armv7a/usr/lib/rustlib/src/rust/library/core/src/slice/iter.rs - manifest_repos/toolchain - Git at Google

 //! Definitions of a bunch of iterators for `[T]`.

 #[macro_use] // import iterator! and forward_iterator!
 mod macros;

 use crate::cmp;
 use crate::cmp::Ordering;
 use crate::fmt;
 use crate::intrinsics::{assume, exact_div, unchecked_sub};
 use crate::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
 use crate::marker::{PhantomData, Send, Sized, Sync};
 use crate::mem::{self, SizedTypeProperties};
 use crate::num::NonZeroUsize;
 use crate::ptr::NonNull;

 use super::{from_raw_parts, from_raw_parts_mut};

 #[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>()
 }

 /// 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`]: slice::iter
 /// [slices]: slice
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct Iter<'a, T: 'a> {
     ptr: NonNull<T>,
     end: *const T, // If T is a ZST, this is actually ptr+len.  This encoding is picked so that
     // ptr == end is a quick test for the Iterator being empty, that works
     // for both ZST and non-ZST.
     _marker: 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> {
     #[inline]
     pub(super) fn new(slice: &'a [T]) -> Self {
         let ptr = slice.as_ptr();
         // SAFETY: Similar to `IterMut::new`.
         unsafe {
             assume(!ptr.is_null());

             let end =
                 if T::IS_ZST { ptr.wrapping_byte_add(slice.len()) } else { ptr.add(slice.len()) };

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

     /// 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());
     /// ```
     #[must_use]
     #[stable(feature = "iter_to_slice", since = "1.4.0")]
     #[inline]
     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> {
     #[inline]
     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> {
     #[inline]
     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`]: slice::iter_mut
 /// [slices]: slice
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct IterMut<'a, T: 'a> {
     ptr: NonNull<T>,
     end: *mut T, // If T is a ZST, this is actually ptr+len.  This encoding is picked so that
     // ptr == end is a quick test for the Iterator being empty, that works
     // for both ZST and non-ZST.
     _marker: 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> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T]) -> Self {
         let ptr = slice.as_mut_ptr();
         // SAFETY: There are several things here:
         //
         // `ptr` has been obtained by `slice.as_ptr()` where `slice` is a valid
         // reference thus it is non-NUL and safe to use and pass to
         // `NonNull::new_unchecked` .
         //
         // Adding `slice.len()` to the starting pointer gives a pointer
         // at the end of `slice`. `end` will never be dereferenced, only checked
         // for direct pointer equality with `ptr` to check if the iterator is
         // done.
         //
         // In the case of a ZST, the end pointer is just the start pointer plus
         // the length, to also allows for the fast `ptr == end` check.
         //
         // See the `next_unchecked!` and `is_empty!` macros as well as the
         // `post_inc_start` method for more information.
         unsafe {
             assume(!ptr.is_null());

             let end =
                 if T::IS_ZST { ptr.wrapping_byte_add(slice.len()) } else { ptr.add(slice.len()) };

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

     /// 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:?}");
     /// ```
     #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "iter_to_slice", since = "1.4.0")]
     pub fn into_slice(self) -> &'a mut [T] {
         // SAFETY: the iterator was created from a mutable slice with pointer
         // `self.ptr` and length `len!(self)`. This guarantees that all the prerequisites
         // for `from_raw_parts_mut` are fulfilled.
         unsafe { from_raw_parts_mut(self.ptr.as_ptr(), len!(self)) }
     }

     /// Views the underlying data as a subslice of the original data.
     ///
     /// To avoid creating `&mut [T]` references that alias, the returned slice
     /// borrows its lifetime from the iterator the method is applied on.
     ///
     /// # Examples
     ///
     /// Basic usage:
     ///
     /// ```
     /// 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]);
     /// ```
     #[must_use]
     #[stable(feature = "slice_iter_mut_as_slice", since = "1.53.0")]
     #[inline]
     pub fn as_slice(&self) -> &[T] {
         self.make_slice()
     }

     /// Views the underlying data as a mutable 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_mut_slice)]
     ///
     /// let mut slice: &mut [usize] = &mut [1, 2, 3];
     ///
     /// // First, we get the iterator:
     /// let mut iter = slice.iter_mut();
     /// // Then, we get a mutable slice from it:
     /// let mut_slice = iter.as_mut_slice();
     /// // So if we check what the `as_mut_slice` method returned, we have "[1, 2, 3]":
     /// assert_eq!(mut_slice, &mut [1, 2, 3]);
     ///
     /// // We can use it to mutate the slice:
     /// mut_slice[0] = 4;
     /// mut_slice[2] = 5;
     ///
     /// // Next, we can move to the second element of the slice, checking that
     /// // it yields the value we just wrote:
     /// assert_eq!(iter.next(), Some(&mut 4));
     /// // Now `as_mut_slice` returns "[2, 5]":
     /// assert_eq!(iter.as_mut_slice(), &mut [2, 5]);
     /// ```
     #[must_use]
     // FIXME: Uncomment the `AsMut<[T]>` impl when this gets stabilized.
     #[unstable(feature = "slice_iter_mut_as_mut_slice", issue = "93079")]
     pub fn as_mut_slice(&mut self) -> &mut [T] {
         // SAFETY: the iterator was created from a mutable slice with pointer
         // `self.ptr` and length `len!(self)`. This guarantees that all the prerequisites
         // for `from_raw_parts_mut` are fulfilled.
         unsafe { from_raw_parts_mut(self.ptr.as_ptr(), len!(self)) }
     }
 }

 #[stable(feature = "slice_iter_mut_as_slice", since = "1.53.0")]
 impl<T> AsRef<[T]> for IterMut<'_, T> {
     #[inline]
     fn as_ref(&self) -> &[T] {
         self.as_slice()
     }
 }

