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

    //! A dynamically-sized view into a contiguous sequence, `[T]`.
    //!
    //! Slices are a view into a block of memory represented as a pointer and a
    //! length.
    //!
    //! ```
    //! // slicing a Vec
    //! let vec = vec![1, 2, 3];
    //! let int_slice = &vec[..];
    //! // coercing an array to a slice
    //! let str_slice: &[&str] = &["one", "two", "three"];
    //! ```
    //!
    //! Slices are either mutable or shared. The shared slice type is `&[T]`,
    //! while the mutable slice type is `&mut [T]`, where `T` represents the element
    //! type. For example, you can mutate the block of memory that a mutable slice
    //! points to:
    //!
    //! ```
    //! let x = &mut [1, 2, 3];
    //! x[1] = 7;
    //! assert_eq!(x, &[1, 7, 3]);
    //! ```
    //!
    //! Here are some of the things this module contains:
    //!
    //! ## Structs
    //!
    //! There are several structs that are useful for slices, such as [`Iter`], which
    //! represents iteration over a slice.
    //!
    //! ## Trait Implementations
    //!
    //! There are several implementations of common traits for slices. Some examples
    //! include:
    //!
    //! * [`Clone`]
    //! * [`Eq`], [`Ord`] - for slices whose element type are [`Eq`] or [`Ord`].
    //! * [`Hash`] - for slices whose element type is [`Hash`].
    //!
    //! ## Iteration
    //!
    //! The slices implement `IntoIterator`. The iterator yields references to the
    //! slice elements.
    //!
    //! ```
    //! let numbers = &[0, 1, 2];
    //! for n in numbers {
    //!     println!("{} is a number!", n);
    //! }
    //! ```
    //!
    //! The mutable slice yields mutable references to the elements:
    //!
    //! ```
    //! let mut scores = [7, 8, 9];
    //! for score in &mut scores[..] {
    //!     *score += 1;
    //! }
    //! ```
    //!
    //! This iterator yields mutable references to the slice's elements, so while
    //! the element type of the slice is `i32`, the element type of the iterator is
    //! `&mut i32`.
    //!
    //! * [`.iter`] and [`.iter_mut`] are the explicit methods to return the default
    //!   iterators.
    //! * Further methods that return iterators are [`.split`], [`.splitn`],
    //!   [`.chunks`], [`.windows`] and more.
    //!
    //! *[See also the slice primitive type](../../std/primitive.slice.html).*
    //!
    //! [`Clone`]: ../../std/clone/trait.Clone.html
    //! [`Eq`]: ../../std/cmp/trait.Eq.html
    //! [`Ord`]: ../../std/cmp/trait.Ord.html
    //! [`Iter`]: struct.Iter.html
    //! [`Hash`]: ../../std/hash/trait.Hash.html
    //! [`.iter`]: ../../std/primitive.slice.html#method.iter
    //! [`.iter_mut`]: ../../std/primitive.slice.html#method.iter_mut
    //! [`.split`]: ../../std/primitive.slice.html#method.split
    //! [`.splitn`]: ../../std/primitive.slice.html#method.splitn
    //! [`.chunks`]: ../../std/primitive.slice.html#method.chunks
    //! [`.windows`]: ../../std/primitive.slice.html#method.windows
    #![stable(feature = "rust1", since = "1.0.0")]

    // Many of the usings in this module are only used in the test configuration.
    // It's cleaner to just turn off the unused_imports warning than to fix them.
    #![cfg_attr(test, allow(unused_imports, dead_code))]

    use core::cmp::Ordering::{self, Less};
    use core::mem::size_of;
    use core::mem;
    use core::ptr;
    use core::slice as core_slice;

    use borrow::{Borrow, BorrowMut, ToOwned};
    use boxed::Box;
    use vec::Vec;

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::slice::{Chunks, Windows};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::slice::{Iter, IterMut};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::slice::{SplitMut, ChunksMut, Split};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::slice::{SplitN, RSplitN, SplitNMut, RSplitNMut};
    #[unstable(feature = "slice_rsplit", issue = "41020")]
    pub use core::slice::{RSplit, RSplitMut};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::slice::{from_raw_parts, from_raw_parts_mut};
    #[unstable(feature = "from_ref", issue = "45703")]
    pub use core::slice::{from_ref, from_ref_mut};
    #[unstable(feature = "slice_get_slice", issue = "35729")]
    pub use core::slice::SliceIndex;

    ////////////////////////////////////////////////////////////////////////////////
    // Basic slice extension methods
    ////////////////////////////////////////////////////////////////////////////////

