7 files changed, 506 insertions, 158 deletions

diff --git a/library/core/src/iter/adapters/array_chunks.rs b/library/core/src/iter/adapters/array_chunks.rs
new file mode 100644
index 000000000..9b479a9f8
--- /dev/null
+++ b/library/core/src/iter/adapters/array_chunks.rs
@@ -0,0 +1,182 @@
+use crate::array;
+use crate::iter::{ByRefSized, FusedIterator, Iterator};
+use crate::ops::{ControlFlow, NeverShortCircuit, Try};
+
+/// An iterator over `N` elements of the iterator at a time.
+///
+/// The chunks do not overlap. If `N` does not divide the length of the
+/// iterator, then the last up to `N-1` elements will be omitted.
+///
+/// This `struct` is created by the [`array_chunks`][Iterator::array_chunks]
+/// method on [`Iterator`]. See its documentation for more.
+#[derive(Debug, Clone)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
+pub struct ArrayChunks<I: Iterator, const N: usize> {
+    iter: I,
+    remainder: Option<array::IntoIter<I::Item, N>>,
+}
+
+impl<I, const N: usize> ArrayChunks<I, N>
+where
+    I: Iterator,
+{
+    #[track_caller]
+    pub(in crate::iter) fn new(iter: I) -> Self {
+        assert!(N != 0, "chunk size must be non-zero");
+        Self { iter, remainder: None }
+    }
+
+    /// Returns an iterator over the remaining elements of the original iterator
+    /// that are not going to be returned by this iterator. The returned
+    /// iterator will yield at most `N-1` elements.
+    #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
+    #[inline]
+    pub fn into_remainder(self) -> Option<array::IntoIter<I::Item, N>> {
+        self.remainder
+    }
+}
+
+#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
+impl<I, const N: usize> Iterator for ArrayChunks<I, N>
+where
+    I: Iterator,
+{
+    type Item = [I::Item; N];
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        self.try_for_each(ControlFlow::Break).break_value()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (lower, upper) = self.iter.size_hint();
+
+        (lower / N, upper.map(|n| n / N))
+    }
+
+    #[inline]
+    fn count(self) -> usize {
+        self.iter.count() / N
+    }
+
+    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Output = B>,
+    {
+        let mut acc = init;
+        loop {
+            match self.iter.next_chunk() {
+                Ok(chunk) => acc = f(acc, chunk)?,
+                Err(remainder) => {
+                    // Make sure to not override `self.remainder` with an empty array
+                    // when `next` is called after `ArrayChunks` exhaustion.
+                    self.remainder.get_or_insert(remainder);
+
+                    break try { acc };
+                }
+            }
+        }
+    }
+
+    fn fold<B, F>(mut self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0
+    }
+}
+
+#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
+impl<I, const N: usize> DoubleEndedIterator for ArrayChunks<I, N>
+where
+    I: DoubleEndedIterator + ExactSizeIterator,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.try_rfold((), |(), x| ControlFlow::Break(x)).break_value()
+    }
+
+    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: Try<Output = B>,
+    {
+        // We are iterating from the back we need to first handle the remainder.
+        self.next_back_remainder();
+
+        let mut acc = init;
+        let mut iter = ByRefSized(&mut self.iter).rev();
+
+        // NB remainder is handled by `next_back_remainder`, so
+        // `next_chunk` can't return `Err` with non-empty remainder
+        // (assuming correct `I as ExactSizeIterator` impl).
+        while let Ok(mut chunk) = iter.next_chunk() {
+            // FIXME: do not do double reverse
+            //        (we could instead add `next_chunk_back` for example)
+            chunk.reverse();
+            acc = f(acc, chunk)?
+        }
+
+        try { acc }
+    }
+
+    fn rfold<B, F>(mut self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.try_rfold(init, NeverShortCircuit::wrap_mut_2(f)).0
+    }
+}
+
+impl<I, const N: usize> ArrayChunks<I, N>
+where
+    I: DoubleEndedIterator + ExactSizeIterator,
+{
+    /// Updates `self.remainder` such that `self.iter.len` is divisible by `N`.
+    fn next_back_remainder(&mut self) {
+        // Make sure to not override `self.remainder` with an empty array
+        // when `next_back` is called after `ArrayChunks` exhaustion.
+        if self.remainder.is_some() {
+            return;
+        }
+
+        // We use the `ExactSizeIterator` implementation of the underlying
+        // iterator to know how many remaining elements there are.
+        let rem = self.iter.len() % N;
+
+        // Take the last `rem` elements out of `self.iter`.
+        let mut remainder =
+            // SAFETY: `unwrap_err` always succeeds because x % N < N for all x.
+            unsafe { self.iter.by_ref().rev().take(rem).next_chunk().unwrap_err_unchecked() };
+
+        // We used `.rev()` above, so we need to re-reverse the reminder
+        remainder.as_mut_slice().reverse();
+        self.remainder = Some(remainder);
+    }
+}
+
+#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
+impl<I, const N: usize> FusedIterator for ArrayChunks<I, N> where I: FusedIterator {}
+
+#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
+impl<I, const N: usize> ExactSizeIterator for ArrayChunks<I, N>
+where
+    I: ExactSizeIterator,
+{
+    #[inline]
+    fn len(&self) -> usize {
+        self.iter.len() / N
+    }
+
+    #[inline]
+    fn is_empty(&self) -> bool {
+        self.iter.len() < N
+    }
+}
diff --git a/library/core/src/iter/adapters/by_ref_sized.rs b/library/core/src/iter/adapters/by_ref_sized.rs
index cc1e8e8a2..477e7117c 100644
--- a/library/core/src/iter/adapters/by_ref_sized.rs
+++ b/library/core/src/iter/adapters/by_ref_sized.rs
@@ -1,4 +1,4 @@
-use crate::ops::Try;
+use crate::ops::{NeverShortCircuit, Try};
 
