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    // Copyright 2013-2016 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use cmp::Ordering;
    use ops::Try;

    use super::LoopState;
    use super::{Chain, Cycle, Cloned, Enumerate, Filter, FilterMap, Fuse};
    use super::{Flatten, FlatMap, flatten_compat};
    use super::{Inspect, Map, Peekable, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile, Rev};
    use super::{Zip, Sum, Product};
    use super::{ChainState, FromIterator, ZipImpl};

    fn _assert_is_object_safe(_: &dyn Iterator<Item=()>) {}

    /// An interface for dealing with iterators.
    ///
    /// This is the main iterator trait. For more about the concept of iterators
    /// generally, please see the [module-level documentation]. In particular, you
    /// may want to know how to [implement `Iterator`][impl].
    ///
    /// [module-level documentation]: index.html
    /// [impl]: index.html#implementing-iterator
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_on_unimplemented(
        on(
            _Self="&str",
            label="`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
        ),
        label="`{Self}` is not an iterator; maybe try calling `.iter()` or a similar method"
    )]
    #[doc(spotlight)]
    pub trait Iterator {
        /// The type of the elements being iterated over.
        #[stable(feature = "rust1", since = "1.0.0")]
        type Item;

        /// Advances the iterator and returns the next value.
        ///
        /// Returns [`None`] when iteration is finished. Individual iterator
        /// implementations may choose to resume iteration, and so calling `next()`
        /// again may or may not eventually start returning [`Some(Item)`] again at some
        /// point.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        /// [`Some(Item)`]: ../../std/option/enum.Option.html#variant.Some
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter();
        ///
        /// // A call to next() returns the next value...
        /// assert_eq!(Some(&1), iter.next());
        /// assert_eq!(Some(&2), iter.next());
        /// assert_eq!(Some(&3), iter.next());
        ///
        /// // ... and then None once it's over.
        /// assert_eq!(None, iter.next());
        ///
        /// // More calls may or may not return None. Here, they always will.
        /// assert_eq!(None, iter.next());
        /// assert_eq!(None, iter.next());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn next(&mut self) -> Option<Self::Item>;

        /// Returns the bounds on the remaining length of the iterator.
        ///
        /// Specifically, `size_hint()` returns a tuple where the first element
        /// is the lower bound, and the second element is the upper bound.
        ///
        /// The second half of the tuple that is returned is an [`Option`]`<`[`usize`]`>`.
        /// A [`None`] here means that either there is no known upper bound, or the
        /// upper bound is larger than [`usize`].
        ///
        /// # Implementation notes
        ///
        /// It is not enforced that an iterator implementation yields the declared
        /// number of elements. A buggy iterator may yield less than the lower bound
        /// or more than the upper bound of elements.
        ///
        /// `size_hint()` is primarily intended to be used for optimizations such as
        /// reserving space for the elements of the iterator, but must not be
        /// trusted to e.g. omit bounds checks in unsafe code. An incorrect
        /// implementation of `size_hint()` should not lead to memory safety
        /// violations.
        ///
        /// That said, the implementation should provide a correct estimation,
        /// because otherwise it would be a violation of the trait's protocol.
        ///
        /// The default implementation returns `(0, None)` which is correct for any
        /// iterator.
        ///
        /// [`usize`]: ../../std/primitive.usize.html
        /// [`Option`]: ../../std/option/enum.Option.html
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        /// let iter = a.iter();
        ///
        /// assert_eq!((3, Some(3)), iter.size_hint());
        /// ```
        ///
        /// A more complex example:
        ///
        /// ```
        /// // The even numbers from zero to ten.
        /// let iter = (0..10).filter(|x| x % 2 == 0);
        ///
        /// // We might iterate from zero to ten times. Knowing that it's five
        /// // exactly wouldn't be possible without executing filter().
        /// assert_eq!((0, Some(10)), iter.size_hint());
        ///
        /// // Let's add five more numbers with chain()
        /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
        ///
        /// // now both bounds are increased by five
        /// assert_eq!((5, Some(15)), iter.size_hint());
        /// ```
        ///
        /// Returning `None` for an upper bound:
        ///
        /// ```
        /// // an infinite iterator has no upper bound
        /// // and the maximum possible lower bound
        /// let iter = 0..;
        ///
        /// assert_eq!((usize::max_value(), None), iter.size_hint());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn size_hint(&self) -> (usize, Option<usize>) { (0, None) }

        /// Consumes the iterator, counting the number of iterations and returning it.
        ///
        /// This method will evaluate the iterator until its [`next`] returns
        /// [`None`]. Once [`None`] is encountered, `count()` returns the number of
        /// times it called [`next`].
        ///
        /// [`next`]: #tymethod.next
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Overflow Behavior
        ///
        /// The method does no guarding against overflows, so counting elements of
        /// an iterator with more than [`usize::MAX`] elements either produces the
        /// wrong result or panics. If debug assertions are enabled, a panic is
        /// guaranteed.
        ///
        /// # Panics
        ///
        /// This function might panic if the iterator has more than [`usize::MAX`]
        /// elements.
        ///
        /// [`usize::MAX`]: ../../std/usize/constant.MAX.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        /// assert_eq!(a.iter().count(), 3);
        ///
        /// let a = [1, 2, 3, 4, 5];
        /// assert_eq!(a.iter().count(), 5);
        /// ```
        #[inline]
        #[rustc_inherit_overflow_checks]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn count(self) -> usize where Self: Sized {
            // Might overflow.
            self.fold(0, |cnt, _| cnt + 1)
        }

        /// Consumes the iterator, returning the last element.
        ///
        /// This method will evaluate the iterator until it returns [`None`]. While
        /// doing so, it keeps track of the current element. After [`None`] is
        /// returned, `last()` will then return the last element it saw.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        /// assert_eq!(a.iter().last(), Some(&3));
        ///
        /// let a = [1, 2, 3, 4, 5];
        /// assert_eq!(a.iter().last(), Some(&5));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn last(self) -> Option<Self::Item> where Self: Sized {
            let mut last = None;
            for x in self { last = Some(x); }
            last
        }

