Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 3784 insertions, 0 deletions

diff --git a/third_party/rust/itertools/src/lib.rs b/third_party/rust/itertools/src/lib.rs
new file mode 100644
index 0000000000..f91968870f
--- /dev/null
+++ b/third_party/rust/itertools/src/lib.rs
@@ -0,0 +1,3784 @@
+#![warn(missing_docs)]
+#![crate_name="itertools"]
+#![cfg_attr(not(feature = "use_std"), no_std)]
+
+//! Extra iterator adaptors, functions and macros.
+//!
+//! To extend [`Iterator`] with methods in this crate, import
+//! the [`Itertools`] trait:
+//!
+//! ```
+//! use itertools::Itertools;
+//! ```
+//!
+//! Now, new methods like [`interleave`](Itertools::interleave)
+//! are available on all iterators:
+//!
+//! ```
+//! use itertools::Itertools;
+//!
+//! let it = (1..3).interleave(vec![-1, -2]);
+//! itertools::assert_equal(it, vec![1, -1, 2, -2]);
+//! ```
+//!
+//! Most iterator methods are also provided as functions (with the benefit
+//! that they convert parameters using [`IntoIterator`]):
+//!
+//! ```
+//! use itertools::interleave;
+//!
+//! for elt in interleave(&[1, 2, 3], &[2, 3, 4]) {
+//!     /* loop body */
+//! }
+//! ```
+//!
+//! ## Crate Features
+//!
+//! - `use_std`
+//!   - Enabled by default.
+//!   - Disable to compile itertools using `#![no_std]`. This disables
+//!     any items that depend on collections (like `group_by`, `unique`,
+//!     `kmerge`, `join` and many more).
+//!
+//! ## Rust Version
+//!
+//! This version of itertools requires Rust 1.32 or later.
+#![doc(html_root_url="https://docs.rs/itertools/0.8/")]
+
+#[cfg(not(feature = "use_std"))]
+extern crate core as std;
+
+#[cfg(feature = "use_alloc")]
+extern crate alloc;
+
+#[cfg(feature = "use_alloc")]
+use alloc::{
+    string::String,
+    vec::Vec,
+};
+
+pub use either::Either;
+
+use core::borrow::Borrow;
+#[cfg(feature = "use_std")]
+use std::collections::HashMap;
+use std::iter::{IntoIterator, once};
+use std::cmp::Ordering;
+use std::fmt;
+#[cfg(feature = "use_std")]
+use std::collections::HashSet;
+#[cfg(feature = "use_std")]
+use std::hash::Hash;
+#[cfg(feature = "use_alloc")]
+use std::fmt::Write;
+#[cfg(feature = "use_alloc")]
+type VecIntoIter<T> = alloc::vec::IntoIter<T>;
+#[cfg(feature = "use_alloc")]
+use std::iter::FromIterator;
+
+#[macro_use]
+mod impl_macros;
+
+// for compatibility with no std and macros
+#[doc(hidden)]
+pub use std::iter as __std_iter;
+
+/// The concrete iterator types.
+pub mod structs {
+    pub use crate::adaptors::{
+        Dedup,
+        DedupBy,
+        DedupWithCount,
+        DedupByWithCount,
+        Interleave,
+        InterleaveShortest,
+        FilterMapOk,
+        FilterOk,
+        Product,
+        PutBack,
+        Batching,
+        MapInto,
+        MapOk,
+        Merge,
+        MergeBy,
+        TakeWhileRef,
+        WhileSome,
+        Coalesce,
+        TupleCombinations,
+        Positions,
+        Update,
+    };
+    #[allow(deprecated)]
+    pub use crate::adaptors::{MapResults, Step};
+    #[cfg(feature = "use_alloc")]
+    pub use crate::adaptors::MultiProduct;
+    #[cfg(feature = "use_alloc")]
+    pub use crate::combinations::Combinations;
+    #[cfg(feature = "use_alloc")]
+    pub use crate::combinations_with_replacement::CombinationsWithReplacement;
+    pub use crate::cons_tuples_impl::ConsTuples;
+    pub use crate::exactly_one_err::ExactlyOneError;
+    pub use crate::format::{Format, FormatWith};
+    pub use crate::flatten_ok::FlattenOk;
+    #[cfg(feature = "use_std")]
+    pub use crate::grouping_map::{GroupingMap, GroupingMapBy};
+    #[cfg(feature = "use_alloc")]
+    pub use crate::groupbylazy::{IntoChunks, Chunk, Chunks, GroupBy, Group, Groups};
+    pub use crate::intersperse::{Intersperse, IntersperseWith};
+    #[cfg(feature = "use_alloc")]
+    pub use crate::kmerge_impl::{KMerge, KMergeBy};
+    pub use crate::merge_join::MergeJoinBy;
+    #[cfg(feature = "use_alloc")]
+    pub use crate::multipeek_impl::MultiPeek;
+    #[cfg(feature = "use_alloc")]
+    pub use crate::peek_nth::PeekNth;
+    pub use crate::pad_tail::PadUsing;
+    pub use crate::peeking_take_while::PeekingTakeWhile;
+    #[cfg(feature = "use_alloc")]
+    pub use crate::permutations::Permutations;
+    pub use crate::process_results_impl::ProcessResults;
+    #[cfg(feature = "use_alloc")]
+    pub use crate::powerset::Powerset;
+    #[cfg(feature = "use_alloc")]
+    pub use crate::put_back_n_impl::PutBackN;
+    #[cfg(feature = "use_alloc")]
+    pub use crate::rciter_impl::RcIter;
+    pub use crate::repeatn::RepeatN;
+    #[allow(deprecated)]
+    pub use crate::sources::{RepeatCall, Unfold, Iterate};
+    #[cfg(feature = "use_alloc")]
+    pub use crate::tee::Tee;
+    pub use crate::tuple_impl::{TupleBuffer, TupleWindows, CircularTupleWindows, Tuples};
+    #[cfg(feature = "use_std")]
+    pub use crate::duplicates_impl::{Duplicates, DuplicatesBy};
+    #[cfg(feature = "use_std")]
+    pub use crate::unique_impl::{Unique, UniqueBy};
+    pub use crate::with_position::WithPosition;
+    pub use crate::zip_eq_impl::ZipEq;
+    pub use crate::zip_longest::ZipLongest;
+    pub use crate::ziptuple::Zip;
+}
+
+/// Traits helpful for using certain `Itertools` methods in generic contexts.
+pub mod traits {
+    pub use crate::tuple_impl::HomogeneousTuple;
+}
+
+#[allow(deprecated)]
+pub use crate::structs::*;
+pub use crate::concat_impl::concat;
+pub use crate::cons_tuples_impl::cons_tuples;
+pub use crate::diff::diff_with;
+pub use crate::diff::Diff;
+#[cfg(feature = "use_alloc")]
+pub use crate::kmerge_impl::{kmerge_by};
+pub use crate::minmax::MinMaxResult;
+pub use crate::peeking_take_while::PeekingNext;
+pub use crate::process_results_impl::process_results;
+pub use crate::repeatn::repeat_n;
+#[allow(deprecated)]
+pub use crate::sources::{repeat_call, unfold, iterate};
+pub use crate::with_position::Position;
+pub use crate::unziptuple::{multiunzip, MultiUnzip};
+pub use crate::ziptuple::multizip;
+mod adaptors;
+mod either_or_both;
+pub use crate::either_or_both::EitherOrBoth;
+#[doc(hidden)]
+pub mod free;
+#[doc(inline)]
+pub use crate::free::*;
+mod concat_impl;
+mod cons_tuples_impl;
+#[cfg(feature = "use_alloc")]
+mod combinations;
+#[cfg(feature = "use_alloc")]
+mod combinations_with_replacement;
+mod exactly_one_err;
+mod diff;
+mod flatten_ok;
+#[cfg(feature = "use_std")]
+mod extrema_set;
+mod format;
+#[cfg(feature = "use_std")]
+mod grouping_map;
+#[cfg(feature = "use_alloc")]
+mod group_map;
+#[cfg(feature = "use_alloc")]
+mod groupbylazy;
+mod intersperse;
+#[cfg(feature = "use_alloc")]
+mod k_smallest;
+#[cfg(feature = "use_alloc")]
+mod kmerge_impl;
+#[cfg(feature = "use_alloc")]
+mod lazy_buffer;
+mod merge_join;
+mod minmax;
+#[cfg(feature = "use_alloc")]
+mod multipeek_impl;
+mod pad_tail;
+#[cfg(feature = "use_alloc")]
+mod peek_nth;
+mod peeking_take_while;
+#[cfg(feature = "use_alloc")]
+mod permutations;
+#[cfg(feature = "use_alloc")]
+mod powerset;
+mod process_results_impl;
+#[cfg(feature = "use_alloc")]
+mod put_back_n_impl;
+#[cfg(feature = "use_alloc")]
+mod rciter_impl;
+mod repeatn;
+mod size_hint;
+mod sources;
+#[cfg(feature = "use_alloc")]
+mod tee;
+mod tuple_impl;
+#[cfg(feature = "use_std")]
+mod duplicates_impl;
+#[cfg(feature = "use_std")]
+mod unique_impl;
+mod unziptuple;
+mod with_position;
+mod zip_eq_impl;
+mod zip_longest;
+mod ziptuple;
+
+#[macro_export]
+/// Create an iterator over the “cartesian product” of iterators.
+///
+/// Iterator element type is like `(A, B, ..., E)` if formed
+/// from iterators `(I, J, ..., M)` with element types `I::Item = A`, `J::Item = B`, etc.
+///
+/// ```
+/// # use itertools::iproduct;
+/// #
+/// # fn main() {
+/// // Iterate over the coordinates of a 4 x 4 x 4 grid
+/// // from (0, 0, 0), (0, 0, 1), .., (0, 1, 0), (0, 1, 1), .. etc until (3, 3, 3)
+/// for (i, j, k) in iproduct!(0..4, 0..4, 0..4) {
+///    // ..
+/// }
+/// # }
+/// ```
+macro_rules! iproduct {
+    (@flatten $I:expr,) => (
+        $I
+    );
+    (@flatten $I:expr, $J:expr, $($K:expr,)*) => (
+        $crate::iproduct!(@flatten $crate::cons_tuples($crate::iproduct!($I, $J)), $($K,)*)
+    );
+    ($I:expr) => (
+        $crate::__std_iter::IntoIterator::into_iter($I)
+    );
+    ($I:expr, $J:expr) => (
+        $crate::Itertools::cartesian_product($crate::iproduct!($I), $crate::iproduct!($J))
+    );
+    ($I:expr, $J:expr, $($K:expr),+) => (
+        $crate::iproduct!(@flatten $crate::iproduct!($I, $J), $($K,)+)
+    );
+}
+
+#[macro_export]
+/// Create an iterator running multiple iterators in lockstep.
+///
+/// The `izip!` iterator yields elements until any subiterator
+/// returns `None`.
+///
+/// This is a version of the standard ``.zip()`` that's supporting more than
+/// two iterators. The iterator element type is a tuple with one element
+/// from each of the input iterators. Just like ``.zip()``, the iteration stops
+/// when the shortest of the inputs reaches its end.
+///
+/// **Note:** The result of this macro is in the general case an iterator
+/// composed of repeated `.zip()` and a `.map()`; it has an anonymous type.
+/// The special cases of one and two arguments produce the equivalent of
+/// `$a.into_iter()` and `$a.into_iter().zip($b)` respectively.
+///
+/// Prefer this macro `izip!()` over [`multizip`] for the performance benefits
+/// of using the standard library `.zip()`.
+///
+/// ```
+/// # use itertools::izip;
+/// #
+/// # fn main() {
+///
+/// // iterate over three sequences side-by-side
+/// let mut results = [0, 0, 0, 0];
+/// let inputs = [3, 7, 9, 6];
+///
+/// for (r, index, input) in izip!(&mut results, 0..10, &inputs) {
+///     *r = index * 10 + input;
+/// }
+///
+/// assert_eq!(results, [0 + 3, 10 + 7, 29, 36]);
+/// # }
+/// ```
+macro_rules! izip {
+    // @closure creates a tuple-flattening closure for .map() call. usage:
+    // @closure partial_pattern => partial_tuple , rest , of , iterators
+    // eg. izip!( @closure ((a, b), c) => (a, b, c) , dd , ee )
+    ( @closure $p:pat => $tup:expr ) => {
+        |$p| $tup
+    };
+
+    // The "b" identifier is a different identifier on each recursion level thanks to hygiene.
+    ( @closure $p:pat => ( $($tup:tt)* ) , $_iter:expr $( , $tail:expr )* ) => {
+        $crate::izip!(@closure ($p, b) => ( $($tup)*, b ) $( , $tail )*)
+    };
+
+    // unary
+    ($first:expr $(,)*) => {
+        $crate::__std_iter::IntoIterator::into_iter($first)
+    };
+
+    // binary
+    ($first:expr, $second:expr $(,)*) => {
+        $crate::izip!($first)
+            .zip($second)
+    };
+
+    // n-ary where n > 2
+    ( $first:expr $( , $rest:expr )* $(,)* ) => {
+        $crate::izip!($first)
+            $(
+                .zip($rest)
+            )*
+            .map(
+                $crate::izip!(@closure a => (a) $( , $rest )*)
+            )
+    };
+}
+
+#[macro_export]
+/// [Chain][`chain`] zero or more iterators together into one sequence.
+///
+/// The comma-separated arguments must implement [`IntoIterator`].
+/// The final argument may be followed by a trailing comma.
+///
+/// [`chain`]: Iterator::chain
+///
+/// # Examples
+///
+/// Empty invocations of `chain!` expand to an invocation of [`std::iter::empty`]:
+/// ```
+/// use std::iter;
+/// use itertools::chain;
+///
+/// let _: iter::Empty<()> = chain!();
+/// let _: iter::Empty<i8> = chain!();
+/// ```
+///
+/// Invocations of `chain!` with one argument expand to [`arg.into_iter()`](IntoIterator):
+/// ```
+/// use std::{ops::Range, slice};
+/// use itertools::chain;
+/// let _: <Range<_> as IntoIterator>::IntoIter = chain!((2..6),); // trailing comma optional!
+/// let _:     <&[_] as IntoIterator>::IntoIter = chain!(&[2, 3, 4]);
+/// ```
+///
+/// Invocations of `chain!` with multiple arguments [`.into_iter()`](IntoIterator) each
+/// argument, and then [`chain`] them together:
+/// ```
+/// use std::{iter::*, ops::Range, slice};
+/// use itertools::{assert_equal, chain};
+///
+/// // e.g., this:
+/// let with_macro:  Chain<Chain<Once<_>, Take<Repeat<_>>>, slice::Iter<_>> =
+///     chain![once(&0), repeat(&1).take(2), &[2, 3, 5],];
+///
+/// // ...is equivalent to this:
+/// let with_method: Chain<Chain<Once<_>, Take<Repeat<_>>>, slice::Iter<_>> =
+///     once(&0)
+///         .chain(repeat(&1).take(2))
+///         .chain(&[2, 3, 5]);
+///
+/// assert_equal(with_macro, with_method);
+/// ```
+macro_rules! chain {
+    () => {
+        core::iter::empty()
+    };
+    ($first:expr $(, $rest:expr )* $(,)?) => {
+        {
+            let iter = core::iter::IntoIterator::into_iter($first);
+            $(
+                let iter =
+                    core::iter::Iterator::chain(
+                        iter,
+                        core::iter::IntoIterator::into_iter($rest));
+            )*
+            iter
+        }
+    };
+}
+
+/// An [`Iterator`] blanket implementation that provides extra adaptors and
+/// methods.
+///
+/// This trait defines a number of methods. They are divided into two groups:
+///
+/// * *Adaptors* take an iterator and parameter as input, and return
+/// a new iterator value. These are listed first in the trait. An example
+/// of an adaptor is [`.interleave()`](Itertools::interleave)
+///
+/// * *Regular methods* are those that don't return iterators and instead
+/// return a regular value of some other kind.
+/// [`.next_tuple()`](Itertools::next_tuple) is an example and the first regular
+/// method in the list.