 /// Like `Iterator::by_ref`, but requiring `Sized` so it can forward generics.
 ///
@@ -8,28 +8,31 @@ use crate::ops::Try;
 #[derive(Debug)]
 pub struct ByRefSized<'a, I>(pub &'a mut I);
 
+// The following implementations use UFCS-style, rather than trusting autoderef,
+// to avoid accidentally calling the `&mut Iterator` implementations.
+
 #[unstable(feature = "std_internals", issue = "none")]
 impl<I: Iterator> Iterator for ByRefSized<'_, I> {
     type Item = I::Item;
 
     #[inline]
     fn next(&mut self) -> Option<Self::Item> {
-        self.0.next()
+        I::next(self.0)
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        self.0.size_hint()
+        I::size_hint(self.0)
     }
 
     #[inline]
     fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        self.0.advance_by(n)
+        I::advance_by(self.0, n)
     }
 
     #[inline]
     fn nth(&mut self, n: usize) -> Option<Self::Item> {
-        self.0.nth(n)
+        I::nth(self.0, n)
     }
 
     #[inline]
@@ -37,7 +40,8 @@ impl<I: Iterator> Iterator for ByRefSized<'_, I> {
     where
         F: FnMut(B, Self::Item) -> B,
     {
-        self.0.fold(init, f)
+        // `fold` needs ownership, so this can't forward directly.
+        I::try_fold(self.0, init, NeverShortCircuit::wrap_mut_2(f)).0
     }
 
     #[inline]
@@ -46,7 +50,7 @@ impl<I: Iterator> Iterator for ByRefSized<'_, I> {
         F: FnMut(B, Self::Item) -> R,
         R: Try<Output = B>,
     {
-        self.0.try_fold(init, f)
+        I::try_fold(self.0, init, f)
     }
 }
 
@@ -54,17 +58,17 @@ impl<I: Iterator> Iterator for ByRefSized<'_, I> {
 impl<I: DoubleEndedIterator> DoubleEndedIterator for ByRefSized<'_, I> {
     #[inline]
     fn next_back(&mut self) -> Option<Self::Item> {
-        self.0.next_back()
+        I::next_back(self.0)
     }
 
     #[inline]
     fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
-        self.0.advance_back_by(n)
+        I::advance_back_by(self.0, n)
     }
 
     #[inline]
     fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
-        self.0.nth_back(n)
+        I::nth_back(self.0, n)
     }
 
     #[inline]
@@ -72,7 +76,8 @@ impl<I: DoubleEndedIterator> DoubleEndedIterator for ByRefSized<'_, I> {
     where
         F: FnMut(B, Self::Item) -> B,
     {
-        self.0.rfold(init, f)
+        // `rfold` needs ownership, so this can't forward directly.
+        I::try_rfold(self.0, init, NeverShortCircuit::wrap_mut_2(f)).0
     }
 