        /// Returns the `n`th element of the iterator.
        ///
        /// Like most indexing operations, the count starts from zero, so `nth(0)`
        /// returns the first value, `nth(1)` the second, and so on.
        ///
        /// Note that all preceding elements, as well as the returned element, will be
        /// consumed from the iterator. That means that the preceding elements will be
        /// discarded, and also that calling `nth(0)` multiple times on the same iterator
        /// will return different elements.
        ///
        /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
        /// iterator.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        /// assert_eq!(a.iter().nth(1), Some(&2));
        /// ```
        ///
        /// Calling `nth()` multiple times doesn't rewind the iterator:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter();
        ///
        /// assert_eq!(iter.nth(1), Some(&2));
        /// assert_eq!(iter.nth(1), None);
        /// ```
        ///
        /// Returning `None` if there are less than `n + 1` elements:
        ///
        /// ```
        /// let a = [1, 2, 3];
        /// assert_eq!(a.iter().nth(10), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
            for x in self {
                if n == 0 { return Some(x) }
                n -= 1;
            }
            None
        }

        /// Creates an iterator starting at the same point, but stepping by
        /// the given amount at each iteration.
        ///
        /// Note 1: The first element of the iterator will always be returned,
        /// regardless of the step given.
        ///
        /// Note 2: The time at which ignored elements are pulled is not fixed.
        /// `StepBy` behaves like the sequence `next(), nth(step-1), nth(step-1), …`,
        /// but is also free to behave like the sequence
        /// `advance_n_and_return_first(step), advance_n_and_return_first(step), …`
        /// Which way is used may change for some iterators for performance reasons.
        /// The second way will advance the iterator earlier and may consume more items.
        ///
        /// `advance_n_and_return_first` is the equivalent of:
        /// ```
        /// fn advance_n_and_return_first<I>(iter: &mut I, total_step: usize) -> Option<I::Item>
        /// where
        ///     I: Iterator,
        /// {
        ///     let next = iter.next();
        ///     if total_step > 1 {
        ///         iter.nth(total_step-2);
        ///     }
        ///     next
        /// }
        /// ```
        ///
        /// # Panics
        ///
        /// The method will panic if the given step is `0`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [0, 1, 2, 3, 4, 5];
        /// let mut iter = a.into_iter().step_by(2);
        ///
        /// assert_eq!(iter.next(), Some(&0));
        /// assert_eq!(iter.next(), Some(&2));
        /// assert_eq!(iter.next(), Some(&4));
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "iterator_step_by", since = "1.28.0")]
        fn step_by(self, step: usize) -> StepBy<Self> where Self: Sized {
            assert!(step != 0);
            StepBy{iter: self, step: step - 1, first_take: true}
        }

        /// Takes two iterators and creates a new iterator over both in sequence.
        ///
        /// `chain()` will return a new iterator which will first iterate over
        /// values from the first iterator and then over values from the second
        /// iterator.
        ///
        /// In other words, it links two iterators together, in a chain. 🔗
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a1 = [1, 2, 3];
        /// let a2 = [4, 5, 6];
        ///
        /// let mut iter = a1.iter().chain(a2.iter());
        ///
        /// assert_eq!(iter.next(), Some(&1));
        /// assert_eq!(iter.next(), Some(&2));
        /// assert_eq!(iter.next(), Some(&3));
        /// assert_eq!(iter.next(), Some(&4));
        /// assert_eq!(iter.next(), Some(&5));
        /// assert_eq!(iter.next(), Some(&6));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
        /// anything that can be converted into an [`Iterator`], not just an
        /// [`Iterator`] itself. For example, slices (`&[T]`) implement
        /// [`IntoIterator`], and so can be passed to `chain()` directly:
        ///
        /// [`IntoIterator`]: trait.IntoIterator.html
        /// [`Iterator`]: trait.Iterator.html
        ///
        /// ```
        /// let s1 = &[1, 2, 3];
        /// let s2 = &[4, 5, 6];
        ///
        /// let mut iter = s1.iter().chain(s2);
        ///
        /// assert_eq!(iter.next(), Some(&1));
        /// assert_eq!(iter.next(), Some(&2));
        /// assert_eq!(iter.next(), Some(&3));
        /// assert_eq!(iter.next(), Some(&4));
        /// assert_eq!(iter.next(), Some(&5));
        /// assert_eq!(iter.next(), Some(&6));
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter> where
            Self: Sized, U: IntoIterator<Item=Self::Item>,
        {
            Chain{a: self, b: other.into_iter(), state: ChainState::Both}
        }

        /// 'Zips up' two iterators into a single iterator of pairs.
        ///
        /// `zip()` returns a new iterator that will iterate over two other
        /// iterators, returning a tuple where the first element comes from the
        /// first iterator, and the second element comes from the second iterator.
        ///
        /// In other words, it zips two iterators together, into a single one.
        ///
        /// If either iterator returns [`None`], [`next`] from the zipped iterator
        /// will return [`None`]. If the first iterator returns [`None`], `zip` will
        /// short-circuit and `next` will not be called on the second iterator.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a1 = [1, 2, 3];
        /// let a2 = [4, 5, 6];
        ///
        /// let mut iter = a1.iter().zip(a2.iter());
        ///
        /// assert_eq!(iter.next(), Some((&1, &4)));
        /// assert_eq!(iter.next(), Some((&2, &5)));
        /// assert_eq!(iter.next(), Some((&3, &6)));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
        /// anything that can be converted into an [`Iterator`], not just an
        /// [`Iterator`] itself. For example, slices (`&[T]`) implement
        /// [`IntoIterator`], and so can be passed to `zip()` directly:
        ///
        /// [`IntoIterator`]: trait.IntoIterator.html
        /// [`Iterator`]: trait.Iterator.html
        ///
        /// ```
        /// let s1 = &[1, 2, 3];
        /// let s2 = &[4, 5, 6];
        ///
        /// let mut iter = s1.iter().zip(s2);
        ///
        /// assert_eq!(iter.next(), Some((&1, &4)));
        /// assert_eq!(iter.next(), Some((&2, &5)));
        /// assert_eq!(iter.next(), Some((&3, &6)));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// `zip()` is often used to zip an infinite iterator to a finite one.
        /// This works because the finite iterator will eventually return [`None`],
        /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
        ///
        /// ```
        /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
        ///
        /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
        ///
        /// assert_eq!((0, 'f'), enumerate[0]);
        /// assert_eq!((0, 'f'), zipper[0]);
        ///
        /// assert_eq!((1, 'o'), enumerate[1]);
        /// assert_eq!((1, 'o'), zipper[1]);
        ///
        /// assert_eq!((2, 'o'), enumerate[2]);
        /// assert_eq!((2, 'o'), zipper[2]);
        /// ```
        ///
        /// [`enumerate`]: trait.Iterator.html#method.enumerate
        /// [`next`]: ../../std/iter/trait.Iterator.html#tymethod.next
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter> where
            Self: Sized, U: IntoIterator
        {
            Zip::new(self, other.into_iter())
        }