+pub trait Itertools : Iterator {
+    // adaptors
+
+    /// Alternate elements from two iterators until both have run out.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let it = (1..7).interleave(vec![-1, -2]);
+    /// itertools::assert_equal(it, vec![1, -1, 2, -2, 3, 4, 5, 6]);
+    /// ```
+    fn interleave<J>(self, other: J) -> Interleave<Self, J::IntoIter>
+        where J: IntoIterator<Item = Self::Item>,
+              Self: Sized
+    {
+        interleave(self, other)
+    }
+
+    /// Alternate elements from two iterators until at least one of them has run
+    /// out.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let it = (1..7).interleave_shortest(vec![-1, -2]);
+    /// itertools::assert_equal(it, vec![1, -1, 2, -2, 3]);
+    /// ```
+    fn interleave_shortest<J>(self, other: J) -> InterleaveShortest<Self, J::IntoIter>
+        where J: IntoIterator<Item = Self::Item>,
+              Self: Sized
+    {
+        adaptors::interleave_shortest(self, other.into_iter())
+    }
+
+    /// An iterator adaptor to insert a particular value
+    /// between each element of the adapted iterator.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// itertools::assert_equal((0..3).intersperse(8), vec![0, 8, 1, 8, 2]);
+    /// ```
+    fn intersperse(self, element: Self::Item) -> Intersperse<Self>
+        where Self: Sized,
+              Self::Item: Clone
+    {
+        intersperse::intersperse(self, element)
+    }
+
+    /// An iterator adaptor to insert a particular value created by a function
+    /// between each element of the adapted iterator.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let mut i = 10;
+    /// itertools::assert_equal((0..3).intersperse_with(|| { i -= 1; i }), vec![0, 9, 1, 8, 2]);
+    /// assert_eq!(i, 8);
+    /// ```
+    fn intersperse_with<F>(self, element: F) -> IntersperseWith<Self, F>
+        where Self: Sized,
+        F: FnMut() -> Self::Item
+    {
+        intersperse::intersperse_with(self, element)
+    }
+
+    /// Create an iterator which iterates over both this and the specified
+    /// iterator simultaneously, yielding pairs of two optional elements.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// As long as neither input iterator is exhausted yet, it yields two values
+    /// via `EitherOrBoth::Both`.
+    ///
+    /// When the parameter iterator is exhausted, it only yields a value from the
+    /// `self` iterator via `EitherOrBoth::Left`.
+    ///
+    /// When the `self` iterator is exhausted, it only yields a value from the
+    /// parameter iterator via `EitherOrBoth::Right`.
+    ///
+    /// When both iterators return `None`, all further invocations of `.next()`
+    /// will return `None`.
+    ///
+    /// Iterator element type is
+    /// [`EitherOrBoth<Self::Item, J::Item>`](EitherOrBoth).
+    ///
+    /// ```rust
+    /// use itertools::EitherOrBoth::{Both, Right};
+    /// use itertools::Itertools;
+    /// let it = (0..1).zip_longest(1..3);
+    /// itertools::assert_equal(it, vec![Both(0, 1), Right(2)]);
+    /// ```
+    #[inline]
+    fn zip_longest<J>(self, other: J) -> ZipLongest<Self, J::IntoIter>
+        where J: IntoIterator,
+              Self: Sized
+    {
+        zip_longest::zip_longest(self, other.into_iter())
+    }
+
+    /// Create an iterator which iterates over both this and the specified
+    /// iterator simultaneously, yielding pairs of elements.
+    ///
+    /// **Panics** if the iterators reach an end and they are not of equal
+    /// lengths.
+    #[inline]
+    fn zip_eq<J>(self, other: J) -> ZipEq<Self, J::IntoIter>
+        where J: IntoIterator,
+              Self: Sized
+    {
+        zip_eq(self, other)
+    }
+
+    /// A “meta iterator adaptor”. Its closure receives a reference to the
+    /// iterator and may pick off as many elements as it likes, to produce the
+    /// next iterator element.
+    ///
+    /// Iterator element type is `B`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // An adaptor that gathers elements in pairs
+    /// let pit = (0..4).batching(|it| {
+    ///            match it.next() {
+    ///                None => None,
+    ///                Some(x) => match it.next() {
+    ///                    None => None,
+    ///                    Some(y) => Some((x, y)),
+    ///                }
+    ///            }
+    ///        });
+    ///
+    /// itertools::assert_equal(pit, vec![(0, 1), (2, 3)]);
+    /// ```
+    ///
+    fn batching<B, F>(self, f: F) -> Batching<Self, F>
+        where F: FnMut(&mut Self) -> Option<B>,
+              Self: Sized
+    {
+        adaptors::batching(self, f)
+    }
+
+    /// Return an *iterable* that can group iterator elements.
+    /// Consecutive elements that map to the same key (“runs”), are assigned
+    /// to the same group.
+    ///
+    /// `GroupBy` is the storage for the lazy grouping operation.
+    ///
+    /// If the groups are consumed in order, or if each group's iterator is
+    /// dropped without keeping it around, then `GroupBy` uses no
+    /// allocations.  It needs allocations only if several group iterators
+    /// are alive at the same time.
+    ///
+    /// This type implements [`IntoIterator`] (it is **not** an iterator
+    /// itself), because the group iterators need to borrow from this
+    /// value. It should be stored in a local variable or temporary and
+    /// iterated.
+    ///
+    /// Iterator element type is `(K, Group)`: the group's key and the
+    /// group iterator.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // group data into runs of larger than zero or not.
+    /// let data = vec![1, 3, -2, -2, 1, 0, 1, 2];
+    /// // groups:     |---->|------>|--------->|
+    ///
+    /// // Note: The `&` is significant here, `GroupBy` is iterable
+    /// // only by reference. You can also call `.into_iter()` explicitly.
+    /// let mut data_grouped = Vec::new();
+    /// for (key, group) in &data.into_iter().group_by(|elt| *elt >= 0) {
+    ///     data_grouped.push((key, group.collect()));
+    /// }
+    /// assert_eq!(data_grouped, vec![(true, vec![1, 3]), (false, vec![-2, -2]), (true, vec![1, 0, 1, 2])]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn group_by<K, F>(self, key: F) -> GroupBy<K, Self, F>
+        where Self: Sized,
+              F: FnMut(&Self::Item) -> K,
+              K: PartialEq,
+    {
+        groupbylazy::new(self, key)
+    }
+
+    /// Return an *iterable* that can chunk the iterator.
+    ///
+    /// Yield subiterators (chunks) that each yield a fixed number elements,
+    /// determined by `size`. The last chunk will be shorter if there aren't
+    /// enough elements.
+    ///
+    /// `IntoChunks` is based on `GroupBy`: it is iterable (implements
+    /// `IntoIterator`, **not** `Iterator`), and it only buffers if several
+    /// chunk iterators are alive at the same time.
+    ///
+    /// Iterator element type is `Chunk`, each chunk's iterator.
+    ///
+    /// **Panics** if `size` is 0.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![1, 1, 2, -2, 6, 0, 3, 1];
+    /// //chunk size=3 |------->|-------->|--->|
+    ///
+    /// // Note: The `&` is significant here, `IntoChunks` is iterable
+    /// // only by reference. You can also call `.into_iter()` explicitly.
+    /// for chunk in &data.into_iter().chunks(3) {
+    ///     // Check that the sum of each chunk is 4.
+    ///     assert_eq!(4, chunk.sum());
+    /// }
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn chunks(self, size: usize) -> IntoChunks<Self>
+        where Self: Sized,
+    {
+        assert!(size != 0);
+        groupbylazy::new_chunks(self, size)
+    }
+
+    /// Return an iterator over all contiguous windows producing tuples of
+    /// a specific size (up to 12).
+    ///
+    /// `tuple_windows` clones the iterator elements so that they can be
+    /// part of successive windows, this makes it most suited for iterators
+    /// of references and other values that are cheap to copy.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// let mut v = Vec::new();
+    ///
+    /// // pairwise iteration
+    /// for (a, b) in (1..5).tuple_windows() {
+    ///     v.push((a, b));
+    /// }
+    /// assert_eq!(v, vec![(1, 2), (2, 3), (3, 4)]);
+    ///
+    /// let mut it = (1..5).tuple_windows();
+    /// assert_eq!(Some((1, 2, 3)), it.next());
+    /// assert_eq!(Some((2, 3, 4)), it.next());
+    /// assert_eq!(None, it.next());
+    ///
+    /// // this requires a type hint
+    /// let it = (1..5).tuple_windows::<(_, _, _)>();
+    /// itertools::assert_equal(it, vec![(1, 2, 3), (2, 3, 4)]);
+    ///
+    /// // you can also specify the complete type
+    /// use itertools::TupleWindows;
+    /// use std::ops::Range;
+    ///
+    /// let it: TupleWindows<Range<u32>, (u32, u32, u32)> = (1..5).tuple_windows();
+    /// itertools::assert_equal(it, vec![(1, 2, 3), (2, 3, 4)]);
+    /// ```
+    fn tuple_windows<T>(self) -> TupleWindows<Self, T>
+        where Self: Sized + Iterator<Item = T::Item>,
+              T: traits::HomogeneousTuple,
+              T::Item: Clone
+    {
+        tuple_impl::tuple_windows(self)
+    }
+
+    /// Return an iterator over all windows, wrapping back to the first
+    /// elements when the window would otherwise exceed the length of the
+    /// iterator, producing tuples of a specific size (up to 12).
+    ///
+    /// `circular_tuple_windows` clones the iterator elements so that they can be
+    /// part of successive windows, this makes it most suited for iterators
+    /// of references and other values that are cheap to copy.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// let mut v = Vec::new();
+    /// for (a, b) in (1..5).circular_tuple_windows() {
+    ///     v.push((a, b));
+    /// }
+    /// assert_eq!(v, vec![(1, 2), (2, 3), (3, 4), (4, 1)]);
+    ///
+    /// let mut it = (1..5).circular_tuple_windows();
+    /// assert_eq!(Some((1, 2, 3)), it.next());
+    /// assert_eq!(Some((2, 3, 4)), it.next());
+    /// assert_eq!(Some((3, 4, 1)), it.next());
+    /// assert_eq!(Some((4, 1, 2)), it.next());
+    /// assert_eq!(None, it.next());
+    ///
+    /// // this requires a type hint
+    /// let it = (1..5).circular_tuple_windows::<(_, _, _)>();
+    /// itertools::assert_equal(it, vec![(1, 2, 3), (2, 3, 4), (3, 4, 1), (4, 1, 2)]);
+    /// ```
+    fn circular_tuple_windows<T>(self) -> CircularTupleWindows<Self, T>
+        where Self: Sized + Clone + Iterator<Item = T::Item> + ExactSizeIterator,
+              T: tuple_impl::TupleCollect + Clone,
+              T::Item: Clone
+    {
+        tuple_impl::circular_tuple_windows(self)
+    }
+    /// Return an iterator that groups the items in tuples of a specific size
+    /// (up to 12).
+    ///
+    /// See also the method [`.next_tuple()`](Itertools::next_tuple).
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// let mut v = Vec::new();
+    /// for (a, b) in (1..5).tuples() {
+    ///     v.push((a, b));
+    /// }
+    /// assert_eq!(v, vec![(1, 2), (3, 4)]);
+    ///
+    /// let mut it = (1..7).tuples();
+    /// assert_eq!(Some((1, 2, 3)), it.next());
+    /// assert_eq!(Some((4, 5, 6)), it.next());
+    /// assert_eq!(None, it.next());
+    ///
+    /// // this requires a type hint
+    /// let it = (1..7).tuples::<(_, _, _)>();
+    /// itertools::assert_equal(it, vec![(1, 2, 3), (4, 5, 6)]);
+    ///
+    /// // you can also specify the complete type
+    /// use itertools::Tuples;
+    /// use std::ops::Range;
+    ///
+    /// let it: Tuples<Range<u32>, (u32, u32, u32)> = (1..7).tuples();
+    /// itertools::assert_equal(it, vec![(1, 2, 3), (4, 5, 6)]);
+    /// ```
+    ///
+    /// See also [`Tuples::into_buffer`].
+    fn tuples<T>(self) -> Tuples<Self, T>
+        where Self: Sized + Iterator<Item = T::Item>,
+              T: traits::HomogeneousTuple
+    {
+        tuple_impl::tuples(self)
+    }
+
+    /// Split into an iterator pair that both yield all elements from
+    /// the original iterator.
+    ///
+    /// **Note:** If the iterator is clonable, prefer using that instead
+    /// of using this method. Cloning is likely to be more efficient.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// let xs = vec![0, 1, 2, 3];
+    ///
+    /// let (mut t1, t2) = xs.into_iter().tee();
+    /// itertools::assert_equal(t1.next(), Some(0));
+    /// itertools::assert_equal(t2, 0..4);
+    /// itertools::assert_equal(t1, 1..4);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn tee(self) -> (Tee<Self>, Tee<Self>)
+        where Self: Sized,
+              Self::Item: Clone
+    {
+        tee::new(self)
+    }
+
+    /// Return an iterator adaptor that steps `n` elements in the base iterator
+    /// for each iteration.
+    ///
+    /// The iterator steps by yielding the next element from the base iterator,
+    /// then skipping forward `n - 1` elements.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// **Panics** if the step is 0.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let it = (0..8).step(3);
+    /// itertools::assert_equal(it, vec![0, 3, 6]);
+    /// ```
+    #[deprecated(note="Use std .step_by() instead", since="0.8.0")]
+    #[allow(deprecated)]
+    fn step(self, n: usize) -> Step<Self>
+        where Self: Sized
+    {
+        adaptors::step(self, n)
+    }
+
+    /// Convert each item of the iterator using the [`Into`] trait.
+    ///
+    /// ```rust
+    /// use itertools::Itertools;
+    ///
+    /// (1i32..42i32).map_into::<f64>().collect_vec();
+    /// ```
+    fn map_into<R>(self) -> MapInto<Self, R>
+        where Self: Sized,
+              Self::Item: Into<R>,
+    {
+        adaptors::map_into(self)
+    }
+
+    /// See [`.map_ok()`](Itertools::map_ok).
+    #[deprecated(note="Use .map_ok() instead", since="0.10.0")]
+    fn map_results<F, T, U, E>(self, f: F) -> MapOk<Self, F>
+        where Self: Iterator<Item = Result<T, E>> + Sized,
+              F: FnMut(T) -> U,
+    {
+        self.map_ok(f)
+    }
+
+    /// Return an iterator adaptor that applies the provided closure
+    /// to every `Result::Ok` value. `Result::Err` values are
+    /// unchanged.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let input = vec![Ok(41), Err(false), Ok(11)];
+    /// let it = input.into_iter().map_ok(|i| i + 1);
+    /// itertools::assert_equal(it, vec![Ok(42), Err(false), Ok(12)]);
+    /// ```
+    fn map_ok<F, T, U, E>(self, f: F) -> MapOk<Self, F>
+        where Self: Iterator<Item = Result<T, E>> + Sized,
+              F: FnMut(T) -> U,
+    {
+        adaptors::map_ok(self, f)
+    }
+
+    /// Return an iterator adaptor that filters every `Result::Ok`
+    /// value with the provided closure. `Result::Err` values are
+    /// unchanged.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let input = vec![Ok(22), Err(false), Ok(11)];
+    /// let it = input.into_iter().filter_ok(|&i| i > 20);
+    /// itertools::assert_equal(it, vec![Ok(22), Err(false)]);
+    /// ```
+    fn filter_ok<F, T, E>(self, f: F) -> FilterOk<Self, F>
+        where Self: Iterator<Item = Result<T, E>> + Sized,
+              F: FnMut(&T) -> bool,
+    {
+        adaptors::filter_ok(self, f)
+    }
+
+    /// Return an iterator adaptor that filters and transforms every
+    /// `Result::Ok` value with the provided closure. `Result::Err`
+    /// values are unchanged.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let input = vec![Ok(22), Err(false), Ok(11)];
+    /// let it = input.into_iter().filter_map_ok(|i| if i > 20 { Some(i * 2) } else { None });
+    /// itertools::assert_equal(it, vec![Ok(44), Err(false)]);
+    /// ```
+    fn filter_map_ok<F, T, U, E>(self, f: F) -> FilterMapOk<Self, F>
+        where Self: Iterator<Item = Result<T, E>> + Sized,
+              F: FnMut(T) -> Option<U>,
+    {
+        adaptors::filter_map_ok(self, f)
+    }
+
+    /// Return an iterator adaptor that flattens every `Result::Ok` value into
+    /// a series of `Result::Ok` values. `Result::Err` values are unchanged.
+    /// 
+    /// This is useful when you have some common error type for your crate and
+    /// need to propagate it upwards, but the `Result::Ok` case needs to be flattened.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let input = vec![Ok(0..2), Err(false), Ok(2..4)];
+    /// let it = input.iter().cloned().flatten_ok();
+    /// itertools::assert_equal(it.clone(), vec![Ok(0), Ok(1), Err(false), Ok(2), Ok(3)]);
+    /// 
+    /// // This can also be used to propagate errors when collecting.