     #[inline]
@@ -81,6 +86,6 @@ impl<I: DoubleEndedIterator> DoubleEndedIterator for ByRefSized<'_, I> {
         F: FnMut(B, Self::Item) -> R,
         R: Try<Output = B>,
     {
-        self.0.try_rfold(init, f)
+        I::try_rfold(self.0, init, f)
     }
 }
diff --git a/library/core/src/iter/adapters/flatten.rs b/library/core/src/iter/adapters/flatten.rs
index 15a120e35..307016c26 100644
--- a/library/core/src/iter/adapters/flatten.rs
+++ b/library/core/src/iter/adapters/flatten.rs
@@ -1,6 +1,6 @@
 use crate::fmt;
 use crate::iter::{DoubleEndedIterator, Fuse, FusedIterator, Iterator, Map, TrustedLen};
-use crate::ops::Try;
+use crate::ops::{ControlFlow, Try};
 
 /// An iterator that maps each element to an iterator, and yields the elements
 /// of the produced iterators.
@@ -73,6 +73,21 @@ where
     {
         self.inner.fold(init, fold)
     }
+
+    #[inline]
+    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+        self.inner.advance_by(n)
+    }
+
+    #[inline]
+    fn count(self) -> usize {
+        self.inner.count()
+    }
+
+    #[inline]
+    fn last(self) -> Option<Self::Item> {
+        self.inner.last()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -103,6 +118,11 @@ where
     {
         self.inner.rfold(init, fold)
     }
+
+    #[inline]
+    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+        self.inner.advance_back_by(n)
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
@@ -214,6 +234,21 @@ where
     {
         self.inner.fold(init, fold)
     }
+
+    #[inline]
+    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+        self.inner.advance_by(n)
+    }
+
+    #[inline]
+    fn count(self) -> usize {
+        self.inner.count()
+    }
+
+    #[inline]
+    fn last(self) -> Option<Self::Item> {
+        self.inner.last()
+    }
 }
 
 #[stable(feature = "iterator_flatten", since = "1.29.0")]
@@ -244,6 +279,11 @@ where
     {
         self.inner.rfold(init, fold)
     }
+
+    #[inline]
+    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+        self.inner.advance_back_by(n)
+    }
 }
 
 #[stable(feature = "iterator_flatten", since = "1.29.0")]
@@ -280,6 +320,144 @@ where
     }
 }
 
