]
            fn next_back(&mut self) -> Option<$elem> {
                // could be implemented with slices, but this avoids bounds checks

                // SAFETY: `assume` calls are safe since a slice's start pointer must be non-null,
                // and slices over non-ZSTs must also have a non-null end pointer.
                // The call to `next_back_unchecked!` is safe since we check if the iterator is
                // empty first.
                unsafe {
                    assume(!self.ptr.as_ptr().is_null());
                    if mem::size_of::<T>() != 0 {
                        assume(!self.end.is_null());
                    }
                    if is_empty!(self) {
                        None
                    } else {
                        Some(next_back_unchecked!(self))
                    }
                }
            }

            #[inline]
            fn nth_back(&mut self, n: usize) -> Option<$elem> {
                if n >= len!(self) {
                    // This iterator is now empty.
                    self.end = self.ptr.as_ptr();
                    return None;
                }
                // SAFETY: We are in bounds. `pre_dec_end` does the right thing even for ZSTs.
                unsafe {
                    self.pre_dec_end(n as isize);
                    Some(next_back_unchecked!(self))
                }
            }
        }

        #[stable(feature = "fused", since = "1.26.0")]
        impl<T> FusedIterator for $name<'_, T> {}

        #[unstable(feature = "trusted_len", issue = "37572")]
        unsafe impl<T> TrustedLen for $name<'_, T> {}
    }
}

/// Immutable slice iterator
///
/// This struct is created by the [`iter`] method on [slices].
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// // First, we declare a type which has `iter` method to get the `Iter` struct (&[usize here]):
/// let slice = &[1, 2, 3];
///
/// // Then, we iterate over it:
/// for element in slice.iter() {
///     println!("{}", element);
/// }
/// ```
///
/// [`iter`]: ../../std/primitive.slice.html#method.iter
/// [slices]: ../../std/primitive.slice.html
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Iter<'a, T: 'a> {
    ptr: NonNull<T>,
    end: *const T, // If T is a ZST, this is actually ptr+len.  This encoding is picked so that
    // ptr == end is a quick test for the Iterator being empty, that works
    // for both ZST and non-ZST.
    _marker: marker::PhantomData<&'a T>,
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("Iter").field(&self.as_slice()).finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Sync> Sync for Iter<'_, T> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Sync> Send for Iter<'_, T> {}

impl<'a, T> Iter<'a, T> {
    /// Views the underlying data as a subslice of the original data.
    ///
    /// This has the same lifetime as the original slice, and so the
    /// iterator can continue to be used while this exists.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// // First, we declare a type which has the `iter` method to get the `Iter`
    /// // struct (&[usize here]):
    /// let slice = &[1, 2, 3];
    ///
    /// // Then, we get the iterator:
    /// let mut iter = slice.iter();
    /// // So if we print what `as_slice` method returns here, we have "[1, 2, 3]":
    /// println!("{:?}", iter.as_slice());
    ///
    /// // Next, we move to the second element of the slice:
    /// iter.next();
    /// // Now `as_slice` returns "[2, 3]":
    /// println!("{:?}", iter.as_slice());
    /// ```
    #[stable(feature = "iter_to_slice", since = "1.4.0")]
    pub fn as_slice(&self) -> &'a [T] {
        self.make_slice()
    }
}

iterator! {struct Iter -> *const T, &'a T, const, {/* no mut */}, {
    fn is_sorted_by<F>(self, mut compare: F) -> bool
    where
        Self: Sized,
        F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
    {
        self.as_slice().windows(2).all(|w| {
            compare(&&w[0], &&w[1]).map(|o| o != Ordering::Greater).unwrap_or(false)
        })
    }
}}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Clone for Iter<'_, T> {
    fn clone(&self) -> Self {
        Iter { ptr: self.ptr, end: self.end, _marker: self._marker }
    }
}

#[stable(feature = "slice_iter_as_ref", since = "1.13.0")]
impl<T> AsRef<[T]> for Iter<'_, T> {
    fn as_ref(&self) -> &[T] {
        self.as_slice()
    }
}

/// Mutable slice iterator.
///
/// This struct is created by the [`iter_mut`] method on [slices].
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// // First, we declare a type which has `iter_mut` method to get the `IterMut`
/// // struct (&[usize here]):
/// let mut slice = &mut [1, 2, 3];
///
/// // Then, we iterate over it and increment each element value:
/// for element in slice.iter_mut() {
///     *element += 1;
/// }
///
/// // We now have "[2, 3, 4]":
/// println!("{:?}", slice);
/// ```
///
/// [`iter_mut`]: ../../std/primitive.slice.html#method.iter_mut
/// [slices]: ../../std/primitive.slice.html
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IterMut<'a, T: 'a> {
    ptr: NonNull<T>,
    end: *mut T, // If T is a ZST, this is actually ptr+len.  This encoding is picked so that
    // ptr == end is a quick test for the Iterator being empty, that works
    // for both ZST and non-ZST.
    _marker: marker::PhantomData<&'a mut T>,
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<T: fmt::Debug> fmt::Debug for IterMut<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("IterMut").field(&self.make_slice()).finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Sync> Sync for IterMut<'_, T> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: Send> Send for IterMut<'_, T> {}

impl<'a, T> IterMut<'a, T> {
    /// Views the underlying data as a subslice of the original data.
    ///
    /// To avoid creating `&mut` references that alias, this is forced
    /// to consume the iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// // First, we declare a type which has `iter_mut` method to get the `IterMut`
    /// // struct (&[usize here]):
    /// let mut slice = &mut [1, 2, 3];
    ///
    /// {
    ///     // Then, we get the iterator:
    ///     let mut iter = slice.iter_mut();
    ///     // We move to next element:
    ///     iter.next();
    ///     // So if we print what `into_slice` method returns here, we have "[2, 3]":
    ///     println!("{:?}", iter.into_slice());
    /// }
    ///
    /// // Now let's modify a value of the slice:
    /// {
    ///     // First we get back the iterator:
    ///     let mut iter = slice.iter_mut();
    ///     // We change the value of the first element of the slice returned by the `next` method:
    ///     *iter.next().unwrap() += 1;
    /// }
    /// // Now slice is "[2, 2, 3]":
    /// println!("{:?}", slice);
    /// ```
    #[stable(feature = "iter_to_slice", since = "1.4.0")]
    pub fn into_slice(self) -> &'a mut [T] {
        // SAFETY: the iterator was created from a mutable slice with pointer
        // `self.ptr` and length `len!(self)`. This guarantees that all the prerequisites
        // for `from_raw_parts_mut` are fulfilled.
        unsafe { from_raw_parts_mut(self.ptr.as_ptr(), len!(self)) }
    }

    /// Views the underlying data as a subslice of the original data.
    ///
    /// To avoid creating `&mut [T]` references that alias, the returned slice
    /// borrows its lifetime from the iterator the method is applied on.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// # #![feature(slice_iter_mut_as_slice)]
    /// let mut slice: &mut [usize] = &mut [1, 2, 3];
    ///
    /// // First, we get the iterator:
    /// let mut iter = slice.iter_mut();
    /// // So if we check what the `as_slice` method returns here, we have "[1, 2, 3]":
    /// assert_eq!(iter.as_slice(), &[1, 2, 3]);
    ///
    /// // Next, we move to the second element of the slice:
    /// iter.next();
    /// // Now `as_slice` returns "[2, 3]":
    /// assert_eq!(iter.as_slice(), &[2, 3]);
    /// ```
    #[unstable(feature = "slice_iter_mut_as_slice", reason = "recently added", issue = "58957")]
    pub fn as_slice(&self) -> &[T] {
        self.make_slice()
    }
}

iterator! {struct IterMut -> *mut T, &'a mut T, mut, {mut}, {}}

/// An internal abstraction over the splitting iterators, so that
/// splitn, splitn_mut etc can be implemented once.
#[doc(hidden)]
trait SplitIter: DoubleEndedIterator {
    /// Marks the underlying iterator as complete, extracting the remaining
    /// portion of the slice.
    fn finish(&mut self) -> Option<Self::Item>;
}

/// An iterator over subslices separated by elements that match a predicate
/// function.
///
/// This struct is created by the [`split`] method on [slices].
///
/// [`split`]: ../../std/primitive.slice.html#method.split
/// [slices]: ../../std/primitive.slice.html
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Split<'a, T: 'a, P>
where
    P: FnMut(&T) -> bool,
{
    v: &'a [T],
    pred: P,
    finished: bool,
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<T: fmt::Debug, P> fmt::Debug for Split<'_, T, P>
where
    P: FnMut(&T) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Split").field("v", &self.v).field("finished", &self.finished).finish()
    }
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[stable(feature = "rust1", since = "1.0.0")]
impl<T, P> Clone for Split<'_, T, P>
where
    P: Clone + FnMut(&T) -> bool,
{
    fn clone(&self) -> Self {
        Split { v: self.v, pred: self.pred.clone(), finished: self.finished }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, P> Iterator for Split<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    type Item = &'a [T];