+    /// let output_result: Result<Vec<i32>, bool> = it.collect();
+    /// assert_eq!(output_result, Err(false));
+    /// ```
+    fn flatten_ok<T, E>(self) -> FlattenOk<Self, T, E>
+        where Self: Iterator<Item = Result<T, E>> + Sized,
+              T: IntoIterator
+    {
+        flatten_ok::flatten_ok(self)
+    }
+
+    /// Return an iterator adaptor that merges the two base iterators in
+    /// ascending order.  If both base iterators are sorted (ascending), the
+    /// result is sorted.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a = (0..11).step(3);
+    /// let b = (0..11).step(5);
+    /// let it = a.merge(b);
+    /// itertools::assert_equal(it, vec![0, 0, 3, 5, 6, 9, 10]);
+    /// ```
+    fn merge<J>(self, other: J) -> Merge<Self, J::IntoIter>
+        where Self: Sized,
+              Self::Item: PartialOrd,
+              J: IntoIterator<Item = Self::Item>
+    {
+        merge(self, other)
+    }
+
+    /// Return an iterator adaptor that merges the two base iterators in order.
+    /// This is much like [`.merge()`](Itertools::merge) but allows for a custom ordering.
+    ///
+    /// This can be especially useful for sequences of tuples.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a = (0..).zip("bc".chars());
+    /// let b = (0..).zip("ad".chars());
+    /// let it = a.merge_by(b, |x, y| x.1 <= y.1);
+    /// itertools::assert_equal(it, vec![(0, 'a'), (0, 'b'), (1, 'c'), (1, 'd')]);
+    /// ```
+
+    fn merge_by<J, F>(self, other: J, is_first: F) -> MergeBy<Self, J::IntoIter, F>
+        where Self: Sized,
+              J: IntoIterator<Item = Self::Item>,
+              F: FnMut(&Self::Item, &Self::Item) -> bool
+    {
+        adaptors::merge_by_new(self, other.into_iter(), is_first)
+    }
+
+    /// Create an iterator that merges items from both this and the specified
+    /// iterator in ascending order.
+    ///
+    /// It chooses whether to pair elements based on the `Ordering` returned by the
+    /// specified compare function. At any point, inspecting the tip of the
+    /// iterators `I` and `J` as items `i` of type `I::Item` and `j` of type
+    /// `J::Item` respectively, the resulting iterator will:
+    ///
+    /// - Emit `EitherOrBoth::Left(i)` when `i < j`,
+    ///   and remove `i` from its source iterator
+    /// - Emit `EitherOrBoth::Right(j)` when `i > j`,
+    ///   and remove `j` from its source iterator
+    /// - Emit `EitherOrBoth::Both(i, j)` when  `i == j`,
+    ///   and remove both `i` and `j` from their respective source iterators
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// use itertools::EitherOrBoth::{Left, Right, Both};
+    ///
+    /// let multiples_of_2 = (0..10).step(2);
+    /// let multiples_of_3 = (0..10).step(3);
+    ///
+    /// itertools::assert_equal(
+    ///     multiples_of_2.merge_join_by(multiples_of_3, |i, j| i.cmp(j)),
+    ///     vec![Both(0, 0), Left(2), Right(3), Left(4), Both(6, 6), Left(8), Right(9)]
+    /// );
+    /// ```
+    #[inline]
+    fn merge_join_by<J, F>(self, other: J, cmp_fn: F) -> MergeJoinBy<Self, J::IntoIter, F>
+        where J: IntoIterator,
+              F: FnMut(&Self::Item, &J::Item) -> std::cmp::Ordering,
+              Self: Sized
+    {
+        merge_join_by(self, other, cmp_fn)
+    }
+
+    /// Return an iterator adaptor that flattens an iterator of iterators by
+    /// merging them in ascending order.
+    ///
+    /// If all base iterators are sorted (ascending), the result is sorted.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a = (0..6).step(3);
+    /// let b = (1..6).step(3);
+    /// let c = (2..6).step(3);
+    /// let it = vec![a, b, c].into_iter().kmerge();
+    /// itertools::assert_equal(it, vec![0, 1, 2, 3, 4, 5]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn kmerge(self) -> KMerge<<Self::Item as IntoIterator>::IntoIter>
+        where Self: Sized,
+              Self::Item: IntoIterator,
+              <Self::Item as IntoIterator>::Item: PartialOrd,
+    {
+        kmerge(self)
+    }
+
+    /// Return an iterator adaptor that flattens an iterator of iterators by
+    /// merging them according to the given closure.
+    ///
+    /// The closure `first` is called with two elements *a*, *b* and should
+    /// return `true` if *a* is ordered before *b*.
+    ///
+    /// If all base iterators are sorted according to `first`, the result is
+    /// sorted.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a = vec![-1f64, 2., 3., -5., 6., -7.];
+    /// let b = vec![0., 2., -4.];
+    /// let mut it = vec![a, b].into_iter().kmerge_by(|a, b| a.abs() < b.abs());
+    /// assert_eq!(it.next(), Some(0.));
+    /// assert_eq!(it.last(), Some(-7.));
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn kmerge_by<F>(self, first: F)
+        -> KMergeBy<<Self::Item as IntoIterator>::IntoIter, F>
+        where Self: Sized,
+              Self::Item: IntoIterator,
+              F: FnMut(&<Self::Item as IntoIterator>::Item,
+                       &<Self::Item as IntoIterator>::Item) -> bool
+    {
+        kmerge_by(self, first)
+    }
+
+    /// Return an iterator adaptor that iterates over the cartesian product of
+    /// the element sets of two iterators `self` and `J`.
+    ///
+    /// Iterator element type is `(Self::Item, J::Item)`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let it = (0..2).cartesian_product("αβ".chars());
+    /// itertools::assert_equal(it, vec![(0, 'α'), (0, 'β'), (1, 'α'), (1, 'β')]);
+    /// ```
+    fn cartesian_product<J>(self, other: J) -> Product<Self, J::IntoIter>
+        where Self: Sized,
+              Self::Item: Clone,
+              J: IntoIterator,
+              J::IntoIter: Clone
+    {
+        adaptors::cartesian_product(self, other.into_iter())
+    }
+
+    /// Return an iterator adaptor that iterates over the cartesian product of
+    /// all subiterators returned by meta-iterator `self`.
+    ///
+    /// All provided iterators must yield the same `Item` type. To generate
+    /// the product of iterators yielding multiple types, use the
+    /// [`iproduct`] macro instead.
+    ///
+    ///
+    /// The iterator element type is `Vec<T>`, where `T` is the iterator element
+    /// of the subiterators.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// let mut multi_prod = (0..3).map(|i| (i * 2)..(i * 2 + 2))
+    ///     .multi_cartesian_product();
+    /// assert_eq!(multi_prod.next(), Some(vec![0, 2, 4]));
+    /// assert_eq!(multi_prod.next(), Some(vec![0, 2, 5]));
+    /// assert_eq!(multi_prod.next(), Some(vec![0, 3, 4]));
+    /// assert_eq!(multi_prod.next(), Some(vec![0, 3, 5]));
+    /// assert_eq!(multi_prod.next(), Some(vec![1, 2, 4]));
+    /// assert_eq!(multi_prod.next(), Some(vec![1, 2, 5]));
+    /// assert_eq!(multi_prod.next(), Some(vec![1, 3, 4]));
+    /// assert_eq!(multi_prod.next(), Some(vec![1, 3, 5]));
+    /// assert_eq!(multi_prod.next(), None);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn multi_cartesian_product(self) -> MultiProduct<<Self::Item as IntoIterator>::IntoIter>
+        where Self: Sized,
+              Self::Item: IntoIterator,
+              <Self::Item as IntoIterator>::IntoIter: Clone,
+              <Self::Item as IntoIterator>::Item: Clone
+    {
+        adaptors::multi_cartesian_product(self)
+    }
+
+    /// Return an iterator adaptor that uses the passed-in closure to
+    /// optionally merge together consecutive elements.
+    ///
+    /// The closure `f` is passed two elements, `previous` and `current` and may
+    /// return either (1) `Ok(combined)` to merge the two values or
+    /// (2) `Err((previous', current'))` to indicate they can't be merged.
+    /// In (2), the value `previous'` is emitted by the iterator.
+    /// Either (1) `combined` or (2) `current'` becomes the previous value
+    /// when coalesce continues with the next pair of elements to merge. The
+    /// value that remains at the end is also emitted by the iterator.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sum same-sign runs together
+    /// let data = vec![-1., -2., -3., 3., 1., 0., -1.];
+    /// itertools::assert_equal(data.into_iter().coalesce(|x, y|
+    ///         if (x >= 0.) == (y >= 0.) {
+    ///             Ok(x + y)
+    ///         } else {
+    ///             Err((x, y))
+    ///         }),
+    ///         vec![-6., 4., -1.]);
+    /// ```
+    fn coalesce<F>(self, f: F) -> Coalesce<Self, F>
+        where Self: Sized,
+              F: FnMut(Self::Item, Self::Item)
+                       -> Result<Self::Item, (Self::Item, Self::Item)>
+    {
+        adaptors::coalesce(self, f)
+    }
+
+    /// Remove duplicates from sections of consecutive identical elements.
+    /// If the iterator is sorted, all elements will be unique.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![1., 1., 2., 3., 3., 2., 2.];
+    /// itertools::assert_equal(data.into_iter().dedup(),
+    ///                         vec![1., 2., 3., 2.]);
+    /// ```
+    fn dedup(self) -> Dedup<Self>
+        where Self: Sized,
+              Self::Item: PartialEq,
+    {
+        adaptors::dedup(self)
+    }
+
+    /// Remove duplicates from sections of consecutive identical elements,
+    /// determining equality using a comparison function.
+    /// If the iterator is sorted, all elements will be unique.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![(0, 1.), (1, 1.), (0, 2.), (0, 3.), (1, 3.), (1, 2.), (2, 2.)];
+    /// itertools::assert_equal(data.into_iter().dedup_by(|x, y| x.1 == y.1),
+    ///                         vec![(0, 1.), (0, 2.), (0, 3.), (1, 2.)]);
+    /// ```
+    fn dedup_by<Cmp>(self, cmp: Cmp) -> DedupBy<Self, Cmp>
+        where Self: Sized,
+              Cmp: FnMut(&Self::Item, &Self::Item)->bool,
+    {
+        adaptors::dedup_by(self, cmp)
+    }
+
+    /// Remove duplicates from sections of consecutive identical elements, while keeping a count of
+    /// how many repeated elements were present.
+    /// If the iterator is sorted, all elements will be unique.
+    ///
+    /// Iterator element type is `(usize, Self::Item)`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec!['a', 'a', 'b', 'c', 'c', 'b', 'b'];
+    /// itertools::assert_equal(data.into_iter().dedup_with_count(),
+    ///                         vec![(2, 'a'), (1, 'b'), (2, 'c'), (2, 'b')]);
+    /// ```
+    fn dedup_with_count(self) -> DedupWithCount<Self>
+    where
+        Self: Sized,
+    {
+        adaptors::dedup_with_count(self)
+    }
+
+    /// Remove duplicates from sections of consecutive identical elements, while keeping a count of
+    /// how many repeated elements were present.
+    /// This will determine equality using a comparison function.
+    /// If the iterator is sorted, all elements will be unique.
+    ///
+    /// Iterator element type is `(usize, Self::Item)`.
+    ///
+    /// This iterator is *fused*.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![(0, 'a'), (1, 'a'), (0, 'b'), (0, 'c'), (1, 'c'), (1, 'b'), (2, 'b')];
+    /// itertools::assert_equal(data.into_iter().dedup_by_with_count(|x, y| x.1 == y.1),
+    ///                         vec![(2, (0, 'a')), (1, (0, 'b')), (2, (0, 'c')), (2, (1, 'b'))]);
+    /// ```
+    fn dedup_by_with_count<Cmp>(self, cmp: Cmp) -> DedupByWithCount<Self, Cmp>
+    where
+        Self: Sized,
+        Cmp: FnMut(&Self::Item, &Self::Item) -> bool,
+    {
+        adaptors::dedup_by_with_count(self, cmp)
+    }
+
+    /// Return an iterator adaptor that produces elements that appear more than once during the
+    /// iteration. Duplicates are detected using hash and equality.
+    ///
+    /// The iterator is stable, returning the duplicate items in the order in which they occur in
+    /// the adapted iterator. Each duplicate item is returned exactly once. If an item appears more
+    /// than twice, the second item is the item retained and the rest are discarded.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![10, 20, 30, 20, 40, 10, 50];
+    /// itertools::assert_equal(data.into_iter().duplicates(),
+    ///                         vec![20, 10]);
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn duplicates(self) -> Duplicates<Self>
+        where Self: Sized,
+              Self::Item: Eq + Hash
+    {
+        duplicates_impl::duplicates(self)
+    }
+
+    /// Return an iterator adaptor that produces elements that appear more than once during the
+    /// iteration. Duplicates are detected using hash and equality.
+    ///
+    /// Duplicates are detected by comparing the key they map to with the keying function `f` by
+    /// hash and equality. The keys are stored in a hash map in the iterator.
+    ///
+    /// The iterator is stable, returning the duplicate items in the order in which they occur in
+    /// the adapted iterator. Each duplicate item is returned exactly once. If an item appears more
+    /// than twice, the second item is the item retained and the rest are discarded.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec!["a", "bb", "aa", "c", "ccc"];
+    /// itertools::assert_equal(data.into_iter().duplicates_by(|s| s.len()),
+    ///                         vec!["aa", "c"]);
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn duplicates_by<V, F>(self, f: F) -> DuplicatesBy<Self, V, F>
+        where Self: Sized,
+              V: Eq + Hash,
+              F: FnMut(&Self::Item) -> V
+    {
+        duplicates_impl::duplicates_by(self, f)
+    }
+
+    /// Return an iterator adaptor that filters out elements that have
+    /// already been produced once during the iteration. Duplicates
+    /// are detected using hash and equality.
+    ///
+    /// Clones of visited elements are stored in a hash set in the
+    /// iterator.
+    ///
+    /// The iterator is stable, returning the non-duplicate items in the order
+    /// in which they occur in the adapted iterator. In a set of duplicate
+    /// items, the first item encountered is the item retained.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![10, 20, 30, 20, 40, 10, 50];
+    /// itertools::assert_equal(data.into_iter().unique(),
+    ///                         vec![10, 20, 30, 40, 50]);
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn unique(self) -> Unique<Self>
+        where Self: Sized,
+              Self::Item: Clone + Eq + Hash
+    {
+        unique_impl::unique(self)
+    }
+
+    /// Return an iterator adaptor that filters out elements that have
+    /// already been produced once during the iteration.
+    ///
+    /// Duplicates are detected by comparing the key they map to
+    /// with the keying function `f` by hash and equality.
+    /// The keys are stored in a hash set in the iterator.
+    ///
+    /// The iterator is stable, returning the non-duplicate items in the order
+    /// in which they occur in the adapted iterator. In a set of duplicate
+    /// items, the first item encountered is the item retained.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec!["a", "bb", "aa", "c", "ccc"];
+    /// itertools::assert_equal(data.into_iter().unique_by(|s| s.len()),
+    ///                         vec!["a", "bb", "ccc"]);
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn unique_by<V, F>(self, f: F) -> UniqueBy<Self, V, F>
+        where Self: Sized,
+              V: Eq + Hash,
+              F: FnMut(&Self::Item) -> V
+    {
+        unique_impl::unique_by(self, f)
+    }
+
+    /// Return an iterator adaptor that borrows from this iterator and
+    /// takes items while the closure `accept` returns `true`.
+    ///
+    /// This adaptor can only be used on iterators that implement `PeekingNext`
+    /// like `.peekable()`, `put_back` and a few other collection iterators.
+    ///
+    /// The last and rejected element (first `false`) is still available when
+    /// `peeking_take_while` is done.
+    ///
+    ///
+    /// See also [`.take_while_ref()`](Itertools::take_while_ref)
+    /// which is a similar adaptor.
+    fn peeking_take_while<F>(&mut self, accept: F) -> PeekingTakeWhile<Self, F>
+        where Self: Sized + PeekingNext,
+              F: FnMut(&Self::Item) -> bool,
+    {
+        peeking_take_while::peeking_take_while(self, accept)
+    }
+
+    /// Return an iterator adaptor that borrows from a `Clone`-able iterator
+    /// to only pick off elements while the predicate `accept` returns `true`.
