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    // ignore-tidy-filelength
    // ignore-tidy-undocumented-unsafe

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

    #[unstable(feature = "slice_internals", issue = "0",
               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")]
        #[cfg_attr(not(bootstrap), 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 {
            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")]
        #[cfg_attr(
            not(bootstrap),
            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> {
            self.get(0)
        }

        /// 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> {
            self.get_mut(0)
        }

        /// 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 self.is_empty() { None } else { Some((&self[0], &self[1..])) }
        }

        /// 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 self.is_empty() { None } else {
                let split = self.split_at_mut(1);
                Some((&mut split.0[0], split.1))
            }
        }

        /// 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])> {
            let len = self.len();
            if len == 0 { None } else { Some((&self[len - 1], &self[..(len - 1)])) }
        }

        /// 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])> {
            let len = self.len();
            if len == 0 { None } else {
                let split = self.split_at_mut(len - 1);
                Some((&mut split.1[0], split.0))
            }

        }

        /// 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> {
            let last_idx = self.len().checked_sub(1)?;
            self.get(last_idx)
        }

        /// 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> {
            let last_idx = self.len().checked_sub(1)?;
            self.get_mut(last_idx)
        }

        /// 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.
        ///
        /// This is generally not recommended, use with caution!
        /// Calling this method with an out-of-bounds index is *[undefined behavior]*
        /// even if the resulting reference is not used.
        /// For a safe alternative see [`get`].
        ///
        /// [`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>
        {
            index.get_unchecked(self)
        }

        /// Returns a mutable reference to an element or subslice, without doing
        /// bounds checking.
        ///
        /// This is generally not recommended, use with caution!
        /// Calling this method with an out-of-bounds index is *[undefined behavior]*
        /// even if the resulting reference is not used.
        /// For a safe alternative see [`get_mut`].
        ///
        /// [`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>
        {
            index.get_unchecked_mut(self)
        }

        /// 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")]
        #[cfg_attr(
            not(bootstrap),
            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 size.
        ///
        /// 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> {
            // 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 start = self.as_ptr();
            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 size.
        ///
        /// 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> {
            // See as_ptr_range() above for why `add` here is safe.
            let start = self.as_mut_ptr();
            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) {
            unsafe {
                // 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];
                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 {
                    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 {
                    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 {
                // Unsafe swap to avoid the bounds check in safe swap.
                unsafe {
                    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> {
            unsafe {
                let ptr = self.as_ptr();
                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,
                    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> {
            unsafe {
                let ptr = self.as_mut_ptr();
                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,
                    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!(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!(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!(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!(chunk_size != 0);
            let rem = self.len() % chunk_size;
            let len = self.len() - rem;
            let (fst, snd) = self.split_at(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!(chunk_size != 0);
            let rem = self.len() % chunk_size;
            let len = self.len() - rem;
            let (fst, snd) = self.split_at_mut(len);
            ChunksExactMut { v: fst, rem: snd, 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 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;
            let (fst, snd) = self.split_at(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;
            let (fst, snd) = self.split_at_mut(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!(left == []);
        ///    assert!(right == [1, 2, 3, 4, 5, 6]);
        /// }
        ///
        /// {
        ///     let (left, right) = v.split_at(2);
        ///     assert!(left == [1, 2]);
        ///     assert!(right == [3, 4, 5, 6]);
        /// }
        ///
        /// {
        ///     let (left, right) = v.split_at(6);
        ///     assert!(left == [1, 2, 3, 4, 5, 6]);
        ///     assert!(right == []);
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn split_at(&self, mid: usize) -> (&[T], &[T]) {
            (&self[..mid], &self[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!(left == [1, 0]);
        ///     assert!(right == [3, 0, 5, 6]);
        ///     left[1] = 2;
        ///     right[1] = 4;
        /// }
        /// assert!(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]) {
            let len = self.len();
            let ptr = self.as_mut_ptr();

            unsafe {
                assert!(mid <= len);

                (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`, 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..]
        }

        /// 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;
                // mid is always in [0, size), that means mid is >= 0 and < size.
                // 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;
            }
            // 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_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(m 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;

            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;

            unsafe {
                let p = self.as_mut_ptr();
                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;

            unsafe {
                let p = self.as_mut_ptr();
                rotate::ptr_rotate(mid, p.add(mid), k);
            }
        }