    // HACK(japaric) needed for the implementation of `vec!` macro during testing
    // NB see the hack module in this file for more details
    #[cfg(test)]
    pub use self::hack::into_vec;

    // HACK(japaric) needed for the implementation of `Vec::clone` during testing
    // NB see the hack module in this file for more details
    #[cfg(test)]
    pub use self::hack::to_vec;

    // HACK(japaric): With cfg(test) `impl [T]` is not available, these three
    // functions are actually methods that are in `impl [T]` but not in
    // `core::slice::SliceExt` - we need to supply these functions for the
    // `test_permutations` test
    mod hack {
        use boxed::Box;
        use core::mem;

        #[cfg(test)]
        use string::ToString;
        use vec::Vec;

        pub fn into_vec<T>(mut b: Box<[T]>) -> Vec<T> {
            unsafe {
                let xs = Vec::from_raw_parts(b.as_mut_ptr(), b.len(), b.len());
                mem::forget(b);
                xs
            }
        }

        #[inline]
        pub fn to_vec<T>(s: &[T]) -> Vec<T>
            where T: Clone
        {
            let mut vector = Vec::with_capacity(s.len());
            vector.extend_from_slice(s);
            vector
        }
    }

    #[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")]
        #[inline]
        pub fn len(&self) -> usize {
            core_slice::SliceExt::len(self)
        }

        /// 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")]
        #[inline]
        pub fn is_empty(&self) -> bool {
            core_slice::SliceExt::is_empty(self)
        }

        /// 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> {
            core_slice::SliceExt::first(self)
        }

        /// 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> {
            core_slice::SliceExt::first_mut(self)
        }

        /// 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])> {
            core_slice::SliceExt::split_first(self)
        }

        /// 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])> {
            core_slice::SliceExt::split_first_mut(self)
        }

        /// 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])> {
            core_slice::SliceExt::split_last(self)

        }

        /// 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])> {
            core_slice::SliceExt::split_last_mut(self)
        }

        /// 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> {
            core_slice::SliceExt::last(self)
        }

        /// 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> {
            core_slice::SliceExt::last_mut(self)
        }

        /// 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>
        {
            core_slice::SliceExt::get(self, index)
        }

        /// 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>
        {
            core_slice::SliceExt::get_mut(self, index)
        }

        /// Returns a reference to an element or subslice, without doing bounds
        /// checking.
        ///
        /// This is generally not recommended, use with caution! For a safe
        /// alternative see [`get`].
        ///
        /// [`get`]: #method.get
        ///
        /// # 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>
        {
            core_slice::SliceExt::get_unchecked(self, index)
        }

        /// Returns a mutable reference to an element or subslice, without doing
        /// bounds checking.
        ///
        /// This is generally not recommended, use with caution! For a safe
        /// alternative see [`get_mut`].
        ///
        /// [`get_mut`]: #method.get_mut
        ///
        /// # 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>
        {
            core_slice::SliceExt::get_unchecked_mut(self, index)
        }

        /// 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.
        ///
        /// 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.offset(i as isize));
        ///     }
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn as_ptr(&self) -> *const T {
            core_slice::SliceExt::as_ptr(self)
        }

        /// 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.offset(i as isize) += 2;
        ///     }
        /// }
        /// assert_eq!(x, &[3, 4, 6]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn as_mut_ptr(&mut self) -> *mut T {
            core_slice::SliceExt::as_mut_ptr(self)
        }

        /// 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) {
            core_slice::SliceExt::swap(self, a, b)
        }

        /// 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) {
            core_slice::SliceExt::reverse(self)
        }

        /// 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> {
            core_slice::SliceExt::iter(self)
        }

        /// 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> {
            core_slice::SliceExt::iter_mut(self)
        }

        /// 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> {
            core_slice::SliceExt::windows(self, size)
        }

        /// Returns an iterator over `size` elements of the slice at a
        /// time. The chunks are slices and do not overlap. If `size` does
        /// not divide the length of the slice, then the last chunk will
        /// not have length `size`.
        ///
        /// # Panics
        ///
        /// Panics if `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());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn chunks(&self, size: usize) -> Chunks<T> {
            core_slice::SliceExt::chunks(self, size)
        }

        /// Returns an iterator over `chunk_size` elements of the slice at a time.
        /// 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`.
        ///
        /// # 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]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn chunks_mut(&mut self, chunk_size: usize) -> ChunksMut<T> {
            core_slice::SliceExt::chunks_mut(self, 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]) {
            core_slice::SliceExt::split_at(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]) {
            core_slice::SliceExt::split_at_mut(self, 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
        {
            core_slice::SliceExt::split(self, pred)
        }