+    ///
+    /// It uses the `Clone` trait to restore the original iterator so that the
+    /// last and rejected element (first `false`) is still available when
+    /// `take_while_ref` is done.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let mut hexadecimals = "0123456789abcdef".chars();
+    ///
+    /// let decimals = hexadecimals.take_while_ref(|c| c.is_numeric())
+    ///                            .collect::<String>();
+    /// assert_eq!(decimals, "0123456789");
+    /// assert_eq!(hexadecimals.next(), Some('a'));
+    ///
+    /// ```
+    fn take_while_ref<F>(&mut self, accept: F) -> TakeWhileRef<Self, F>
+        where Self: Clone,
+              F: FnMut(&Self::Item) -> bool
+    {
+        adaptors::take_while_ref(self, accept)
+    }
+
+    /// Return an iterator adaptor that filters `Option<A>` iterator elements
+    /// and produces `A`. Stops on the first `None` encountered.
+    ///
+    /// Iterator element type is `A`, the unwrapped element.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // List all hexadecimal digits
+    /// itertools::assert_equal(
+    ///     (0..).map(|i| std::char::from_digit(i, 16)).while_some(),
+    ///     "0123456789abcdef".chars());
+    ///
+    /// ```
+    fn while_some<A>(self) -> WhileSome<Self>
+        where Self: Sized + Iterator<Item = Option<A>>
+    {
+        adaptors::while_some(self)
+    }
+
+    /// Return an iterator adaptor that iterates over the combinations of the
+    /// elements from an iterator.
+    ///
+    /// Iterator element can be any homogeneous tuple of type `Self::Item` with
+    /// size up to 12.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let mut v = Vec::new();
+    /// for (a, b) in (1..5).tuple_combinations() {
+    ///     v.push((a, b));
+    /// }
+    /// assert_eq!(v, vec![(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]);
+    ///
+    /// let mut it = (1..5).tuple_combinations();
+    /// assert_eq!(Some((1, 2, 3)), it.next());
+    /// assert_eq!(Some((1, 2, 4)), it.next());
+    /// assert_eq!(Some((1, 3, 4)), it.next());
+    /// assert_eq!(Some((2, 3, 4)), it.next());
+    /// assert_eq!(None, it.next());
+    ///
+    /// // this requires a type hint
+    /// let it = (1..5).tuple_combinations::<(_, _, _)>();
+    /// itertools::assert_equal(it, vec![(1, 2, 3), (1, 2, 4), (1, 3, 4), (2, 3, 4)]);
+    ///
+    /// // you can also specify the complete type
+    /// use itertools::TupleCombinations;
+    /// use std::ops::Range;
+    ///
+    /// let it: TupleCombinations<Range<u32>, (u32, u32, u32)> = (1..5).tuple_combinations();
+    /// itertools::assert_equal(it, vec![(1, 2, 3), (1, 2, 4), (1, 3, 4), (2, 3, 4)]);
+    /// ```
+    fn tuple_combinations<T>(self) -> TupleCombinations<Self, T>
+        where Self: Sized + Clone,
+              Self::Item: Clone,
+              T: adaptors::HasCombination<Self>,
+    {
+        adaptors::tuple_combinations(self)
+    }
+
+    /// Return an iterator adaptor that iterates over the `k`-length combinations of
+    /// the elements from an iterator.
+    ///
+    /// Iterator element type is `Vec<Self::Item>`. The iterator produces a new Vec per iteration,
+    /// and clones the iterator elements.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let it = (1..5).combinations(3);
+    /// itertools::assert_equal(it, vec![
+    ///     vec![1, 2, 3],
+    ///     vec![1, 2, 4],
+    ///     vec![1, 3, 4],
+    ///     vec![2, 3, 4],
+    /// ]);
+    /// ```
+    ///
+    /// Note: Combinations does not take into account the equality of the iterated values.
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let it = vec![1, 2, 2].into_iter().combinations(2);
+    /// itertools::assert_equal(it, vec![
+    ///     vec![1, 2], // Note: these are the same
+    ///     vec![1, 2], // Note: these are the same
+    ///     vec![2, 2],
+    /// ]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn combinations(self, k: usize) -> Combinations<Self>
+        where Self: Sized,
+              Self::Item: Clone
+    {
+        combinations::combinations(self, k)
+    }
+
+    /// Return an iterator that iterates over the `k`-length combinations of
+    /// the elements from an iterator, with replacement.
+    ///
+    /// Iterator element type is `Vec<Self::Item>`. The iterator produces a new Vec per iteration,
+    /// and clones the iterator elements.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let it = (1..4).combinations_with_replacement(2);
+    /// itertools::assert_equal(it, vec![
+    ///     vec![1, 1],
+    ///     vec![1, 2],
+    ///     vec![1, 3],
+    ///     vec![2, 2],
+    ///     vec![2, 3],
+    ///     vec![3, 3],
+    /// ]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn combinations_with_replacement(self, k: usize) -> CombinationsWithReplacement<Self>
+    where
+        Self: Sized,
+        Self::Item: Clone,
+    {
+        combinations_with_replacement::combinations_with_replacement(self, k)
+    }
+
+    /// Return an iterator adaptor that iterates over all k-permutations of the
+    /// elements from an iterator.
+    ///
+    /// Iterator element type is `Vec<Self::Item>` with length `k`. The iterator
+    /// produces a new Vec per iteration, and clones the iterator elements.
+    ///
+    /// If `k` is greater than the length of the input iterator, the resultant
+    /// iterator adaptor will be empty.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let perms = (5..8).permutations(2);
+    /// itertools::assert_equal(perms, vec![
+    ///     vec![5, 6],
+    ///     vec![5, 7],
+    ///     vec![6, 5],
+    ///     vec![6, 7],
+    ///     vec![7, 5],
+    ///     vec![7, 6],
+    /// ]);
+    /// ```
+    ///
+    /// Note: Permutations does not take into account the equality of the iterated values.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let it = vec![2, 2].into_iter().permutations(2);
+    /// itertools::assert_equal(it, vec![
+    ///     vec![2, 2], // Note: these are the same
+    ///     vec![2, 2], // Note: these are the same
+    /// ]);
+    /// ```
+    ///
+    /// Note: The source iterator is collected lazily, and will not be
+    /// re-iterated if the permutations adaptor is completed and re-iterated.
+    #[cfg(feature = "use_alloc")]
+    fn permutations(self, k: usize) -> Permutations<Self>
+        where Self: Sized,
+              Self::Item: Clone
+    {
+        permutations::permutations(self, k)
+    }
+
+    /// Return an iterator that iterates through the powerset of the elements from an
+    /// iterator.
+    ///
+    /// Iterator element type is `Vec<Self::Item>`. The iterator produces a new `Vec`
+    /// per iteration, and clones the iterator elements.
+    ///
+    /// The powerset of a set contains all subsets including the empty set and the full
+    /// input set. A powerset has length _2^n_ where _n_ is the length of the input
+    /// set.
+    ///
+    /// Each `Vec` produced by this iterator represents a subset of the elements
+    /// produced by the source iterator.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let sets = (1..4).powerset().collect::<Vec<_>>();
+    /// itertools::assert_equal(sets, vec![
+    ///     vec![],
+    ///     vec![1],
+    ///     vec![2],
+    ///     vec![3],
+    ///     vec![1, 2],
+    ///     vec![1, 3],
+    ///     vec![2, 3],
+    ///     vec![1, 2, 3],
+    /// ]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn powerset(self) -> Powerset<Self>
+        where Self: Sized,
+              Self::Item: Clone,
+    {
+        powerset::powerset(self)
+    }
+
+    /// Return an iterator adaptor that pads the sequence to a minimum length of
+    /// `min` by filling missing elements using a closure `f`.
+    ///
+    /// Iterator element type is `Self::Item`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let it = (0..5).pad_using(10, |i| 2*i);
+    /// itertools::assert_equal(it, vec![0, 1, 2, 3, 4, 10, 12, 14, 16, 18]);
+    ///
+    /// let it = (0..10).pad_using(5, |i| 2*i);
+    /// itertools::assert_equal(it, vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+    ///
+    /// let it = (0..5).pad_using(10, |i| 2*i).rev();
+    /// itertools::assert_equal(it, vec![18, 16, 14, 12, 10, 4, 3, 2, 1, 0]);
+    /// ```
+    fn pad_using<F>(self, min: usize, f: F) -> PadUsing<Self, F>
+        where Self: Sized,
+              F: FnMut(usize) -> Self::Item
+    {
+        pad_tail::pad_using(self, min, f)
+    }
+
+    /// Return an iterator adaptor that wraps each element in a `Position` to
+    /// ease special-case handling of the first or last elements.
+    ///
+    /// Iterator element type is
+    /// [`Position<Self::Item>`](Position)
+    ///
+    /// ```
+    /// use itertools::{Itertools, Position};
+    ///
+    /// let it = (0..4).with_position();
+    /// itertools::assert_equal(it,
+    ///                         vec![Position::First(0),
+    ///                              Position::Middle(1),
+    ///                              Position::Middle(2),
+    ///                              Position::Last(3)]);
+    ///
+    /// let it = (0..1).with_position();
+    /// itertools::assert_equal(it, vec![Position::Only(0)]);
+    /// ```
+    fn with_position(self) -> WithPosition<Self>
+        where Self: Sized,
+    {
+        with_position::with_position(self)
+    }
+
+    /// Return an iterator adaptor that yields the indices of all elements
+    /// satisfying a predicate, counted from the start of the iterator.
+    ///
+    /// Equivalent to `iter.enumerate().filter(|(_, v)| predicate(v)).map(|(i, _)| i)`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![1, 2, 3, 3, 4, 6, 7, 9];
+    /// itertools::assert_equal(data.iter().positions(|v| v % 2 == 0), vec![1, 4, 5]);
+    ///
+    /// itertools::assert_equal(data.iter().positions(|v| v % 2 == 1).rev(), vec![7, 6, 3, 2, 0]);
+    /// ```
+    fn positions<P>(self, predicate: P) -> Positions<Self, P>
+        where Self: Sized,
+              P: FnMut(Self::Item) -> bool,
+    {
+        adaptors::positions(self, predicate)
+    }
+
+    /// Return an iterator adaptor that applies a mutating function
+    /// to each element before yielding it.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let input = vec![vec![1], vec![3, 2, 1]];
+    /// let it = input.into_iter().update(|mut v| v.push(0));
+    /// itertools::assert_equal(it, vec![vec![1, 0], vec![3, 2, 1, 0]]);
+    /// ```
+    fn update<F>(self, updater: F) -> Update<Self, F>
+        where Self: Sized,
+              F: FnMut(&mut Self::Item),
+    {
+        adaptors::update(self, updater)
+    }
+
+    // non-adaptor methods
+    /// Advances the iterator and returns the next items grouped in a tuple of
+    /// a specific size (up to 12).
+    ///
+    /// If there are enough elements to be grouped in a tuple, then the tuple is
+    /// returned inside `Some`, otherwise `None` is returned.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let mut iter = 1..5;
+    ///
+    /// assert_eq!(Some((1, 2)), iter.next_tuple());
+    /// ```
+    fn next_tuple<T>(&mut self) -> Option<T>
+        where Self: Sized + Iterator<Item = T::Item>,
+              T: traits::HomogeneousTuple
+    {
+        T::collect_from_iter_no_buf(self)
+    }
+
+    /// Collects all items from the iterator into a tuple of a specific size
+    /// (up to 12).
+    ///
+    /// If the number of elements inside the iterator is **exactly** equal to
+    /// the tuple size, then the tuple is returned inside `Some`, otherwise
+    /// `None` is returned.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let iter = 1..3;
+    ///
+    /// if let Some((x, y)) = iter.collect_tuple() {
+    ///     assert_eq!((x, y), (1, 2))
+    /// } else {
+    ///     panic!("Expected two elements")
+    /// }
+    /// ```
+    fn collect_tuple<T>(mut self) -> Option<T>
+        where Self: Sized + Iterator<Item = T::Item>,
+              T: traits::HomogeneousTuple
+    {
+        match self.next_tuple() {
+            elt @ Some(_) => match self.next() {
+                Some(_) => None,
+                None => elt,
+            },
+            _ => None
+        }
+    }
+
+
+    /// Find the position and value of the first element satisfying a predicate.
+    ///
+    /// The iterator is not advanced past the first element found.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let text = "Hα";
+    /// assert_eq!(text.chars().find_position(|ch| ch.is_lowercase()), Some((1, 'α')));
+    /// ```
+    fn find_position<P>(&mut self, mut pred: P) -> Option<(usize, Self::Item)>
+        where P: FnMut(&Self::Item) -> bool
+    {
+        for (index, elt) in self.enumerate() {
+            if pred(&elt) {
+                return Some((index, elt));
+            }
+        }
+        None
+    }
+    /// Find the value of the first element satisfying a predicate or return the last element, if any.
+    ///
+    /// The iterator is not advanced past the first element found.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let numbers = [1, 2, 3, 4];
+    /// assert_eq!(numbers.iter().find_or_last(|&&x| x > 5), Some(&4));
+    /// assert_eq!(numbers.iter().find_or_last(|&&x| x > 2), Some(&3));
+    /// assert_eq!(std::iter::empty::<i32>().find_or_last(|&x| x > 5), None);
+    /// ```
+    fn find_or_last<P>(mut self, mut predicate: P) -> Option<Self::Item>
+        where Self: Sized,
+              P: FnMut(&Self::Item) -> bool,
+    {
+        let mut prev = None;
+        self.find_map(|x| if predicate(&x) { Some(x) } else { prev = Some(x); None })
+            .or(prev)
+    }
+    /// Find the value of the first element satisfying a predicate or return the first element, if any.
+    ///
+    /// The iterator is not advanced past the first element found.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let numbers = [1, 2, 3, 4];
+    /// assert_eq!(numbers.iter().find_or_first(|&&x| x > 5), Some(&1));
+    /// assert_eq!(numbers.iter().find_or_first(|&&x| x > 2), Some(&3));
+    /// assert_eq!(std::iter::empty::<i32>().find_or_first(|&x| x > 5), None);
+    /// ```
+    fn find_or_first<P>(mut self, mut predicate: P) -> Option<Self::Item>
+        where Self: Sized,
+              P: FnMut(&Self::Item) -> bool,
+    {
+        let first = self.next()?;
+        Some(if predicate(&first) {
+            first
+        } else {
+            self.find(|x| predicate(x)).unwrap_or(first)
+        })
+    }
+    /// Returns `true` if the given item is present in this iterator.
+    ///
+    /// This method is short-circuiting. If the given item is present in this
+    /// iterator, this method will consume the iterator up-to-and-including
+    /// the item. If the given item is not present in this iterator, the
+    /// iterator will be exhausted.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// #[derive(PartialEq, Debug)]
+    /// enum Enum { A, B, C, D, E, }
+    /// 
+    /// let mut iter = vec![Enum::A, Enum::B, Enum::C, Enum::D].into_iter();
+    /// 
+    /// // search `iter` for `B`
+    /// assert_eq!(iter.contains(&Enum::B), true);
+    /// // `B` was found, so the iterator now rests at the item after `B` (i.e, `C`).
+    /// assert_eq!(iter.next(), Some(Enum::C));
+    /// 
+    /// // search `iter` for `E`
+    /// assert_eq!(iter.contains(&Enum::E), false);
+    /// // `E` wasn't found, so `iter` is now exhausted
+    /// assert_eq!(iter.next(), None);
+    /// ```
+    fn contains<Q>(&mut self, query: &Q) -> bool
+    where
+        Self: Sized,
+        Self::Item: Borrow<Q>,
+        Q: PartialEq,
+    {
+        self.any(|x| x.borrow() == query)
+    }
+
+    /// Check whether all elements compare equal.
+    ///
+    /// Empty iterators are considered to have equal elements:
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![1, 1, 1, 2, 2, 3, 3, 3, 4, 5, 5];
+    /// assert!(!data.iter().all_equal());
+    /// assert!(data[0..3].iter().all_equal());
+    /// assert!(data[3..5].iter().all_equal());
+    /// assert!(data[5..8].iter().all_equal());
+    ///
+    /// let data : Option<usize> = None;
+    /// assert!(data.into_iter().all_equal());
+    /// ```
+    fn all_equal(&mut self) -> bool
+        where Self: Sized,
+              Self::Item: PartialEq,
+    {
+        match self.next() {
+            None => true,
+            Some(a) => self.all(|x| a == x),
+        }
+    }
+
+    /// Check whether all elements are unique (non equal).