    #[inline]
    fn next(&mut self) -> Option<&'a [T]> {
        if self.finished {
            return None;
        }

        match self.v.iter().position(|x| (self.pred)(x)) {
            None => self.finish(),
            Some(idx) => {
                let ret = Some(&self.v[..idx]);
                self.v = &self.v[idx + 1..];
                ret
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.finished { (0, Some(0)) } else { (1, Some(self.v.len() + 1)) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, P> DoubleEndedIterator for Split<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    #[inline]
    fn next_back(&mut self) -> Option<&'a [T]> {
        if self.finished {
            return None;
        }

        match self.v.iter().rposition(|x| (self.pred)(x)) {
            None => self.finish(),
            Some(idx) => {
                let ret = Some(&self.v[idx + 1..]);
                self.v = &self.v[..idx];
                ret
            }
        }
    }
}

impl<'a, T, P> SplitIter for Split<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    #[inline]
    fn finish(&mut self) -> Option<&'a [T]> {
        if self.finished {
            None
        } else {
            self.finished = true;
            Some(self.v)
        }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<T, P> FusedIterator for Split<'_, T, P> where P: FnMut(&T) -> bool {}

/// An iterator over subslices separated by elements that match a predicate
/// function. Unlike `Split`, it contains the matched part as a terminator
/// of the subslice.
///
/// This struct is created by the [`split_inclusive`] method on [slices].
///
/// [`split_inclusive`]: ../../std/primitive.slice.html#method.split_inclusive
/// [slices]: ../../std/primitive.slice.html
#[unstable(feature = "split_inclusive", issue = "72360")]
pub struct SplitInclusive<'a, T: 'a, P>
where
    P: FnMut(&T) -> bool,
{
    v: &'a [T],
    pred: P,
    finished: bool,
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<T: fmt::Debug, P> fmt::Debug for SplitInclusive<'_, T, P>
where
    P: FnMut(&T) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitInclusive")
            .field("v", &self.v)
            .field("finished", &self.finished)
            .finish()
    }
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[unstable(feature = "split_inclusive", issue = "72360")]
impl<T, P> Clone for SplitInclusive<'_, T, P>
where
    P: Clone + FnMut(&T) -> bool,
{
    fn clone(&self) -> Self {
        SplitInclusive { v: self.v, pred: self.pred.clone(), finished: self.finished }
    }
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<'a, T, P> Iterator for SplitInclusive<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    type Item = &'a [T];

    #[inline]
    fn next(&mut self) -> Option<&'a [T]> {
        if self.finished {
            return None;
        }

        let idx =
            self.v.iter().position(|x| (self.pred)(x)).map(|idx| idx + 1).unwrap_or(self.v.len());
        if idx == self.v.len() {
            self.finished = true;
        }
        let ret = Some(&self.v[..idx]);
        self.v = &self.v[idx..];
        ret
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.finished { (0, Some(0)) } else { (1, Some(self.v.len() + 1)) }
    }
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<'a, T, P> DoubleEndedIterator for SplitInclusive<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    #[inline]
    fn next_back(&mut self) -> Option<&'a [T]> {
        if self.finished {
            return None;
        }

        // The last index of self.v is already checked and found to match
        // by the last iteration, so we start searching a new match
        // one index to the left.
        let remainder = if self.v.is_empty() { &[] } else { &self.v[..(self.v.len() - 1)] };
        let idx = remainder.iter().rposition(|x| (self.pred)(x)).map(|idx| idx + 1).unwrap_or(0);
        if idx == 0 {
            self.finished = true;
        }
        let ret = Some(&self.v[idx..]);
        self.v = &self.v[..idx];
        ret
    }
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<T, P> FusedIterator for SplitInclusive<'_, T, P> where P: FnMut(&T) -> bool {}

/// An iterator over the mutable subslices of the vector which are separated
/// by elements that match `pred`.
///
/// This struct is created by the [`split_mut`] method on [slices].
///
/// [`split_mut`]: ../../std/primitive.slice.html#method.split_mut
/// [slices]: ../../std/primitive.slice.html
#[stable(feature = "rust1", since = "1.0.0")]
pub struct SplitMut<'a, T: 'a, P>
where
    P: FnMut(&T) -> bool,
{
    v: &'a mut [T],
    pred: P,
    finished: bool,
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<T: fmt::Debug, P> fmt::Debug for SplitMut<'_, T, P>
where
    P: FnMut(&T) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitMut").field("v", &self.v).field("finished", &self.finished).finish()
    }
}

impl<'a, T, P> SplitIter for SplitMut<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    #[inline]
    fn finish(&mut self) -> Option<&'a mut [T]> {
        if self.finished {
            None
        } else {
            self.finished = true;
            Some(mem::replace(&mut self.v, &mut []))
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, P> Iterator for SplitMut<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    type Item = &'a mut [T];

    #[inline]
    fn next(&mut self) -> Option<&'a mut [T]> {
        if self.finished {
            return None;
        }

        let idx_opt = {
            // work around borrowck limitations
            let pred = &mut self.pred;
            self.v.iter().position(|x| (*pred)(x))
        };
        match idx_opt {
            None => self.finish(),
            Some(idx) => {
                let tmp = mem::replace(&mut self.v, &mut []);
                let (head, tail) = tmp.split_at_mut(idx);
                self.v = &mut tail[1..];
                Some(head)
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.finished {
            (0, Some(0))
        } else {
            // if the predicate doesn't match anything, we yield one slice
            // if it matches every element, we yield len+1 empty slices.
            (1, Some(self.v.len() + 1))
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, P> DoubleEndedIterator for SplitMut<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut [T]> {
        if self.finished {
            return None;
        }

        let idx_opt = {
            // work around borrowck limitations
            let pred = &mut self.pred;
            self.v.iter().rposition(|x| (*pred)(x))
        };
        match idx_opt {
            None => self.finish(),
            Some(idx) => {
                let tmp = mem::replace(&mut self.v, &mut []);
                let (head, tail) = tmp.split_at_mut(idx);
                self.v = head;
                Some(&mut tail[1..])
            }
        }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<T, P> FusedIterator for SplitMut<'_, T, P> where P: FnMut(&T) -> bool {}

/// An iterator over the mutable subslices of the vector which are separated
/// by elements that match `pred`. Unlike `SplitMut`, it contains the matched
/// parts in the ends of the subslices.
///
/// This struct is created by the [`split_inclusive_mut`] method on [slices].
///
/// [`split_inclusive_mut`]: ../../std/primitive.slice.html#method.split_inclusive_mut
/// [slices]: ../../std/primitive.slice.html
#[unstable(feature = "split_inclusive", issue = "72360")]
pub struct SplitInclusiveMut<'a, T: 'a, P>
where
    P: FnMut(&T) -> bool,
{
    v: &'a mut [T],
    pred: P,
    finished: bool,
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<T: fmt::Debug, P> fmt::Debug for SplitInclusiveMut<'_, T, P>
where
    P: FnMut(&T) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitInclusiveMut")
            .field("v", &self.v)
            .field("finished", &self.finished)
            .finish()
    }
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<'a, T, P> Iterator for SplitInclusiveMut<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    type Item = &'a mut [T];

    #[inline]
    fn next(&mut self) -> Option<&'a mut [T]> {
        if self.finished {
            return None;
        }

        let idx_opt = {
            // work around borrowck limitations
            let pred = &mut self.pred;
            self.v.iter().position(|x| (*pred)(x))
        };
        let idx = idx_opt.map(|idx| idx + 1).unwrap_or(self.v.len());
        if idx == self.v.len() {
            self.finished = true;
        }
        let tmp = mem::replace(&mut self.v, &mut []);
        let (head, tail) = tmp.split_at_mut(idx);
        self.v = tail;
        Some(head)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.finished {
            (0, Some(0))
        } else {
            // if the predicate doesn't match anything, we yield one slice
            // if it matches every element, we yield len+1 empty slices.
            (1, Some(self.v.len() + 1))
        }
    }
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<'a, T, P> DoubleEndedIterator for SplitInclusiveMut<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut [T]> {
        if self.finished {
            return None;
        }

        let idx_opt = if self.v.is_empty() {
            None
        } else {
            // work around borrowck limitations
            let pred = &mut self.pred;

            // The last index of self.v is already checked and found to match
            // by the last iteration, so we start searching a new match
            // one index to the left.
            let remainder = &self.v[..(self.v.len() - 1)];
            remainder.iter().rposition(|x| (*pred)(x))
        };
        let idx = idx_opt.map(|idx| idx + 1).unwrap_or(0);
        if idx == 0 {
            self.finished = true;
        }
        let tmp = mem::replace(&mut self.v, &mut []);
        let (head, tail) = tmp.split_at_mut(idx);
        self.v = head;
        Some(tail)
    }
}

#[unstable(feature = "split_inclusive", issue = "72360")]
impl<T, P> FusedIterator for SplitInclusiveMut<'_, T, P> where P: FnMut(&T) -> bool {}

/// An iterator over subslices separated by elements that match a predicate
/// function, starting from the end of the slice.
///
/// This struct is created by the [`rsplit`] method on [slices].
///
/// [`rsplit`]: ../../std/primitive.slice.html#method.rsplit
/// [slices]: ../../std/primitive.slice.html
#[stable(feature = "slice_rsplit", since = "1.27.0")]
#[derive(Clone)] // Is this correct, or does it incorrectly require `T: Clone`?
pub struct RSplit<'a, T: 'a, P>
where
    P: FnMut(&T) -> bool,
{
    inner: Split<'a, T, P>,
}

#[stable(feature = "slice_rsplit", since = "1.27.0")]
impl<T: fmt::Debug, P> fmt::Debug for RSplit<'_, T, P>
where
    P: FnMut(&T) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RSplit")
            .field("v", &self.inner.v)
            .field("finished", &self.inner.finished)
            .finish()
    }
}

#[stable(feature = "slice_rsplit", since = "1.27.0")]
impl<'a, T, P> Iterator for RSplit<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    type Item = &'a [T];