        /// Copies the elements from `src` into `self`.
        ///
        /// The length of `src` must be the same as `self`.
        ///
        /// If `src` 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 `src` 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 {
            assert_eq!(self.len(), src.len(),
                       "destination and source slices have different lengths");
            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")]
        pub fn copy_within<R: ops::RangeBounds<usize>>(&mut self, src: R, dest: usize)
        where
            T: Copy,
        {
            let src_start = match src.start_bound() {
                ops::Bound::Included(&n) => n,
                ops::Bound::Excluded(&n) => n
                    .checked_add(1)
                    .unwrap_or_else(|| slice_index_overflow_fail()),
                ops::Bound::Unbounded => 0,
            };
            let src_end = match src.end_bound() {
                ops::Bound::Included(&n) => n
                    .checked_add(1)
                    .unwrap_or_else(|| slice_index_overflow_fail()),
                ops::Bound::Excluded(&n) => n,
                ops::Bound::Unbounded => self.len(),
            };
            assert!(src_start <= src_end, "src end is before src start");
            assert!(src_end <= self.len(), "src is out of bounds");
            let count = src_end - src_start;
            assert!(dest <= self.len() - count, "dest is out of bounds");
            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");
            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.
                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;
                    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();
            let offset = crate::ptr::align_offset(ptr, mem::align_of::<U>());
            if offset > self.len() {
                (self, &[], &[])
            } else {
                let (left, rest) = self.split_at(offset);
                // now `rest` is definitely aligned, so `from_raw_parts_mut` below is okay
                let (us_len, ts_len) = rest.align_to_offsets::<U>();
                (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();
            let offset = 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);
                // now `rest` is definitely aligned, so `from_raw_parts_mut` below is okay
                let (us_len, ts_len) = rest.align_to_offsets::<U>();
                let mut_ptr = rest.as_mut_ptr();
                (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, std::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)
        }
    }

    #[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 {
            self.iter().all(|b| b.is_ascii())
        }

        /// 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();
            }
        }

    }

    #[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]
    fn slice_index_len_fail(index: usize, len: usize) -> ! {
        panic!("index {} out of range for slice of length {}", index, len);
    }

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

    #[inline(never)]
    #[cold]
    fn slice_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.
    #[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 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 = "0")]
        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 = "0")]
        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 is *[undefined behavior]*
        /// even if the resulting reference is not used.
        /// [undefined behavior]: ../../reference/behavior-considered-undefined.html
        #[unstable(feature = "slice_index_methods", issue = "0")]
        unsafe fn get_unchecked(self, slice: &T) -> &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 is *[undefined behavior]*
        /// even if the resulting reference is not used.
        /// [undefined behavior]: ../../reference/behavior-considered-undefined.html
        #[unstable(feature = "slice_index_methods", issue = "0")]
        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 = "0")]
        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 = "0")]
        fn index_mut(self, slice: &mut T) -> &mut Self::Output;
    }

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

        #[inline]
        fn get(self, slice: &[T]) -> Option<&T> {
            if self < slice.len() {
                unsafe {
                    Some(self.get_unchecked(slice))
                }
            } else {
                None
            }
        }

        #[inline]
        fn get_mut(self, slice: &mut [T]) -> Option<&mut T> {
            if self < slice.len() {
                unsafe {
                    Some(self.get_unchecked_mut(slice))
                }
            } else {
                None
            }
        }

        #[inline]
        unsafe fn get_unchecked(self, slice: &[T]) -> &T {
            &*slice.as_ptr().add(self)
        }

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

        #[inline]
        unsafe fn get_unchecked(self, slice: &[T]) -> &[T] {
            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] {
            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_index_len_fail(self.end, slice.len());
            }
            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_index_len_fail(self.end, slice.len());
            }
            unsafe {
                self.get_unchecked_mut(slice)
            }
        }
    }

    #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
    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: &[T]) -> &[T] {
            (0..self.end).get_unchecked(slice)
        }

        #[inline]
        unsafe fn get_unchecked_mut(self, slice: &mut [T]) -> &mut [T] {
            (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")]
    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: &[T]) -> &[T] {
            (self.start..slice.len()).get_unchecked(slice)
        }

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

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

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

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

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

        #[inline]
        unsafe fn get_unchecked(self, slice: &[T]) -> &[T] {
            (*self.start()..self.end() + 1).get_unchecked(slice)
        }

        #[inline]
        unsafe fn get_unchecked_mut(self, slice: &mut [T]) -> &mut [T] {
            (*self.start()..self.end() + 1).get_unchecked_mut(slice)
        }