+    ///
+    /// Empty iterators are considered to have unique elements:
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![1, 2, 3, 4, 1, 5];
+    /// assert!(!data.iter().all_unique());
+    /// assert!(data[0..4].iter().all_unique());
+    /// assert!(data[1..6].iter().all_unique());
+    ///
+    /// let data : Option<usize> = None;
+    /// assert!(data.into_iter().all_unique());
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn all_unique(&mut self) -> bool
+        where Self: Sized,
+              Self::Item: Eq + Hash
+    {
+        let mut used = HashSet::new();
+        self.all(move |elt| used.insert(elt))
+    }
+
+    /// Consume the first `n` elements from the iterator eagerly,
+    /// and return the same iterator again.
+    ///
+    /// It works similarly to *.skip(* `n` *)* except it is eager and
+    /// preserves the iterator type.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let mut iter = "αβγ".chars().dropping(2);
+    /// itertools::assert_equal(iter, "γ".chars());
+    /// ```
+    ///
+    /// *Fusing notes: if the iterator is exhausted by dropping,
+    /// the result of calling `.next()` again depends on the iterator implementation.*
+    fn dropping(mut self, n: usize) -> Self
+        where Self: Sized
+    {
+        if n > 0 {
+            self.nth(n - 1);
+        }
+        self
+    }
+
+    /// Consume the last `n` elements from the iterator eagerly,
+    /// and return the same iterator again.
+    ///
+    /// This is only possible on double ended iterators. `n` may be
+    /// larger than the number of elements.
+    ///
+    /// Note: This method is eager, dropping the back elements immediately and
+    /// preserves the iterator type.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let init = vec![0, 3, 6, 9].into_iter().dropping_back(1);
+    /// itertools::assert_equal(init, vec![0, 3, 6]);
+    /// ```
+    fn dropping_back(mut self, n: usize) -> Self
+        where Self: Sized,
+              Self: DoubleEndedIterator
+    {
+        if n > 0 {
+            (&mut self).rev().nth(n - 1);
+        }
+        self
+    }
+
+    /// Run the closure `f` eagerly on each element of the iterator.
+    ///
+    /// Consumes the iterator until its end.
+    ///
+    /// ```
+    /// use std::sync::mpsc::channel;
+    /// use itertools::Itertools;
+    ///
+    /// let (tx, rx) = channel();
+    ///
+    /// // use .foreach() to apply a function to each value -- sending it
+    /// (0..5).map(|x| x * 2 + 1).foreach(|x| { tx.send(x).unwrap(); } );
+    ///
+    /// drop(tx);
+    ///
+    /// itertools::assert_equal(rx.iter(), vec![1, 3, 5, 7, 9]);
+    /// ```
+    #[deprecated(note="Use .for_each() instead", since="0.8.0")]
+    fn foreach<F>(self, f: F)
+        where F: FnMut(Self::Item),
+              Self: Sized,
+    {
+        self.for_each(f);
+    }
+
+    /// Combine all an iterator's elements into one element by using [`Extend`].
+    ///
+    /// This combinator will extend the first item with each of the rest of the
+    /// items of the iterator. If the iterator is empty, the default value of
+    /// `I::Item` is returned.
+    ///
+    /// ```rust
+    /// use itertools::Itertools;
+    ///
+    /// let input = vec![vec![1], vec![2, 3], vec![4, 5, 6]];
+    /// assert_eq!(input.into_iter().concat(),
+    ///            vec![1, 2, 3, 4, 5, 6]);
+    /// ```
+    fn concat(self) -> Self::Item
+        where Self: Sized,
+              Self::Item: Extend<<<Self as Iterator>::Item as IntoIterator>::Item> + IntoIterator + Default
+    {
+        concat(self)
+    }
+
+    /// `.collect_vec()` is simply a type specialization of [`Iterator::collect`],
+    /// for convenience.
+    #[cfg(feature = "use_alloc")]
+    fn collect_vec(self) -> Vec<Self::Item>
+        where Self: Sized
+    {
+        self.collect()
+    }
+
+    /// `.try_collect()` is more convenient way of writing
+    /// `.collect::<Result<_, _>>()`
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use std::{fs, io};
+    /// use itertools::Itertools;
+    ///
+    /// fn process_dir_entries(entries: &[fs::DirEntry]) {
+    ///     // ...
+    /// }
+    ///
+    /// fn do_stuff() -> std::io::Result<()> {
+    ///     let entries: Vec<_> = fs::read_dir(".")?.try_collect()?;
+    ///     process_dir_entries(&entries);
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn try_collect<T, U, E>(self) -> Result<U, E>
+    where
+        Self: Sized + Iterator<Item = Result<T, E>>,
+        Result<U, E>: FromIterator<Result<T, E>>,
+    {
+        self.collect()
+    }
+
+    /// Assign to each reference in `self` from the `from` iterator,
+    /// stopping at the shortest of the two iterators.
+    ///
+    /// The `from` iterator is queried for its next element before the `self`
+    /// iterator, and if either is exhausted the method is done.
+    ///
+    /// Return the number of elements written.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let mut xs = [0; 4];
+    /// xs.iter_mut().set_from(1..);
+    /// assert_eq!(xs, [1, 2, 3, 4]);
+    /// ```
+    #[inline]
+    fn set_from<'a, A: 'a, J>(&mut self, from: J) -> usize
+        where Self: Iterator<Item = &'a mut A>,
+              J: IntoIterator<Item = A>
+    {
+        let mut count = 0;
+        for elt in from {
+            match self.next() {
+                None => break,
+                Some(ptr) => *ptr = elt,
+            }
+            count += 1;
+        }
+        count
+    }
+
+    /// Combine all iterator elements into one String, separated by `sep`.
+    ///
+    /// Use the `Display` implementation of each element.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// assert_eq!(["a", "b", "c"].iter().join(", "), "a, b, c");
+    /// assert_eq!([1, 2, 3].iter().join(", "), "1, 2, 3");
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn join(&mut self, sep: &str) -> String
+        where Self::Item: std::fmt::Display
+    {
+        match self.next() {
+            None => String::new(),
+            Some(first_elt) => {
+                // estimate lower bound of capacity needed
+                let (lower, _) = self.size_hint();
+                let mut result = String::with_capacity(sep.len() * lower);
+                write!(&mut result, "{}", first_elt).unwrap();
+                self.for_each(|elt| {
+                    result.push_str(sep);
+                    write!(&mut result, "{}", elt).unwrap();
+                });
+                result
+            }
+        }
+    }
+
+    /// Format all iterator elements, separated by `sep`.
+    ///
+    /// All elements are formatted (any formatting trait)
+    /// with `sep` inserted between each element.
+    ///
+    /// **Panics** if the formatter helper is formatted more than once.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = [1.1, 2.71828, -3.];
+    /// assert_eq!(
+    ///     format!("{:.2}", data.iter().format(", ")),
+    ///            "1.10, 2.72, -3.00");
+    /// ```
+    fn format(self, sep: &str) -> Format<Self>
+        where Self: Sized,
+    {
+        format::new_format_default(self, sep)
+    }
+
+    /// Format all iterator elements, separated by `sep`.
+    ///
+    /// This is a customizable version of [`.format()`](Itertools::format).
+    ///
+    /// The supplied closure `format` is called once per iterator element,
+    /// with two arguments: the element and a callback that takes a
+    /// `&Display` value, i.e. any reference to type that implements `Display`.
+    ///
+    /// Using `&format_args!(...)` is the most versatile way to apply custom
+    /// element formatting. The callback can be called multiple times if needed.
+    ///
+    /// **Panics** if the formatter helper is formatted more than once.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = [1.1, 2.71828, -3.];
+    /// let data_formatter = data.iter().format_with(", ", |elt, f| f(&format_args!("{:.2}", elt)));
+    /// assert_eq!(format!("{}", data_formatter),
+    ///            "1.10, 2.72, -3.00");
+    ///
+    /// // .format_with() is recursively composable
+    /// let matrix = [[1., 2., 3.],
+    ///               [4., 5., 6.]];
+    /// let matrix_formatter = matrix.iter().format_with("\n", |row, f| {
+    ///                                 f(&row.iter().format_with(", ", |elt, g| g(&elt)))
+    ///                              });
+    /// assert_eq!(format!("{}", matrix_formatter),
+    ///            "1, 2, 3\n4, 5, 6");
+    ///
+    ///
+    /// ```
+    fn format_with<F>(self, sep: &str, format: F) -> FormatWith<Self, F>
+        where Self: Sized,
+              F: FnMut(Self::Item, &mut dyn FnMut(&dyn fmt::Display) -> fmt::Result) -> fmt::Result,
+    {
+        format::new_format(self, sep, format)
+    }
+
+    /// See [`.fold_ok()`](Itertools::fold_ok).
+    #[deprecated(note="Use .fold_ok() instead", since="0.10.0")]
+    fn fold_results<A, E, B, F>(&mut self, start: B, f: F) -> Result<B, E>
+        where Self: Iterator<Item = Result<A, E>>,
+              F: FnMut(B, A) -> B
+    {
+        self.fold_ok(start, f)
+    }
+
+    /// Fold `Result` values from an iterator.
+    ///
+    /// Only `Ok` values are folded. If no error is encountered, the folded
+    /// value is returned inside `Ok`. Otherwise, the operation terminates
+    /// and returns the first `Err` value it encounters. No iterator elements are
+    /// consumed after the first error.
+    ///
+    /// The first accumulator value is the `start` parameter.
+    /// Each iteration passes the accumulator value and the next value inside `Ok`
+    /// to the fold function `f` and its return value becomes the new accumulator value.
+    ///
+    /// For example the sequence *Ok(1), Ok(2), Ok(3)* will result in a
+    /// computation like this:
+    ///
+    /// ```ignore
+    /// let mut accum = start;
+    /// accum = f(accum, 1);
+    /// accum = f(accum, 2);
+    /// accum = f(accum, 3);
+    /// ```
+    ///
+    /// With a `start` value of 0 and an addition as folding function,
+    /// this effectively results in *((0 + 1) + 2) + 3*
+    ///
+    /// ```
+    /// use std::ops::Add;
+    /// use itertools::Itertools;
+    ///
+    /// let values = [1, 2, -2, -1, 2, 1];
+    /// assert_eq!(
+    ///     values.iter()
+    ///           .map(Ok::<_, ()>)
+    ///           .fold_ok(0, Add::add),
+    ///     Ok(3)
+    /// );
+    /// assert!(
+    ///     values.iter()
+    ///           .map(|&x| if x >= 0 { Ok(x) } else { Err("Negative number") })
+    ///           .fold_ok(0, Add::add)
+    ///           .is_err()
+    /// );
+    /// ```
+    fn fold_ok<A, E, B, F>(&mut self, mut start: B, mut f: F) -> Result<B, E>
+        where Self: Iterator<Item = Result<A, E>>,
+              F: FnMut(B, A) -> B
+    {
+        for elt in self {
+            match elt {
+                Ok(v) => start = f(start, v),
+                Err(u) => return Err(u),
+            }
+        }
+        Ok(start)
+    }
+
+    /// Fold `Option` values from an iterator.
+    ///
+    /// Only `Some` values are folded. If no `None` is encountered, the folded
+    /// value is returned inside `Some`. Otherwise, the operation terminates
+    /// and returns `None`. No iterator elements are consumed after the `None`.
+    ///
+    /// This is the `Option` equivalent to [`fold_ok`](Itertools::fold_ok).
+    ///
+    /// ```
+    /// use std::ops::Add;
+    /// use itertools::Itertools;
+    ///
+    /// let mut values = vec![Some(1), Some(2), Some(-2)].into_iter();
+    /// assert_eq!(values.fold_options(5, Add::add), Some(5 + 1 + 2 - 2));
+    ///
+    /// let mut more_values = vec![Some(2), None, Some(0)].into_iter();
+    /// assert!(more_values.fold_options(0, Add::add).is_none());
+    /// assert_eq!(more_values.next().unwrap(), Some(0));
+    /// ```
+    fn fold_options<A, B, F>(&mut self, mut start: B, mut f: F) -> Option<B>
+        where Self: Iterator<Item = Option<A>>,
+              F: FnMut(B, A) -> B
+    {
+        for elt in self {
+            match elt {
+                Some(v) => start = f(start, v),
+                None => return None,
+            }
+        }
+        Some(start)
+    }
+
+    /// Accumulator of the elements in the iterator.
+    ///
+    /// Like `.fold()`, without a base case. If the iterator is
+    /// empty, return `None`. With just one element, return it.
+    /// Otherwise elements are accumulated in sequence using the closure `f`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// assert_eq!((0..10).fold1(|x, y| x + y).unwrap_or(0), 45);
+    /// assert_eq!((0..0).fold1(|x, y| x * y), None);
+    /// ```
+    #[deprecated(since = "0.10.2", note = "Use `Iterator::reduce` instead")]
+    fn fold1<F>(mut self, f: F) -> Option<Self::Item>
+        where F: FnMut(Self::Item, Self::Item) -> Self::Item,
+              Self: Sized,
+    {
+        self.next().map(move |x| self.fold(x, f))
+    }
+
+    /// Accumulate the elements in the iterator in a tree-like manner.
+    ///
+    /// You can think of it as, while there's more than one item, repeatedly
+    /// combining adjacent items.  It does so in bottom-up-merge-sort order,
+    /// however, so that it needs only logarithmic stack space.
+    ///
+    /// This produces a call tree like the following (where the calls under
+    /// an item are done after reading that item):
+    ///
+    /// ```text
+    /// 1 2 3 4 5 6 7
+    /// │ │ │ │ │ │ │
+    /// └─f └─f └─f │
+    ///   │   │   │ │
+    ///   └───f   └─f
+    ///       │     │
+    ///       └─────f
+    /// ```
+    ///
+    /// Which, for non-associative functions, will typically produce a different
+    /// result than the linear call tree used by [`Iterator::reduce`]:
+    ///
+    /// ```text
+    /// 1 2 3 4 5 6 7
+    /// │ │ │ │ │ │ │
+    /// └─f─f─f─f─f─f
+    /// ```
+    ///
+    /// If `f` is associative, prefer the normal [`Iterator::reduce`] instead.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // The same tree as above
+    /// let num_strings = (1..8).map(|x| x.to_string());
+    /// assert_eq!(num_strings.tree_fold1(|x, y| format!("f({}, {})", x, y)),
+    ///     Some(String::from("f(f(f(1, 2), f(3, 4)), f(f(5, 6), 7))")));
+    ///
+    /// // Like fold1, an empty iterator produces None
+    /// assert_eq!((0..0).tree_fold1(|x, y| x * y), None);
+    ///
+    /// // tree_fold1 matches fold1 for associative operations...
+    /// assert_eq!((0..10).tree_fold1(|x, y| x + y),
+    ///     (0..10).fold1(|x, y| x + y));
+    /// // ...but not for non-associative ones
+    /// assert_ne!((0..10).tree_fold1(|x, y| x - y),
+    ///     (0..10).fold1(|x, y| x - y));
+    /// ```
+    fn tree_fold1<F>(mut self, mut f: F) -> Option<Self::Item>
+        where F: FnMut(Self::Item, Self::Item) -> Self::Item,
+              Self: Sized,
+    {
+        type State<T> = Result<T, Option<T>>;
+
+        fn inner0<T, II, FF>(it: &mut II, f: &mut FF) -> State<T>
+            where
+                II: Iterator<Item = T>,
+                FF: FnMut(T, T) -> T
+        {
+            // This function could be replaced with `it.next().ok_or(None)`,
+            // but half the useful tree_fold1 work is combining adjacent items,
+            // so put that in a form that LLVM is more likely to optimize well.
+
+            let a =
+                if let Some(v) = it.next() { v }
+                else { return Err(None) };
+            let b =
+                if let Some(v) = it.next() { v }
+                else { return Err(Some(a)) };
+            Ok(f(a, b))
+        }
+
+        fn inner<T, II, FF>(stop: usize, it: &mut II, f: &mut FF) -> State<T>
+            where
+                II: Iterator<Item = T>,
+                FF: FnMut(T, T) -> T
+        {
+            let mut x = inner0(it, f)?;
+            for height in 0..stop {
+                // Try to get another tree the same size with which to combine it,
+                // creating a new tree that's twice as big for next time around.
+                let next =
+                    if height == 0 {
+                        inner0(it, f)
+                    } else {
+                        inner(height, it, f)
+                    };
+                match next {
+                    Ok(y) => x = f(x, y),
+
+                    // If we ran out of items, combine whatever we did manage
+                    // to get.  It's better combined with the current value
+                    // than something in a parent frame, because the tree in
+                    // the parent is always as least as big as this one.