+impl<I, U> FlattenCompat<I, U>
+where
+    I: Iterator<Item: IntoIterator<IntoIter = U>>,
+{
+    /// Folds the inner iterators into an accumulator by applying an operation.
+    ///
+    /// Folds over the inner iterators, not over their elements. Is used by the `fold`, `count`,
+    /// and `last` methods.
+    #[inline]
+    fn iter_fold<Acc, Fold>(self, mut acc: Acc, mut fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, U) -> Acc,
+    {
+        #[inline]
+        fn flatten<T: IntoIterator, Acc>(
+            fold: &mut impl FnMut(Acc, T::IntoIter) -> Acc,
+        ) -> impl FnMut(Acc, T) -> Acc + '_ {
+            move |acc, iter| fold(acc, iter.into_iter())
+        }
+
+        if let Some(iter) = self.frontiter {
+            acc = fold(acc, iter);
+        }
+
+        acc = self.iter.fold(acc, flatten(&mut fold));
+
+        if let Some(iter) = self.backiter {
+            acc = fold(acc, iter);
+        }
+
+        acc
+    }
+
+    /// Folds over the inner iterators as long as the given function returns successfully,
+    /// always storing the most recent inner iterator in `self.frontiter`.
+    ///
+    /// Folds over the inner iterators, not over their elements. Is used by the `try_fold` and
+    /// `advance_by` methods.
+    #[inline]
+    fn iter_try_fold<Acc, Fold, R>(&mut self, mut acc: Acc, mut fold: Fold) -> R
+    where
+        Fold: FnMut(Acc, &mut U) -> R,
+        R: Try<Output = Acc>,
+    {
+        #[inline]
+        fn flatten<'a, T: IntoIterator, Acc, R: Try<Output = Acc>>(
+            frontiter: &'a mut Option<T::IntoIter>,
+            fold: &'a mut impl FnMut(Acc, &mut T::IntoIter) -> R,
+        ) -> impl FnMut(Acc, T) -> R + 'a {
+            move |acc, iter| fold(acc, frontiter.insert(iter.into_iter()))
+        }
+
+        if let Some(iter) = &mut self.frontiter {
+            acc = fold(acc, iter)?;
+        }
+        self.frontiter = None;
+
+        acc = self.iter.try_fold(acc, flatten(&mut self.frontiter, &mut fold))?;
+        self.frontiter = None;
+
+        if let Some(iter) = &mut self.backiter {
+            acc = fold(acc, iter)?;
+        }
+        self.backiter = None;
+
+        try { acc }
+    }
+}
+
+impl<I, U> FlattenCompat<I, U>
+where
+    I: DoubleEndedIterator<Item: IntoIterator<IntoIter = U>>,
+{
+    /// Folds the inner iterators into an accumulator by applying an operation, starting form the
+    /// back.
+    ///
+    /// Folds over the inner iterators, not over their elements. Is used by the `rfold` method.
+    #[inline]
+    fn iter_rfold<Acc, Fold>(self, mut acc: Acc, mut fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, U) -> Acc,
+    {
+        #[inline]
+        fn flatten<T: IntoIterator, Acc>(
+            fold: &mut impl FnMut(Acc, T::IntoIter) -> Acc,
+        ) -> impl FnMut(Acc, T) -> Acc + '_ {
+            move |acc, iter| fold(acc, iter.into_iter())
+        }
+
+        if let Some(iter) = self.backiter {
+            acc = fold(acc, iter);
+        }
+
+        acc = self.iter.rfold(acc, flatten(&mut fold));
+
+        if let Some(iter) = self.frontiter {
+            acc = fold(acc, iter);
+        }
+
+        acc
+    }
+
+    /// Folds over the inner iterators in reverse order as long as the given function returns
+    /// successfully, always storing the most recent inner iterator in `self.backiter`.
+    ///
+    /// Folds over the inner iterators, not over their elements. Is used by the `try_rfold` and
+    /// `advance_back_by` methods.
+    #[inline]
+    fn iter_try_rfold<Acc, Fold, R>(&mut self, mut acc: Acc, mut fold: Fold) -> R
+    where
+        Fold: FnMut(Acc, &mut U) -> R,
+        R: Try<Output = Acc>,
+    {
+        #[inline]
+        fn flatten<'a, T: IntoIterator, Acc, R: Try>(
+            backiter: &'a mut Option<T::IntoIter>,
+            fold: &'a mut impl FnMut(Acc, &mut T::IntoIter) -> R,
+        ) -> impl FnMut(Acc, T) -> R + 'a {
+            move |acc, iter| fold(acc, backiter.insert(iter.into_iter()))
+        }
+
+        if let Some(iter) = &mut self.backiter {
+            acc = fold(acc, iter)?;
+        }
+        self.backiter = None;
+
+        acc = self.iter.try_rfold(acc, flatten(&mut self.backiter, &mut fold))?;
+        self.backiter = None;
+
+        if let Some(iter) = &mut self.frontiter {
+            acc = fold(acc, iter)?;
+        }
+        self.frontiter = None;
+
+        try { acc }
+    }
+}
+
 impl<I, U> Iterator for FlattenCompat<I, U>
 where
     I: Iterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
@@ -323,99 +501,74 @@ where
     }
 
     #[inline]
-    fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R
+    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
     where
         Self: Sized,
         Fold: FnMut(Acc, Self::Item) -> R,
         R: Try<Output = Acc>,
     {
         #[inline]
-        fn flatten<'a, T: IntoIterator, Acc, R: Try<Output = Acc>>(
-            frontiter: &'a mut Option<T::IntoIter>,
-            fold: &'a mut impl FnMut(Acc, T::Item) -> R,
-        ) -> impl FnMut(Acc, T) -> R + 'a {
-            move |acc, x| {
-                let mut mid = x.into_iter();
-                let r = mid.try_fold(acc, &mut *fold);
-                *frontiter = Some(mid);
-                r
-            }
-        }
-
-        if let Some(ref mut front) = self.frontiter {
-            init = front.try_fold(init, &mut fold)?;
+        fn flatten<U: Iterator, Acc, R: Try<Output = Acc>>(
+            mut fold: impl FnMut(Acc, U::Item) -> R,
+        ) -> impl FnMut(Acc, &mut U) -> R {
+            move |acc, iter| iter.try_fold(acc, &mut fold)
         }
-        self.frontiter = None;
 