    #[inline]
    fn next(&mut self) -> Option<&'a [T]> {
        self.inner.next_back()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}

#[stable(feature = "slice_rsplit", since = "1.27.0")]
impl<'a, T, P> DoubleEndedIterator for RSplit<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    #[inline]
    fn next_back(&mut self) -> Option<&'a [T]> {
        self.inner.next()
    }
}

#[stable(feature = "slice_rsplit", since = "1.27.0")]
impl<'a, T, P> SplitIter for RSplit<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    #[inline]
    fn finish(&mut self) -> Option<&'a [T]> {
        self.inner.finish()
    }
}

#[stable(feature = "slice_rsplit", since = "1.27.0")]
impl<T, P> FusedIterator for RSplit<'_, T, P> where P: FnMut(&T) -> bool {}

/// An iterator over the subslices of the vector which are separated
/// by elements that match `pred`, starting from the end of the slice.
///
/// This struct is created by the [`rsplit_mut`] method on [slices].
///
/// [`rsplit_mut`]: ../../std/primitive.slice.html#method.rsplit_mut
/// [slices]: ../../std/primitive.slice.html
#[stable(feature = "slice_rsplit", since = "1.27.0")]
pub struct RSplitMut<'a, T: 'a, P>
where
    P: FnMut(&T) -> bool,
{
    inner: SplitMut<'a, T, P>,
}

#[stable(feature = "slice_rsplit", since = "1.27.0")]
impl<T: fmt::Debug, P> fmt::Debug for RSplitMut<'_, T, P>
where
    P: FnMut(&T) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RSplitMut")
            .field("v", &self.inner.v)
            .field("finished", &self.inner.finished)
            .finish()
    }
}

#[stable(feature = "slice_rsplit", since = "1.27.0")]
impl<'a, T, P> SplitIter for RSplitMut<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    #[inline]
    fn finish(&mut self) -> Option<&'a mut [T]> {
        self.inner.finish()
    }
}

#[stable(feature = "slice_rsplit", since = "1.27.0")]
impl<'a, T, P> Iterator for RSplitMut<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    type Item = &'a mut [T];

    #[inline]
    fn next(&mut self) -> Option<&'a mut [T]> {
        self.inner.next_back()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.inner.size_hint()
    }
}

#[stable(feature = "slice_rsplit", since = "1.27.0")]
impl<'a, T, P> DoubleEndedIterator for RSplitMut<'a, T, P>
where
    P: FnMut(&T) -> bool,
{
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut [T]> {
        self.inner.next()
    }
}

#[stable(feature = "slice_rsplit", since = "1.27.0")]
impl<T, P> FusedIterator for RSplitMut<'_, T, P> where P: FnMut(&T) -> bool {}

/// An private iterator over subslices separated by elements that
/// match a predicate function, splitting at most a fixed number of
/// times.
#[derive(Debug)]
struct GenericSplitN<I> {
    iter: I,
    count: usize,
}

impl<T, I: SplitIter<Item = T>> Iterator for GenericSplitN<I> {
    type Item = T;

    #[inline]
    fn next(&mut self) -> Option<T> {
        match self.count {
            0 => None,
            1 => {
                self.count -= 1;
                self.iter.finish()
            }
            _ => {
                self.count -= 1;
                self.iter.next()
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (lower, upper_opt) = self.iter.size_hint();
        (lower, upper_opt.map(|upper| cmp::min(self.count, upper)))
    }
}

/// An iterator over subslices separated by elements that match a predicate
/// function, limited to a given number of splits.
///
/// This struct is created by the [`splitn`] method on [slices].
///
/// [`splitn`]: ../../std/primitive.slice.html#method.splitn
/// [slices]: ../../std/primitive.slice.html
#[stable(feature = "rust1", since = "1.0.0")]
pub struct SplitN<'a, T: 'a, P>
where
    P: FnMut(&T) -> bool,
{
    inner: GenericSplitN<Split<'a, T, P>>,
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<T: fmt::Debug, P> fmt::Debug for SplitN<'_, T, P>
where
    P: FnMut(&T) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitN").field("inner", &self.inner).finish()
    }
}

/// An iterator over subslices separated by elements that match a
/// predicate function, limited to a given number of splits, starting
/// from the end of the slice.
///
/// This struct is created by the [`rsplitn`] method on [slices].
///
/// [`rsplitn`]: ../../std/primitive.slice.html#method.rsplitn
/// [slices]: ../../std/primitive.slice.html
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RSplitN<'a, T: 'a, P>
where
    P: FnMut(&T) -> bool,
{
    inner: GenericSplitN<RSplit<'a, T, P>>,
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<T: fmt::Debug, P> fmt::Debug for RSplitN<'_, T, P>
where
    P: FnMut(&T) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RSplitN").field("inner", &self.inner).finish()
    }
}

/// An iterator over subslices separated by elements that match a predicate
/// function, limited to a given number of splits.
///
/// This struct is created by the [`splitn_mut`] method on [slices].
///
/// [`splitn_mut`]: ../../std/primitive.slice.html#method.splitn_mut
/// [slices]: ../../std/primitive.slice.html
#[stable(feature = "rust1", since = "1.0.0")]
pub struct SplitNMut<'a, T: 'a, P>
where
    P: FnMut(&T) -> bool,
{
    inner: GenericSplitN<SplitMut<'a, T, P>>,
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<T: fmt::Debug, P> fmt::Debug for SplitNMut<'_, T, P>
where
    P: FnMut(&T) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SplitNMut").field("inner", &self.inner).finish()
    }
}

/// An iterator over subslices separated by elements that match a
/// predicate function, limited to a given number of splits, starting
/// from the end of the slice.
///
/// This struct is created by the [`rsplitn_mut`] method on [slices].
///
/// [`rsplitn_mut`]: ../../std/primitive.slice.html#method.rsplitn_mut
/// [slices]: ../../std/primitive.slice.html
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RSplitNMut<'a, T: 'a, P>
where
    P: FnMut(&T) -> bool,
{
    inner: GenericSplitN<RSplitMut<'a, T, P>>,
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<T: fmt::Debug, P> fmt::Debug for RSplitNMut<'_, T, P>
where
    P: FnMut(&T) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RSplitNMut").field("inner", &self.inner).finish()
    }
}

macro_rules! forward_iterator {
    ($name:ident: $elem:ident, $iter_of:ty) => {
        #[stable(feature = "rust1", since = "1.0.0")]
        impl<'a, $elem, P> Iterator for $name<'a, $elem, P>
        where
            P: FnMut(&T) -> bool,
        {
            type Item = $iter_of;

            #[inline]
            fn next(&mut self) -> Option<$iter_of> {
                self.inner.next()
            }

            #[inline]
            fn size_hint(&self) -> (usize, Option<usize>) {
                self.inner.size_hint()
            }
        }

        #[stable(feature = "fused", since = "1.26.0")]
        impl<'a, $elem, P> FusedIterator for $name<'a, $elem, P> where P: FnMut(&T) -> bool {}
    };
}

forward_iterator! { SplitN: T, &'a [T] }
forward_iterator! { RSplitN: T, &'a [T] }
forward_iterator! { SplitNMut: T, &'a mut [T] }
forward_iterator! { RSplitNMut: T, &'a mut [T] }

/// An iterator over overlapping subslices of length `size`.
///
/// This struct is created by the [`windows`] method on [slices].
///
/// [`windows`]: ../../std/primitive.slice.html#method.windows
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Windows<'a, T: 'a> {
    v: &'a [T],
    size: usize,
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Clone for Windows<'_, T> {
    fn clone(&self) -> Self {
        Windows { v: self.v, size: self.size }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Iterator for Windows<'a, T> {
    type Item = &'a [T];

    #[inline]
    fn next(&mut self) -> Option<&'a [T]> {
        if self.size > self.v.len() {
            None
        } else {
            let ret = Some(&self.v[..self.size]);
            self.v = &self.v[1..];
            ret
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.size > self.v.len() {
            (0, Some(0))
        } else {
            let size = self.v.len() - self.size + 1;
            (size, Some(size))
        }
    }

    #[inline]
    fn count(self) -> usize {
        self.len()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        let (end, overflow) = self.size.overflowing_add(n);
        if end > self.v.len() || overflow {
            self.v = &[];
            None
        } else {
            let nth = &self.v[n..end];
            self.v = &self.v[n + 1..];
            Some(nth)
        }
    }

    #[inline]
    fn last(self) -> Option<Self::Item> {
        if self.size > self.v.len() {
            None
        } else {
            let start = self.v.len() - self.size;
            Some(&self.v[start..])
        }
    }

    #[doc(hidden)]
    unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
        // SAFETY: since the caller guarantees that `i` is in bounds,
        // which means that `i` cannot overflow an `isize`, and the
        // slice created by `from_raw_parts` is a subslice of `self.v`
        // thus is guaranteed to be valid for the lifetime `'a` of `self.v`.
        unsafe { from_raw_parts(self.v.as_ptr().add(idx), self.size) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> DoubleEndedIterator for Windows<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a [T]> {
        if self.size > self.v.len() {
            None
        } else {
            let ret = Some(&self.v[self.v.len() - self.size..]);
            self.v = &self.v[..self.v.len() - 1];
            ret
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let (end, overflow) = self.v.len().overflowing_sub(n);
        if end < self.size || overflow {
            self.v = &[];
            None
        } else {
            let ret = &self.v[end - self.size..end];
            self.v = &self.v[..end - 1];
            Some(ret)
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> ExactSizeIterator for Windows<'_, T> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for Windows<'_, T> {}