        /// 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
        {
            core_slice::SliceExt::split_mut(self, pred)
        }

        /// 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
        ///
        /// ```
        /// #![feature(slice_rsplit)]
        ///
        /// 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.
        ///
        /// ```
        /// #![feature(slice_rsplit)]
        ///
        /// 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);
        /// ```
        #[unstable(feature = "slice_rsplit", issue = "41020")]
        #[inline]
        pub fn rsplit<F>(&self, pred: F) -> RSplit<T, F>
            where F: FnMut(&T) -> bool
        {
            core_slice::SliceExt::rsplit(self, 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
        ///
        /// ```
        /// #![feature(slice_rsplit)]
        ///
        /// 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]);
        /// ```
        ///
        #[unstable(feature = "slice_rsplit", issue = "41020")]
        #[inline]
        pub fn rsplit_mut<F>(&mut self, pred: F) -> RSplitMut<T, F>
            where F: FnMut(&T) -> bool
        {
            core_slice::SliceExt::rsplit_mut(self, 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
        {
            core_slice::SliceExt::splitn(self, n, 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
        ///
        /// ```
        /// 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
        {
            core_slice::SliceExt::splitn_mut(self, n, pred)
        }

        /// 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
        {
            core_slice::SliceExt::rsplitn(self, n, pred)
        }

        /// 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
        {
            core_slice::SliceExt::rsplitn_mut(self, n, pred)
        }

        /// 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));
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn contains(&self, x: &T) -> bool
            where T: PartialEq
        {
            core_slice::SliceExt::contains(self, x)
        }

        /// 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
        {
            core_slice::SliceExt::starts_with(self, needle)
        }

        /// 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
        {
            core_slice::SliceExt::ends_with(self, needle)
        }

        /// Binary searches this sorted slice for a given element.
        ///
        /// If the value is found then `Ok` is returned, containing the
        /// index of the matching element; if the value is not found then
        /// `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, });
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn binary_search(&self, x: &T) -> Result<usize, usize>
            where T: Ord
        {
            core_slice::SliceExt::binary_search(self, 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 a matching value is found then returns `Ok`, containing
        /// the index for the matched element; if no match is found then
        /// `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, f: F) -> Result<usize, usize>
            where F: FnMut(&'a T) -> Ordering
        {
            core_slice::SliceExt::binary_search_by(self, f)
        }

        /// 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 a matching value is found then returns `Ok`, containing the
        /// index for the matched element; if no match is found then `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, f: F) -> Result<usize, usize>
            where F: FnMut(&'a T) -> B,
                  B: Ord
        {
            core_slice::SliceExt::binary_search_by_key(self, b, f)
        }

        /// Sorts the slice.
        ///
        /// This sort is stable (i.e. does not reorder equal elements) and `O(n log n)` worst-case.
        ///
        /// When applicable, unstable sorting is preferred because it is generally faster than stable
        /// sorting and it doesn't allocate auxiliary memory.
        /// See [`sort_unstable`](#method.sort_unstable).
        ///
        /// # Current implementation
        ///
        /// The current algorithm is an adaptive, iterative merge sort inspired by
        /// [timsort](https://en.wikipedia.org/wiki/Timsort).
        /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
        /// two or more sorted sequences concatenated one after another.
        ///
        /// Also, it allocates temporary storage half the size of `self`, but for short slices a
        /// non-allocating insertion sort is used instead.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut v = [-5, 4, 1, -3, 2];
        ///
        /// v.sort();
        /// assert!(v == [-5, -3, 1, 2, 4]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn sort(&mut self)
            where T: Ord
        {
            merge_sort(self, |a, b| a.lt(b));
        }

        /// Sorts the slice with a comparator function.
        ///
        /// This sort is stable (i.e. does not reorder equal elements) and `O(n log n)` worst-case.
        ///
        /// When applicable, unstable sorting is preferred because it is generally faster than stable
        /// sorting and it doesn't allocate auxiliary memory.
        /// See [`sort_unstable_by`](#method.sort_unstable_by).
        ///
        /// # Current implementation
        ///
        /// The current algorithm is an adaptive, iterative merge sort inspired by
        /// [timsort](https://en.wikipedia.org/wiki/Timsort).
        /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
        /// two or more sorted sequences concatenated one after another.
        ///
        /// Also, it allocates temporary storage half the size of `self`, but for short slices a
        /// non-allocating insertion sort is used instead.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut v = [5, 4, 1, 3, 2];
        /// v.sort_by(|a, b| a.cmp(b));
        /// assert!(v == [1, 2, 3, 4, 5]);
        ///
        /// // reverse sorting
        /// v.sort_by(|a, b| b.cmp(a));
        /// assert!(v == [5, 4, 3, 2, 1]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn sort_by<F>(&mut self, mut compare: F)
            where F: FnMut(&T, &T) -> Ordering
        {
            merge_sort(self, |a, b| compare(a, b) == Less);
        }