-        init = self.iter.try_fold(init, flatten(&mut self.frontiter, &mut fold))?;
-        self.frontiter = None;
-
-        if let Some(ref mut back) = self.backiter {
-            init = back.try_fold(init, &mut fold)?;
-        }
-        self.backiter = None;
-
-        try { init }
+        self.iter_try_fold(init, flatten(fold))
     }
 
     #[inline]
-    fn fold<Acc, Fold>(self, mut init: Acc, mut fold: Fold) -> Acc
+    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
     where
         Fold: FnMut(Acc, Self::Item) -> Acc,
     {
         #[inline]
-        fn flatten<T: IntoIterator, Acc>(
-            fold: &mut impl FnMut(Acc, T::Item) -> Acc,
-        ) -> impl FnMut(Acc, T) -> Acc + '_ {
-            move |acc, x| x.into_iter().fold(acc, &mut *fold)
-        }
-
-        if let Some(front) = self.frontiter {
-            init = front.fold(init, &mut fold);
-        }
-
-        init = self.iter.fold(init, flatten(&mut fold));
-
-        if let Some(back) = self.backiter {
-            init = back.fold(init, &mut fold);
+        fn flatten<U: Iterator, Acc>(
+            mut fold: impl FnMut(Acc, U::Item) -> Acc,
+        ) -> impl FnMut(Acc, U) -> Acc {
+            move |acc, iter| iter.fold(acc, &mut fold)
         }
 
-        init
+        self.iter_fold(init, flatten(fold))
     }
 
     #[inline]
     #[rustc_inherit_overflow_checks]
     fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        let mut rem = n;
-        loop {
-            if let Some(ref mut front) = self.frontiter {
-                match front.advance_by(rem) {
-                    ret @ Ok(_) => return ret,
-                    Err(advanced) => rem -= advanced,
-                }
-            }
-            self.frontiter = match self.iter.next() {
-                Some(iterable) => Some(iterable.into_iter()),
-                _ => break,
+        #[inline]
+        #[rustc_inherit_overflow_checks]
+        fn advance<U: Iterator>(n: usize, iter: &mut U) -> ControlFlow<(), usize> {
+            match iter.advance_by(n) {
+                Ok(()) => ControlFlow::BREAK,
+                Err(advanced) => ControlFlow::Continue(n - advanced),
             }
         }
 
-        self.frontiter = None;
-
-        if let Some(ref mut back) = self.backiter {
-            match back.advance_by(rem) {
-                ret @ Ok(_) => return ret,
-                Err(advanced) => rem -= advanced,
-            }
+        match self.iter_try_fold(n, advance) {
+            ControlFlow::Continue(remaining) if remaining > 0 => Err(n - remaining),
+            _ => Ok(()),
         }
+    }
 
-        if rem > 0 {
-            return Err(n - rem);
+    #[inline]
+    fn count(self) -> usize {
+        #[inline]
+        #[rustc_inherit_overflow_checks]
+        fn count<U: Iterator>(acc: usize, iter: U) -> usize {
+            acc + iter.count()
         }
 
-        self.backiter = None;
+        self.iter_fold(0, count)
+    }
+
+    #[inline]
+    fn last(self) -> Option<Self::Item> {
+        #[inline]
+        fn last<U: Iterator>(last: Option<U::Item>, iter: U) -> Option<U::Item> {
+            iter.last().or(last)
+        }
 
-        Ok(())
+        self.iter_fold(None, last)
     }
 }
 
@@ -438,105 +591,53 @@ where
     }
 
     #[inline]
-    fn try_rfold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R
+    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
     where
         Self: Sized,
         Fold: FnMut(Acc, Self::Item) -> R,
         R: Try<Output = Acc>,
     {
         #[inline]
-        fn flatten<'a, T: IntoIterator, Acc, R: Try<Output = Acc>>(
-            backiter: &'a mut Option<T::IntoIter>,
-            fold: &'a mut impl FnMut(Acc, T::Item) -> R,
-        ) -> impl FnMut(Acc, T) -> R + 'a
-        where
-            T::IntoIter: DoubleEndedIterator,
-        {
-            move |acc, x| {
-                let mut mid = x.into_iter();
-                let r = mid.try_rfold(acc, &mut *fold);
-                *backiter = Some(mid);
-                r
-            }
+        fn flatten<U: DoubleEndedIterator, Acc, R: Try<Output = Acc>>(
+            mut fold: impl FnMut(Acc, U::Item) -> R,
+        ) -> impl FnMut(Acc, &mut U) -> R {
+            move |acc, iter| iter.try_rfold(acc, &mut fold)
         }
 