#[stable(feature = "fused", since = "1.26.0")]
impl<T> FusedIterator for Windows<'_, T> {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for Windows<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

/// An iterator over a slice in (non-overlapping) chunks (`chunk_size` elements at a
/// time), starting at the beginning of the slice.
///
/// When the slice len is not evenly divided by the chunk size, the last slice
/// of the iteration will be the remainder.
///
/// This struct is created by the [`chunks`] method on [slices].
///
/// [`chunks`]: ../../std/primitive.slice.html#method.chunks
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Chunks<'a, T: 'a> {
    v: &'a [T],
    chunk_size: usize,
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Clone for Chunks<'_, T> {
    fn clone(&self) -> Self {
        Chunks { v: self.v, chunk_size: self.chunk_size }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Iterator for Chunks<'a, T> {
    type Item = &'a [T];

    #[inline]
    fn next(&mut self) -> Option<&'a [T]> {
        if self.v.is_empty() {
            None
        } else {
            let chunksz = cmp::min(self.v.len(), self.chunk_size);
            let (fst, snd) = self.v.split_at(chunksz);
            self.v = snd;
            Some(fst)
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.v.is_empty() {
            (0, Some(0))
        } else {
            let n = self.v.len() / self.chunk_size;
            let rem = self.v.len() % self.chunk_size;
            let n = if rem > 0 { n + 1 } else { n };
            (n, Some(n))
        }
    }

    #[inline]
    fn count(self) -> usize {
        self.len()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        let (start, overflow) = n.overflowing_mul(self.chunk_size);
        if start >= self.v.len() || overflow {
            self.v = &[];
            None
        } else {
            let end = match start.checked_add(self.chunk_size) {
                Some(sum) => cmp::min(self.v.len(), sum),
                None => self.v.len(),
            };
            let nth = &self.v[start..end];
            self.v = &self.v[end..];
            Some(nth)
        }
    }

    #[inline]
    fn last(self) -> Option<Self::Item> {
        if self.v.is_empty() {
            None
        } else {
            let start = (self.v.len() - 1) / self.chunk_size * self.chunk_size;
            Some(&self.v[start..])
        }
    }

    #[doc(hidden)]
    unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
        let start = idx * self.chunk_size;
        let end = match start.checked_add(self.chunk_size) {
            None => self.v.len(),
            Some(end) => cmp::min(end, self.v.len()),
        };
        // SAFETY: the caller guarantees that `i` is in bounds,
        // which means that `start` must be in bounds of the
        // underlying `self.v` slice, and we made sure that `end`
        // is also in bounds of `self.v`. Thus, `start` cannot overflow
        // an `isize`, and the slice constructed by `from_raw_parts`
        // is a subslice of `self.v` which is guaranteed to be valid
        // for the lifetime `'a` of `self.v`.
        unsafe { from_raw_parts(self.v.as_ptr().add(start), end - start) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> DoubleEndedIterator for Chunks<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a [T]> {
        if self.v.is_empty() {
            None
        } else {
            let remainder = self.v.len() % self.chunk_size;
            let chunksz = if remainder != 0 { remainder } else { self.chunk_size };
            let (fst, snd) = self.v.split_at(self.v.len() - chunksz);
            self.v = fst;
            Some(snd)
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.len();
        if n >= len {
            self.v = &[];
            None
        } else {
            let start = (len - 1 - n) * self.chunk_size;
            let end = match start.checked_add(self.chunk_size) {
                Some(res) => cmp::min(res, self.v.len()),
                None => self.v.len(),
            };
            let nth_back = &self.v[start..end];
            self.v = &self.v[..start];
            Some(nth_back)
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> ExactSizeIterator for Chunks<'_, T> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for Chunks<'_, T> {}

#[stable(feature = "fused", since = "1.26.0")]
impl<T> FusedIterator for Chunks<'_, T> {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for Chunks<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

/// An iterator over a slice in (non-overlapping) mutable chunks (`chunk_size`
/// elements at a time), starting at the beginning of the slice.
///
/// When the slice len is not evenly divided by the chunk size, the last slice
/// of the iteration will be the remainder.
///
/// This struct is created by the [`chunks_mut`] method on [slices].
///
/// [`chunks_mut`]: ../../std/primitive.slice.html#method.chunks_mut
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct ChunksMut<'a, T: 'a> {
    v: &'a mut [T],
    chunk_size: usize,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Iterator for ChunksMut<'a, T> {
    type Item = &'a mut [T];

    #[inline]
    fn next(&mut self) -> Option<&'a mut [T]> {
        if self.v.is_empty() {
            None
        } else {
            let sz = cmp::min(self.v.len(), self.chunk_size);
            let tmp = mem::replace(&mut self.v, &mut []);
            let (head, tail) = tmp.split_at_mut(sz);
            self.v = tail;
            Some(head)
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.v.is_empty() {
            (0, Some(0))
        } else {
            let n = self.v.len() / self.chunk_size;
            let rem = self.v.len() % self.chunk_size;
            let n = if rem > 0 { n + 1 } else { n };
            (n, Some(n))
        }
    }

    #[inline]
    fn count(self) -> usize {
        self.len()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<&'a mut [T]> {
        let (start, overflow) = n.overflowing_mul(self.chunk_size);
        if start >= self.v.len() || overflow {
            self.v = &mut [];
            None
        } else {
            let end = match start.checked_add(self.chunk_size) {
                Some(sum) => cmp::min(self.v.len(), sum),
                None => self.v.len(),
            };
            let tmp = mem::replace(&mut self.v, &mut []);
            let (head, tail) = tmp.split_at_mut(end);
            let (_, nth) = head.split_at_mut(start);
            self.v = tail;
            Some(nth)
        }
    }

    #[inline]
    fn last(self) -> Option<Self::Item> {
        if self.v.is_empty() {
            None
        } else {
            let start = (self.v.len() - 1) / self.chunk_size * self.chunk_size;
            Some(&mut self.v[start..])
        }
    }

    #[doc(hidden)]
    unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
        let start = idx * self.chunk_size;
        let end = match start.checked_add(self.chunk_size) {
            None => self.v.len(),
            Some(end) => cmp::min(end, self.v.len()),
        };
        // SAFETY: see comments for `Chunks::get_unchecked`.
        //
        // Also note that the caller also guarantees that we're never called
        // with the same index again, and that no other methods that will
        // access this subslice are called, so it is valid for the returned
        // slice to be mutable.
        unsafe { from_raw_parts_mut(self.v.as_mut_ptr().add(start), end - start) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> DoubleEndedIterator for ChunksMut<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut [T]> {
        if self.v.is_empty() {
            None
        } else {
            let remainder = self.v.len() % self.chunk_size;
            let sz = if remainder != 0 { remainder } else { self.chunk_size };
            let tmp = mem::replace(&mut self.v, &mut []);
            let tmp_len = tmp.len();
            let (head, tail) = tmp.split_at_mut(tmp_len - sz);
            self.v = head;
            Some(tail)
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.len();
        if n >= len {
            self.v = &mut [];
            None
        } else {
            let start = (len - 1 - n) * self.chunk_size;
            let end = match start.checked_add(self.chunk_size) {
                Some(res) => cmp::min(res, self.v.len()),
                None => self.v.len(),
            };
            let (temp, _tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
            let (head, nth_back) = temp.split_at_mut(start);
            self.v = head;
            Some(nth_back)
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> ExactSizeIterator for ChunksMut<'_, T> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for ChunksMut<'_, T> {}

#[stable(feature = "fused", since = "1.26.0")]
impl<T> FusedIterator for ChunksMut<'_, T> {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for ChunksMut<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

/// An iterator over a slice in (non-overlapping) chunks (`chunk_size` elements at a
/// time), starting at the beginning of the slice.
///
/// When the slice len is not evenly divided by the chunk size, the last
/// up to `chunk_size-1` elements will be omitted but can be retrieved from
/// the [`remainder`] function from the iterator.
///
/// This struct is created by the [`chunks_exact`] method on [slices].
///
/// [`chunks_exact`]: ../../std/primitive.slice.html#method.chunks_exact
/// [`remainder`]: ChunksExact::remainder
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug)]
#[stable(feature = "chunks_exact", since = "1.31.0")]
pub struct ChunksExact<'a, T: 'a> {
    v: &'a [T],
    rem: &'a [T],
    chunk_size: usize,
}

impl<'a, T> ChunksExact<'a, T> {
    /// Returns the remainder of the original slice that is not going to be
    /// returned by the iterator. The returned slice has at most `chunk_size-1`
    /// elements.
    #[stable(feature = "chunks_exact", since = "1.31.0")]
    pub fn remainder(&self) -> &'a [T] {
        self.rem
    }
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[stable(feature = "chunks_exact", since = "1.31.0")]
impl<T> Clone for ChunksExact<'_, T> {
    fn clone(&self) -> Self {
        ChunksExact { v: self.v, rem: self.rem, chunk_size: self.chunk_size }
    }
}

#[stable(feature = "chunks_exact", since = "1.31.0")]
impl<'a, T> Iterator for ChunksExact<'a, T> {
    type Item = &'a [T];

    #[inline]
    fn next(&mut self) -> Option<&'a [T]> {
        if self.v.len() < self.chunk_size {
            None
        } else {
            let (fst, snd) = self.v.split_at(self.chunk_size);
            self.v = snd;
            Some(fst)
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let n = self.v.len() / self.chunk_size;
        (n, Some(n))
    }