        /// Takes a closure and creates an iterator which calls that closure on each
        /// element.
        ///
        /// `map()` transforms one iterator into another, by means of its argument:
        /// something that implements `FnMut`. It produces a new iterator which
        /// calls this closure on each element of the original iterator.
        ///
        /// If you are good at thinking in types, you can think of `map()` like this:
        /// If you have an iterator that gives you elements of some type `A`, and
        /// you want an iterator of some other type `B`, you can use `map()`,
        /// passing a closure that takes an `A` and returns a `B`.
        ///
        /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
        /// lazy, it is best used when you're already working with other iterators.
        /// If you're doing some sort of looping for a side effect, it's considered
        /// more idiomatic to use [`for`] than `map()`.
        ///
        /// [`for`]: ../../book/first-edition/loops.html#for
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.into_iter().map(|x| 2 * x);
        ///
        /// assert_eq!(iter.next(), Some(2));
        /// assert_eq!(iter.next(), Some(4));
        /// assert_eq!(iter.next(), Some(6));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
        ///
        /// ```
        /// # #![allow(unused_must_use)]
        /// // don't do this:
        /// (0..5).map(|x| println!("{}", x));
        ///
        /// // it won't even execute, as it is lazy. Rust will warn you about this.
        ///
        /// // Instead, use for:
        /// for x in 0..5 {
        ///     println!("{}", x);
        /// }
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn map<B, F>(self, f: F) -> Map<Self, F> where
            Self: Sized, F: FnMut(Self::Item) -> B,
        {
            Map{iter: self, f: f}
        }

        /// Calls a closure on each element of an iterator.
        ///
        /// This is equivalent to using a [`for`] loop on the iterator, although
        /// `break` and `continue` are not possible from a closure.  It's generally
        /// more idiomatic to use a `for` loop, but `for_each` may be more legible
        /// when processing items at the end of longer iterator chains.  In some
        /// cases `for_each` may also be faster than a loop, because it will use
        /// internal iteration on adaptors like `Chain`.
        ///
        /// [`for`]: ../../book/first-edition/loops.html#for
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::sync::mpsc::channel;
        ///
        /// let (tx, rx) = channel();
        /// (0..5).map(|x| x * 2 + 1)
        ///       .for_each(move |x| tx.send(x).unwrap());
        ///
        /// let v: Vec<_> =  rx.iter().collect();
        /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
        /// ```
        ///
        /// For such a small example, a `for` loop may be cleaner, but `for_each`
        /// might be preferable to keep a functional style with longer iterators:
        ///
        /// ```
        /// (0..5).flat_map(|x| x * 100 .. x * 110)
        ///       .enumerate()
        ///       .filter(|&(i, x)| (i + x) % 3 == 0)
        ///       .for_each(|(i, x)| println!("{}:{}", i, x));
        /// ```
        #[inline]
        #[stable(feature = "iterator_for_each", since = "1.21.0")]
        fn for_each<F>(self, mut f: F) where
            Self: Sized, F: FnMut(Self::Item),
        {
            self.fold((), move |(), item| f(item));
        }

        /// Creates an iterator which uses a closure to determine if an element
        /// should be yielded.
        ///
        /// The closure must return `true` or `false`. `filter()` creates an
        /// iterator which calls this closure on each element. If the closure
        /// returns `true`, then the element is returned. If the closure returns
        /// `false`, it will try again, and call the closure on the next element,
        /// seeing if it passes the test.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [0i32, 1, 2];
        ///
        /// let mut iter = a.into_iter().filter(|x| x.is_positive());
        ///
        /// assert_eq!(iter.next(), Some(&1));
        /// assert_eq!(iter.next(), Some(&2));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// Because the closure passed to `filter()` takes a reference, and many
        /// iterators iterate over references, this leads to a possibly confusing
        /// situation, where the type of the closure is a double reference:
        ///
        /// ```
        /// let a = [0, 1, 2];
        ///
        /// let mut iter = a.into_iter().filter(|x| **x > 1); // need two *s!
        ///
        /// assert_eq!(iter.next(), Some(&2));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// It's common to instead use destructuring on the argument to strip away
        /// one:
        ///
        /// ```
        /// let a = [0, 1, 2];
        ///
        /// let mut iter = a.into_iter().filter(|&x| *x > 1); // both & and *
        ///
        /// assert_eq!(iter.next(), Some(&2));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// or both:
        ///
        /// ```
        /// let a = [0, 1, 2];
        ///
        /// let mut iter = a.into_iter().filter(|&&x| x > 1); // two &s
        ///
        /// assert_eq!(iter.next(), Some(&2));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// of these layers.
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn filter<P>(self, predicate: P) -> Filter<Self, P> where
            Self: Sized, P: FnMut(&Self::Item) -> bool,
        {
            Filter{iter: self, predicate: predicate}
        }