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

    #[stable(feature = "inclusive_range", since = "1.26.0")]
    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: &[T]) -> &[T] {
            (0..=self.end).get_unchecked(slice)
        }

        #[inline]
        unsafe fn get_unchecked_mut(self, slice: &mut [T]) -> &mut [T] {
            (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 == $self.end}
    }
    // To get rid of some bounds checks (see `position`), we compute the length in a somewhat
    // unexpected way. (Tested by `codegen/slice-position-bounds-check`.)
    macro_rules! len {
        ($self: ident) => {{
            #![allow(unused_unsafe)] // we're sometimes used within an unsafe block

            let start = $self.ptr;
            let size = size_from_ptr(start);
            if size == 0 {
                // This _cannot_ use `unchecked_sub` because we depend on wrapping
                // to represent the length of long ZST slice iterators.
                ($self.end as usize).wrapping_sub(start as 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 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] {
                    unsafe { from_raw_parts(self.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
                    } else {
                        let old = self.ptr;
                        self.ptr = self.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
                    } else {
                        self.end = 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
                    unsafe {
                        assume(!self.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;
                        } else {
                            self.ptr = self.end;
                        }
                        return None;
                    }
                    // 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()
                }

                #[inline]
                #[rustc_inherit_overflow_checks]
                fn position<P>(&mut self, mut predicate: P) -> Option<usize> where
                    Self: Sized,
                    P: FnMut(Self::Item) -> bool,
                {
                    // The addition might panic on overflow.
                    let n = len!(self);
                    self.try_fold(0, move |i, x| {
                        if predicate(x) { Err(i) }
                        else { Ok(i + 1) }
                    }).err()
                        .map(|i| {
                            unsafe { assume(i < n) };
                            i
                        })
                }

                #[inline]
                fn rposition<P>(&mut self, mut predicate: P) -> Option<usize> where
                    P: FnMut(Self::Item) -> bool,
                    Self: Sized + ExactSizeIterator + DoubleEndedIterator
                {
                    // No need for an overflow check here, because `ExactSizeIterator`
                    let n = len!(self);
                    self.try_rfold(n, move |i, x| {
                        let i = i - 1;
                        if predicate(x) { Err(i) }
                        else { Ok(i) }
                    }).err()
                        .map(|i| {
                            unsafe { assume(i < n) };
                            i
                        })
                }

                $($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
                    unsafe {
                        assume(!self.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;
                        return None;
                    }
                    // 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: *const 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: *mut 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] {
            unsafe { from_raw_parts_mut(self.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 the 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 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..])
            }
        }
    }

    #[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)]
    unsafe impl<'a, T> TrustedRandomAccess for Windows<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a [T] {
            from_raw_parts(self.v.as_ptr().add(i), self.size)
        }
        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..])
            }
        }
    }

    #[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)]
    unsafe impl<'a, T> TrustedRandomAccess for Chunks<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a [T] {
            let start = i * self.chunk_size;
            let end = match start.checked_add(self.chunk_size) {
                None => self.v.len(),
                Some(end) => cmp::min(end, self.v.len()),
            };
            from_raw_parts(self.v.as_ptr().add(start), end - start)
        }
        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..])
            }
        }
    }

    #[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)]
    unsafe impl<'a, T> TrustedRandomAccess for ChunksMut<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a mut [T] {
            let start = i * self.chunk_size;
            let end = match start.checked_add(self.chunk_size) {
                None => self.v.len(),
                Some(end) => cmp::min(end, self.v.len()),
            };
            from_raw_parts_mut(self.v.as_mut_ptr().add(start), end - start)
        }
        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`]: ../../std/slice/struct.ChunksExact.html#method.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()
        }
    }

    #[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)]
    #[stable(feature = "chunks_exact", since = "1.31.0")]
    unsafe impl<'a, T> TrustedRandomAccess for ChunksExact<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a [T] {
            let start = i * self.chunk_size;
            from_raw_parts(self.v.as_ptr().add(start), self.chunk_size)
        }
        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`]: ../../std/slice/struct.ChunksExactMut.html#method.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()
        }
    }

    #[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)]
    #[stable(feature = "chunks_exact", since = "1.31.0")]
    unsafe impl<'a, T> TrustedRandomAccess for ChunksExactMut<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a mut [T] {
            let start = i * self.chunk_size;
            from_raw_parts_mut(self.v.as_mut_ptr().add(start), self.chunk_size)
        }
        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])
            }
        }
    }