 // #[stable(feature = "slice_iter_mut_as_mut_slice", since = "FIXME")]
 // impl<T> AsMut<[T]> for IterMut<'_, T> {
 //     fn as_mut(&mut self) -> &mut [T] {
 //         self.as_mut_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)]
 pub(super) 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].
 ///
 /// # Example
 ///
 /// ```
 /// let slice = [10, 40, 33, 20];
 /// let mut iter = slice.split(|num| num % 3 == 0);
 /// ```
 ///
 /// [`split`]: slice::split
 /// [slices]: slice
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct Split<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
     // Used for `SplitWhitespace` and `SplitAsciiWhitespace` `as_str` methods
     pub(crate) v: &'a [T],
     pred: P,
     // Used for `SplitAsciiWhitespace` `as_str` method
     pub(crate) finished: bool,
 }

 impl<'a, T: 'a, P: FnMut(&T) -> bool> Split<'a, T, P> {
     #[inline]
     pub(super) fn new(slice: &'a [T], pred: P) -> Self {
         Self { v: slice, pred, finished: false }
     }
     /// Returns a slice which contains items not yet handled by split.
     /// # Example
     ///
     /// ```
     /// #![feature(split_as_slice)]
     /// let slice = [1,2,3,4,5];
     /// let mut split = slice.split(|v| v % 2 == 0);
     /// assert!(split.next().is_some());
     /// assert_eq!(split.as_slice(), &[3,4,5]);
     /// ```
     #[unstable(feature = "split_as_slice", issue = "96137")]
     pub fn as_slice(&self) -> &'a [T] {
         if self.finished { &[] } else { &self.v }
     }
 }

 #[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 {
             // 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 Split<'a, T, P>
 where
     P: FnMut(&T) -> bool,
 {
     #[inline]
     fn next_back(&mut self) -> Option<&'a [T]> {
         if self.finished {
             return None;
         }

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

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

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

 /// An iterator over subslices separated by elements that match a predicate
 /// function. Unlike `Split`, it contains the matched part as a terminator
 /// of the subslice.
 ///
 /// This struct is created by the [`split_inclusive`] method on [slices].
 ///
 /// # Example
 ///
 /// ```
 /// let slice = [10, 40, 33, 20];
 /// let mut iter = slice.split_inclusive(|num| num % 3 == 0);
 /// ```
 ///
 /// [`split_inclusive`]: slice::split_inclusive
 /// [slices]: slice
 #[stable(feature = "split_inclusive", since = "1.51.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct SplitInclusive<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
     v: &'a [T],
     pred: P,
     finished: bool,
 }

 impl<'a, T: 'a, P: FnMut(&T) -> bool> SplitInclusive<'a, T, P> {
     #[inline]
     pub(super) fn new(slice: &'a [T], pred: P) -> Self {
         let finished = slice.is_empty();
         Self { v: slice, pred, finished }
     }
 }

 #[stable(feature = "split_inclusive", since = "1.51.0")]
 impl<T: fmt::Debug, P> fmt::Debug for SplitInclusive<'_, T, P>
 where
     P: FnMut(&T) -> bool,
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("SplitInclusive")
             .field("v", &self.v)
             .field("finished", &self.finished)
             .finish()
     }
 }

 // FIXME(#26925) Remove in favor of `#[derive(Clone)]`
 #[stable(feature = "split_inclusive", since = "1.51.0")]
 impl<T, P> Clone for SplitInclusive<'_, T, P>
 where
     P: Clone + FnMut(&T) -> bool,
 {
     fn clone(&self) -> Self {
         SplitInclusive { v: self.v, pred: self.pred.clone(), finished: self.finished }
     }
 }

 #[stable(feature = "split_inclusive", since = "1.51.0")]
 impl<'a, T, P> Iterator for SplitInclusive<'a, T, P>
 where
     P: FnMut(&T) -> bool,
 {
     type Item = &'a [T];

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

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

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

 #[stable(feature = "split_inclusive", since = "1.51.0")]
 impl<'a, T, P> DoubleEndedIterator for SplitInclusive<'a, T, P>
 where
     P: FnMut(&T) -> bool,
 {
     #[inline]
     fn next_back(&mut self) -> Option<&'a [T]> {
         if self.finished {
             return None;
         }

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

 #[stable(feature = "split_inclusive", since = "1.51.0")]
 impl<T, P> FusedIterator for SplitInclusive<'_, T, P> where P: FnMut(&T) -> bool {}

 /// An iterator over the mutable subslices of the vector which are separated
 /// by elements that match `pred`.
 ///
 /// This struct is created by the [`split_mut`] method on [slices].
 ///
 /// # Example
 ///
 /// ```
 /// let mut v = [10, 40, 30, 20, 60, 50];
 /// let iter = v.split_mut(|num| *num % 3 == 0);
 /// ```
 ///
 /// [`split_mut`]: slice::split_mut
 /// [slices]: slice
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct SplitMut<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
     v: &'a mut [T],
     pred: P,
     finished: bool,
 }

 impl<'a, T: 'a, P: FnMut(&T) -> bool> SplitMut<'a, T, P> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T], pred: P) -> Self {
         Self { v: slice, pred, finished: false }
     }
 }

 #[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;
         }

         match self.v.iter().position(|x| (self.pred)(x)) {
             None => self.finish(),
             Some(idx) => {
                 let tmp = mem::take(&mut self.v);
                 // idx is the index of the element we are splitting on. We want to set self to the
                 // region after idx, and return the subslice before and not including idx.
                 // So first we split after idx
                 let (head, tail) = tmp.split_at_mut(idx + 1);
                 self.v = tail;
                 // Then return the subslice up to but not including the found element
                 Some(&mut head[..idx])
             }
         }
     }

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

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

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

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

 /// An iterator over the mutable subslices of the vector which are separated
 /// by elements that match `pred`. Unlike `SplitMut`, it contains the matched
 /// parts in the ends of the subslices.
 ///
 /// This struct is created by the [`split_inclusive_mut`] method on [slices].
 ///
 /// # Example
 ///
 /// ```
 /// let mut v = [10, 40, 30, 20, 60, 50];
 /// let iter = v.split_inclusive_mut(|num| *num % 3 == 0);
 /// ```
 ///
 /// [`split_inclusive_mut`]: slice::split_inclusive_mut
 /// [slices]: slice
 #[stable(feature = "split_inclusive", since = "1.51.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct SplitInclusiveMut<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
     v: &'a mut [T],
     pred: P,
     finished: bool,
 }

 impl<'a, T: 'a, P: FnMut(&T) -> bool> SplitInclusiveMut<'a, T, P> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T], pred: P) -> Self {
         let finished = slice.is_empty();
         Self { v: slice, pred, finished }
     }
 }

 #[stable(feature = "split_inclusive", since = "1.51.0")]
 impl<T: fmt::Debug, P> fmt::Debug for SplitInclusiveMut<'_, T, P>
 where
     P: FnMut(&T) -> bool,
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("SplitInclusiveMut")
             .field("v", &self.v)
             .field("finished", &self.finished)
             .finish()
     }
 }