+                    Err(None) => return Err(Some(x)),
+                    Err(Some(y)) => return Err(Some(f(x, y))),
+                }
+            }
+            Ok(x)
+        }
+
+        match inner(usize::max_value(), &mut self, &mut f) {
+            Err(x) => x,
+            _ => unreachable!(),
+        }
+    }
+
+    /// An iterator method that applies a function, producing a single, final value.
+    ///
+    /// `fold_while()` is basically equivalent to [`Iterator::fold`] but with additional support for
+    /// early exit via short-circuiting.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// use itertools::FoldWhile::{Continue, Done};
+    ///
+    /// let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+    ///
+    /// let mut result = 0;
+    ///
+    /// // for loop:
+    /// for i in &numbers {
+    ///     if *i > 5 {
+    ///         break;
+    ///     }
+    ///     result = result + i;
+    /// }
+    ///
+    /// // fold:
+    /// let result2 = numbers.iter().fold(0, |acc, x| {
+    ///     if *x > 5 { acc } else { acc + x }
+    /// });
+    ///
+    /// // fold_while:
+    /// let result3 = numbers.iter().fold_while(0, |acc, x| {
+    ///     if *x > 5 { Done(acc) } else { Continue(acc + x) }
+    /// }).into_inner();
+    ///
+    /// // they're the same
+    /// assert_eq!(result, result2);
+    /// assert_eq!(result2, result3);
+    /// ```
+    ///
+    /// The big difference between the computations of `result2` and `result3` is that while
+    /// `fold()` called the provided closure for every item of the callee iterator,
+    /// `fold_while()` actually stopped iterating as soon as it encountered `Fold::Done(_)`.
+    fn fold_while<B, F>(&mut self, init: B, mut f: F) -> FoldWhile<B>
+        where Self: Sized,
+              F: FnMut(B, Self::Item) -> FoldWhile<B>
+    {
+        use Result::{
+            Ok as Continue,
+            Err as Break,
+        };
+
+        let result = self.try_fold(init, #[inline(always)] |acc, v|
+            match f(acc, v) {
+              FoldWhile::Continue(acc) => Continue(acc),
+              FoldWhile::Done(acc) => Break(acc),
+            }
+        );
+
+        match result {
+            Continue(acc) => FoldWhile::Continue(acc),
+            Break(acc) => FoldWhile::Done(acc),
+        }
+    }
+
+    /// Iterate over the entire iterator and add all the elements.
+    ///
+    /// An empty iterator returns `None`, otherwise `Some(sum)`.
+    ///
+    /// # Panics
+    ///
+    /// When calling `sum1()` and a primitive integer type is being returned, this
+    /// method will panic if the computation overflows and debug assertions are
+    /// enabled.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let empty_sum = (1..1).sum1::<i32>();
+    /// assert_eq!(empty_sum, None);
+    ///
+    /// let nonempty_sum = (1..11).sum1::<i32>();
+    /// assert_eq!(nonempty_sum, Some(55));
+    /// ```
+    fn sum1<S>(mut self) -> Option<S>
+        where Self: Sized,
+              S: std::iter::Sum<Self::Item>,
+    {
+        self.next()
+            .map(|first| once(first).chain(self).sum())
+    }
+
+    /// Iterate over the entire iterator and multiply all the elements.
+    ///
+    /// An empty iterator returns `None`, otherwise `Some(product)`.
+    ///
+    /// # Panics
+    ///
+    /// When calling `product1()` and a primitive integer type is being returned,
+    /// method will panic if the computation overflows and debug assertions are
+    /// enabled.
+    ///
+    /// # Examples
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let empty_product = (1..1).product1::<i32>();
+    /// assert_eq!(empty_product, None);
+    ///
+    /// let nonempty_product = (1..11).product1::<i32>();
+    /// assert_eq!(nonempty_product, Some(3628800));
+    /// ```
+    fn product1<P>(mut self) -> Option<P>
+        where Self: Sized,
+              P: std::iter::Product<Self::Item>,
+    {
+        self.next()
+            .map(|first| once(first).chain(self).product())
+    }
+
+    /// Sort all iterator elements into a new iterator in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, uses the
+    /// [`slice::sort_unstable`] method and returns the result as a new
+    /// iterator that owns its elements.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sort the letters of the text in ascending order
+    /// let text = "bdacfe";
+    /// itertools::assert_equal(text.chars().sorted_unstable(),
+    ///                         "abcdef".chars());
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn sorted_unstable(self) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              Self::Item: Ord
+    {
+        // Use .sort_unstable() directly since it is not quite identical with
+        // .sort_by(Ord::cmp)
+        let mut v = Vec::from_iter(self);
+        v.sort_unstable();
+        v.into_iter()
+    }
+
+    /// Sort all iterator elements into a new iterator in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, uses the
+    /// [`slice::sort_unstable_by`] method and returns the result as a new
+    /// iterator that owns its elements.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sort people in descending order by age
+    /// let people = vec![("Jane", 20), ("John", 18), ("Jill", 30), ("Jack", 27)];
+    ///
+    /// let oldest_people_first = people
+    ///     .into_iter()
+    ///     .sorted_unstable_by(|a, b| Ord::cmp(&b.1, &a.1))
+    ///     .map(|(person, _age)| person);
+    ///
+    /// itertools::assert_equal(oldest_people_first,
+    ///                         vec!["Jill", "Jack", "Jane", "John"]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn sorted_unstable_by<F>(self, cmp: F) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              F: FnMut(&Self::Item, &Self::Item) -> Ordering,
+    {
+        let mut v = Vec::from_iter(self);
+        v.sort_unstable_by(cmp);
+        v.into_iter()
+    }
+
+    /// Sort all iterator elements into a new iterator in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, uses the
+    /// [`slice::sort_unstable_by_key`] method and returns the result as a new
+    /// iterator that owns its elements.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sort people in descending order by age
+    /// let people = vec![("Jane", 20), ("John", 18), ("Jill", 30), ("Jack", 27)];
+    ///
+    /// let oldest_people_first = people
+    ///     .into_iter()
+    ///     .sorted_unstable_by_key(|x| -x.1)
+    ///     .map(|(person, _age)| person);
+    ///
+    /// itertools::assert_equal(oldest_people_first,
+    ///                         vec!["Jill", "Jack", "Jane", "John"]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn sorted_unstable_by_key<K, F>(self, f: F) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              K: Ord,
+              F: FnMut(&Self::Item) -> K,
+    {
+        let mut v = Vec::from_iter(self);
+        v.sort_unstable_by_key(f);
+        v.into_iter()
+    }
+
+    /// Sort all iterator elements into a new iterator in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, uses the
+    /// [`slice::sort`] method and returns the result as a new
+    /// iterator that owns its elements.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sort the letters of the text in ascending order
+    /// let text = "bdacfe";
+    /// itertools::assert_equal(text.chars().sorted(),
+    ///                         "abcdef".chars());
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn sorted(self) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              Self::Item: Ord
+    {
+        // Use .sort() directly since it is not quite identical with
+        // .sort_by(Ord::cmp)
+        let mut v = Vec::from_iter(self);
+        v.sort();
+        v.into_iter()
+    }
+
+    /// Sort all iterator elements into a new iterator in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, uses the
+    /// [`slice::sort_by`] method and returns the result as a new
+    /// iterator that owns its elements.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sort people in descending order by age
+    /// let people = vec![("Jane", 20), ("John", 18), ("Jill", 30), ("Jack", 27)];
+    ///
+    /// let oldest_people_first = people
+    ///     .into_iter()
+    ///     .sorted_by(|a, b| Ord::cmp(&b.1, &a.1))
+    ///     .map(|(person, _age)| person);
+    ///
+    /// itertools::assert_equal(oldest_people_first,
+    ///                         vec!["Jill", "Jack", "Jane", "John"]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn sorted_by<F>(self, cmp: F) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              F: FnMut(&Self::Item, &Self::Item) -> Ordering,
+    {
+        let mut v = Vec::from_iter(self);
+        v.sort_by(cmp);
+        v.into_iter()
+    }
+
+    /// Sort all iterator elements into a new iterator in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, uses the
+    /// [`slice::sort_by_key`] method and returns the result as a new
+    /// iterator that owns its elements.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sort people in descending order by age
+    /// let people = vec![("Jane", 20), ("John", 18), ("Jill", 30), ("Jack", 27)];
+    ///
+    /// let oldest_people_first = people
+    ///     .into_iter()
+    ///     .sorted_by_key(|x| -x.1)
+    ///     .map(|(person, _age)| person);
+    ///
+    /// itertools::assert_equal(oldest_people_first,
+    ///                         vec!["Jill", "Jack", "Jane", "John"]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn sorted_by_key<K, F>(self, f: F) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              K: Ord,
+              F: FnMut(&Self::Item) -> K,
+    {
+        let mut v = Vec::from_iter(self);
+        v.sort_by_key(f);
+        v.into_iter()
+    }
+
+    /// Sort all iterator elements into a new iterator in ascending order. The key function is
+    /// called exactly once per key.
+    ///
+    /// **Note:** This consumes the entire iterator, uses the
+    /// [`slice::sort_by_cached_key`] method and returns the result as a new
+    /// iterator that owns its elements.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // sort people in descending order by age
+    /// let people = vec![("Jane", 20), ("John", 18), ("Jill", 30), ("Jack", 27)];
+    ///
+    /// let oldest_people_first = people
+    ///     .into_iter()
+    ///     .sorted_by_cached_key(|x| -x.1)
+    ///     .map(|(person, _age)| person);
+    ///
+    /// itertools::assert_equal(oldest_people_first,
+    ///                         vec!["Jill", "Jack", "Jane", "John"]);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn sorted_by_cached_key<K, F>(self, f: F) -> VecIntoIter<Self::Item>
+    where
+        Self: Sized,
+        K: Ord,
+        F: FnMut(&Self::Item) -> K,
+    {
+        let mut v = Vec::from_iter(self);
+        v.sort_by_cached_key(f);
+        v.into_iter()
+    }
+
+    /// Sort the k smallest elements into a new iterator, in ascending order.
+    ///
+    /// **Note:** This consumes the entire iterator, and returns the result
+    /// as a new iterator that owns its elements.  If the input contains
+    /// less than k elements, the result is equivalent to `self.sorted()`.
+    ///
+    /// This is guaranteed to use `k * sizeof(Self::Item) + O(1)` memory
+    /// and `O(n log k)` time, with `n` the number of elements in the input.
+    ///
+    /// The sorted iterator, if directly collected to a `Vec`, is converted
+    /// without any extra copying or allocation cost.
+    ///
+    /// **Note:** This is functionally-equivalent to `self.sorted().take(k)`
+    /// but much more efficient.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// // A random permutation of 0..15
+    /// let numbers = vec![6, 9, 1, 14, 0, 4, 8, 7, 11, 2, 10, 3, 13, 12, 5];
+    ///
+    /// let five_smallest = numbers
+    ///     .into_iter()
+    ///     .k_smallest(5);
+    ///
+    /// itertools::assert_equal(five_smallest, 0..5);
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn k_smallest(self, k: usize) -> VecIntoIter<Self::Item>
+        where Self: Sized,
+              Self::Item: Ord
+    {
+        crate::k_smallest::k_smallest(self, k)
+            .into_sorted_vec()
+            .into_iter()
+    }
+
+    /// Collect all iterator elements into one of two
+    /// partitions. Unlike [`Iterator::partition`], each partition may
+    /// have a distinct type.
+    ///
+    /// ```
+    /// use itertools::{Itertools, Either};
+    ///
+    /// let successes_and_failures = vec![Ok(1), Err(false), Err(true), Ok(2)];
+    ///
+    /// let (successes, failures): (Vec<_>, Vec<_>) = successes_and_failures
+    ///     .into_iter()
+    ///     .partition_map(|r| {
+    ///         match r {
+    ///             Ok(v) => Either::Left(v),
+    ///             Err(v) => Either::Right(v),
+    ///         }
+    ///     });
+    ///
+    /// assert_eq!(successes, [1, 2]);
+    /// assert_eq!(failures, [false, true]);
+    /// ```
+    fn partition_map<A, B, F, L, R>(self, mut predicate: F) -> (A, B)
+        where Self: Sized,
+              F: FnMut(Self::Item) -> Either<L, R>,
+              A: Default + Extend<L>,
+              B: Default + Extend<R>,
+    {
+        let mut left = A::default();
+        let mut right = B::default();
+
+        self.for_each(|val| match predicate(val) {
+            Either::Left(v) => left.extend(Some(v)),
+            Either::Right(v) => right.extend(Some(v)),
+        });
+
+        (left, right)
+    }
+
+    /// Partition a sequence of `Result`s into one list of all the `Ok` elements
+    /// and another list of all the `Err` elements.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let successes_and_failures = vec![Ok(1), Err(false), Err(true), Ok(2)];
+    ///
+    /// let (successes, failures): (Vec<_>, Vec<_>) = successes_and_failures
+    ///     .into_iter()
+    ///     .partition_result();
+    ///
+    /// assert_eq!(successes, [1, 2]);
+    /// assert_eq!(failures, [false, true]);
+    /// ```
+    fn partition_result<A, B, T, E>(self) -> (A, B)
+        where
+            Self: Iterator<Item = Result<T, E>> + Sized,
+            A: Default + Extend<T>,
+            B: Default + Extend<E>,
+    {
+        self.partition_map(|r| match r {
+            Ok(v) => Either::Left(v),
+            Err(v) => Either::Right(v),
+        })
+    }
+
+    /// Return a `HashMap` of keys mapped to `Vec`s of values. Keys and values
+    /// are taken from `(Key, Value)` tuple pairs yielded by the input iterator.
+    ///
+    /// Essentially a shorthand for `.into_grouping_map().collect::<Vec<_>>()`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let data = vec![(0, 10), (2, 12), (3, 13), (0, 20), (3, 33), (2, 42)];
+    /// let lookup = data.into_iter().into_group_map();
+    ///
+    /// assert_eq!(lookup[&0], vec![10, 20]);
+    /// assert_eq!(lookup.get(&1), None);
+    /// assert_eq!(lookup[&2], vec![12, 42]);
+    /// assert_eq!(lookup[&3], vec![13, 33]);
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn into_group_map<K, V>(self) -> HashMap<K, Vec<V>>
+        where Self: Iterator<Item=(K, V)> + Sized,
+              K: Hash + Eq,
+    {
+        group_map::into_group_map(self)
+    }
+
+    /// Return an `Iterator` on a `HashMap`. Keys mapped to `Vec`s of values. The key is specified
+    /// in the closure.
+    ///
+    /// Essentially a shorthand for `.into_grouping_map_by(f).collect::<Vec<_>>()`.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// use std::collections::HashMap;
+    ///
+    /// let data = vec![(0, 10), (2, 12), (3, 13), (0, 20), (3, 33), (2, 42)];
+    /// let lookup: HashMap<u32,Vec<(u32, u32)>> =
+    ///     data.clone().into_iter().into_group_map_by(|a| a.0);
+    ///
+    /// assert_eq!(lookup[&0], vec![(0,10),(0,20)]);
+    /// assert_eq!(lookup.get(&1), None);
+    /// assert_eq!(lookup[&2], vec![(2,12), (2,42)]);
+    /// assert_eq!(lookup[&3], vec![(3,13), (3,33)]);
+    ///
+    /// assert_eq!(
+    ///     data.into_iter()
+    ///         .into_group_map_by(|x| x.0)
+    ///         .into_iter()
+    ///         .map(|(key, values)| (key, values.into_iter().fold(0,|acc, (_,v)| acc + v )))
+    ///         .collect::<HashMap<u32,u32>>()[&0],
+    ///     30,
+    /// );
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn into_group_map_by<K, V, F>(self, f: F) -> HashMap<K, Vec<V>>
+        where
+            Self: Iterator<Item=V> + Sized,
+            K: Hash + Eq,
+            F: Fn(&V) -> K,
+    {
+        group_map::into_group_map_by(self, f)
+    }
+
+    /// Constructs a `GroupingMap` to be used later with one of the efficient 
+    /// group-and-fold operations it allows to perform.
+    /// 
+    /// The input iterator must yield item in the form of `(K, V)` where the
+    /// value of type `K` will be used as key to identify the groups and the
+    /// value of type `V` as value for the folding operation.
+    /// 
+    /// See [`GroupingMap`] for more informations
+    /// on what operations are available.