    #[inline]
    fn count(self) -> usize {
        self.len()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        let (start, overflow) = n.overflowing_mul(self.chunk_size);
        if start >= self.v.len() || overflow {
            self.v = &[];
            None
        } else {
            let (_, snd) = self.v.split_at(start);
            self.v = snd;
            self.next()
        }
    }

    #[inline]
    fn last(mut self) -> Option<Self::Item> {
        self.next_back()
    }

    #[doc(hidden)]
    unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
        let start = idx * self.chunk_size;
        // SAFETY: mostly identical to `Chunks::get_unchecked`.
        unsafe { from_raw_parts(self.v.as_ptr().add(start), self.chunk_size) }
    }
}

#[stable(feature = "chunks_exact", since = "1.31.0")]
impl<'a, T> DoubleEndedIterator for ChunksExact<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a [T]> {
        if self.v.len() < self.chunk_size {
            None
        } else {
            let (fst, snd) = self.v.split_at(self.v.len() - self.chunk_size);
            self.v = fst;
            Some(snd)
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.len();
        if n >= len {
            self.v = &[];
            None
        } else {
            let start = (len - 1 - n) * self.chunk_size;
            let end = start + self.chunk_size;
            let nth_back = &self.v[start..end];
            self.v = &self.v[..start];
            Some(nth_back)
        }
    }
}

#[stable(feature = "chunks_exact", since = "1.31.0")]
impl<T> ExactSizeIterator for ChunksExact<'_, T> {
    fn is_empty(&self) -> bool {
        self.v.is_empty()
    }
}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for ChunksExact<'_, T> {}

#[stable(feature = "chunks_exact", since = "1.31.0")]
impl<T> FusedIterator for ChunksExact<'_, T> {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for ChunksExact<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

/// An iterator over a slice in (non-overlapping) mutable chunks (`chunk_size`
/// elements at a time), starting at the beginning of the slice.
///
/// When the slice len is not evenly divided by the chunk size, the last up to
/// `chunk_size-1` elements will be omitted but can be retrieved from the
/// [`into_remainder`] function from the iterator.
///
/// This struct is created by the [`chunks_exact_mut`] method on [slices].
///
/// [`chunks_exact_mut`]: ../../std/primitive.slice.html#method.chunks_exact_mut
/// [`into_remainder`]: ChunksExactMut::into_remainder
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug)]
#[stable(feature = "chunks_exact", since = "1.31.0")]
pub struct ChunksExactMut<'a, T: 'a> {
    v: &'a mut [T],
    rem: &'a mut [T],
    chunk_size: usize,
}

impl<'a, T> ChunksExactMut<'a, T> {
    /// Returns the remainder of the original slice that is not going to be
    /// returned by the iterator. The returned slice has at most `chunk_size-1`
    /// elements.
    #[stable(feature = "chunks_exact", since = "1.31.0")]
    pub fn into_remainder(self) -> &'a mut [T] {
        self.rem
    }
}

#[stable(feature = "chunks_exact", since = "1.31.0")]
impl<'a, T> Iterator for ChunksExactMut<'a, T> {
    type Item = &'a mut [T];

    #[inline]
    fn next(&mut self) -> Option<&'a mut [T]> {
        if self.v.len() < self.chunk_size {
            None
        } else {
            let tmp = mem::replace(&mut self.v, &mut []);
            let (head, tail) = tmp.split_at_mut(self.chunk_size);
            self.v = tail;
            Some(head)
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let n = self.v.len() / self.chunk_size;
        (n, Some(n))
    }

    #[inline]
    fn count(self) -> usize {
        self.len()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<&'a mut [T]> {
        let (start, overflow) = n.overflowing_mul(self.chunk_size);
        if start >= self.v.len() || overflow {
            self.v = &mut [];
            None
        } else {
            let tmp = mem::replace(&mut self.v, &mut []);
            let (_, snd) = tmp.split_at_mut(start);
            self.v = snd;
            self.next()
        }
    }

    #[inline]
    fn last(mut self) -> Option<Self::Item> {
        self.next_back()
    }

    #[doc(hidden)]
    unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
        let start = idx * self.chunk_size;
        // SAFETY: see comments for `ChunksMut::get_unchecked`.
        unsafe { from_raw_parts_mut(self.v.as_mut_ptr().add(start), self.chunk_size) }
    }
}

#[stable(feature = "chunks_exact", since = "1.31.0")]
impl<'a, T> DoubleEndedIterator for ChunksExactMut<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut [T]> {
        if self.v.len() < self.chunk_size {
            None
        } else {
            let tmp = mem::replace(&mut self.v, &mut []);
            let tmp_len = tmp.len();
            let (head, tail) = tmp.split_at_mut(tmp_len - self.chunk_size);
            self.v = head;
            Some(tail)
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.len();
        if n >= len {
            self.v = &mut [];
            None
        } else {
            let start = (len - 1 - n) * self.chunk_size;
            let end = start + self.chunk_size;
            let (temp, _tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
            let (head, nth_back) = temp.split_at_mut(start);
            self.v = head;
            Some(nth_back)
        }
    }
}

#[stable(feature = "chunks_exact", since = "1.31.0")]
impl<T> ExactSizeIterator for ChunksExactMut<'_, T> {
    fn is_empty(&self) -> bool {
        self.v.is_empty()
    }
}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for ChunksExactMut<'_, T> {}

#[stable(feature = "chunks_exact", since = "1.31.0")]
impl<T> FusedIterator for ChunksExactMut<'_, T> {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for ChunksExactMut<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

/// An iterator over a slice in (non-overlapping) chunks (`N` elements at a
/// time), starting at the beginning of the slice.
///
/// When the slice len is not evenly divided by the chunk size, the last
/// up to `chunk_size-1` elements will be omitted but can be retrieved from
/// the [`remainder`] function from the iterator.
///
/// This struct is created by the [`array_chunks`] method on [slices].
///
/// [`array_chunks`]: ../../std/primitive.slice.html#method.array_chunks
/// [`remainder`]: ArrayChunks::remainder
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug)]
#[unstable(feature = "array_chunks", issue = "74985")]
pub struct ArrayChunks<'a, T: 'a, const N: usize> {
    iter: Iter<'a, [T; N]>,
    rem: &'a [T],
}

impl<'a, T, const N: usize> ArrayChunks<'a, T, N> {
    /// Returns the remainder of the original slice that is not going to be
    /// returned by the iterator. The returned slice has at most `chunk_size-1`
    /// elements.
    #[unstable(feature = "array_chunks", issue = "74985")]
    pub fn remainder(&self) -> &'a [T] {
        self.rem
    }
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[unstable(feature = "array_chunks", issue = "74985")]
impl<T, const N: usize> Clone for ArrayChunks<'_, T, N> {
    fn clone(&self) -> Self {
        ArrayChunks { iter: self.iter.clone(), rem: self.rem }
    }
}

#[unstable(feature = "array_chunks", issue = "74985")]
impl<'a, T, const N: usize> Iterator for ArrayChunks<'a, T, N> {
    type Item = &'a [T; N];

    #[inline]
    fn next(&mut self) -> Option<&'a [T; N]> {
        self.iter.next()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }

    #[inline]
    fn count(self) -> usize {
        self.iter.count()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        self.iter.nth(n)
    }

    #[inline]
    fn last(self) -> Option<Self::Item> {
        self.iter.last()
    }

    unsafe fn get_unchecked(&mut self, i: usize) -> &'a [T; N] {
        // SAFETY: The safety guarantees of `get_unchecked` are transferred to
        // the caller.
        unsafe { self.iter.get_unchecked(i) }
    }
}

#[unstable(feature = "array_chunks", issue = "74985")]
impl<'a, T, const N: usize> DoubleEndedIterator for ArrayChunks<'a, T, N> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a [T; N]> {
        self.iter.next_back()
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        self.iter.nth_back(n)
    }
}

#[unstable(feature = "array_chunks", issue = "74985")]
impl<T, const N: usize> ExactSizeIterator for ArrayChunks<'_, T, N> {
    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T, const N: usize> TrustedLen for ArrayChunks<'_, T, N> {}

#[unstable(feature = "array_chunks", issue = "74985")]
impl<T, const N: usize> FusedIterator for ArrayChunks<'_, T, N> {}

#[doc(hidden)]
#[unstable(feature = "array_chunks", issue = "74985")]
unsafe impl<'a, T, const N: usize> TrustedRandomAccess for ArrayChunks<'a, T, N> {
    fn may_have_side_effect() -> bool {
        false
    }
}

/// An iterator over a slice in (non-overlapping) chunks (`chunk_size` elements at a
/// time), starting at the end of the slice.
///
/// When the slice len is not evenly divided by the chunk size, the last slice
/// of the iteration will be the remainder.
///
/// This struct is created by the [`rchunks`] method on [slices].
///
/// [`rchunks`]: ../../std/primitive.slice.html#method.rchunks
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug)]
#[stable(feature = "rchunks", since = "1.31.0")]
pub struct RChunks<'a, T: 'a> {
    v: &'a [T],
    chunk_size: usize,
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[stable(feature = "rchunks", since = "1.31.0")]
impl<T> Clone for RChunks<'_, T> {
    fn clone(&self) -> Self {
        RChunks { v: self.v, chunk_size: self.chunk_size }
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<'a, T> Iterator for RChunks<'a, T> {
    type Item = &'a [T];