        /// Sorts the slice with a key extraction function.
        ///
        /// This sort is stable (i.e. does not reorder equal elements) and `O(n log n)` worst-case.
        ///
        /// When applicable, unstable sorting is preferred because it is generally faster than stable
        /// sorting and it doesn't allocate auxiliary memory.
        /// See [`sort_unstable_by_key`](#method.sort_unstable_by_key).
        ///
        /// # Current implementation
        ///
        /// The current algorithm is an adaptive, iterative merge sort inspired by
        /// [timsort](https://en.wikipedia.org/wiki/Timsort).
        /// It is designed to be very fast in cases where the slice is nearly sorted, or consists of
        /// two or more sorted sequences concatenated one after another.
        ///
        /// Also, it allocates temporary storage half the size of `self`, but for short slices a
        /// non-allocating insertion sort is used instead.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut v = [-5i32, 4, 1, -3, 2];
        ///
        /// v.sort_by_key(|k| k.abs());
        /// assert!(v == [1, 2, -3, 4, -5]);
        /// ```
        #[stable(feature = "slice_sort_by_key", since = "1.7.0")]
        #[inline]
        pub fn sort_by_key<B, F>(&mut self, mut f: F)
            where F: FnMut(&T) -> B, B: Ord
        {
            merge_sort(self, |a, b| f(a).lt(&f(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
        {
            core_slice::SliceExt::sort_unstable(self);
        }

        /// 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.
        ///
        /// # 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, compare: F)
            where F: FnMut(&T, &T) -> Ordering
        {
            core_slice::SliceExt::sort_unstable_by(self, compare);
        }

        /// 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(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 = [-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<B, F>(&mut self, f: F)
            where F: FnMut(&T) -> B,
                  B: Ord
        {
            core_slice::SliceExt::sort_unstable_by_key(self, f);
        }

        /// Permutes the slice in-place such that `self[mid..]` moves to the
        /// beginning of the slice while `self[..mid]` moves to the end of the
        /// slice.  Equivalently, rotates the slice `mid` places to the left
        /// or `k = self.len() - mid` places to the right.
        ///
        /// This is a "k-rotation", a permutation in which item `i` moves to
        /// position `i + k`, modulo the length of the slice.  See _Elements
        /// of Programming_ [§10.4][eop].
        ///
        /// Rotation by `mid` and rotation by `k` are inverse operations.
        ///
        /// [eop]: https://books.google.com/books?id=CO9ULZGINlsC&pg=PA178&q=k-rotation
        ///
        /// # Panics
        ///
        /// This function will panic if `mid` is greater than the length of the
        /// slice.  (Note that `mid == self.len()` does _not_ panic; it's a nop
        /// rotation with `k == 0`, the inverse of a rotation with `mid == 0`.)
        ///
        /// # Complexity
        ///
        /// Takes linear (in `self.len()`) time.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(slice_rotate)]
        ///
        /// let mut a = [1, 2, 3, 4, 5, 6, 7];
        /// let mid = 2;
        /// a.rotate(mid);
        /// assert_eq!(&a, &[3, 4, 5, 6, 7, 1, 2]);
        /// let k = a.len() - mid;
        /// a.rotate(k);
        /// assert_eq!(&a, &[1, 2, 3, 4, 5, 6, 7]);
        ///
        /// use std::ops::Range;
        /// fn slide<T>(slice: &mut [T], range: Range<usize>, to: usize) {
        ///     if to < range.start {
        ///         slice[to..range.end].rotate(range.start-to);
        ///     } else if to > range.end {
        ///         slice[range.start..to].rotate(range.end-range.start);
        ///     }
        /// }
        /// let mut v: Vec<_> = (0..10).collect();
        /// slide(&mut v, 1..4, 7);
        /// assert_eq!(&v, &[0, 4, 5, 6, 1, 2, 3, 7, 8, 9]);
        /// slide(&mut v, 6..8, 1);
        /// assert_eq!(&v, &[0, 3, 7, 4, 5, 6, 1, 2, 8, 9]);
        /// ```
        #[unstable(feature = "slice_rotate", issue = "41891")]
        pub fn rotate(&mut self, mid: usize) {
            core_slice::SliceExt::rotate(self, mid);
        }