 #[stable(feature = "split_inclusive", since = "1.51.0")]
 impl<'a, T, P> Iterator for SplitInclusiveMut<'a, T, P>
 where
     P: FnMut(&T) -> bool,
 {
     type Item = &'a mut [T];

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

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

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

 #[stable(feature = "split_inclusive", since = "1.51.0")]
 impl<'a, T, P> DoubleEndedIterator for SplitInclusiveMut<'a, T, P>
 where
     P: FnMut(&T) -> bool,
 {
     #[inline]
     fn next_back(&mut self) -> Option<&'a mut [T]> {
         if self.finished {
             return None;
         }

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

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

 #[stable(feature = "split_inclusive", since = "1.51.0")]
 impl<T, P> FusedIterator for SplitInclusiveMut<'_, T, P> where P: FnMut(&T) -> bool {}

 /// An iterator over subslices separated by elements that match a predicate
 /// function, starting from the end of the slice.
 ///
 /// This struct is created by the [`rsplit`] method on [slices].
 ///
 /// # Example
 ///
 /// ```
 /// let slice = [11, 22, 33, 0, 44, 55];
 /// let iter = slice.rsplit(|num| *num == 0);
 /// ```
 ///
 /// [`rsplit`]: slice::rsplit
 /// [slices]: slice
 #[stable(feature = "slice_rsplit", since = "1.27.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct RSplit<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
     inner: Split<'a, T, P>,
 }

 impl<'a, T: 'a, P: FnMut(&T) -> bool> RSplit<'a, T, P> {
     #[inline]
     pub(super) fn new(slice: &'a [T], pred: P) -> Self {
         Self { inner: Split::new(slice, pred) }
     }
 }

 #[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()
     }
 }

 // FIXME(#26925) Remove in favor of `#[derive(Clone)]`
 #[stable(feature = "slice_rsplit", since = "1.27.0")]
 impl<T, P> Clone for RSplit<'_, T, P>
 where
     P: Clone + FnMut(&T) -> bool,
 {
     fn clone(&self) -> Self {
         RSplit { inner: self.inner.clone() }
     }
 }

 #[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].
 ///
 /// # Example
 ///
 /// ```
 /// let mut slice = [11, 22, 33, 0, 44, 55];
 /// let iter = slice.rsplit_mut(|num| *num == 0);
 /// ```
 ///
 /// [`rsplit_mut`]: slice::rsplit_mut
 /// [slices]: slice
 #[stable(feature = "slice_rsplit", since = "1.27.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct RSplitMut<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
     inner: SplitMut<'a, T, P>,
 }

 impl<'a, T: 'a, P: FnMut(&T) -> bool> RSplitMut<'a, T, P> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T], pred: P) -> Self {
         Self { inner: SplitMut::new(slice, pred) }
     }
 }

 #[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();
         (
             cmp::min(self.count, lower),
             Some(upper_opt.map_or(self.count, |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].
 ///
 /// # Example
 ///
 /// ```
 /// let slice = [10, 40, 30, 20, 60, 50];
 /// let iter = slice.splitn(2, |num| *num % 3 == 0);
 /// ```
 ///
 /// [`splitn`]: slice::splitn
 /// [slices]: slice
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct SplitN<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
     inner: GenericSplitN<Split<'a, T, P>>,
 }

 impl<'a, T: 'a, P: FnMut(&T) -> bool> SplitN<'a, T, P> {
     #[inline]
     pub(super) fn new(s: Split<'a, T, P>, n: usize) -> Self {
         Self { inner: GenericSplitN { iter: s, count: n } }
     }
 }

 #[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].
 ///
 /// # Example
 ///
 /// ```
 /// let slice = [10, 40, 30, 20, 60, 50];
 /// let iter = slice.rsplitn(2, |num| *num % 3 == 0);
 /// ```
 ///
 /// [`rsplitn`]: slice::rsplitn
 /// [slices]: slice
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct RSplitN<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
     inner: GenericSplitN<RSplit<'a, T, P>>,
 }

 impl<'a, T: 'a, P: FnMut(&T) -> bool> RSplitN<'a, T, P> {
     #[inline]
     pub(super) fn new(s: RSplit<'a, T, P>, n: usize) -> Self {
         Self { inner: GenericSplitN { iter: s, count: n } }
     }
 }

 #[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].
 ///
 /// # Example
 ///
 /// ```
 /// let mut slice = [10, 40, 30, 20, 60, 50];
 /// let iter = slice.splitn_mut(2, |num| *num % 3 == 0);
 /// ```
 ///
 /// [`splitn_mut`]: slice::splitn_mut
 /// [slices]: slice
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct SplitNMut<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
     inner: GenericSplitN<SplitMut<'a, T, P>>,
 }

 impl<'a, T: 'a, P: FnMut(&T) -> bool> SplitNMut<'a, T, P> {
     #[inline]
     pub(super) fn new(s: SplitMut<'a, T, P>, n: usize) -> Self {
         Self { inner: GenericSplitN { iter: s, count: n } }
     }
 }

 #[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].
 ///
 /// # Example
 ///
 /// ```
 /// let mut slice = [10, 40, 30, 20, 60, 50];
 /// let iter = slice.rsplitn_mut(2, |num| *num % 3 == 0);
 /// ```
 ///
 /// [`rsplitn_mut`]: slice::rsplitn_mut
 /// [slices]: slice
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct RSplitNMut<'a, T: 'a, P>
 where
     P: FnMut(&T) -> bool,
 {
     inner: GenericSplitN<RSplitMut<'a, T, P>>,
 }

 impl<'a, T: 'a, P: FnMut(&T) -> bool> RSplitNMut<'a, T, P> {
     #[inline]
     pub(super) fn new(s: RSplitMut<'a, T, P>, n: usize) -> Self {
         Self { inner: GenericSplitN { iter: s, count: n } }
     }
 }

 #[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()
     }
 }

 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].
 ///
 /// # Example
 ///
 /// ```
 /// let slice = ['r', 'u', 's', 't'];
 /// let iter = slice.windows(2);
 /// ```
 ///
 /// [`windows`]: slice::windows
 /// [slices]: slice
 #[derive(Debug)]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct Windows<'a, T: 'a> {
     v: &'a [T],
     size: NonZeroUsize,
 }

 impl<'a, T: 'a> Windows<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a [T], size: NonZeroUsize) -> Self {
         Self { v: slice, size }
     }
 }