    #[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)]
    #[stable(feature = "rchunks", since = "1.31.0")]
    unsafe impl<'a, T> TrustedRandomAccess for RChunks<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a [T] {
            let end = self.v.len() - i * self.chunk_size;
            let start = match end.checked_sub(self.chunk_size) {
                None => 0,
                Some(start) => start,
            };
            from_raw_parts(self.v.as_ptr().add(start), end - start)
        }
        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])
            }
        }
    }

    #[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)]
    #[stable(feature = "rchunks", since = "1.31.0")]
    unsafe impl<'a, T> TrustedRandomAccess for RChunksMut<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a mut [T] {
            let end = self.v.len() - i * self.chunk_size;
            let start = match end.checked_sub(self.chunk_size) {
                None => 0,
                Some(start) => start,
            };
            from_raw_parts_mut(self.v.as_mut_ptr().add(start), end - start)
        }
        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`]: ../../std/slice/struct.ChunksExact.html#method.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()
        }
    }

    #[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)]
    #[stable(feature = "rchunks", since = "1.31.0")]
    unsafe impl<'a, T> TrustedRandomAccess for RChunksExact<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a [T] {
            let end = self.v.len() - i * self.chunk_size;
            let start = end - self.chunk_size;
            from_raw_parts(self.v.as_ptr().add(start), self.chunk_size)
        }
        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`]: ../../std/slice/struct.ChunksExactMut.html#method.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()
        }
    }

    #[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)]
    #[stable(feature = "rchunks", since = "1.31.0")]
    unsafe impl<'a, T> TrustedRandomAccess for RChunksExactMut<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a mut [T] {
            let end = self.v.len() - i * self.chunk_size;
            let start = end - self.chunk_size;
            from_raw_parts_mut(self.v.as_mut_ptr().add(start), self.chunk_size)
        }
        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.
    ///     * `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);
    /// ```
    ///
    /// [valid]: ../../std/ptr/index.html#safety
    /// [`NonNull::dangling()`]: ../../std/ptr/struct.NonNull.html#method.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");
        &*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 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]: ../../std/ptr/index.html#safety
    /// [`NonNull::dangling()`]: ../../std/ptr/struct.NonNull.html#method.dangling
    /// [`pointer::offset`]: ../../std/primitive.pointer.html#method.offset
    /// [`from_raw_parts`]: ../../std/slice/fn.from_raw_parts.html
    #[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");
        &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] {
        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] {
        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 = "0")]
    #[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 {
        /// 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`
    impl<A> SlicePartialEq<A> for [A]
        where A: PartialEq<A> + Eq
    {
        default fn equal(&self, other: &[A]) -> bool {
            if self.len() != other.len() {
                return false;
            }

            if self.as_ptr() == other.as_ptr() {
                return true;
            }

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

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

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

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

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

            self.len().partial_cmp(&other.len())
        }
    }

    impl<A> SlicePartialOrd<A> for [A]
        where A: Ord
    {
        default fn partial_compare(&self, other: &[A]) -> Option<Ordering> {
            Some(SliceOrd::compare(self, other))
        }
    }

    #[doc(hidden)]
    // intermediate trait for specialization of slice's Ord
    trait SliceOrd<B> {
        fn compare(&self, other: &[B]) -> Ordering;
    }

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

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

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

            self.len().cmp(&other.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<u8> for [u8] {
        #[inline]
        fn compare(&self, other: &[u8]) -> Ordering {
            let order = unsafe {
                memcmp(self.as_ptr(), other.as_ptr(),
                       cmp::min(self.len(), other.len()))
            };
            if order == 0 {
                self.len().cmp(&other.len())
            } else if order < 0 {
                Less
            } else {
                Greater
            }
        }
    }

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

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

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

    #[doc(hidden)]
    unsafe impl<'a, T> TrustedRandomAccess for Iter<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a T {
            &*self.ptr.add(i)
        }
        fn may_have_side_effect() -> bool { false }
    }

    #[doc(hidden)]
    unsafe impl<'a, T> TrustedRandomAccess for IterMut<'a, T> {
        unsafe fn get_unchecked(&mut self, i: usize) -> &'a mut T {
            &mut *self.ptr.add(i)
        }
        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;
            let bytes: &[u8] = unsafe { from_raw_parts(x.as_ptr() as *const u8, x.len()) };
            memchr::memchr(byte, bytes).is_some()
        }
    }