        /// Creates an iterator that both filters and maps.
        ///
        /// The closure must return an [`Option<T>`]. `filter_map` creates an
        /// iterator which calls this closure on each element. If the closure
        /// returns [`Some(element)`][`Some`], then that element is returned. If the
        /// closure returns [`None`], it will try again, and call the closure on the
        /// next element, seeing if it will return [`Some`].
        ///
        /// Why `filter_map` and not just [`filter`] and [`map`]? The key is in this
        /// part:
        ///
        /// [`filter`]: #method.filter
        /// [`map`]: #method.map
        ///
        /// > If the closure returns [`Some(element)`][`Some`], then that element is returned.
        ///
        /// In other words, it removes the [`Option<T>`] layer automatically. If your
        /// mapping is already returning an [`Option<T>`] and you want to skip over
        /// [`None`]s, then `filter_map` is much, much nicer to use.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = ["1", "lol", "3", "NaN", "5"];
        ///
        /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
        ///
        /// assert_eq!(iter.next(), Some(1));
        /// assert_eq!(iter.next(), Some(3));
        /// assert_eq!(iter.next(), Some(5));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// Here's the same example, but with [`filter`] and [`map`]:
        ///
        /// ```
        /// let a = ["1", "lol", "3", "NaN", "5"];
        /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
        /// assert_eq!(iter.next(), Some(1));
        /// assert_eq!(iter.next(), Some(3));
        /// assert_eq!(iter.next(), Some(5));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// [`Option<T>`]: ../../std/option/enum.Option.html
        /// [`Some`]: ../../std/option/enum.Option.html#variant.Some
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F> where
            Self: Sized, F: FnMut(Self::Item) -> Option<B>,
        {
            FilterMap { iter: self, f: f }
        }

        /// Creates an iterator which gives the current iteration count as well as
        /// the next value.
        ///
        /// The iterator returned yields pairs `(i, val)`, where `i` is the
        /// current index of iteration and `val` is the value returned by the
        /// iterator.
        ///
        /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
        /// different sized integer, the [`zip`] function provides similar
        /// functionality.
        ///
        /// # Overflow Behavior
        ///
        /// The method does no guarding against overflows, so enumerating more than
        /// [`usize::MAX`] elements either produces the wrong result or panics. If
        /// debug assertions are enabled, a panic is guaranteed.
        ///
        /// # Panics
        ///
        /// The returned iterator might panic if the to-be-returned index would
        /// overflow a [`usize`].
        ///
        /// [`usize::MAX`]: ../../std/usize/constant.MAX.html
        /// [`usize`]: ../../std/primitive.usize.html
        /// [`zip`]: #method.zip
        ///
        /// # Examples
        ///
        /// ```
        /// let a = ['a', 'b', 'c'];
        ///
        /// let mut iter = a.iter().enumerate();
        ///
        /// assert_eq!(iter.next(), Some((0, &'a')));
        /// assert_eq!(iter.next(), Some((1, &'b')));
        /// assert_eq!(iter.next(), Some((2, &'c')));
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn enumerate(self) -> Enumerate<Self> where Self: Sized {
            Enumerate { iter: self, count: 0 }
        }

        /// Creates an iterator which can use `peek` to look at the next element of
        /// the iterator without consuming it.
        ///
        /// Adds a [`peek`] method to an iterator. See its documentation for
        /// more information.
        ///
        /// Note that the underlying iterator is still advanced when [`peek`] is
        /// called for the first time: In order to retrieve the next element,
        /// [`next`] is called on the underlying iterator, hence any side effects (i.e.
        /// anything other than fetching the next value) of the [`next`] method
        /// will occur.
        ///
        /// [`peek`]: struct.Peekable.html#method.peek
        /// [`next`]: ../../std/iter/trait.Iterator.html#tymethod.next
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let xs = [1, 2, 3];
        ///
        /// let mut iter = xs.iter().peekable();
        ///
        /// // peek() lets us see into the future
        /// assert_eq!(iter.peek(), Some(&&1));
        /// assert_eq!(iter.next(), Some(&1));
        ///
        /// assert_eq!(iter.next(), Some(&2));
        ///
        /// // we can peek() multiple times, the iterator won't advance
        /// assert_eq!(iter.peek(), Some(&&3));
        /// assert_eq!(iter.peek(), Some(&&3));
        ///
        /// assert_eq!(iter.next(), Some(&3));
        ///
        /// // after the iterator is finished, so is peek()
        /// assert_eq!(iter.peek(), None);
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn peekable(self) -> Peekable<Self> where Self: Sized {
            Peekable{iter: self, peeked: None}
        }

        /// Creates an iterator that [`skip`]s elements based on a predicate.
        ///
        /// [`skip`]: #method.skip
        ///
        /// `skip_while()` takes a closure as an argument. It will call this
        /// closure on each element of the iterator, and ignore elements
        /// until it returns `false`.
        ///
        /// After `false` is returned, `skip_while()`'s job is over, and the
        /// rest of the elements are yielded.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [-1i32, 0, 1];
        ///
        /// let mut iter = a.into_iter().skip_while(|x| x.is_negative());
        ///
        /// assert_eq!(iter.next(), Some(&0));
        /// assert_eq!(iter.next(), Some(&1));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// Because the closure passed to `skip_while()` takes a reference, and many
        /// iterators iterate over references, this leads to a possibly confusing
        /// situation, where the type of the closure is a double reference:
        ///
        /// ```
        /// let a = [-1, 0, 1];
        ///
        /// let mut iter = a.into_iter().skip_while(|x| **x < 0); // need two *s!
        ///
        /// assert_eq!(iter.next(), Some(&0));
        /// assert_eq!(iter.next(), Some(&1));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// Stopping after an initial `false`:
        ///
        /// ```
        /// let a = [-1, 0, 1, -2];
        ///
        /// let mut iter = a.into_iter().skip_while(|x| **x < 0);
        ///
        /// assert_eq!(iter.next(), Some(&0));
        /// assert_eq!(iter.next(), Some(&1));
        ///
        /// // while this would have been false, since we already got a false,
        /// // skip_while() isn't used any more
        /// assert_eq!(iter.next(), Some(&-2));
        ///
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P> where
            Self: Sized, P: FnMut(&Self::Item) -> bool,
        {
            SkipWhile{iter: self, flag: false, predicate: predicate}
        }