        /// 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];
        ///
        /// 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 {
            core_slice::SliceExt::clone_from_slice(self, src)
        }

        /// 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];
        ///
        /// 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 {
            core_slice::SliceExt::copy_from_slice(self, src)
        }

        /// 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:
        ///
        /// ```
        /// #![feature(swap_with_slice)]
        ///
        /// 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
        /// #![feature(swap_with_slice)]
        ///
        /// 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:
        ///
        /// ```
        /// #![feature(swap_with_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
        #[unstable(feature = "swap_with_slice", issue = "44030")]
        pub fn swap_with_slice(&mut self, other: &mut [T]) {
            core_slice::SliceExt::swap_with_slice(self, other)
        }

        /// Copies `self` into a new `Vec`.
        ///
        /// # Examples
        ///
        /// ```
        /// let s = [10, 40, 30];
        /// let x = s.to_vec();
        /// // Here, `s` and `x` can be modified independently.
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn to_vec(&self) -> Vec<T>
            where T: Clone
        {
            // NB see hack module in this file
            hack::to_vec(self)
        }

        /// Converts `self` into a vector without clones or allocation.
        ///
        /// The resulting vector can be converted back into a box via
        /// `Vec<T>`'s `into_boxed_slice` method.
        ///
        /// # Examples
        ///
        /// ```
        /// let s: Box<[i32]> = Box::new([10, 40, 30]);
        /// let x = s.into_vec();
        /// // `s` cannot be used anymore because it has been converted into `x`.
        ///
        /// assert_eq!(x, vec![10, 40, 30]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn into_vec(self: Box<Self>) -> Vec<T> {
            // NB see hack module in this file
            hack::into_vec(self)
        }
    }

    #[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())
        }

        /// Returns a vector containing a copy of this slice where each byte
        /// is mapped to its ASCII upper case equivalent.
        ///
        /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
        /// but non-ASCII letters are unchanged.
        ///
        /// To uppercase the value in-place, use [`make_ascii_uppercase`].
        ///
        /// [`make_ascii_uppercase`]: #method.make_ascii_uppercase
        #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
        #[inline]
        pub fn to_ascii_uppercase(&self) -> Vec<u8> {
            let mut me = self.to_vec();
            me.make_ascii_uppercase();
            me
        }

        /// Returns a vector containing a copy of this slice where each byte
        /// is mapped to its ASCII lower case equivalent.
        ///
        /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
        /// but non-ASCII letters are unchanged.
        ///
        /// To lowercase the value in-place, use [`make_ascii_lowercase`].
        ///
        /// [`make_ascii_lowercase`]: #method.make_ascii_lowercase
        #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
        #[inline]
        pub fn to_ascii_lowercase(&self) -> Vec<u8> {
            let mut me = self.to_vec();
            me.make_ascii_lowercase();
            me
        }

        /// 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();
            }
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Extension traits for slices over specific kinds of data
    ////////////////////////////////////////////////////////////////////////////////
    #[unstable(feature = "slice_concat_ext",
               reason = "trait should not have to exist",
               issue = "27747")]
    /// An extension trait for concatenating slices
    pub trait SliceConcatExt<T: ?Sized> {
        #[unstable(feature = "slice_concat_ext",
                   reason = "trait should not have to exist",
                   issue = "27747")]
        /// The resulting type after concatenation
        type Output;

        /// Flattens a slice of `T` into a single value `Self::Output`.
        ///
        /// # Examples
        ///
        /// ```
        /// assert_eq!(["hello", "world"].concat(), "helloworld");
        /// assert_eq!([[1, 2], [3, 4]].concat(), [1, 2, 3, 4]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn concat(&self) -> Self::Output;

        /// Flattens a slice of `T` into a single value `Self::Output`, placing a
        /// given separator between each.
        ///
        /// # Examples
        ///
        /// ```
        /// assert_eq!(["hello", "world"].join(" "), "hello world");
        /// assert_eq!([[1, 2], [3, 4]].join(&0), [1, 2, 0, 3, 4]);
        /// ```
        #[stable(feature = "rename_connect_to_join", since = "1.3.0")]
        fn join(&self, sep: &T) -> Self::Output;

        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_deprecated(since = "1.3.0", reason = "renamed to join")]
        fn connect(&self, sep: &T) -> Self::Output;
    }