+    #[cfg(feature = "use_std")]
+    fn into_grouping_map<K, V>(self) -> GroupingMap<Self>
+        where Self: Iterator<Item=(K, V)> + Sized,
+              K: Hash + Eq,
+    {
+        grouping_map::new(self)
+    }
+
+    /// Constructs a `GroupingMap` to be used later with one of the efficient 
+    /// group-and-fold operations it allows to perform.
+    /// 
+    /// The values from this iterator will be used as values for the folding operation
+    /// while the keys will be obtained from the values by calling `key_mapper`.
+    /// 
+    /// See [`GroupingMap`] for more informations
+    /// on what operations are available.
+    #[cfg(feature = "use_std")]
+    fn into_grouping_map_by<K, V, F>(self, key_mapper: F) -> GroupingMapBy<Self, F>
+        where Self: Iterator<Item=V> + Sized,
+              K: Hash + Eq,
+              F: FnMut(&V) -> K
+    {
+        grouping_map::new(grouping_map::MapForGrouping::new(self, key_mapper))
+    }
+
+    /// Return all minimum elements of an iterator.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().min_set(), Vec::<&i32>::new());
+    ///
+    /// let a = [1];
+    /// assert_eq!(a.iter().min_set(), vec![&1]);
+    ///
+    /// let a = [1, 2, 3, 4, 5];
+    /// assert_eq!(a.iter().min_set(), vec![&1]);
+    ///
+    /// let a = [1, 1, 1, 1];
+    /// assert_eq!(a.iter().min_set(), vec![&1, &1, &1, &1]);
+    /// ```
+    ///
+    /// The elements can be floats but no particular result is guaranteed
+    /// if an element is NaN.
+    #[cfg(feature = "use_std")]
+    fn min_set(self) -> Vec<Self::Item>
+        where Self: Sized, Self::Item: Ord
+    {
+        extrema_set::min_set_impl(self, |_| (), |x, y, _, _| x.cmp(y))
+    }
+
+    /// Return all minimum elements of an iterator, as determined by
+    /// the specified function.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use std::cmp::Ordering;
+    /// use itertools::Itertools;
+    ///
+    /// let a: [(i32, i32); 0] = [];
+    /// assert_eq!(a.iter().min_set_by(|_, _| Ordering::Equal), Vec::<&(i32, i32)>::new());
+    ///
+    /// let a = [(1, 2)];
+    /// assert_eq!(a.iter().min_set_by(|&&(k1,_), &&(k2, _)| k1.cmp(&k2)), vec![&(1, 2)]);
+    ///
+    /// let a = [(1, 2), (2, 2), (3, 9), (4, 8), (5, 9)];
+    /// assert_eq!(a.iter().min_set_by(|&&(_,k1), &&(_,k2)| k1.cmp(&k2)), vec![&(1, 2), &(2, 2)]);
+    ///
+    /// let a = [(1, 2), (1, 3), (1, 4), (1, 5)];
+    /// assert_eq!(a.iter().min_set_by(|&&(k1,_), &&(k2, _)| k1.cmp(&k2)), vec![&(1, 2), &(1, 3), &(1, 4), &(1, 5)]);
+    /// ```
+    ///
+    /// The elements can be floats but no particular result is guaranteed
+    /// if an element is NaN.
+    #[cfg(feature = "use_std")]
+    fn min_set_by<F>(self, mut compare: F) -> Vec<Self::Item>
+        where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering
+    {
+        extrema_set::min_set_impl(
+            self,
+            |_| (),
+            |x, y, _, _| compare(x, y)
+        )
+    }
+
+    /// Return all minimum elements of an iterator, as determined by
+    /// the specified function.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a: [(i32, i32); 0] = [];
+    /// assert_eq!(a.iter().min_set_by_key(|_| ()), Vec::<&(i32, i32)>::new());
+    ///
+    /// let a = [(1, 2)];
+    /// assert_eq!(a.iter().min_set_by_key(|&&(k,_)| k), vec![&(1, 2)]);
+    ///
+    /// let a = [(1, 2), (2, 2), (3, 9), (4, 8), (5, 9)];
+    /// assert_eq!(a.iter().min_set_by_key(|&&(_, k)| k), vec![&(1, 2), &(2, 2)]);
+    ///
+    /// let a = [(1, 2), (1, 3), (1, 4), (1, 5)];
+    /// assert_eq!(a.iter().min_set_by_key(|&&(k, _)| k), vec![&(1, 2), &(1, 3), &(1, 4), &(1, 5)]);
+    /// ```
+    ///
+    /// The elements can be floats but no particular result is guaranteed
+    /// if an element is NaN.
+    #[cfg(feature = "use_std")]
+    fn min_set_by_key<K, F>(self, key: F) -> Vec<Self::Item>
+        where Self: Sized, K: Ord, F: FnMut(&Self::Item) -> K
+    {
+        extrema_set::min_set_impl(self, key, |_, _, kx, ky| kx.cmp(ky))
+    }
+
+    /// Return all maximum elements of an iterator.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().max_set(), Vec::<&i32>::new());
+    ///
+    /// let a = [1];
+    /// assert_eq!(a.iter().max_set(), vec![&1]);
+    ///
+    /// let a = [1, 2, 3, 4, 5];
+    /// assert_eq!(a.iter().max_set(), vec![&5]);
+    ///
+    /// let a = [1, 1, 1, 1];
+    /// assert_eq!(a.iter().max_set(), vec![&1, &1, &1, &1]);
+    /// ```
+    ///
+    /// The elements can be floats but no particular result is guaranteed
+    /// if an element is NaN.
+    #[cfg(feature = "use_std")]
+    fn max_set(self) -> Vec<Self::Item>
+        where Self: Sized, Self::Item: Ord
+    {
+        extrema_set::max_set_impl(self, |_| (), |x, y, _, _| x.cmp(y))
+    }
+
+    /// Return all maximum elements of an iterator, as determined by
+    /// the specified function.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use std::cmp::Ordering;
+    /// use itertools::Itertools;
+    ///
+    /// let a: [(i32, i32); 0] = [];
+    /// assert_eq!(a.iter().max_set_by(|_, _| Ordering::Equal), Vec::<&(i32, i32)>::new());
+    ///
+    /// let a = [(1, 2)];
+    /// assert_eq!(a.iter().max_set_by(|&&(k1,_), &&(k2, _)| k1.cmp(&k2)), vec![&(1, 2)]);
+    ///
+    /// let a = [(1, 2), (2, 2), (3, 9), (4, 8), (5, 9)];
+    /// assert_eq!(a.iter().max_set_by(|&&(_,k1), &&(_,k2)| k1.cmp(&k2)), vec![&(3, 9), &(5, 9)]);
+    ///
+    /// let a = [(1, 2), (1, 3), (1, 4), (1, 5)];
+    /// assert_eq!(a.iter().max_set_by(|&&(k1,_), &&(k2, _)| k1.cmp(&k2)), vec![&(1, 2), &(1, 3), &(1, 4), &(1, 5)]);
+    /// ```
+    ///
+    /// The elements can be floats but no particular result is guaranteed
+    /// if an element is NaN.
+    #[cfg(feature = "use_std")]
+    fn max_set_by<F>(self, mut compare: F) -> Vec<Self::Item>
+        where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering
+    {
+        extrema_set::max_set_impl(
+            self,
+            |_| (),
+            |x, y, _, _| compare(x, y)
+        )
+    }
+
+    /// Return all minimum elements of an iterator, as determined by
+    /// the specified function.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a: [(i32, i32); 0] = [];
+    /// assert_eq!(a.iter().max_set_by_key(|_| ()), Vec::<&(i32, i32)>::new());
+    ///
+    /// let a = [(1, 2)];
+    /// assert_eq!(a.iter().max_set_by_key(|&&(k,_)| k), vec![&(1, 2)]);
+    ///
+    /// let a = [(1, 2), (2, 2), (3, 9), (4, 8), (5, 9)];
+    /// assert_eq!(a.iter().max_set_by_key(|&&(_, k)| k), vec![&(3, 9), &(5, 9)]);
+    ///
+    /// let a = [(1, 2), (1, 3), (1, 4), (1, 5)];
+    /// assert_eq!(a.iter().max_set_by_key(|&&(k, _)| k), vec![&(1, 2), &(1, 3), &(1, 4), &(1, 5)]);
+    /// ```
+    ///
+    /// The elements can be floats but no particular result is guaranteed
+    /// if an element is NaN.
+    #[cfg(feature = "use_std")]
+    fn max_set_by_key<K, F>(self, key: F) -> Vec<Self::Item>
+        where Self: Sized, K: Ord, F: FnMut(&Self::Item) -> K
+    {
+        extrema_set::max_set_impl(self, key, |_, _, kx, ky| kx.cmp(ky))
+    }
+
+    /// Return the minimum and maximum elements in the iterator.
+    ///
+    /// The return type `MinMaxResult` is an enum of three variants:
+    ///
+    /// - `NoElements` if the iterator is empty.
+    /// - `OneElement(x)` if the iterator has exactly one element.
+    /// - `MinMax(x, y)` is returned otherwise, where `x <= y`. Two
+    ///    values are equal if and only if there is more than one
+    ///    element in the iterator and all elements are equal.
+    ///
+    /// On an iterator of length `n`, `minmax` does `1.5 * n` comparisons,
+    /// and so is faster than calling `min` and `max` separately which does
+    /// `2 * n` comparisons.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// use itertools::MinMaxResult::{NoElements, OneElement, MinMax};
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().minmax(), NoElements);
+    ///
+    /// let a = [1];
+    /// assert_eq!(a.iter().minmax(), OneElement(&1));
+    ///
+    /// let a = [1, 2, 3, 4, 5];
+    /// assert_eq!(a.iter().minmax(), MinMax(&1, &5));
+    ///
+    /// let a = [1, 1, 1, 1];
+    /// assert_eq!(a.iter().minmax(), MinMax(&1, &1));
+    /// ```
+    ///
+    /// The elements can be floats but no particular result is guaranteed
+    /// if an element is NaN.
+    fn minmax(self) -> MinMaxResult<Self::Item>
+        where Self: Sized, Self::Item: PartialOrd
+    {
+        minmax::minmax_impl(self, |_| (), |x, y, _, _| x < y)
+    }
+
+    /// Return the minimum and maximum element of an iterator, as determined by
+    /// the specified function.
+    ///
+    /// The return value is a variant of [`MinMaxResult`] like for [`.minmax()`](Itertools::minmax).
+    ///
+    /// For the minimum, the first minimal element is returned.  For the maximum,
+    /// the last maximal element wins.  This matches the behavior of the standard
+    /// [`Iterator::min`] and [`Iterator::max`] methods.
+    ///
+    /// The keys can be floats but no particular result is guaranteed
+    /// if a key is NaN.
+    fn minmax_by_key<K, F>(self, key: F) -> MinMaxResult<Self::Item>
+        where Self: Sized, K: PartialOrd, F: FnMut(&Self::Item) -> K
+    {
+        minmax::minmax_impl(self, key, |_, _, xk, yk| xk < yk)
+    }
+
+    /// Return the minimum and maximum element of an iterator, as determined by
+    /// the specified comparison function.
+    ///
+    /// The return value is a variant of [`MinMaxResult`] like for [`.minmax()`](Itertools::minmax).
+    ///
+    /// For the minimum, the first minimal element is returned.  For the maximum,
+    /// the last maximal element wins.  This matches the behavior of the standard
+    /// [`Iterator::min`] and [`Iterator::max`] methods.
+    fn minmax_by<F>(self, mut compare: F) -> MinMaxResult<Self::Item>
+        where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering
+    {
+        minmax::minmax_impl(
+            self,
+            |_| (),
+            |x, y, _, _| Ordering::Less == compare(x, y)
+        )
+    }
+
+    /// Return the position of the maximum element in the iterator.
+    ///
+    /// If several elements are equally maximum, the position of the
+    /// last of them is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().position_max(), None);
+    ///
+    /// let a = [-3, 0, 1, 5, -10];
+    /// assert_eq!(a.iter().position_max(), Some(3));
+    ///
+    /// let a = [1, 1, -1, -1];
+    /// assert_eq!(a.iter().position_max(), Some(1));
+    /// ```
+    fn position_max(self) -> Option<usize>
+        where Self: Sized, Self::Item: Ord
+    {
+        self.enumerate()
+            .max_by(|x, y| Ord::cmp(&x.1, &y.1))
+            .map(|x| x.0)
+    }
+
+    /// Return the position of the maximum element in the iterator, as
+    /// determined by the specified function.
+    ///
+    /// If several elements are equally maximum, the position of the
+    /// last of them is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().position_max_by_key(|x| x.abs()), None);
+    ///
+    /// let a = [-3_i32, 0, 1, 5, -10];
+    /// assert_eq!(a.iter().position_max_by_key(|x| x.abs()), Some(4));
+    ///
+    /// let a = [1_i32, 1, -1, -1];
+    /// assert_eq!(a.iter().position_max_by_key(|x| x.abs()), Some(3));
+    /// ```
+    fn position_max_by_key<K, F>(self, mut key: F) -> Option<usize>
+        where Self: Sized, K: Ord, F: FnMut(&Self::Item) -> K
+    {
+        self.enumerate()
+            .max_by(|x, y| Ord::cmp(&key(&x.1), &key(&y.1)))
+            .map(|x| x.0)
+    }
+
+    /// Return the position of the maximum element in the iterator, as
+    /// determined by the specified comparison function.
+    ///
+    /// If several elements are equally maximum, the position of the
+    /// last of them is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().position_max_by(|x, y| x.cmp(y)), None);
+    ///
+    /// let a = [-3_i32, 0, 1, 5, -10];
+    /// assert_eq!(a.iter().position_max_by(|x, y| x.cmp(y)), Some(3));
+    ///
+    /// let a = [1_i32, 1, -1, -1];
+    /// assert_eq!(a.iter().position_max_by(|x, y| x.cmp(y)), Some(1));
+    /// ```
+    fn position_max_by<F>(self, mut compare: F) -> Option<usize>
+        where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering
+    {
+        self.enumerate()
+            .max_by(|x, y| compare(&x.1, &y.1))
+            .map(|x| x.0)
+    }
+
+    /// Return the position of the minimum element in the iterator.
+    ///
+    /// If several elements are equally minimum, the position of the
+    /// first of them is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().position_min(), None);
+    ///
+    /// let a = [-3, 0, 1, 5, -10];
+    /// assert_eq!(a.iter().position_min(), Some(4));
+    ///
+    /// let a = [1, 1, -1, -1];
+    /// assert_eq!(a.iter().position_min(), Some(2));
+    /// ```
+    fn position_min(self) -> Option<usize>
+        where Self: Sized, Self::Item: Ord
+    {
+        self.enumerate()
+            .min_by(|x, y| Ord::cmp(&x.1, &y.1))
+            .map(|x| x.0)
+    }
+
+    /// Return the position of the minimum element in the iterator, as
+    /// determined by the specified function.
+    ///
+    /// If several elements are equally minimum, the position of the
+    /// first of them is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().position_min_by_key(|x| x.abs()), None);
+    ///
+    /// let a = [-3_i32, 0, 1, 5, -10];
+    /// assert_eq!(a.iter().position_min_by_key(|x| x.abs()), Some(1));
+    ///
+    /// let a = [1_i32, 1, -1, -1];
+    /// assert_eq!(a.iter().position_min_by_key(|x| x.abs()), Some(0));
+    /// ```
+    fn position_min_by_key<K, F>(self, mut key: F) -> Option<usize>
+        where Self: Sized, K: Ord, F: FnMut(&Self::Item) -> K
+    {
+        self.enumerate()
+            .min_by(|x, y| Ord::cmp(&key(&x.1), &key(&y.1)))
+            .map(|x| x.0)
+    }
+
+    /// Return the position of the minimum element in the iterator, as
+    /// determined by the specified comparison function.
+    ///
+    /// If several elements are equally minimum, the position of the
+    /// first of them is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().position_min_by(|x, y| x.cmp(y)), None);
+    ///
+    /// let a = [-3_i32, 0, 1, 5, -10];
+    /// assert_eq!(a.iter().position_min_by(|x, y| x.cmp(y)), Some(4));
+    ///
+    /// let a = [1_i32, 1, -1, -1];
+    /// assert_eq!(a.iter().position_min_by(|x, y| x.cmp(y)), Some(2));
+    /// ```
+    fn position_min_by<F>(self, mut compare: F) -> Option<usize>
+        where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering
+    {
+        self.enumerate()
+            .min_by(|x, y| compare(&x.1, &y.1))
+            .map(|x| x.0)
+    }
+
+    /// Return the positions of the minimum and maximum elements in
+    /// the iterator.