        /// Creates an iterator that yields elements based on a predicate.
        ///
        /// `take_while()` takes a closure as an argument. It will call this
        /// closure on each element of the iterator, and yield elements
        /// while it returns `true`.
        ///
        /// After `false` is returned, `take_while()`'s job is over, and the
        /// rest of the elements are ignored.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [-1i32, 0, 1];
        ///
        /// let mut iter = a.into_iter().take_while(|x| x.is_negative());
        ///
        /// assert_eq!(iter.next(), Some(&-1));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// Because the closure passed to `take_while()` takes a reference, and many
        /// iterators iterate over references, this leads to a possibly confusing
        /// situation, where the type of the closure is a double reference:
        ///
        /// ```
        /// let a = [-1, 0, 1];
        ///
        /// let mut iter = a.into_iter().take_while(|x| **x < 0); // need two *s!
        ///
        /// assert_eq!(iter.next(), Some(&-1));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// Stopping after an initial `false`:
        ///
        /// ```
        /// let a = [-1, 0, 1, -2];
        ///
        /// let mut iter = a.into_iter().take_while(|x| **x < 0);
        ///
        /// assert_eq!(iter.next(), Some(&-1));
        ///
        /// // We have more elements that are less than zero, but since we already
        /// // got a false, take_while() isn't used any more
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// Because `take_while()` needs to look at the value in order to see if it
        /// should be included or not, consuming iterators will see that it is
        /// removed:
        ///
        /// ```
        /// let a = [1, 2, 3, 4];
        /// let mut iter = a.into_iter();
        ///
        /// let result: Vec<i32> = iter.by_ref()
        ///                            .take_while(|n| **n != 3)
        ///                            .cloned()
        ///                            .collect();
        ///
        /// assert_eq!(result, &[1, 2]);
        ///
        /// let result: Vec<i32> = iter.cloned().collect();
        ///
        /// assert_eq!(result, &[4]);
        /// ```
        ///
        /// The `3` is no longer there, because it was consumed in order to see if
        /// the iteration should stop, but wasn't placed back into the iterator or
        /// some similar thing.
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P> where
            Self: Sized, P: FnMut(&Self::Item) -> bool,
        {
            TakeWhile{iter: self, flag: false, predicate: predicate}
        }

        /// Creates an iterator that skips the first `n` elements.
        ///
        /// After they have been consumed, the rest of the elements are yielded.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter().skip(2);
        ///
        /// assert_eq!(iter.next(), Some(&3));
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn skip(self, n: usize) -> Skip<Self> where Self: Sized {
            Skip{iter: self, n: n}
        }

        /// Creates an iterator that yields its first `n` elements.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter().take(2);
        ///
        /// assert_eq!(iter.next(), Some(&1));
        /// assert_eq!(iter.next(), Some(&2));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// `take()` is often used with an infinite iterator, to make it finite:
        ///
        /// ```
        /// let mut iter = (0..).take(3);
        ///
        /// assert_eq!(iter.next(), Some(0));
        /// assert_eq!(iter.next(), Some(1));
        /// assert_eq!(iter.next(), Some(2));
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn take(self, n: usize) -> Take<Self> where Self: Sized, {
            Take{iter: self, n: n}
        }

        /// An iterator adaptor similar to [`fold`] that holds internal state and
        /// produces a new iterator.
        ///
        /// [`fold`]: #method.fold
        ///
        /// `scan()` takes two arguments: an initial value which seeds the internal
        /// state, and a closure with two arguments, the first being a mutable
        /// reference to the internal state and the second an iterator element.
        /// The closure can assign to the internal state to share state between
        /// iterations.
        ///
        /// On iteration, the closure will be applied to each element of the
        /// iterator and the return value from the closure, an [`Option`], is
        /// yielded by the iterator.
        ///
        /// [`Option`]: ../../std/option/enum.Option.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter().scan(1, |state, &x| {
        ///     // each iteration, we'll multiply the state by the element
        ///     *state = *state * x;
        ///
        ///     // then, we'll yield the negation of the state
        ///     Some(-*state)
        /// });
        ///
        /// assert_eq!(iter.next(), Some(-1));
        /// assert_eq!(iter.next(), Some(-2));
        /// assert_eq!(iter.next(), Some(-6));
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
            where Self: Sized, F: FnMut(&mut St, Self::Item) -> Option<B>,
        {
            Scan{iter: self, f: f, state: initial_state}
        }

        /// Creates an iterator that works like map, but flattens nested structure.
        ///
        /// The [`map`] adapter is very useful, but only when the closure
        /// argument produces values. If it produces an iterator instead, there's
        /// an extra layer of indirection. `flat_map()` will remove this extra layer
        /// on its own.
        ///
        /// You can think of `flat_map(f)` as the semantic equivalent
        /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
        ///
        /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
        /// one item for each element, and `flat_map()`'s closure returns an
        /// iterator for each element.
        ///
        /// [`map`]: #method.map
        /// [`flatten`]: #method.flatten
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let words = ["alpha", "beta", "gamma"];
        ///
        /// // chars() returns an iterator
        /// let merged: String = words.iter()
        ///                           .flat_map(|s| s.chars())
        ///                           .collect();
        /// assert_eq!(merged, "alphabetagamma");
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
            where Self: Sized, U: IntoIterator, F: FnMut(Self::Item) -> U,
        {
            FlatMap { inner: flatten_compat(self.map(f)) }
        }

        /// Creates an iterator that flattens nested structure.
        ///
        /// This is useful when you have an iterator of iterators or an iterator of
        /// things that can be turned into iterators and you want to remove one
        /// level of indirection.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
        /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
        /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
        /// ```
        ///
        /// Mapping and then flattening:
        ///
        /// ```
        /// let words = ["alpha", "beta", "gamma"];
        ///
        /// // chars() returns an iterator
        /// let merged: String = words.iter()
        ///                           .map(|s| s.chars())
        ///                           .flatten()
        ///                           .collect();
        /// assert_eq!(merged, "alphabetagamma");
        /// ```
        ///
        /// You can also rewrite this in terms of [`flat_map()`], which is preferable
        /// in this case since it conveys intent more clearly:
        ///
        /// ```
        /// let words = ["alpha", "beta", "gamma"];
        ///
        /// // chars() returns an iterator
        /// let merged: String = words.iter()
        ///                           .flat_map(|s| s.chars())
        ///                           .collect();
        /// assert_eq!(merged, "alphabetagamma");
        /// ```
        ///
        /// Flattening once only removes one level of nesting:
        ///
        /// ```
        /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
        ///
        /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
        /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
        ///
        /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
        /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
        /// ```
        ///
        /// Here we see that `flatten()` does not perform a "deep" flatten.
        /// Instead, only one level of nesting is removed. That is, if you
        /// `flatten()` a three-dimensional array the result will be
        /// two-dimensional and not one-dimensional. To get a one-dimensional
        /// structure, you have to `flatten()` again.
        ///
        /// [`flat_map()`]: #method.flat_map
        #[inline]
        #[stable(feature = "iterator_flatten", since = "1.29")]
        fn flatten(self) -> Flatten<Self>
        where Self: Sized, Self::Item: IntoIterator {
            Flatten { inner: flatten_compat(self) }
        }