    #[inline]
    fn next(&mut self) -> Option<&'a [T]> {
        if self.v.is_empty() {
            None
        } else {
            let chunksz = cmp::min(self.v.len(), self.chunk_size);
            let (fst, snd) = self.v.split_at(self.v.len() - chunksz);
            self.v = fst;
            Some(snd)
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.v.is_empty() {
            (0, Some(0))
        } else {
            let n = self.v.len() / self.chunk_size;
            let rem = self.v.len() % self.chunk_size;
            let n = if rem > 0 { n + 1 } else { n };
            (n, Some(n))
        }
    }

    #[inline]
    fn count(self) -> usize {
        self.len()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        let (end, overflow) = n.overflowing_mul(self.chunk_size);
        if end >= self.v.len() || overflow {
            self.v = &[];
            None
        } else {
            // Can't underflow because of the check above
            let end = self.v.len() - end;
            let start = match end.checked_sub(self.chunk_size) {
                Some(sum) => sum,
                None => 0,
            };
            let nth = &self.v[start..end];
            self.v = &self.v[0..start];
            Some(nth)
        }
    }

    #[inline]
    fn last(self) -> Option<Self::Item> {
        if self.v.is_empty() {
            None
        } else {
            let rem = self.v.len() % self.chunk_size;
            let end = if rem == 0 { self.chunk_size } else { rem };
            Some(&self.v[0..end])
        }
    }

    #[doc(hidden)]
    unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
        let end = self.v.len() - idx * self.chunk_size;
        let start = match end.checked_sub(self.chunk_size) {
            None => 0,
            Some(start) => start,
        };
        // SAFETY: mostly identical to `Chunks::get_unchecked`.
        unsafe { from_raw_parts(self.v.as_ptr().add(start), end - start) }
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<'a, T> DoubleEndedIterator for RChunks<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a [T]> {
        if self.v.is_empty() {
            None
        } else {
            let remainder = self.v.len() % self.chunk_size;
            let chunksz = if remainder != 0 { remainder } else { self.chunk_size };
            let (fst, snd) = self.v.split_at(chunksz);
            self.v = snd;
            Some(fst)
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.len();
        if n >= len {
            self.v = &[];
            None
        } else {
            // can't underflow because `n < len`
            let offset_from_end = (len - 1 - n) * self.chunk_size;
            let end = self.v.len() - offset_from_end;
            let start = end.saturating_sub(self.chunk_size);
            let nth_back = &self.v[start..end];
            self.v = &self.v[end..];
            Some(nth_back)
        }
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<T> ExactSizeIterator for RChunks<'_, T> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for RChunks<'_, T> {}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<T> FusedIterator for RChunks<'_, T> {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for RChunks<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

/// An iterator over a slice in (non-overlapping) mutable chunks (`chunk_size`
/// elements at a time), starting at the end of the slice.
///
/// When the slice len is not evenly divided by the chunk size, the last slice
/// of the iteration will be the remainder.
///
/// This struct is created by the [`rchunks_mut`] method on [slices].
///
/// [`rchunks_mut`]: ../../std/primitive.slice.html#method.rchunks_mut
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug)]
#[stable(feature = "rchunks", since = "1.31.0")]
pub struct RChunksMut<'a, T: 'a> {
    v: &'a mut [T],
    chunk_size: usize,
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<'a, T> Iterator for RChunksMut<'a, T> {
    type Item = &'a mut [T];

    #[inline]
    fn next(&mut self) -> Option<&'a mut [T]> {
        if self.v.is_empty() {
            None
        } else {
            let sz = cmp::min(self.v.len(), self.chunk_size);
            let tmp = mem::replace(&mut self.v, &mut []);
            let tmp_len = tmp.len();
            let (head, tail) = tmp.split_at_mut(tmp_len - sz);
            self.v = head;
            Some(tail)
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.v.is_empty() {
            (0, Some(0))
        } else {
            let n = self.v.len() / self.chunk_size;
            let rem = self.v.len() % self.chunk_size;
            let n = if rem > 0 { n + 1 } else { n };
            (n, Some(n))
        }
    }

    #[inline]
    fn count(self) -> usize {
        self.len()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<&'a mut [T]> {
        let (end, overflow) = n.overflowing_mul(self.chunk_size);
        if end >= self.v.len() || overflow {
            self.v = &mut [];
            None
        } else {
            // Can't underflow because of the check above
            let end = self.v.len() - end;
            let start = match end.checked_sub(self.chunk_size) {
                Some(sum) => sum,
                None => 0,
            };
            let tmp = mem::replace(&mut self.v, &mut []);
            let (head, tail) = tmp.split_at_mut(start);
            let (nth, _) = tail.split_at_mut(end - start);
            self.v = head;
            Some(nth)
        }
    }

    #[inline]
    fn last(self) -> Option<Self::Item> {
        if self.v.is_empty() {
            None
        } else {
            let rem = self.v.len() % self.chunk_size;
            let end = if rem == 0 { self.chunk_size } else { rem };
            Some(&mut self.v[0..end])
        }
    }

    #[doc(hidden)]
    unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
        let end = self.v.len() - idx * self.chunk_size;
        let start = match end.checked_sub(self.chunk_size) {
            None => 0,
            Some(start) => start,
        };
        // SAFETY: see comments for `RChunks::get_unchecked` and `ChunksMut::get_unchecked`
        unsafe { from_raw_parts_mut(self.v.as_mut_ptr().add(start), end - start) }
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<'a, T> DoubleEndedIterator for RChunksMut<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut [T]> {
        if self.v.is_empty() {
            None
        } else {
            let remainder = self.v.len() % self.chunk_size;
            let sz = if remainder != 0 { remainder } else { self.chunk_size };
            let tmp = mem::replace(&mut self.v, &mut []);
            let (head, tail) = tmp.split_at_mut(sz);
            self.v = tail;
            Some(head)
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.len();
        if n >= len {
            self.v = &mut [];
            None
        } else {
            // can't underflow because `n < len`
            let offset_from_end = (len - 1 - n) * self.chunk_size;
            let end = self.v.len() - offset_from_end;
            let start = end.saturating_sub(self.chunk_size);
            let (tmp, tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
            let (_, nth_back) = tmp.split_at_mut(start);
            self.v = tail;
            Some(nth_back)
        }
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<T> ExactSizeIterator for RChunksMut<'_, T> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for RChunksMut<'_, T> {}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<T> FusedIterator for RChunksMut<'_, T> {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for RChunksMut<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

/// An iterator over a slice in (non-overlapping) chunks (`chunk_size` elements at a
/// time), starting at the end of the slice.
///
/// When the slice len is not evenly divided by the chunk size, the last
/// up to `chunk_size-1` elements will be omitted but can be retrieved from
/// the [`remainder`] function from the iterator.
///
/// This struct is created by the [`rchunks_exact`] method on [slices].
///
/// [`rchunks_exact`]: ../../std/primitive.slice.html#method.rchunks_exact
/// [`remainder`]: ChunksExact::remainder
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug)]
#[stable(feature = "rchunks", since = "1.31.0")]
pub struct RChunksExact<'a, T: 'a> {
    v: &'a [T],
    rem: &'a [T],
    chunk_size: usize,
}

impl<'a, T> RChunksExact<'a, T> {
    /// Returns the remainder of the original slice that is not going to be
    /// returned by the iterator. The returned slice has at most `chunk_size-1`
    /// elements.
    #[stable(feature = "rchunks", since = "1.31.0")]
    pub fn remainder(&self) -> &'a [T] {
        self.rem
    }
}

// FIXME(#26925) Remove in favor of `#[derive(Clone)]`
#[stable(feature = "rchunks", since = "1.31.0")]
impl<'a, T> Clone for RChunksExact<'a, T> {
    fn clone(&self) -> RChunksExact<'a, T> {
        RChunksExact { v: self.v, rem: self.rem, chunk_size: self.chunk_size }
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<'a, T> Iterator for RChunksExact<'a, T> {
    type Item = &'a [T];

    #[inline]
    fn next(&mut self) -> Option<&'a [T]> {
        if self.v.len() < self.chunk_size {
            None
        } else {
            let (fst, snd) = self.v.split_at(self.v.len() - self.chunk_size);
            self.v = fst;
            Some(snd)
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let n = self.v.len() / self.chunk_size;
        (n, Some(n))
    }

    #[inline]
    fn count(self) -> usize {
        self.len()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<Self::Item> {
        let (end, overflow) = n.overflowing_mul(self.chunk_size);
        if end >= self.v.len() || overflow {
            self.v = &[];
            None
        } else {
            let (fst, _) = self.v.split_at(self.v.len() - end);
            self.v = fst;
            self.next()
        }
    }

    #[inline]
    fn last(mut self) -> Option<Self::Item> {
        self.next_back()
    }

    #[doc(hidden)]
    unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
        let end = self.v.len() - idx * self.chunk_size;
        let start = end - self.chunk_size;
        // SAFETY:
        // SAFETY: mostmy identical to `Chunks::get_unchecked`.
        unsafe { from_raw_parts(self.v.as_ptr().add(start), self.chunk_size) }
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<'a, T> DoubleEndedIterator for RChunksExact<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a [T]> {
        if self.v.len() < self.chunk_size {
            None
        } else {
            let (fst, snd) = self.v.split_at(self.chunk_size);
            self.v = snd;
            Some(fst)
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.len();
        if n >= len {
            self.v = &[];
            None
        } else {
            // now that we know that `n` corresponds to a chunk,
            // none of these operations can underflow/overflow
            let offset = (len - n) * self.chunk_size;
            let start = self.v.len() - offset;
            let end = start + self.chunk_size;
            let nth_back = &self.v[start..end];
            self.v = &self.v[end..];
            Some(nth_back)
        }
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<'a, T> ExactSizeIterator for RChunksExact<'a, T> {
    fn is_empty(&self) -> bool {
        self.v.is_empty()
    }
}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for RChunksExact<'_, T> {}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<T> FusedIterator for RChunksExact<'_, T> {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for RChunksExact<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