    #[unstable(feature = "slice_concat_ext",
               reason = "trait should not have to exist",
               issue = "27747")]
    impl<T: Clone, V: Borrow<[T]>> SliceConcatExt<T> for [V] {
        type Output = Vec<T>;

        fn concat(&self) -> Vec<T> {
            let size = self.iter().fold(0, |acc, v| acc + v.borrow().len());
            let mut result = Vec::with_capacity(size);
            for v in self {
                result.extend_from_slice(v.borrow())
            }
            result
        }

        fn join(&self, sep: &T) -> Vec<T> {
            let size = self.iter().fold(0, |acc, v| acc + v.borrow().len());
            let mut result = Vec::with_capacity(size + self.len());
            let mut first = true;
            for v in self {
                if first {
                    first = false
                } else {
                    result.push(sep.clone())
                }
                result.extend_from_slice(v.borrow())
            }
            result
        }

        fn connect(&self, sep: &T) -> Vec<T> {
            self.join(sep)
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Standard trait implementations for slices
    ////////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Borrow<[T]> for Vec<T> {
        fn borrow(&self) -> &[T] {
            &self[..]
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> BorrowMut<[T]> for Vec<T> {
        fn borrow_mut(&mut self) -> &mut [T] {
            &mut self[..]
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Clone> ToOwned for [T] {
        type Owned = Vec<T>;
        #[cfg(not(test))]
        fn to_owned(&self) -> Vec<T> {
            self.to_vec()
        }

        #[cfg(test)]
        fn to_owned(&self) -> Vec<T> {
            hack::to_vec(self)
        }

        fn clone_into(&self, target: &mut Vec<T>) {
            // drop anything in target that will not be overwritten
            target.truncate(self.len());
            let len = target.len();

            // reuse the contained values' allocations/resources.
            target.clone_from_slice(&self[..len]);

            // target.len <= self.len due to the truncate above, so the
            // slice here is always in-bounds.
            target.extend_from_slice(&self[len..]);
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Sorting
    ////////////////////////////////////////////////////////////////////////////////

    /// Inserts `v[0]` into pre-sorted sequence `v[1..]` so that whole `v[..]` becomes sorted.
    ///
    /// This is the integral subroutine of insertion sort.
    fn insert_head<T, F>(v: &mut [T], is_less: &mut F)
        where F: FnMut(&T, &T) -> bool
    {
        if v.len() >= 2 && is_less(&v[1], &v[0]) {
            unsafe {
                // There are three ways to implement insertion here:
                //
                // 1. Swap adjacent elements until the first one gets to its final destination.
                //    However, this way we copy data around more than is necessary. If elements are big
                //    structures (costly to copy), this method will be slow.
                //
                // 2. Iterate until the right place for the first element is found. Then shift the
                //    elements succeeding it to make room for it and finally place it into the
                //    remaining hole. This is a good method.
                //
                // 3. Copy the first element into a temporary variable. Iterate until the right place
                //    for it is found. As we go along, copy every traversed element into the slot
                //    preceding it. Finally, copy data from the temporary variable into the remaining
                //    hole. This method is very good. Benchmarks demonstrated slightly better
                //    performance than with the 2nd method.
                //
                // All methods were benchmarked, and the 3rd showed best results. So we chose that one.
                let mut tmp = mem::ManuallyDrop::new(ptr::read(&v[0]));

                // Intermediate state of the insertion process is always tracked by `hole`, which
                // serves two purposes:
                // 1. Protects integrity of `v` from panics in `is_less`.
                // 2. Fills the remaining hole in `v` in the end.
                //
                // Panic safety:
                //
                // If `is_less` panics at any point during the process, `hole` will get dropped and
                // fill the hole in `v` with `tmp`, thus ensuring that `v` still holds every object it
                // initially held exactly once.
                let mut hole = InsertionHole {
                    src: &mut *tmp,
                    dest: &mut v[1],
                };
                ptr::copy_nonoverlapping(&v[1], &mut v[0], 1);

                for i in 2..v.len() {
                    if !is_less(&v[i], &*tmp) {
                        break;
                    }
                    ptr::copy_nonoverlapping(&v[i], &mut v[i - 1], 1);
                    hole.dest = &mut v[i];
                }
                // `hole` gets dropped and thus copies `tmp` into the remaining hole in `v`.
            }
        }

        // When dropped, copies from `src` into `dest`.
        struct InsertionHole<T> {
            src: *mut T,
            dest: *mut T,
        }

        impl<T> Drop for InsertionHole<T> {
            fn drop(&mut self) {
                unsafe { ptr::copy_nonoverlapping(self.src, self.dest, 1); }
            }
        }
    }