        /// Creates an iterator which ends after the first [`None`].
        ///
        /// After an iterator returns [`None`], future calls may or may not yield
        /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
        /// [`None`] is given, it will always return [`None`] forever.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        /// [`Some(T)`]: ../../std/option/enum.Option.html#variant.Some
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // an iterator which alternates between Some and None
        /// struct Alternate {
        ///     state: i32,
        /// }
        ///
        /// impl Iterator for Alternate {
        ///     type Item = i32;
        ///
        ///     fn next(&mut self) -> Option<i32> {
        ///         let val = self.state;
        ///         self.state = self.state + 1;
        ///
        ///         // if it's even, Some(i32), else None
        ///         if val % 2 == 0 {
        ///             Some(val)
        ///         } else {
        ///             None
        ///         }
        ///     }
        /// }
        ///
        /// let mut iter = Alternate { state: 0 };
        ///
        /// // we can see our iterator going back and forth
        /// assert_eq!(iter.next(), Some(0));
        /// assert_eq!(iter.next(), None);
        /// assert_eq!(iter.next(), Some(2));
        /// assert_eq!(iter.next(), None);
        ///
        /// // however, once we fuse it...
        /// let mut iter = iter.fuse();
        ///
        /// assert_eq!(iter.next(), Some(4));
        /// assert_eq!(iter.next(), None);
        ///
        /// // it will always return None after the first time.
        /// assert_eq!(iter.next(), None);
        /// assert_eq!(iter.next(), None);
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fuse(self) -> Fuse<Self> where Self: Sized {
            Fuse{iter: self, done: false}
        }

        /// Do something with each element of an iterator, passing the value on.
        ///
        /// When using iterators, you'll often chain several of them together.
        /// While working on such code, you might want to check out what's
        /// happening at various parts in the pipeline. To do that, insert
        /// a call to `inspect()`.
        ///
        /// It's more common for `inspect()` to be used as a debugging tool than to
        /// exist in your final code, but applications may find it useful in certain
        /// situations when errors need to be logged before being discarded.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 4, 2, 3];
        ///
        /// // this iterator sequence is complex.
        /// let sum = a.iter()
        ///     .cloned()
        ///     .filter(|x| x % 2 == 0)
        ///     .fold(0, |sum, i| sum + i);
        ///
        /// println!("{}", sum);
        ///
        /// // let's add some inspect() calls to investigate what's happening
        /// let sum = a.iter()
        ///     .cloned()
        ///     .inspect(|x| println!("about to filter: {}", x))
        ///     .filter(|x| x % 2 == 0)
        ///     .inspect(|x| println!("made it through filter: {}", x))
        ///     .fold(0, |sum, i| sum + i);
        ///
        /// println!("{}", sum);
        /// ```
        ///
        /// This will print:
        ///
        /// ```text
        /// 6
        /// about to filter: 1
        /// about to filter: 4
        /// made it through filter: 4
        /// about to filter: 2
        /// made it through filter: 2
        /// about to filter: 3
        /// 6
        /// ```
        ///
        /// Logging errors before discarding them:
        ///
        /// ```
        /// let lines = ["1", "2", "a"];
        ///
        /// let sum: i32 = lines
        ///     .iter()
        ///     .map(|line| line.parse::<i32>())
        ///     .inspect(|num| {
        ///         if let Err(ref e) = *num {
        ///             println!("Parsing error: {}", e);
        ///         }
        ///     })
        ///     .filter_map(Result::ok)
        ///     .sum();
        ///
        /// println!("Sum: {}", sum);
        /// ```
        ///
        /// This will print:
        ///
        /// ```text
        /// Parsing error: invalid digit found in string
        /// Sum: 3
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn inspect<F>(self, f: F) -> Inspect<Self, F> where
            Self: Sized, F: FnMut(&Self::Item),
        {
            Inspect{iter: self, f: f}
        }

        /// Borrows an iterator, rather than consuming it.
        ///
        /// This is useful to allow applying iterator adaptors while still
        /// retaining ownership of the original iterator.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let iter = a.into_iter();
        ///
        /// let sum: i32 = iter.take(5).fold(0, |acc, i| acc + i );
        ///
        /// assert_eq!(sum, 6);
        ///
        /// // if we try to use iter again, it won't work. The following line
        /// // gives "error: use of moved value: `iter`
        /// // assert_eq!(iter.next(), None);
        ///
        /// // let's try that again
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.into_iter();
        ///
        /// // instead, we add in a .by_ref()
        /// let sum: i32 = iter.by_ref().take(2).fold(0, |acc, i| acc + i );
        ///
        /// assert_eq!(sum, 3);
        ///
        /// // now this is just fine:
        /// assert_eq!(iter.next(), Some(&3));
        /// assert_eq!(iter.next(), None);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn by_ref(&mut self) -> &mut Self where Self: Sized { self }