/// An iterator over a slice in (non-overlapping) mutable chunks (`chunk_size`
/// elements at a time), starting at the end of the slice.
///
/// When the slice len is not evenly divided by the chunk size, the last up to
/// `chunk_size-1` elements will be omitted but can be retrieved from the
/// [`into_remainder`] function from the iterator.
///
/// This struct is created by the [`rchunks_exact_mut`] method on [slices].
///
/// [`rchunks_exact_mut`]: ../../std/primitive.slice.html#method.rchunks_exact_mut
/// [`into_remainder`]: ChunksExactMut::into_remainder
/// [slices]: ../../std/primitive.slice.html
#[derive(Debug)]
#[stable(feature = "rchunks", since = "1.31.0")]
pub struct RChunksExactMut<'a, T: 'a> {
    v: &'a mut [T],
    rem: &'a mut [T],
    chunk_size: usize,
}

impl<'a, T> RChunksExactMut<'a, T> {
    /// Returns the remainder of the original slice that is not going to be
    /// returned by the iterator. The returned slice has at most `chunk_size-1`
    /// elements.
    #[stable(feature = "rchunks", since = "1.31.0")]
    pub fn into_remainder(self) -> &'a mut [T] {
        self.rem
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<'a, T> Iterator for RChunksExactMut<'a, T> {
    type Item = &'a mut [T];

    #[inline]
    fn next(&mut self) -> Option<&'a mut [T]> {
        if self.v.len() < self.chunk_size {
            None
        } else {
            let tmp = mem::replace(&mut self.v, &mut []);
            let tmp_len = tmp.len();
            let (head, tail) = tmp.split_at_mut(tmp_len - self.chunk_size);
            self.v = head;
            Some(tail)
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let n = self.v.len() / self.chunk_size;
        (n, Some(n))
    }

    #[inline]
    fn count(self) -> usize {
        self.len()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<&'a mut [T]> {
        let (end, overflow) = n.overflowing_mul(self.chunk_size);
        if end >= self.v.len() || overflow {
            self.v = &mut [];
            None
        } else {
            let tmp = mem::replace(&mut self.v, &mut []);
            let tmp_len = tmp.len();
            let (fst, _) = tmp.split_at_mut(tmp_len - end);
            self.v = fst;
            self.next()
        }
    }

    #[inline]
    fn last(mut self) -> Option<Self::Item> {
        self.next_back()
    }

    #[doc(hidden)]
    unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
        let end = self.v.len() - idx * self.chunk_size;
        let start = end - self.chunk_size;
        // SAFETY: see comments for `RChunksMut::get_unchecked`.
        unsafe { from_raw_parts_mut(self.v.as_mut_ptr().add(start), self.chunk_size) }
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<'a, T> DoubleEndedIterator for RChunksExactMut<'a, T> {
    #[inline]
    fn next_back(&mut self) -> Option<&'a mut [T]> {
        if self.v.len() < self.chunk_size {
            None
        } else {
            let tmp = mem::replace(&mut self.v, &mut []);
            let (head, tail) = tmp.split_at_mut(self.chunk_size);
            self.v = tail;
            Some(head)
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.len();
        if n >= len {
            self.v = &mut [];
            None
        } else {
            // now that we know that `n` corresponds to a chunk,
            // none of these operations can underflow/overflow
            let offset = (len - n) * self.chunk_size;
            let start = self.v.len() - offset;
            let end = start + self.chunk_size;
            let (tmp, tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
            let (_, nth_back) = tmp.split_at_mut(start);
            self.v = tail;
            Some(nth_back)
        }
    }
}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<T> ExactSizeIterator for RChunksExactMut<'_, T> {
    fn is_empty(&self) -> bool {
        self.v.is_empty()
    }
}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for RChunksExactMut<'_, T> {}

#[stable(feature = "rchunks", since = "1.31.0")]
impl<T> FusedIterator for RChunksExactMut<'_, T> {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for RChunksExactMut<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

//
// Free functions
//

/// Forms a slice from a pointer and a length.
///
/// The `len` argument is the number of **elements**, not the number of bytes.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `data` must be [valid] for reads for `len * mem::size_of::<T>()` many bytes,
///   and it must be properly aligned. This means in particular:
///
///     * The entire memory range of this slice must be contained within a single allocated object!
///       Slices can never span across multiple allocated objects. See [below](#incorrect-usage)
///       for an example incorrectly not taking this into account.
///     * `data` must be non-null and aligned even for zero-length slices. One
///       reason for this is that enum layout optimizations may rely on references
///       (including slices of any length) being aligned and non-null to distinguish
///       them from other data. You can obtain a pointer that is usable as `data`
///       for zero-length slices using [`NonNull::dangling()`].
///
/// * The memory referenced by the returned slice must not be mutated for the duration
///   of lifetime `'a`, except inside an `UnsafeCell`.
///
/// * The total size `len * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
///   See the safety documentation of [`pointer::offset`].
///
/// # Caveat
///
/// The lifetime for the returned slice is inferred from its usage. To
/// prevent accidental misuse, it's suggested to tie the lifetime to whichever
/// source lifetime is safe in the context, such as by providing a helper
/// function taking the lifetime of a host value for the slice, or by explicit
/// annotation.
///
/// # Examples
///
/// ```
/// use std::slice;
///
/// // manifest a slice for a single element
/// let x = 42;
/// let ptr = &x as *const _;
/// let slice = unsafe { slice::from_raw_parts(ptr, 1) };
/// assert_eq!(slice[0], 42);
/// ```
///
/// ### Incorrect usage
///
/// The following `join_slices` function is **unsound** ⚠️
///
/// ```rust,no_run
/// use std::slice;
///
/// fn join_slices<'a, T>(fst: &'a [T], snd: &'a [T]) -> &'a [T] {
///     let fst_end = fst.as_ptr().wrapping_add(fst.len());
///     let snd_start = snd.as_ptr();
///     assert_eq!(fst_end, snd_start, "Slices must be contiguous!");
///     unsafe {
///         // The assertion above ensures `fst` and `snd` are contiguous, but they might
///         // still be contained within _different allocated objects_, in which case
///         // creating this slice is undefined behavior.
///         slice::from_raw_parts(fst.as_ptr(), fst.len() + snd.len())
///     }
/// }
///
/// fn main() {
///     // `a` and `b` are different allocated objects...
///     let a = 42;
///     let b = 27;
///     // ... which may nevertheless be laid out contiguously in memory: | a | b |
///     let _ = join_slices(slice::from_ref(&a), slice::from_ref(&b)); // UB
/// }
/// ```
///
/// [valid]: ptr#safety
/// [`NonNull::dangling()`]: ptr::NonNull::dangling
/// [`pointer::offset`]: ../../std/primitive.pointer.html#method.offset
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] {
    debug_assert!(is_aligned_and_not_null(data), "attempt to create unaligned or null slice");
    debug_assert!(
        mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize,
        "attempt to create slice covering at least half the address space"
    );
    // SAFETY: the caller must uphold the safety contract for `from_raw_parts`.
    unsafe { &*ptr::slice_from_raw_parts(data, len) }
}

/// Performs the same functionality as [`from_raw_parts`], except that a
/// mutable slice is returned.
///
/// # Safety
///
/// Behavior is undefined if any of the following conditions are violated:
///
/// * `data` must be [valid] for boths reads and writes for `len * mem::size_of::<T>()` many bytes,
///   and it must be properly aligned. This means in particular:
///
///     * The entire memory range of this slice must be contained within a single allocated object!
///       Slices can never span across multiple allocated objects.
///     * `data` must be non-null and aligned even for zero-length slices. One
///       reason for this is that enum layout optimizations may rely on references
///       (including slices of any length) being aligned and non-null to distinguish
///       them from other data. You can obtain a pointer that is usable as `data`
///       for zero-length slices using [`NonNull::dangling()`].
///
/// * The memory referenced by the returned slice must not be accessed through any other pointer
///   (not derived from the return value) for the duration of lifetime `'a`.
///   Both read and write accesses are forbidden.
///
/// * The total size `len * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
///   See the safety documentation of [`pointer::offset`].
///
/// [valid]: ptr#safety
/// [`NonNull::dangling()`]: ptr::NonNull::dangling
/// [`pointer::offset`]: ../../std/primitive.pointer.html#method.offset
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T] {
    debug_assert!(is_aligned_and_not_null(data), "attempt to create unaligned or null slice");
    debug_assert!(
        mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize,
        "attempt to create slice covering at least half the address space"
    );
    // SAFETY: the caller must uphold the safety contract for `from_raw_parts_mut`.
    unsafe { &mut *ptr::slice_from_raw_parts_mut(data, len) }
}

/// Converts a reference to T into a slice of length 1 (without copying).
#[stable(feature = "from_ref", since = "1.28.0")]
pub fn from_ref<T>(s: &T) -> &[T] {
    // SAFETY: a reference is guaranteed to be valid for reads. The returned
    // reference cannot be mutated as it is an immutable reference.
    // `mem::size_of::<T>()` cannot be larger than `isize::MAX`.
    // Thus the call to `from_raw_parts` is safe.
    unsafe { from_raw_parts(s, 1) }
}