+    ///
+    /// The return type [`MinMaxResult`] is an enum of three variants:
+    ///
+    /// - `NoElements` if the iterator is empty.
+    /// - `OneElement(xpos)` if the iterator has exactly one element.
+    /// - `MinMax(xpos, ypos)` is returned otherwise, where the
+    ///    element at `xpos` ≤ the element at `ypos`. While the
+    ///    referenced elements themselves may be equal, `xpos` cannot
+    ///    be equal to `ypos`.
+    ///
+    /// On an iterator of length `n`, `position_minmax` does `1.5 * n`
+    /// comparisons, and so is faster than calling `position_min` and
+    /// `position_max` separately which does `2 * n` comparisons.
+    ///
+    /// For the minimum, if several elements are equally minimum, the
+    /// position of the first of them is returned. For the maximum, if
+    /// several elements are equally maximum, the position of the last
+    /// of them is returned.
+    ///
+    /// The elements can be floats but no particular result is
+    /// guaranteed if an element is NaN.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// use itertools::MinMaxResult::{NoElements, OneElement, MinMax};
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().position_minmax(), NoElements);
+    ///
+    /// let a = [10];
+    /// assert_eq!(a.iter().position_minmax(), OneElement(0));
+    ///
+    /// let a = [-3, 0, 1, 5, -10];
+    /// assert_eq!(a.iter().position_minmax(), MinMax(4, 3));
+    ///
+    /// let a = [1, 1, -1, -1];
+    /// assert_eq!(a.iter().position_minmax(), MinMax(2, 1));
+    /// ```
+    fn position_minmax(self) -> MinMaxResult<usize>
+        where Self: Sized, Self::Item: PartialOrd
+    {
+        use crate::MinMaxResult::{NoElements, OneElement, MinMax};
+        match minmax::minmax_impl(self.enumerate(), |_| (), |x, y, _, _| x.1 < y.1) {
+            NoElements => NoElements,
+            OneElement(x) => OneElement(x.0),
+            MinMax(x, y) => MinMax(x.0, y.0),
+        }
+    }
+
+    /// Return the postions of the minimum and maximum elements of an
+    /// iterator, as determined by the specified function.
+    ///
+    /// The return value is a variant of [`MinMaxResult`] like for
+    /// [`position_minmax`].
+    ///
+    /// For the minimum, if several elements are equally minimum, the
+    /// position of the first of them is returned. For the maximum, if
+    /// several elements are equally maximum, the position of the last
+    /// of them is returned.
+    ///
+    /// The keys can be floats but no particular result is guaranteed
+    /// if a key is NaN.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// use itertools::MinMaxResult::{NoElements, OneElement, MinMax};
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().position_minmax_by_key(|x| x.abs()), NoElements);
+    ///
+    /// let a = [10_i32];
+    /// assert_eq!(a.iter().position_minmax_by_key(|x| x.abs()), OneElement(0));
+    ///
+    /// let a = [-3_i32, 0, 1, 5, -10];
+    /// assert_eq!(a.iter().position_minmax_by_key(|x| x.abs()), MinMax(1, 4));
+    ///
+    /// let a = [1_i32, 1, -1, -1];
+    /// assert_eq!(a.iter().position_minmax_by_key(|x| x.abs()), MinMax(0, 3));
+    /// ```
+    ///
+    /// [`position_minmax`]: Self::position_minmax
+    fn position_minmax_by_key<K, F>(self, mut key: F) -> MinMaxResult<usize>
+        where Self: Sized, K: PartialOrd, F: FnMut(&Self::Item) -> K
+    {
+        use crate::MinMaxResult::{NoElements, OneElement, MinMax};
+        match self.enumerate().minmax_by_key(|e| key(&e.1)) {
+            NoElements => NoElements,
+            OneElement(x) => OneElement(x.0),
+            MinMax(x, y) => MinMax(x.0, y.0),
+        }
+    }
+
+    /// Return the postions of the minimum and maximum elements of an
+    /// iterator, as determined by the specified comparison function.
+    ///
+    /// The return value is a variant of [`MinMaxResult`] like for
+    /// [`position_minmax`].
+    ///
+    /// For the minimum, if several elements are equally minimum, the
+    /// position of the first of them is returned. For the maximum, if
+    /// several elements are equally maximum, the position of the last
+    /// of them is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    /// use itertools::MinMaxResult::{NoElements, OneElement, MinMax};
+    ///
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.iter().position_minmax_by(|x, y| x.cmp(y)), NoElements);
+    ///
+    /// let a = [10_i32];
+    /// assert_eq!(a.iter().position_minmax_by(|x, y| x.cmp(y)), OneElement(0));
+    ///
+    /// let a = [-3_i32, 0, 1, 5, -10];
+    /// assert_eq!(a.iter().position_minmax_by(|x, y| x.cmp(y)), MinMax(4, 3));
+    ///
+    /// let a = [1_i32, 1, -1, -1];
+    /// assert_eq!(a.iter().position_minmax_by(|x, y| x.cmp(y)), MinMax(2, 1));
+    /// ```
+    ///
+    /// [`position_minmax`]: Self::position_minmax
+    fn position_minmax_by<F>(self, mut compare: F) -> MinMaxResult<usize>
+        where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering
+    {
+        use crate::MinMaxResult::{NoElements, OneElement, MinMax};
+        match self.enumerate().minmax_by(|x, y| compare(&x.1, &y.1)) {
+            NoElements => NoElements,
+            OneElement(x) => OneElement(x.0),
+            MinMax(x, y) => MinMax(x.0, y.0),
+        }
+    }
+
+    /// If the iterator yields exactly one element, that element will be returned, otherwise
+    /// an error will be returned containing an iterator that has the same output as the input
+    /// iterator.
+    ///
+    /// This provides an additional layer of validation over just calling `Iterator::next()`.
+    /// If your assumption that there should only be one element yielded is false this provides
+    /// the opportunity to detect and handle that, preventing errors at a distance.
+    ///
+    /// # Examples
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// assert_eq!((0..10).filter(|&x| x == 2).exactly_one().unwrap(), 2);
+    /// assert!((0..10).filter(|&x| x > 1 && x < 4).exactly_one().unwrap_err().eq(2..4));
+    /// assert!((0..10).filter(|&x| x > 1 && x < 5).exactly_one().unwrap_err().eq(2..5));
+    /// assert!((0..10).filter(|&_| false).exactly_one().unwrap_err().eq(0..0));
+    /// ```
+    fn exactly_one(mut self) -> Result<Self::Item, ExactlyOneError<Self>>
+    where
+        Self: Sized,
+    {
+        match self.next() {
+            Some(first) => {
+                match self.next() {
+                    Some(second) => {
+                        Err(ExactlyOneError::new(Some(Either::Left([first, second])), self))
+                    }
+                    None => {
+                        Ok(first)
+                    }
+                }
+            }
+            None => Err(ExactlyOneError::new(None, self)),
+        }
+    }
+
+    /// If the iterator yields no elements, Ok(None) will be returned. If the iterator yields
+    /// exactly one element, that element will be returned, otherwise an error will be returned
+    /// containing an iterator that has the same output as the input iterator.
+    ///
+    /// This provides an additional layer of validation over just calling `Iterator::next()`.
+    /// If your assumption that there should be at most one element yielded is false this provides
+    /// the opportunity to detect and handle that, preventing errors at a distance.
+    ///
+    /// # Examples
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// assert_eq!((0..10).filter(|&x| x == 2).at_most_one().unwrap(), Some(2));
+    /// assert!((0..10).filter(|&x| x > 1 && x < 4).at_most_one().unwrap_err().eq(2..4));
+    /// assert!((0..10).filter(|&x| x > 1 && x < 5).at_most_one().unwrap_err().eq(2..5));
+    /// assert_eq!((0..10).filter(|&_| false).at_most_one().unwrap(), None);
+    /// ```
+    fn at_most_one(mut self) -> Result<Option<Self::Item>, ExactlyOneError<Self>>
+    where
+        Self: Sized,
+    {
+        match self.next() {
+            Some(first) => {
+                match self.next() {
+                    Some(second) => {
+                        Err(ExactlyOneError::new(Some(Either::Left([first, second])), self))
+                    }
+                    None => {
+                        Ok(Some(first))
+                    }
+                }
+            }
+            None => Ok(None),
+        }
+    }
+
+    /// An iterator adaptor that allows the user to peek at multiple `.next()`
+    /// values without advancing the base iterator.
+    ///
+    /// # Examples
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let mut iter = (0..10).multipeek();
+    /// assert_eq!(iter.peek(), Some(&0));
+    /// assert_eq!(iter.peek(), Some(&1));
+    /// assert_eq!(iter.peek(), Some(&2));
+    /// assert_eq!(iter.next(), Some(0));
+    /// assert_eq!(iter.peek(), Some(&1));
+    /// ```
+    #[cfg(feature = "use_alloc")]
+    fn multipeek(self) -> MultiPeek<Self>
+    where
+        Self: Sized,
+    {
+        multipeek_impl::multipeek(self)
+    }
+
+    /// Collect the items in this iterator and return a `HashMap` which
+    /// contains each item that appears in the iterator and the number
+    /// of times it appears.
+    ///
+    /// # Examples
+    /// ```
+    /// # use itertools::Itertools;
+    /// let counts = [1, 1, 1, 3, 3, 5].into_iter().counts();
+    /// assert_eq!(counts[&1], 3);
+    /// assert_eq!(counts[&3], 2);
+    /// assert_eq!(counts[&5], 1);
+    /// assert_eq!(counts.get(&0), None);
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn counts(self) -> HashMap<Self::Item, usize>
+    where
+        Self: Sized,
+        Self::Item: Eq + Hash,
+    {
+        let mut counts = HashMap::new();
+        self.for_each(|item| *counts.entry(item).or_default() += 1);
+        counts
+    }
+
+    /// Collect the items in this iterator and return a `HashMap` which
+    /// contains each item that appears in the iterator and the number
+    /// of times it appears,
+    /// determining identity using a keying function.
+    ///
+    /// ```
+    /// # use itertools::Itertools;
+    /// struct Character {
+    ///   first_name: &'static str,
+    ///   last_name:  &'static str,
+    /// }
+    /// 
+    /// let characters =
+    ///     vec![
+    ///         Character { first_name: "Amy",   last_name: "Pond"      },
+    ///         Character { first_name: "Amy",   last_name: "Wong"      },
+    ///         Character { first_name: "Amy",   last_name: "Santiago"  },
+    ///         Character { first_name: "James", last_name: "Bond"      },
+    ///         Character { first_name: "James", last_name: "Sullivan"  },
+    ///         Character { first_name: "James", last_name: "Norington" },
+    ///         Character { first_name: "James", last_name: "Kirk"      },
+    ///     ];
+    /// 
+    /// let first_name_frequency = 
+    ///     characters
+    ///         .into_iter()
+    ///         .counts_by(|c| c.first_name);
+    ///     
+    /// assert_eq!(first_name_frequency["Amy"], 3);
+    /// assert_eq!(first_name_frequency["James"], 4);
+    /// assert_eq!(first_name_frequency.contains_key("Asha"), false);
+    /// ```
+    #[cfg(feature = "use_std")]
+    fn counts_by<K, F>(self, f: F) -> HashMap<K, usize>
+    where
+        Self: Sized,
+        K: Eq + Hash,
+        F: FnMut(Self::Item) -> K,
+    {
+        self.map(f).counts()
+    }
+
+    /// Converts an iterator of tuples into a tuple of containers.
+    ///
+    /// `unzip()` consumes an entire iterator of n-ary tuples, producing `n` collections, one for each
+    /// column.
+    ///
+    /// This function is, in some sense, the opposite of [`multizip`].
+    /// 
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// let inputs = vec![(1, 2, 3), (4, 5, 6), (7, 8, 9)];
+    ///
+    /// let (a, b, c): (Vec<_>, Vec<_>, Vec<_>) = inputs
+    ///     .into_iter()
+    ///     .multiunzip();
+    ///
+    /// assert_eq!(a, vec![1, 4, 7]);
+    /// assert_eq!(b, vec![2, 5, 8]);
+    /// assert_eq!(c, vec![3, 6, 9]);
+    /// ```
+    fn multiunzip<FromI>(self) -> FromI
+    where
+        Self: Sized + MultiUnzip<FromI>,
+    {
+        MultiUnzip::multiunzip(self)
+    }
+}
+
+impl<T: ?Sized> Itertools for T where T: Iterator { }
+
+/// Return `true` if both iterables produce equal sequences
+/// (elements pairwise equal and sequences of the same length),
+/// `false` otherwise.
+///
+/// [`IntoIterator`] enabled version of [`Iterator::eq`].
+///
+/// ```
+/// assert!(itertools::equal(vec![1, 2, 3], 1..4));
+/// assert!(!itertools::equal(&[0, 0], &[0, 0, 0]));
+/// ```
+pub fn equal<I, J>(a: I, b: J) -> bool
+    where I: IntoIterator,
+          J: IntoIterator,
+          I::Item: PartialEq<J::Item>
+{
+    a.into_iter().eq(b)
+}
+
+/// Assert that two iterables produce equal sequences, with the same
+/// semantics as [`equal(a, b)`](equal).
+///
+/// **Panics** on assertion failure with a message that shows the
+/// two iteration elements.
+///
+/// ```ignore
+/// assert_equal("exceed".split('c'), "excess".split('c'));
+/// // ^PANIC: panicked at 'Failed assertion Some("eed") == Some("ess") for iteration 1',
+/// ```
+pub fn assert_equal<I, J>(a: I, b: J)
+    where I: IntoIterator,
+          J: IntoIterator,
+          I::Item: fmt::Debug + PartialEq<J::Item>,
+          J::Item: fmt::Debug,
+{
+    let mut ia = a.into_iter();
+    let mut ib = b.into_iter();
+    let mut i = 0;
+    loop {
+        match (ia.next(), ib.next()) {
+            (None, None) => return,
+            (a, b) => {
+                let equal = match (&a, &b) {
+                    (&Some(ref a), &Some(ref b)) => a == b,
+                    _ => false,
+                };
+                assert!(equal, "Failed assertion {a:?} == {b:?} for iteration {i}",
+                        i=i, a=a, b=b);
+                i += 1;
+            }
+        }
+    }
+}
+
+/// Partition a sequence using predicate `pred` so that elements
+/// that map to `true` are placed before elements which map to `false`.
+///
+/// The order within the partitions is arbitrary.
+///
+/// Return the index of the split point.
+///
+/// ```
+/// use itertools::partition;
+///
+/// # // use repeated numbers to not promise any ordering
+/// let mut data = [7, 1, 1, 7, 1, 1, 7];
+/// let split_index = partition(&mut data, |elt| *elt >= 3);
+///
+/// assert_eq!(data, [7, 7, 7, 1, 1, 1, 1]);
+/// assert_eq!(split_index, 3);
+/// ```
+pub fn partition<'a, A: 'a, I, F>(iter: I, mut pred: F) -> usize
+    where I: IntoIterator<Item = &'a mut A>,
+          I::IntoIter: DoubleEndedIterator,
+          F: FnMut(&A) -> bool
+{
+    let mut split_index = 0;
+    let mut iter = iter.into_iter();
+    'main: while let Some(front) = iter.next() {
+        if !pred(front) {
+            loop {
+                match iter.next_back() {
+                    Some(back) => if pred(back) {
+                        std::mem::swap(front, back);
+                        break;
+                    },
+                    None => break 'main,
+                }
+            }
+        }
+        split_index += 1;
+    }
+    split_index
+}
+
+/// An enum used for controlling the execution of `fold_while`.
+///
+/// See [`.fold_while()`](Itertools::fold_while) for more information.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum FoldWhile<T> {
+    /// Continue folding with this value
+    Continue(T),
+    /// Fold is complete and will return this value
+    Done(T),
+}
+
+impl<T> FoldWhile<T> {
+    /// Return the value in the continue or done.
+    pub fn into_inner(self) -> T {
+        match self {
+            FoldWhile::Continue(x) | FoldWhile::Done(x) => x,
+        }
+    }
+
+    /// Return true if `self` is `Done`, false if it is `Continue`.
+    pub fn is_done(&self) -> bool {
+        match *self {
+            FoldWhile::Continue(_) => false,
+            FoldWhile::Done(_) => true,
+        }
+    }
+}