 // 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.get() > self.v.len() {
             None
         } else {
             let ret = Some(&self.v[..self.size.get()]);
             self.v = &self.v[1..];
             ret
         }
     }

     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         if self.size.get() > self.v.len() {
             (0, Some(0))
         } else {
             let size = self.v.len() - self.size.get() + 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.get().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.get() > self.v.len() {
             None
         } else {
             let start = self.v.len() - self.size.get();
             Some(&self.v[start..])
         }
     }

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

 #[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.get() > self.v.len() {
             None
         } else {
             let ret = Some(&self.v[self.v.len() - self.size.get()..]);
             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.get() || overflow {
             self.v = &[];
             None
         } else {
             let ret = &self.v[end - self.size.get()..end];
             self.v = &self.v[..end - 1];
             Some(ret)
         }
     }
 }

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

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for Windows<'a, T> {
     const 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].
 ///
 /// # Example
 ///
 /// ```
 /// let slice = ['l', 'o', 'r', 'e', 'm'];
 /// let iter = slice.chunks(2);
 /// ```
 ///
 /// [`chunks`]: slice::chunks
 /// [slices]: slice
 #[derive(Debug)]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct Chunks<'a, T: 'a> {
     v: &'a [T],
     chunk_size: usize,
 }

 impl<'a, T: 'a> Chunks<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a [T], size: usize) -> Self {
         Self { v: slice, chunk_size: size }
     }
 }

 // 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..])
         }
     }

     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
         let start = idx * self.chunk_size;
         // SAFETY: the caller guarantees that `i` is in bounds,
         // which means that `start` must be in bounds of the
         // underlying `self.v` slice, and we made sure that `len`
         // is also in bounds of `self.v`. Thus, `start` cannot overflow
         // an `isize`, and the slice constructed by `from_raw_parts`
         // is a subslice of `self.v` which is guaranteed to be valid
         // for the lifetime `'a` of `self.v`.
         unsafe {
             let len = cmp::min(self.v.len().unchecked_sub(start), self.chunk_size);
             from_raw_parts(self.v.as_ptr().add(start), len)
         }
     }
 }

 #[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 };
             // SAFETY: split_at_unchecked requires the argument be less than or
             // equal to the length. This is guaranteed, but subtle: `chunksz`
             // will always either be `self.v.len() % self.chunk_size`, which
             // will always evaluate to strictly less than `self.v.len()` (or
             // panic, in the case that `self.chunk_size` is zero), or it can be
             // `self.chunk_size`, in the case that the length is exactly
             // divisible by the chunk size.
             //
             // While it seems like using `self.chunk_size` in this case could
             // lead to a value greater than `self.v.len()`, it cannot: if
             // `self.chunk_size` were greater than `self.v.len()`, then
             // `self.v.len() % self.chunk_size` would return nonzero (note that
             // in this branch of the `if`, we already know that `self.v` is
             // non-empty).
             let (fst, snd) = unsafe { self.v.split_at_unchecked(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(self.v.len(), res),
                 None => self.v.len(),
             };
             let nth_back = &self.v[start..end];
             self.v = &self.v[..start];
             Some(nth_back)
         }
     }
 }

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

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for Chunks<'a, T> {
     const 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].
 ///
 /// # Example
 ///
 /// ```
 /// let mut slice = ['l', 'o', 'r', 'e', 'm'];
 /// let iter = slice.chunks_mut(2);
 /// ```
 ///
 /// [`chunks_mut`]: slice::chunks_mut
 /// [slices]: slice
 #[derive(Debug)]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct ChunksMut<'a, T: 'a> {
     /// # Safety
     /// This slice pointer must point at a valid region of `T` with at least length `v.len()`. Normally,
     /// those requirements would mean that we could instead use a `&mut [T]` here, but we cannot
     /// because `__iterator_get_unchecked` needs to return `&mut [T]`, which guarantees certain aliasing
     /// properties that we cannot uphold if we hold on to the full original `&mut [T]`. Wrapping a raw
     /// slice instead lets us hand out non-overlapping `&mut [T]` subslices of the slice we wrap.
     v: *mut [T],
     chunk_size: usize,
     _marker: PhantomData<&'a mut T>,
 }

 impl<'a, T: 'a> ChunksMut<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T], size: usize) -> Self {
         Self { v: slice, chunk_size: size, _marker: PhantomData }
     }
 }

 #[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);
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (head, tail) = unsafe { self.v.split_at_mut(sz) };
             self.v = tail;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *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(),
             };
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (head, tail) = unsafe { self.v.split_at_mut(end) };
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (_, nth) = unsafe { head.split_at_mut(start) };
             self.v = tail;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *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;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *self.v.get_unchecked_mut(start..) })
         }
     }

     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
         let start = idx * self.chunk_size;
         // SAFETY: see comments for `Chunks::__iterator_get_unchecked` and `self.v`.
         //
         // Also note that the caller also guarantees that we're never called
         // with the same index again, and that no other methods that will
         // access this subslice are called, so it is valid for the returned
         // slice to be mutable.
         unsafe {
             let len = cmp::min(self.v.len().unchecked_sub(start), self.chunk_size);
             from_raw_parts_mut(self.v.as_mut_ptr().add(start), len)
         }
     }
 }

 #[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 len = self.v.len();
             // SAFETY: Similar to `Chunks::next_back`
             let (head, tail) = unsafe { self.v.split_at_mut_unchecked(len - sz) };
             self.v = head;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *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(self.v.len(), res),
                 None => self.v.len(),
             };
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (temp, _tail) = unsafe { self.v.split_at_mut(end) };
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (head, nth_back) = unsafe { temp.split_at_mut(start) };
             self.v = head;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *nth_back })
         }
     }
 }

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

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for ChunksMut<'a, T> {
     const MAY_HAVE_SIDE_EFFECT: bool = false;
 }

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

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

 /// 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].
 ///
 /// # Example
 ///
 /// ```
 /// let slice = ['l', 'o', 'r', 'e', 'm'];
 /// let iter = slice.chunks_exact(2);
 /// ```
 ///
 /// [`chunks_exact`]: slice::chunks_exact
 /// [`remainder`]: ChunksExact::remainder
 /// [slices]: slice
 #[derive(Debug)]
 #[stable(feature = "chunks_exact", since = "1.31.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct ChunksExact<'a, T: 'a> {
     v: &'a [T],
     rem: &'a [T],
     chunk_size: usize,
 }

 impl<'a, T> ChunksExact<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a [T], chunk_size: usize) -> Self {
         let rem = slice.len() % chunk_size;
         let fst_len = slice.len() - rem;
         // SAFETY: 0 <= fst_len <= slice.len() by construction above
         let (fst, snd) = unsafe { slice.split_at_unchecked(fst_len) };
         Self { v: fst, rem: snd, chunk_size }
     }