/// Converts a reference to T into a slice of length 1 (without copying).
#[stable(feature = "from_ref", since = "1.28.0")]
pub fn from_mut<T>(s: &mut T) -> &mut [T] {
    // SAFETY: a mutable reference is guaranteed to be valid for writes.
    // The reference cannot be accessed by another pointer as it is an mutable reference.
    // `mem::size_of::<T>()` cannot be larger than `isize::MAX`.
    // Thus the call to `from_raw_parts_mut` is safe.
    unsafe { from_raw_parts_mut(s, 1) }
}

// This function is public only because there is no other way to unit test heapsort.
#[unstable(feature = "sort_internals", reason = "internal to sort module", issue = "none")]
#[doc(hidden)]
pub fn heapsort<T, F>(v: &mut [T], mut is_less: F)
where
    F: FnMut(&T, &T) -> bool,
{
    sort::heapsort(v, &mut is_less);
}

//
// Comparison traits
//

extern "C" {
    /// Calls implementation provided memcmp.
    ///
    /// Interprets the data as u8.
    ///
    /// Returns 0 for equal, < 0 for less than and > 0 for greater
    /// than.
    // FIXME(#32610): Return type should be c_int
    fn memcmp(s1: *const u8, s2: *const u8, n: usize) -> i32;
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<A, B> PartialEq<[B]> for [A]
where
    A: PartialEq<B>,
{
    fn eq(&self, other: &[B]) -> bool {
        SlicePartialEq::equal(self, other)
    }

    fn ne(&self, other: &[B]) -> bool {
        SlicePartialEq::not_equal(self, other)
    }
}

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

/// Implements comparison of vectors lexicographically.
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Ord> Ord for [T] {
    fn cmp(&self, other: &[T]) -> Ordering {
        SliceOrd::compare(self, other)
    }
}

/// Implements comparison of vectors lexicographically.
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: PartialOrd> PartialOrd for [T] {
    fn partial_cmp(&self, other: &[T]) -> Option<Ordering> {
        SlicePartialOrd::partial_compare(self, other)
    }
}

#[doc(hidden)]
// intermediate trait for specialization of slice's PartialEq
trait SlicePartialEq<B> {
    fn equal(&self, other: &[B]) -> bool;

    fn not_equal(&self, other: &[B]) -> bool {
        !self.equal(other)
    }
}

// Generic slice equality
impl<A, B> SlicePartialEq<B> for [A]
where
    A: PartialEq<B>,
{
    default fn equal(&self, other: &[B]) -> bool {
        if self.len() != other.len() {
            return false;
        }

        self.iter().zip(other.iter()).all(|(x, y)| x == y)
    }
}

// Use an equal-pointer optimization when types are `Eq`
// We can't make `A` and `B` the same type because `min_specialization` won't
// allow it.
impl<A, B> SlicePartialEq<B> for [A]
where
    A: MarkerEq<B>,
{
    default fn equal(&self, other: &[B]) -> bool {
        if self.len() != other.len() {
            return false;
        }

        // While performance would suffer if `guaranteed_eq` just returned `false`
        // for all arguments, correctness and return value of this function are not affected.
        if self.as_ptr().guaranteed_eq(other.as_ptr() as *const A) {
            return true;
        }

        self.iter().zip(other.iter()).all(|(x, y)| x == y)
    }
}

// Use memcmp for bytewise equality when the types allow
impl<A, B> SlicePartialEq<B> for [A]
where
    A: BytewiseEquality<B>,
{
    fn equal(&self, other: &[B]) -> bool {
        if self.len() != other.len() {
            return false;
        }

        // While performance would suffer if `guaranteed_eq` just returned `false`
        // for all arguments, correctness and return value of this function are not affected.
        if self.as_ptr().guaranteed_eq(other.as_ptr() as *const A) {
            return true;
        }
        // SAFETY: `self` and `other` are references and are thus guaranteed to be valid.
        // The two slices have been checked to have the same size above.
        unsafe {
            let size = mem::size_of_val(self);
            memcmp(self.as_ptr() as *const u8, other.as_ptr() as *const u8, size) == 0
        }
    }
}

#[doc(hidden)]
// intermediate trait for specialization of slice's PartialOrd
trait SlicePartialOrd: Sized {
    fn partial_compare(left: &[Self], right: &[Self]) -> Option<Ordering>;
}

impl<A: PartialOrd> SlicePartialOrd for A {
    default fn partial_compare(left: &[A], right: &[A]) -> Option<Ordering> {
        let l = cmp::min(left.len(), right.len());

        // Slice to the loop iteration range to enable bound check
        // elimination in the compiler
        let lhs = &left[..l];
        let rhs = &right[..l];

        for i in 0..l {
            match lhs[i].partial_cmp(&rhs[i]) {
                Some(Ordering::Equal) => (),
                non_eq => return non_eq,
            }
        }

        left.len().partial_cmp(&right.len())
    }
}

// This is the impl that we would like to have. Unfortunately it's not sound.
// See `partial_ord_slice.rs`.
/*
impl<A> SlicePartialOrd for A
where
    A: Ord,
{
    default fn partial_compare(left: &[A], right: &[A]) -> Option<Ordering> {
        Some(SliceOrd::compare(left, right))
    }
}
*/

impl<A: AlwaysApplicableOrd> SlicePartialOrd for A {
    fn partial_compare(left: &[A], right: &[A]) -> Option<Ordering> {
        Some(SliceOrd::compare(left, right))
    }
}

#[rustc_specialization_trait]
trait AlwaysApplicableOrd: SliceOrd + Ord {}

macro_rules! always_applicable_ord {
    ($([$($p:tt)*] $t:ty,)*) => {
        $(impl<$($p)*> AlwaysApplicableOrd for $t {})*
    }
}

always_applicable_ord! {
    [] u8, [] u16, [] u32, [] u64, [] u128, [] usize,
    [] i8, [] i16, [] i32, [] i64, [] i128, [] isize,
    [] bool, [] char,
    [T: ?Sized] *const T, [T: ?Sized] *mut T,
    [T: AlwaysApplicableOrd] &T,
    [T: AlwaysApplicableOrd] &mut T,
    [T: AlwaysApplicableOrd] Option<T>,
}

#[doc(hidden)]
// intermediate trait for specialization of slice's Ord
trait SliceOrd: Sized {
    fn compare(left: &[Self], right: &[Self]) -> Ordering;
}

impl<A: Ord> SliceOrd for A {
    default fn compare(left: &[Self], right: &[Self]) -> Ordering {
        let l = cmp::min(left.len(), right.len());

        // Slice to the loop iteration range to enable bound check
        // elimination in the compiler
        let lhs = &left[..l];
        let rhs = &right[..l];

        for i in 0..l {
            match lhs[i].cmp(&rhs[i]) {
                Ordering::Equal => (),
                non_eq => return non_eq,
            }
        }

        left.len().cmp(&right.len())
    }
}

// memcmp compares a sequence of unsigned bytes lexicographically.
// this matches the order we want for [u8], but no others (not even [i8]).
impl SliceOrd for u8 {
    #[inline]
    fn compare(left: &[Self], right: &[Self]) -> Ordering {
        let order =
            // SAFETY: `left` and `right` are references and are thus guaranteed to be valid.
            // We use the minimum of both lengths which guarantees that both regions are
            // valid for reads in that interval.
            unsafe { memcmp(left.as_ptr(), right.as_ptr(), cmp::min(left.len(), right.len())) };
        if order == 0 {
            left.len().cmp(&right.len())
        } else if order < 0 {
            Less
        } else {
            Greater
        }
    }
}

// Hack to allow specializing on `Eq` even though `Eq` has a method.
#[rustc_unsafe_specialization_marker]
trait MarkerEq<T>: PartialEq<T> {}

impl<T: Eq> MarkerEq<T> for T {}

#[doc(hidden)]
/// Trait implemented for types that can be compared for equality using
/// their bytewise representation
#[rustc_specialization_trait]
trait BytewiseEquality<T>: MarkerEq<T> + Copy {}

macro_rules! impl_marker_for {
    ($traitname:ident, $($ty:ty)*) => {
        $(
            impl $traitname<$ty> for $ty { }
        )*
    }
}

impl_marker_for!(BytewiseEquality,
                 u8 i8 u16 i16 u32 i32 u64 i64 u128 i128 usize isize char bool);

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for Iter<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<'a, T> TrustedRandomAccess for IterMut<'a, T> {
    fn may_have_side_effect() -> bool {
        false
    }
}

trait SliceContains: Sized {
    fn slice_contains(&self, x: &[Self]) -> bool;
}

impl<T> SliceContains for T
where
    T: PartialEq,
{
    default fn slice_contains(&self, x: &[Self]) -> bool {
        x.iter().any(|y| *y == *self)
    }
}

impl SliceContains for u8 {
    fn slice_contains(&self, x: &[Self]) -> bool {
        memchr::memchr(*self, x).is_some()
    }
}

impl SliceContains for i8 {
    fn slice_contains(&self, x: &[Self]) -> bool {
        let byte = *self as u8;
        // SAFETY: `i8` and `u8` have the same memory layout, thus casting `x.as_ptr()`
        // as `*const u8` is safe. The `x.as_ptr()` comes from a reference and is thus guaranteed
        // to be valid for reads for the length of the slice `x.len()`, which cannot be larger
        // than `isize::MAX`. The returned slice is never mutated.
        let bytes: &[u8] = unsafe { from_raw_parts(x.as_ptr() as *const u8, x.len()) };
        memchr::memchr(byte, bytes).is_some()
    }
}