        /// Transforms an iterator into a collection.
        ///
        /// `collect()` can take anything iterable, and turn it into a relevant
        /// collection. This is one of the more powerful methods in the standard
        /// library, used in a variety of contexts.
        ///
        /// The most basic pattern in which `collect()` is used is to turn one
        /// collection into another. You take a collection, call [`iter`] on it,
        /// do a bunch of transformations, and then `collect()` at the end.
        ///
        /// One of the keys to `collect()`'s power is that many things you might
        /// not think of as 'collections' actually are. For example, a [`String`]
        /// is a collection of [`char`]s. And a collection of
        /// [`Result<T, E>`][`Result`] can be thought of as single
        /// [`Result`]`<Collection<T>, E>`. See the examples below for more.
        ///
        /// Because `collect()` is so general, it can cause problems with type
        /// inference. As such, `collect()` is one of the few times you'll see
        /// the syntax affectionately known as the 'turbofish': `::<>`. This
        /// helps the inference algorithm understand specifically which collection
        /// you're trying to collect into.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let doubled: Vec<i32> = a.iter()
        ///                          .map(|&x| x * 2)
        ///                          .collect();
        ///
        /// assert_eq!(vec![2, 4, 6], doubled);
        /// ```
        ///
        /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
        /// we could collect into, for example, a [`VecDeque<T>`] instead:
        ///
        /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
        ///
        /// ```
        /// use std::collections::VecDeque;
        ///
        /// let a = [1, 2, 3];
        ///
        /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
        ///
        /// assert_eq!(2, doubled[0]);
        /// assert_eq!(4, doubled[1]);
        /// assert_eq!(6, doubled[2]);
        /// ```
        ///
        /// Using the 'turbofish' instead of annotating `doubled`:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
        ///
        /// assert_eq!(vec![2, 4, 6], doubled);
        /// ```
        ///
        /// Because `collect()` only cares about what you're collecting into, you can
        /// still use a partial type hint, `_`, with the turbofish:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
        ///
        /// assert_eq!(vec![2, 4, 6], doubled);
        /// ```
        ///
        /// Using `collect()` to make a [`String`]:
        ///
        /// ```
        /// let chars = ['g', 'd', 'k', 'k', 'n'];
        ///
        /// let hello: String = chars.iter()
        ///     .map(|&x| x as u8)
        ///     .map(|x| (x + 1) as char)
        ///     .collect();
        ///
        /// assert_eq!("hello", hello);
        /// ```
        ///
        /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
        /// see if any of them failed:
        ///
        /// ```
        /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
        ///
        /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
        ///
        /// // gives us the first error
        /// assert_eq!(Err("nope"), result);
        ///
        /// let results = [Ok(1), Ok(3)];
        ///
        /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
        ///
        /// // gives us the list of answers
        /// assert_eq!(Ok(vec![1, 3]), result);
        /// ```
        ///
        /// [`iter`]: ../../std/iter/trait.Iterator.html#tymethod.next
        /// [`String`]: ../../std/string/struct.String.html
        /// [`char`]: ../../std/primitive.char.html
        /// [`Result`]: ../../std/result/enum.Result.html
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
        fn collect<B: FromIterator<Self::Item>>(self) -> B where Self: Sized {
            FromIterator::from_iter(self)
        }

        /// Consumes an iterator, creating two collections from it.
        ///
        /// The predicate passed to `partition()` can return `true`, or `false`.
        /// `partition()` returns a pair, all of the elements for which it returned
        /// `true`, and all of the elements for which it returned `false`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let (even, odd): (Vec<i32>, Vec<i32>) = a
        ///     .into_iter()
        ///     .partition(|&n| n % 2 == 0);
        ///
        /// assert_eq!(even, vec![2]);
        /// assert_eq!(odd, vec![1, 3]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn partition<B, F>(self, mut f: F) -> (B, B) where
            Self: Sized,
            B: Default + Extend<Self::Item>,
            F: FnMut(&Self::Item) -> bool
        {
            let mut left: B = Default::default();
            let mut right: B = Default::default();

            for x in self {
                if f(&x) {
                    left.extend(Some(x))
                } else {
                    right.extend(Some(x))
                }
            }

            (left, right)
        }

        /// An iterator method that applies a function as long as it returns
        /// successfully, producing a single, final value.
        ///
        /// `try_fold()` takes two arguments: an initial value, and a closure with
        /// two arguments: an 'accumulator', and an element. The closure either
        /// returns successfully, with the value that the accumulator should have
        /// for the next iteration, or it returns failure, with an error value that
        /// is propagated back to the caller immediately (short-circuiting).
        ///
        /// The initial value is the value the accumulator will have on the first
        /// call.  If applying the closure succeeded against every element of the
        /// iterator, `try_fold()` returns the final accumulator as success.
        ///
        /// Folding is useful whenever you have a collection of something, and want
        /// to produce a single value from it.
        ///
        /// # Note to Implementors
        ///
        /// Most of the other (forward) methods have default implementations in
        /// terms of this one, so try to implement this explicitly if it can
        /// do something better than the default `for` loop implementation.
        ///
        /// In particular, try to have this call `try_fold()` on the internal parts
        /// from which this iterator is composed.  If multiple calls are needed,
        /// the `?` operator may be convenient for chaining the accumulator value
        /// along, but beware any invariants that need to be upheld before those
        /// early returns.  This is a `&mut self` method, so iteration needs to be
        /// resumable after hitting an error here.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// // the checked sum of all of the elements of the array
        /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
        ///
        /// assert_eq!(sum, Some(6));
        /// ```
        ///
        /// Short-circuiting:
        ///
        /// ```
        /// let a = [10, 20, 30, 100, 40, 50];
        /// let mut it = a.iter();
        ///
        /// // This sum overflows when adding the 100 element
        /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
        /// assert_eq!(sum, None);
        ///
        /// // Because it short-circuited, the remaining elements are still
        /// // available through the iterator.
        /// assert_eq!(it.len(), 2);
        /// assert_eq!(it.next(), Some(&40));
        /// ```
        #[inline]
        #[stable(feature = "iterator_try_fold", since = "1.27.0")]
        fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R where
            Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
        {
            let mut accum = init;
            while let Some(x) = self.next() {
                accum = f(accum, x)?;
            }
            Try::from_ok(accum)
        }