    /// Merges non-decreasing runs `v[..mid]` and `v[mid..]` using `buf` as temporary storage, and
    /// stores the result into `v[..]`.
    ///
    /// # Safety
    ///
    /// The two slices must be non-empty and `mid` must be in bounds. Buffer `buf` must be long enough
    /// to hold a copy of the shorter slice. Also, `T` must not be a zero-sized type.
    unsafe fn merge<T, F>(v: &mut [T], mid: usize, buf: *mut T, is_less: &mut F)
        where F: FnMut(&T, &T) -> bool
    {
        let len = v.len();
        let v = v.as_mut_ptr();
        let v_mid = v.offset(mid as isize);
        let v_end = v.offset(len as isize);

        // The merge process first copies the shorter run into `buf`. Then it traces the newly copied
        // run and the longer run forwards (or backwards), comparing their next unconsumed elements and
        // copying the lesser (or greater) one into `v`.
        //
        // As soon as the shorter run is fully consumed, the process is done. If the longer run gets
        // consumed first, then we must copy whatever is left of the shorter run into the remaining
        // hole in `v`.
        //
        // Intermediate state of the process is always tracked by `hole`, which serves two purposes:
        // 1. Protects integrity of `v` from panics in `is_less`.
        // 2. Fills the remaining hole in `v` if the longer run gets consumed first.
        //
        // Panic safety:
        //
        // If `is_less` panics at any point during the process, `hole` will get dropped and fill the
        // hole in `v` with the unconsumed range in `buf`, thus ensuring that `v` still holds every
        // object it initially held exactly once.
        let mut hole;

        if mid <= len - mid {
            // The left run is shorter.
            ptr::copy_nonoverlapping(v, buf, mid);
            hole = MergeHole {
                start: buf,
                end: buf.offset(mid as isize),
                dest: v,
            };

            // Initially, these pointers point to the beginnings of their arrays.
            let left = &mut hole.start;
            let mut right = v_mid;
            let out = &mut hole.dest;

            while *left < hole.end && right < v_end {
                // Consume the lesser side.
                // If equal, prefer the left run to maintain stability.
                let to_copy = if is_less(&*right, &**left) {
                    get_and_increment(&mut right)
                } else {
                    get_and_increment(left)
                };
                ptr::copy_nonoverlapping(to_copy, get_and_increment(out), 1);
            }
        } else {
            // The right run is shorter.
            ptr::copy_nonoverlapping(v_mid, buf, len - mid);
            hole = MergeHole {
                start: buf,
                end: buf.offset((len - mid) as isize),
                dest: v_mid,
            };

            // Initially, these pointers point past the ends of their arrays.
            let left = &mut hole.dest;
            let right = &mut hole.end;
            let mut out = v_end;

            while v < *left && buf < *right {
                // Consume the greater side.
                // If equal, prefer the right run to maintain stability.
                let to_copy = if is_less(&*right.offset(-1), &*left.offset(-1)) {
                    decrement_and_get(left)
                } else {
                    decrement_and_get(right)
                };
                ptr::copy_nonoverlapping(to_copy, decrement_and_get(&mut out), 1);
            }
        }
        // Finally, `hole` gets dropped. If the shorter run was not fully consumed, whatever remains of
        // it will now be copied into the hole in `v`.

        unsafe fn get_and_increment<T>(ptr: &mut *mut T) -> *mut T {
            let old = *ptr;
            *ptr = ptr.offset(1);
            old
        }

        unsafe fn decrement_and_get<T>(ptr: &mut *mut T) -> *mut T {
            *ptr = ptr.offset(-1);
            *ptr
        }

        // When dropped, copies the range `start..end` into `dest..`.
        struct MergeHole<T> {
            start: *mut T,
            end: *mut T,
            dest: *mut T,
        }

        impl<T> Drop for MergeHole<T> {
            fn drop(&mut self) {
                // `T` is not a zero-sized type, so it's okay to divide by its size.
                let len = (self.end as usize - self.start as usize) / mem::size_of::<T>();
                unsafe { ptr::copy_nonoverlapping(self.start, self.dest, len); }
            }
        }
    }

    /// This merge sort borrows some (but not all) ideas from TimSort, which is described in detail
    /// [here](http://svn.python.org/projects/python/trunk/Objects/listsort.txt).
    ///
    /// The algorithm identifies strictly descending and non-descending subsequences, which are called
    /// natural runs. There is a stack of pending runs yet to be merged. Each newly found run is pushed
    /// onto the stack, and then some pairs of adjacent runs are merged until these two invariants are
    /// satisfied:
    ///
    /// 1. for every `i` in `1..runs.len()`: `runs[i - 1].len > runs[i].len`
    /// 2. for every `i` in `2..runs.len()`: `runs[i - 2].len > runs[i - 1].len + runs[i].len`
    ///
    /// The invariants ensure that the total running time is `O(n log n)` worst-case.
    fn merge_sort<T, F>(v: &mut [T], mut is_less: F)
        where F: FnMut(&T, &T) -> bool
    {
        // Slices of up to this length get sorted using insertion sort.
        const MAX_INSERTION: usize = 20;
        // Very short runs are extended using insertion sort to span at least this many elements.
        const MIN_RUN: usize = 10;