     /// 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.
     ///
     /// # Example
     ///
     /// ```
     /// let slice = ['l', 'o', 'r', 'e', 'm'];
     /// let mut iter = slice.chunks_exact(2);
     /// assert_eq!(iter.remainder(), &['m'][..]);
     /// assert_eq!(iter.next(), Some(&['l', 'o'][..]));
     /// assert_eq!(iter.remainder(), &['m'][..]);
     /// assert_eq!(iter.next(), Some(&['r', 'e'][..]));
     /// assert_eq!(iter.remainder(), &['m'][..]);
     /// assert_eq!(iter.next(), None);
     /// assert_eq!(iter.remainder(), &['m'][..]);
     /// ```
     #[must_use]
     #[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()
     }

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

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

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

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

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for ChunksExact<'a, T> {
     const 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].
 ///
 /// # Example
 ///
 /// ```
 /// let mut slice = ['l', 'o', 'r', 'e', 'm'];
 /// let iter = slice.chunks_exact_mut(2);
 /// ```
 ///
 /// [`chunks_exact_mut`]: slice::chunks_exact_mut
 /// [`into_remainder`]: ChunksExactMut::into_remainder
 /// [slices]: slice
 #[derive(Debug)]
 #[stable(feature = "chunks_exact", since = "1.31.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct ChunksExactMut<'a, T: 'a> {
     /// # Safety
     /// This slice pointer must point at a valid region of `T` with at least length `v.len()`. Normally,
     /// those requirements would mean that we could instead use a `&mut [T]` here, but we cannot
     /// because `__iterator_get_unchecked` needs to return `&mut [T]`, which guarantees certain aliasing
     /// properties that we cannot uphold if we hold on to the full original `&mut [T]`. Wrapping a raw
     /// slice instead lets us hand out non-overlapping `&mut [T]` subslices of the slice we wrap.
     v: *mut [T],
     rem: &'a mut [T], // The iterator never yields from here, so this can be unique
     chunk_size: usize,
     _marker: PhantomData<&'a mut T>,
 }

 impl<'a, T> ChunksExactMut<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T], chunk_size: usize) -> Self {
         let rem = slice.len() % chunk_size;
         let fst_len = slice.len() - rem;
         // SAFETY: 0 <= fst_len <= slice.len() by construction above
         let (fst, snd) = unsafe { slice.split_at_mut_unchecked(fst_len) };
         Self { v: fst, rem: snd, chunk_size, _marker: PhantomData }
     }

     /// 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.
     #[must_use = "`self` will be dropped if the result is not used"]
     #[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 {
             // SAFETY: self.chunk_size is inbounds because we compared above against self.v.len()
             let (head, tail) = unsafe { self.v.split_at_mut(self.chunk_size) };
             self.v = tail;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *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 {
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (_, snd) = unsafe { self.v.split_at_mut(start) };
             self.v = snd;
             self.next()
         }
     }

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

     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
         let start = idx * self.chunk_size;
         // SAFETY: see comments for `Chunks::__iterator_get_unchecked` and `self.v`.
         unsafe { from_raw_parts_mut(self.v.as_mut_ptr().add(start), self.chunk_size) }
     }
 }

 #[stable(feature = "chunks_exact", since = "1.31.0")]
 impl<'a, T> DoubleEndedIterator for ChunksExactMut<'a, T> {
     #[inline]
     fn next_back(&mut self) -> Option<&'a mut [T]> {
         if self.v.len() < self.chunk_size {
             None
         } else {
             // SAFETY: This subtraction is inbounds because of the check above
             let (head, tail) = unsafe { self.v.split_at_mut(self.v.len() - self.chunk_size) };
             self.v = head;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *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;
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (temp, _tail) = unsafe { mem::replace(&mut self.v, &mut []).split_at_mut(end) };
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (head, nth_back) = unsafe { temp.split_at_mut(start) };
             self.v = head;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *nth_back })
         }
     }
 }

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

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for ChunksExactMut<'a, T> {
     const MAY_HAVE_SIDE_EFFECT: bool = false;
 }

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

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

 /// A windowed iterator over a slice in overlapping chunks (`N` elements at a
 /// time), starting at the beginning of the slice
 ///
 /// This struct is created by the [`array_windows`] method on [slices].
 ///
 /// # Example
 ///
 /// ```
 /// #![feature(array_windows)]
 ///
 /// let slice = [0, 1, 2, 3];
 /// let iter = slice.array_windows::<2>();
 /// ```
 ///
 /// [`array_windows`]: slice::array_windows
 /// [slices]: slice
 #[derive(Debug, Clone, Copy)]
 #[unstable(feature = "array_windows", issue = "75027")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct ArrayWindows<'a, T: 'a, const N: usize> {
     slice_head: *const T,
     num: usize,
     marker: PhantomData<&'a [T; N]>,
 }

 impl<'a, T: 'a, const N: usize> ArrayWindows<'a, T, N> {
     #[inline]
     pub(super) fn new(slice: &'a [T]) -> Self {
         let num_windows = slice.len().saturating_sub(N - 1);
         Self { slice_head: slice.as_ptr(), num: num_windows, marker: PhantomData }
     }
 }

 #[unstable(feature = "array_windows", issue = "75027")]
 impl<'a, T, const N: usize> Iterator for ArrayWindows<'a, T, N> {
     type Item = &'a [T; N];

     #[inline]
     fn next(&mut self) -> Option<Self::Item> {
         if self.num == 0 {
             return None;
         }
         // SAFETY:
         // This is safe because it's indexing into a slice guaranteed to be length > N.
         let ret = unsafe { &*self.slice_head.cast::<[T; N]>() };
         // SAFETY: Guaranteed that there are at least 1 item remaining otherwise
         // earlier branch would've been hit
         self.slice_head = unsafe { self.slice_head.add(1) };

         self.num -= 1;
         Some(ret)
     }

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

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

     #[inline]
     fn nth(&mut self, n: usize) -> Option<Self::Item> {
         if self.num <= n {
             self.num = 0;
             return None;
         }
         // SAFETY:
         // This is safe because it's indexing into a slice guaranteed to be length > N.
         let ret = unsafe { &*self.slice_head.add(n).cast::<[T; N]>() };
         // SAFETY: Guaranteed that there are at least n items remaining
         self.slice_head = unsafe { self.slice_head.add(n + 1) };

         self.num -= n + 1;
         Some(ret)
     }

     #[inline]
     fn last(mut self) -> Option<Self::Item> {
         self.nth(self.num.checked_sub(1)?)
     }
 }