-        if let Some(ref mut back) = self.backiter {
-            init = back.try_rfold(init, &mut fold)?;
-        }
-        self.backiter = None;
-
-        init = self.iter.try_rfold(init, flatten(&mut self.backiter, &mut fold))?;
-        self.backiter = None;
-
-        if let Some(ref mut front) = self.frontiter {
-            init = front.try_rfold(init, &mut fold)?;
-        }
-        self.frontiter = None;
-
-        try { init }
+        self.iter_try_rfold(init, flatten(fold))
     }
 
     #[inline]
-    fn rfold<Acc, Fold>(self, mut init: Acc, mut fold: Fold) -> Acc
+    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
     where
         Fold: FnMut(Acc, Self::Item) -> Acc,
     {
         #[inline]
-        fn flatten<T: IntoIterator, Acc>(
-            fold: &mut impl FnMut(Acc, T::Item) -> Acc,
-        ) -> impl FnMut(Acc, T) -> Acc + '_
-        where
-            T::IntoIter: DoubleEndedIterator,
-        {
-            move |acc, x| x.into_iter().rfold(acc, &mut *fold)
-        }
-
-        if let Some(back) = self.backiter {
-            init = back.rfold(init, &mut fold);
+        fn flatten<U: DoubleEndedIterator, Acc>(
+            mut fold: impl FnMut(Acc, U::Item) -> Acc,
+        ) -> impl FnMut(Acc, U) -> Acc {
+            move |acc, iter| iter.rfold(acc, &mut fold)
         }
 
-        init = self.iter.rfold(init, flatten(&mut fold));
-
-        if let Some(front) = self.frontiter {
-            init = front.rfold(init, &mut fold);
-        }
-
-        init
+        self.iter_rfold(init, flatten(fold))
     }
 
     #[inline]
     #[rustc_inherit_overflow_checks]
     fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
-        let mut rem = n;
-        loop {
-            if let Some(ref mut back) = self.backiter {
-                match back.advance_back_by(rem) {
-                    ret @ Ok(_) => return ret,
-                    Err(advanced) => rem -= advanced,
-                }
-            }
-            match self.iter.next_back() {
-                Some(iterable) => self.backiter = Some(iterable.into_iter()),
-                _ => break,
-            }
-        }
-
-        self.backiter = None;
-
-        if let Some(ref mut front) = self.frontiter {
-            match front.advance_back_by(rem) {
-                ret @ Ok(_) => return ret,
-                Err(advanced) => rem -= advanced,
+        #[inline]
+        #[rustc_inherit_overflow_checks]
+        fn advance<U: DoubleEndedIterator>(n: usize, iter: &mut U) -> ControlFlow<(), usize> {
+            match iter.advance_back_by(n) {
+                Ok(()) => ControlFlow::BREAK,
+                Err(advanced) => ControlFlow::Continue(n - advanced),
             }
         }
 
-        if rem > 0 {
-            return Err(n - rem);
+        match self.iter_try_rfold(n, advance) {
+            ControlFlow::Continue(remaining) if remaining > 0 => Err(n - remaining),
+            _ => Ok(()),
         }
-
-        self.frontiter = None;
-
-        Ok(())
     }
 }
 
diff --git a/library/core/src/iter/adapters/mod.rs b/library/core/src/iter/adapters/mod.rs
index 916a26e24..bf4fabad3 100644
--- a/library/core/src/iter/adapters/mod.rs
+++ b/library/core/src/iter/adapters/mod.rs
@@ -1,6 +1,7 @@
 use crate::iter::{InPlaceIterable, Iterator};
 use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, NeverShortCircuit, Residual, Try};
 
+mod array_chunks;
 mod by_ref_sized;
 mod chain;
 mod cloned;
@@ -32,6 +33,9 @@ pub use self::{
     scan::Scan, skip::Skip, skip_while::SkipWhile, take::Take, take_while::TakeWhile, zip::Zip,
 };
 