        // Sorting has no meaningful behavior on zero-sized types.
        if size_of::<T>() == 0 {
            return;
        }

        let len = v.len();

        // Short arrays get sorted in-place via insertion sort to avoid allocations.
        if len <= MAX_INSERTION {
            if len >= 2 {
                for i in (0..len-1).rev() {
                    insert_head(&mut v[i..], &mut is_less);
                }
            }
            return;
        }

        // Allocate a buffer to use as scratch memory. We keep the length 0 so we can keep in it
        // shallow copies of the contents of `v` without risking the dtors running on copies if
        // `is_less` panics. When merging two sorted runs, this buffer holds a copy of the shorter run,
        // which will always have length at most `len / 2`.
        let mut buf = Vec::with_capacity(len / 2);

        // In order to identify natural runs in `v`, we traverse it backwards. That might seem like a
        // strange decision, but consider the fact that merges more often go in the opposite direction
        // (forwards). According to benchmarks, merging forwards is slightly faster than merging
        // backwards. To conclude, identifying runs by traversing backwards improves performance.
        let mut runs = vec![];
        let mut end = len;
        while end > 0 {
            // Find the next natural run, and reverse it if it's strictly descending.
            let mut start = end - 1;
            if start > 0 {
                start -= 1;
                unsafe {
                    if is_less(v.get_unchecked(start + 1), v.get_unchecked(start)) {
                        while start > 0 && is_less(v.get_unchecked(start),
                                                   v.get_unchecked(start - 1)) {
                            start -= 1;
                        }
                        v[start..end].reverse();
                    } else {
                        while start > 0 && !is_less(v.get_unchecked(start),
                                                    v.get_unchecked(start - 1)) {
                            start -= 1;
                        }
                    }
                }
            }

            // Insert some more elements into the run if it's too short. Insertion sort is faster than
            // merge sort on short sequences, so this significantly improves performance.
            while start > 0 && end - start < MIN_RUN {
                start -= 1;
                insert_head(&mut v[start..end], &mut is_less);
            }

            // Push this run onto the stack.
            runs.push(Run {
                start,
                len: end - start,
            });
            end = start;

            // Merge some pairs of adjacent runs to satisfy the invariants.
            while let Some(r) = collapse(&runs) {
                let left = runs[r + 1];
                let right = runs[r];
                unsafe {
                    merge(&mut v[left.start .. right.start + right.len], left.len, buf.as_mut_ptr(),
                          &mut is_less);
                }
                runs[r] = Run {
                    start: left.start,
                    len: left.len + right.len,
                };
                runs.remove(r + 1);
            }
        }

        // Finally, exactly one run must remain in the stack.
        debug_assert!(runs.len() == 1 && runs[0].start == 0 && runs[0].len == len);

        // Examines the stack of runs and identifies the next pair of runs to merge. More specifically,
        // if `Some(r)` is returned, that means `runs[r]` and `runs[r + 1]` must be merged next. If the
        // algorithm should continue building a new run instead, `None` is returned.
        //
        // TimSort is infamous for its buggy implementations, as described here:
        // http://envisage-project.eu/timsort-specification-and-verification/
        //
        // The gist of the story is: we must enforce the invariants on the top four runs on the stack.
        // Enforcing them on just top three is not sufficient to ensure that the invariants will still
        // hold for *all* runs in the stack.
        //
        // This function correctly checks invariants for the top four runs. Additionally, if the top
        // run starts at index 0, it will always demand a merge operation until the stack is fully
        // collapsed, in order to complete the sort.
        #[inline]
        fn collapse(runs: &[Run]) -> Option<usize> {
            let n = runs.len();
            if n >= 2 && (runs[n - 1].start == 0 ||
                          runs[n - 2].len <= runs[n - 1].len ||
                          (n >= 3 && runs[n - 3].len <= runs[n - 2].len + runs[n - 1].len) ||
                          (n >= 4 && runs[n - 4].len <= runs[n - 3].len + runs[n - 2].len)) {
                if n >= 3 && runs[n - 3].len < runs[n - 1].len {
                    Some(n - 3)
                } else {
                    Some(n - 2)
                }
            } else {
                None
            }
        }

        #[derive(Clone, Copy)]
        struct Run {
            start: usize,
            len: usize,
        }
    }

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