 #[unstable(feature = "array_windows", issue = "75027")]
 impl<'a, T, const N: usize> DoubleEndedIterator for ArrayWindows<'a, T, N> {
     #[inline]
     fn next_back(&mut self) -> Option<&'a [T; N]> {
         if self.num == 0 {
             return None;
         }
         // SAFETY: Guaranteed that there are n items remaining, n-1 for 0-indexing.
         let ret = unsafe { &*self.slice_head.add(self.num - 1).cast::<[T; N]>() };
         self.num -= 1;
         Some(ret)
     }

     #[inline]
     fn nth_back(&mut self, n: usize) -> Option<&'a [T; N]> {
         if self.num <= n {
             self.num = 0;
             return None;
         }
         // SAFETY: Guaranteed that there are n items remaining, n-1 for 0-indexing.
         let ret = unsafe { &*self.slice_head.add(self.num - (n + 1)).cast::<[T; N]>() };
         self.num -= n + 1;
         Some(ret)
     }
 }

 #[unstable(feature = "array_windows", issue = "75027")]
 impl<T, const N: usize> ExactSizeIterator for ArrayWindows<'_, T, N> {
     fn is_empty(&self) -> bool {
         self.num == 0
     }
 }

 /// An iterator over a slice in (non-overlapping) chunks (`N` elements at a
 /// time), starting at the beginning of the slice.
 ///
 /// When the slice len is not evenly divided by the chunk size, the last
 /// up to `N-1` elements will be omitted but can be retrieved from
 /// the [`remainder`] function from the iterator.
 ///
 /// This struct is created by the [`array_chunks`] method on [slices].
 ///
 /// # Example
 ///
 /// ```
 /// #![feature(array_chunks)]
 ///
 /// let slice = ['l', 'o', 'r', 'e', 'm'];
 /// let iter = slice.array_chunks::<2>();
 /// ```
 ///
 /// [`array_chunks`]: slice::array_chunks
 /// [`remainder`]: ArrayChunks::remainder
 /// [slices]: slice
 #[derive(Debug)]
 #[unstable(feature = "array_chunks", issue = "74985")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct ArrayChunks<'a, T: 'a, const N: usize> {
     iter: Iter<'a, [T; N]>,
     rem: &'a [T],
 }

 impl<'a, T, const N: usize> ArrayChunks<'a, T, N> {
     #[inline]
     pub(super) fn new(slice: &'a [T]) -> Self {
         let (array_slice, rem) = slice.as_chunks();
         Self { iter: array_slice.iter(), rem }
     }

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 /// An iterator over a slice in (non-overlapping) mutable chunks (`N` elements
 /// at a time), starting at the beginning of the slice.
 ///
 /// When the slice len is not evenly divided by the chunk size, the last
 /// up to `N-1` elements will be omitted but can be retrieved from
 /// the [`into_remainder`] function from the iterator.
 ///
 /// This struct is created by the [`array_chunks_mut`] method on [slices].
 ///
 /// # Example
 ///
 /// ```
 /// #![feature(array_chunks)]
 ///
 /// let mut slice = ['l', 'o', 'r', 'e', 'm'];
 /// let iter = slice.array_chunks_mut::<2>();
 /// ```
 ///
 /// [`array_chunks_mut`]: slice::array_chunks_mut
 /// [`into_remainder`]: ../../std/slice/struct.ArrayChunksMut.html#method.into_remainder
 /// [slices]: slice
 #[derive(Debug)]
 #[unstable(feature = "array_chunks", issue = "74985")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct ArrayChunksMut<'a, T: 'a, const N: usize> {
     iter: IterMut<'a, [T; N]>,
     rem: &'a mut [T],
 }

 impl<'a, T, const N: usize> ArrayChunksMut<'a, T, N> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T]) -> Self {
         let (array_slice, rem) = slice.as_chunks_mut();
         Self { iter: array_slice.iter_mut(), rem }
     }

     /// Returns the remainder of the original slice that is not going to be
     /// returned by the iterator. The returned slice has at most `N-1`
     /// elements.
     #[must_use = "`self` will be dropped if the result is not used"]
     #[unstable(feature = "array_chunks", issue = "74985")]
     pub fn into_remainder(self) -> &'a mut [T] {
         self.rem
     }
 }

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

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

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

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

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

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

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

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

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

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

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "array_chunks", issue = "74985")]
 unsafe impl<'a, T, const N: usize> TrustedRandomAccessNoCoerce for ArrayChunksMut<'a, T, N> {
     const 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].
 ///
 /// # Example
 ///
 /// ```
 /// let slice = ['l', 'o', 'r', 'e', 'm'];
 /// let iter = slice.rchunks(2);
 /// ```
 ///
 /// [`rchunks`]: slice::rchunks
 /// [slices]: slice
 #[derive(Debug)]
 #[stable(feature = "rchunks", since = "1.31.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct RChunks<'a, T: 'a> {
     v: &'a [T],
     chunk_size: usize,
 }

 impl<'a, T: 'a> RChunks<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a [T], size: usize) -> Self {
         Self { v: slice, chunk_size: size }
     }
 }

 // 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 len = self.v.len();
             let chunksz = cmp::min(len, self.chunk_size);
             // SAFETY: split_at_unchecked just requires the argument be less
             // than the length. This could only happen if the expression `len -
             // chunksz` overflows. This could only happen if `chunksz > len`,
             // which is impossible as we initialize it as the `min` of `len` and
             // `self.chunk_size`.
             let (fst, snd) = unsafe { self.v.split_at_unchecked(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])
         }
     }