+#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
+pub use self::array_chunks::ArrayChunks;
+
 #[unstable(feature = "std_internals", issue = "none")]
 pub use self::by_ref_sized::ByRefSized;
 
diff --git a/library/core/src/iter/adapters/skip.rs b/library/core/src/iter/adapters/skip.rs
index 2c283100f..dbf0ae9ec 100644
--- a/library/core/src/iter/adapters/skip.rs
+++ b/library/core/src/iter/adapters/skip.rs
@@ -33,21 +33,32 @@ where
     #[inline]
     fn next(&mut self) -> Option<I::Item> {
         if unlikely(self.n > 0) {
-            self.iter.nth(crate::mem::take(&mut self.n) - 1)?;
+            self.iter.nth(crate::mem::take(&mut self.n))
+        } else {
+            self.iter.next()
         }
-        self.iter.next()
     }
 
     #[inline]
     fn nth(&mut self, n: usize) -> Option<I::Item> {
-        // Can't just add n + self.n due to overflow.
         if self.n > 0 {
-            let to_skip = self.n;
-            self.n = 0;
-            // nth(n) skips n+1
-            self.iter.nth(to_skip - 1)?;
+            let skip: usize = crate::mem::take(&mut self.n);
+            // Checked add to handle overflow case.
+            let n = match skip.checked_add(n) {
+                Some(nth) => nth,
+                None => {
+                    // In case of overflow, load skip value, before loading `n`.
+                    // Because the amount of elements to iterate is beyond `usize::MAX`, this
+                    // is split into two `nth` calls where the `skip` `nth` call is discarded.
+                    self.iter.nth(skip - 1)?;
+                    n
+                }
+            };
+            // Load nth element including skip.
+            self.iter.nth(n)
+        } else {
+            self.iter.nth(n)
         }
-        self.iter.nth(n)
     }
 
     #[inline]
diff --git a/library/core/src/iter/mod.rs b/library/core/src/iter/mod.rs
index d5c6aed5b..9514466bd 100644
--- a/library/core/src/iter/mod.rs
+++ b/library/core/src/iter/mod.rs
@@ -398,6 +398,8 @@ pub use self::traits::{
 
 #[stable(feature = "iter_zip", since = "1.59.0")]
 pub use self::adapters::zip;
+#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
+pub use self::adapters::ArrayChunks;
 #[unstable(feature = "std_internals", issue = "none")]
 pub use self::adapters::ByRefSized;
 #[stable(feature = "iter_cloned", since = "1.1.0")]
diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs
index 275412b57..b2d08f4b0 100644
--- a/library/core/src/iter/traits/iterator.rs
+++ b/library/core/src/iter/traits/iterator.rs
@@ -5,7 +5,7 @@ use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
 use super::super::try_process;
 use super::super::ByRefSized;
 use super::super::TrustedRandomAccessNoCoerce;
-use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
+use super::super::{ArrayChunks, Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
 use super::super::{FlatMap, Flatten};
 use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip};
 use super::super::{
@@ -3316,6 +3316,49 @@ pub trait Iterator {
         Cycle::new(self)
     }
 
+    /// Returns an iterator over `N` elements of the iterator at a time.
+    ///
+    /// The chunks do not overlap. If `N` does not divide the length of the
+    /// iterator, then the last up to `N-1` elements will be omitted and can be
+    /// retrieved from the [`.into_remainder()`][ArrayChunks::into_remainder]
+    /// function of the iterator.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `N` is 0.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(iter_array_chunks)]
+    ///
+    /// let mut iter = "lorem".chars().array_chunks();
+    /// assert_eq!(iter.next(), Some(['l', 'o']));
+    /// assert_eq!(iter.next(), Some(['r', 'e']));
+    /// assert_eq!(iter.next(), None);
+    /// assert_eq!(iter.into_remainder().unwrap().as_slice(), &['m']);
+    /// ```
+    ///
+    /// ```
+    /// #![feature(iter_array_chunks)]
+    ///
+    /// let data = [1, 1, 2, -2, 6, 0, 3, 1];
+    /// //          ^-----^  ^------^
+    /// for [x, y, z] in data.iter().array_chunks() {
+    ///     assert_eq!(x + y + z, 4);
+    /// }
+    /// ```
+    #[track_caller]
+    #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
+    fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N>
+    where
+        Self: Sized,
+    {
+        ArrayChunks::new(self)
+    }
+
     /// Sums the elements of an iterator.
     ///
     /// Takes each element, adds them together, and returns the result.