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

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

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

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

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for RChunks<'a, T> {
     const 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].
 ///
 /// # Example
 ///
 /// ```
 /// let mut slice = ['l', 'o', 'r', 'e', 'm'];
 /// let iter = slice.rchunks_mut(2);
 /// ```
 ///
 /// [`rchunks_mut`]: slice::rchunks_mut
 /// [slices]: slice
 #[derive(Debug)]
 #[stable(feature = "rchunks", since = "1.31.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct RChunksMut<'a, T: 'a> {
     /// # Safety
     /// This slice pointer must point at a valid region of `T` with at least length `v.len()`. Normally,
     /// those requirements would mean that we could instead use a `&mut [T]` here, but we cannot
     /// because `__iterator_get_unchecked` needs to return `&mut [T]`, which guarantees certain aliasing
     /// properties that we cannot uphold if we hold on to the full original `&mut [T]`. Wrapping a raw
     /// slice instead lets us hand out non-overlapping `&mut [T]` subslices of the slice we wrap.
     v: *mut [T],
     chunk_size: usize,
     _marker: PhantomData<&'a mut T>,
 }

 impl<'a, T: 'a> RChunksMut<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T], size: usize) -> Self {
         Self { v: slice, chunk_size: size, _marker: PhantomData }
     }
 }

 #[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 len = self.v.len();
             // SAFETY: split_at_mut_unchecked just requires the argument be less
             // than the length. This could only happen if the expression
             // `len - sz` overflows. This could only happen if `sz >
             // len`, which is impossible as we initialize it as the `min` of
             // `self.v.len()` (e.g. `len`) and `self.chunk_size`.
             let (head, tail) = unsafe { self.v.split_at_mut_unchecked(len - sz) };
             self.v = head;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *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,
             };
             // SAFETY: This type ensures that self.v is a valid pointer with a correct len.
             // Therefore the bounds check in split_at_mut guarantees the split point is inbounds.
             let (head, tail) = unsafe { self.v.split_at_mut(start) };
             // SAFETY: This type ensures that self.v is a valid pointer with a correct len.
             // Therefore the bounds check in split_at_mut guarantees the split point is inbounds.
             let (nth, _) = unsafe { tail.split_at_mut(end - start) };
             self.v = head;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *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 };
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *self.v.get_unchecked_mut(0..end) })
         }
     }

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

 #[stable(feature = "rchunks", since = "1.31.0")]
 impl<'a, T> DoubleEndedIterator for RChunksMut<'a, T> {
     #[inline]
     fn next_back(&mut self) -> Option<&'a mut [T]> {
         if self.v.is_empty() {
             None
         } else {
             let remainder = self.v.len() % self.chunk_size;
             let sz = if remainder != 0 { remainder } else { self.chunk_size };
             // SAFETY: Similar to `Chunks::next_back`
             let (head, tail) = unsafe { self.v.split_at_mut_unchecked(sz) };
             self.v = tail;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *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);
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (tmp, tail) = unsafe { self.v.split_at_mut(end) };
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (_, nth_back) = unsafe { tmp.split_at_mut(start) };
             self.v = tail;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *nth_back })
         }
     }
 }

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

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for RChunksMut<'a, T> {
     const MAY_HAVE_SIDE_EFFECT: bool = false;
 }

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

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

 /// 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].
 ///
 /// # Example
 ///
 /// ```
 /// let slice = ['l', 'o', 'r', 'e', 'm'];
 /// let iter = slice.rchunks_exact(2);
 /// ```
 ///
 /// [`rchunks_exact`]: slice::rchunks_exact
 /// [`remainder`]: RChunksExact::remainder
 /// [slices]: slice
 #[derive(Debug)]
 #[stable(feature = "rchunks", since = "1.31.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct RChunksExact<'a, T: 'a> {
     v: &'a [T],
     rem: &'a [T],
     chunk_size: usize,
 }

 impl<'a, T> RChunksExact<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a [T], chunk_size: usize) -> Self {
         let rem = slice.len() % chunk_size;
         // SAFETY: 0 <= rem <= slice.len() by construction above
         let (fst, snd) = unsafe { slice.split_at_unchecked(rem) };
         Self { v: snd, rem: fst, chunk_size }
     }

     /// 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.
     ///
     /// # Example
     ///
     /// ```
     /// let slice = ['l', 'o', 'r', 'e', 'm'];
     /// let mut iter = slice.rchunks_exact(2);
     /// assert_eq!(iter.remainder(), &['l'][..]);
     /// assert_eq!(iter.next(), Some(&['e', 'm'][..]));
     /// assert_eq!(iter.remainder(), &['l'][..]);
     /// assert_eq!(iter.next(), Some(&['o', 'r'][..]));
     /// assert_eq!(iter.remainder(), &['l'][..]);
     /// assert_eq!(iter.next(), None);
     /// assert_eq!(iter.remainder(), &['l'][..]);
     /// ```
     #[must_use]
     #[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()
     }

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

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

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

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

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for RChunksExact<'a, T> {
     const 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].
 ///
 /// # Example
 ///
 /// ```
 /// let mut slice = ['l', 'o', 'r', 'e', 'm'];
 /// let iter = slice.rchunks_exact_mut(2);
 /// ```
 ///
 /// [`rchunks_exact_mut`]: slice::rchunks_exact_mut
 /// [`into_remainder`]: RChunksExactMut::into_remainder
 /// [slices]: slice
 #[derive(Debug)]
 #[stable(feature = "rchunks", since = "1.31.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct RChunksExactMut<'a, T: 'a> {
     /// # Safety
     /// This slice pointer must point at a valid region of `T` with at least length `v.len()`. Normally,
     /// those requirements would mean that we could instead use a `&mut [T]` here, but we cannot
     /// because `__iterator_get_unchecked` needs to return `&mut [T]`, which guarantees certain aliasing
     /// properties that we cannot uphold if we hold on to the full original `&mut [T]`. Wrapping a raw
     /// slice instead lets us hand out non-overlapping `&mut [T]` subslices of the slice we wrap.
     v: *mut [T],
     rem: &'a mut [T],
     chunk_size: usize,
 }

 impl<'a, T> RChunksExactMut<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T], chunk_size: usize) -> Self {
         let rem = slice.len() % chunk_size;
         // SAFETY: 0 <= rem <= slice.len() by construction above
         let (fst, snd) = unsafe { slice.split_at_mut_unchecked(rem) };
         Self { v: snd, rem: fst, chunk_size }
     }

     /// 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.
     #[must_use = "`self` will be dropped if the result is not used"]
     #[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 len = self.v.len();
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (head, tail) = unsafe { self.v.split_at_mut(len - self.chunk_size) };
             self.v = head;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *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 len = self.v.len();
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (fst, _) = unsafe { self.v.split_at_mut(len - end) };
             self.v = fst;
             self.next()
         }
     }

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

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

 #[stable(feature = "rchunks", since = "1.31.0")]
 impl<'a, T> DoubleEndedIterator for RChunksExactMut<'a, T> {
     #[inline]
     fn next_back(&mut self) -> Option<&'a mut [T]> {
         if self.v.len() < self.chunk_size {
             None
         } else {
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (head, tail) = unsafe { self.v.split_at_mut(self.chunk_size) };
             self.v = tail;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *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;
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (tmp, tail) = unsafe { self.v.split_at_mut(end) };
             // SAFETY: The self.v contract ensures that any split_at_mut is valid.
             let (_, nth_back) = unsafe { tmp.split_at_mut(start) };
             self.v = tail;
             // SAFETY: Nothing else points to or will point to the contents of this slice.
             Some(unsafe { &mut *nth_back })
         }
     }
 }

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

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for RChunksExactMut<'a, T> {
     const MAY_HAVE_SIDE_EFFECT: bool = false;
 }

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

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

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for Iter<'a, T> {
     const MAY_HAVE_SIDE_EFFECT: bool = false;
 }

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

 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<'a, T> TrustedRandomAccessNoCoerce for IterMut<'a, T> {
     const MAY_HAVE_SIDE_EFFECT: bool = false;
 }

 /// An iterator over slice in (non-overlapping) chunks separated by a predicate.
 ///
 /// This struct is created by the [`group_by`] method on [slices].
 ///
 /// [`group_by`]: slice::group_by
 /// [slices]: slice
 #[unstable(feature = "slice_group_by", issue = "80552")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct GroupBy<'a, T: 'a, P> {
     slice: &'a [T],
     predicate: P,
 }

 #[unstable(feature = "slice_group_by", issue = "80552")]
 impl<'a, T: 'a, P> GroupBy<'a, T, P> {
     pub(super) fn new(slice: &'a [T], predicate: P) -> Self {
         GroupBy { slice, predicate }
     }
 }

 #[unstable(feature = "slice_group_by", issue = "80552")]
 impl<'a, T: 'a, P> Iterator for GroupBy<'a, T, P>
 where
     P: FnMut(&T, &T) -> bool,
 {
     type Item = &'a [T];

     #[inline]
     fn next(&mut self) -> Option<Self::Item> {
         if self.slice.is_empty() {
             None
         } else {
             let mut len = 1;
             let mut iter = self.slice.windows(2);
             while let Some([l, r]) = iter.next() {
                 if (self.predicate)(l, r) { len += 1 } else { break }
             }
             let (head, tail) = self.slice.split_at(len);
             self.slice = tail;
             Some(head)
         }
     }

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

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

 #[unstable(feature = "slice_group_by", issue = "80552")]
 impl<'a, T: 'a, P> DoubleEndedIterator for GroupBy<'a, T, P>
 where
     P: FnMut(&T, &T) -> bool,
 {
     #[inline]
     fn next_back(&mut self) -> Option<Self::Item> {
         if self.slice.is_empty() {
             None
         } else {
             let mut len = 1;
             let mut iter = self.slice.windows(2);
             while let Some([l, r]) = iter.next_back() {
                 if (self.predicate)(l, r) { len += 1 } else { break }
             }
             let (head, tail) = self.slice.split_at(self.slice.len() - len);
             self.slice = head;
             Some(tail)
         }
     }
 }

 #[unstable(feature = "slice_group_by", issue = "80552")]
 impl<'a, T: 'a, P> FusedIterator for GroupBy<'a, T, P> where P: FnMut(&T, &T) -> bool {}

 #[unstable(feature = "slice_group_by", issue = "80552")]
 impl<'a, T: 'a + fmt::Debug, P> fmt::Debug for GroupBy<'a, T, P> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("GroupBy").field("slice", &self.slice).finish()
     }
 }

 /// An iterator over slice in (non-overlapping) mutable chunks separated
 /// by a predicate.
 ///
 /// This struct is created by the [`group_by_mut`] method on [slices].
 ///
 /// [`group_by_mut`]: slice::group_by_mut
 /// [slices]: slice
 #[unstable(feature = "slice_group_by", issue = "80552")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct GroupByMut<'a, T: 'a, P> {
     slice: &'a mut [T],
     predicate: P,
 }

 #[unstable(feature = "slice_group_by", issue = "80552")]
 impl<'a, T: 'a, P> GroupByMut<'a, T, P> {
     pub(super) fn new(slice: &'a mut [T], predicate: P) -> Self {
         GroupByMut { slice, predicate }
     }
 }

 #[unstable(feature = "slice_group_by", issue = "80552")]
 impl<'a, T: 'a, P> Iterator for GroupByMut<'a, T, P>
 where
     P: FnMut(&T, &T) -> bool,
 {
     type Item = &'a mut [T];

     #[inline]
     fn next(&mut self) -> Option<Self::Item> {
         if self.slice.is_empty() {
             None
         } else {
             let mut len = 1;
             let mut iter = self.slice.windows(2);
             while let Some([l, r]) = iter.next() {
                 if (self.predicate)(l, r) { len += 1 } else { break }
             }
             let slice = mem::take(&mut self.slice);
             let (head, tail) = slice.split_at_mut(len);
             self.slice = tail;
             Some(head)
         }
     }

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

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

 #[unstable(feature = "slice_group_by", issue = "80552")]
 impl<'a, T: 'a, P> DoubleEndedIterator for GroupByMut<'a, T, P>
 where
     P: FnMut(&T, &T) -> bool,
 {
     #[inline]
     fn next_back(&mut self) -> Option<Self::Item> {
         if self.slice.is_empty() {
             None
         } else {
             let mut len = 1;
             let mut iter = self.slice.windows(2);
             while let Some([l, r]) = iter.next_back() {
                 if (self.predicate)(l, r) { len += 1 } else { break }
             }
             let slice = mem::take(&mut self.slice);
             let (head, tail) = slice.split_at_mut(slice.len() - len);
             self.slice = head;
             Some(tail)
         }
     }
 }

 #[unstable(feature = "slice_group_by", issue = "80552")]
 impl<'a, T: 'a, P> FusedIterator for GroupByMut<'a, T, P> where P: FnMut(&T, &T) -> bool {}

 #[unstable(feature = "slice_group_by", issue = "80552")]
 impl<'a, T: 'a + fmt::Debug, P> fmt::Debug for GroupByMut<'a, T, P> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("GroupByMut").field("slice", &self.slice).finish()
     }
 }