        /// An iterator method that applies a fallible function to each item in the
        /// iterator, stopping at the first error and returning that error.
        ///
        /// This can also be thought of as the fallible form of [`for_each()`]
        /// or as the stateless version of [`try_fold()`].
        ///
        /// [`for_each()`]: #method.for_each
        /// [`try_fold()`]: #method.try_fold
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fs::rename;
        /// use std::io::{stdout, Write};
        /// use std::path::Path;
        ///
        /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
        ///
        /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
        /// assert!(res.is_ok());
        ///
        /// let mut it = data.iter().cloned();
        /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
        /// assert!(res.is_err());
        /// // It short-circuited, so the remaining items are still in the iterator:
        /// assert_eq!(it.next(), Some("stale_bread.json"));
        /// ```
        #[inline]
        #[stable(feature = "iterator_try_fold", since = "1.27.0")]
        fn try_for_each<F, R>(&mut self, mut f: F) -> R where
            Self: Sized, F: FnMut(Self::Item) -> R, R: Try<Ok=()>
        {
            self.try_fold((), move |(), x| f(x))
        }

        /// An iterator method that applies a function, producing a single, final value.
        ///
        /// `fold()` takes two arguments: an initial value, and a closure with two
        /// arguments: an 'accumulator', and an element. The closure returns the value that
        /// the accumulator should have for the next iteration.
        ///
        /// The initial value is the value the accumulator will have on the first
        /// call.
        ///
        /// After applying this closure to every element of the iterator, `fold()`
        /// returns the accumulator.
        ///
        /// This operation is sometimes called 'reduce' or 'inject'.
        ///
        /// Folding is useful whenever you have a collection of something, and want
        /// to produce a single value from it.
        ///
        /// Note: `fold()`, and similar methods that traverse the entire iterator,
        /// may not terminate for infinite iterators, even on traits for which a
        /// result is determinable in finite time.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// // the sum of all of the elements of the array
        /// let sum = a.iter().fold(0, |acc, x| acc + x);
        ///
        /// assert_eq!(sum, 6);
        /// ```
        ///
        /// Let's walk through each step of the iteration here:
        ///
        /// | element | acc | x | result |
        /// |---------|-----|---|--------|
        /// |         | 0   |   |        |
        /// | 1       | 0   | 1 | 1      |
        /// | 2       | 1   | 2 | 3      |
        /// | 3       | 3   | 3 | 6      |
        ///
        /// And so, our final result, `6`.
        ///
        /// It's common for people who haven't used iterators a lot to
        /// use a `for` loop with a list of things to build up a result. Those
        /// can be turned into `fold()`s:
        ///
        /// [`for`]: ../../book/first-edition/loops.html#for
        ///
        /// ```
        /// let numbers = [1, 2, 3, 4, 5];
        ///
        /// let mut result = 0;
        ///
        /// // for loop:
        /// for i in &numbers {
        ///     result = result + i;
        /// }
        ///
        /// // fold:
        /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
        ///
        /// // they're the same
        /// assert_eq!(result, result2);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fold<B, F>(mut self, init: B, mut f: F) -> B where
            Self: Sized, F: FnMut(B, Self::Item) -> B,
        {
            self.try_fold(init, move |acc, x| Ok::<B, !>(f(acc, x))).unwrap()
        }

        /// Tests if every element of the iterator matches a predicate.
        ///
        /// `all()` takes a closure that returns `true` or `false`. It applies
        /// this closure to each element of the iterator, and if they all return
        /// `true`, then so does `all()`. If any of them return `false`, it
        /// returns `false`.
        ///
        /// `all()` is short-circuiting; in other words, it will stop processing
        /// as soon as it finds a `false`, given that no matter what else happens,
        /// the result will also be `false`.
        ///
        /// An empty iterator returns `true`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// assert!(a.iter().all(|&x| x > 0));
        ///
        /// assert!(!a.iter().all(|&x| x > 2));
        /// ```
        ///
        /// Stopping at the first `false`:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter();
        ///
        /// assert!(!iter.all(|&x| x != 2));
        ///
        /// // we can still use `iter`, as there are more elements.
        /// assert_eq!(iter.next(), Some(&3));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn all<F>(&mut self, mut f: F) -> bool where
            Self: Sized, F: FnMut(Self::Item) -> bool
        {
            self.try_for_each(move |x| {
                if f(x) { LoopState::Continue(()) }
                else { LoopState::Break(()) }
            }) == LoopState::Continue(())
        }

        /// Tests if any element of the iterator matches a predicate.
        ///
        /// `any()` takes a closure that returns `true` or `false`. It applies
        /// this closure to each element of the iterator, and if any of them return
        /// `true`, then so does `any()`. If they all return `false`, it
        /// returns `false`.
        ///
        /// `any()` is short-circuiting; in other words, it will stop processing
        /// as soon as it finds a `true`, given that no matter what else happens,
        /// the result will also be `true`.
        ///
        /// An empty iterator returns `false`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// assert!(a.iter().any(|&x| x > 0));
        ///
        /// assert!(!a.iter().any(|&x| x > 5));
        /// ```
        ///
        /// Stopping at the first `true`:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter();
        ///
        /// assert!(iter.any(|&x| x != 2));
        ///
        /// // we can still use `iter`, as there are more elements.
        /// assert_eq!(iter.next(), Some(&2));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn any<F>(&mut self, mut f: F) -> bool where
            Self: Sized,
            F: FnMut(Self::Item) -> bool
        {
            self.try_for_each(move |x| {
                if f(x) { LoopState::Break(()) }
                else { LoopState::Continue(()) }
            }) == LoopState::Break(())
        }

        /// Searches for an element of an iterator that satisfies a predicate.
        ///
        /// `find()` takes a closure that returns `true` or `false`. It applies
        /// this closure to each element of the iterator, and if any of them return
        /// `true`, then `find()` returns [`Some(element)`]. If they all return
        /// `false`, it returns [`None`].
        ///
        /// `find()` is short-circuiting; in other words, it will stop processing
        /// as soon as the closure returns `true`.
        ///
        /// Because `find()` takes a reference, and many iterators iterate over
        /// references, this leads to a possibly confusing situation where the
        /// argument is a double reference. You can see this effect in the
        /// examples below, with `&&x`.
        ///
        /// [`Some(element)`]: ../../std/option/enum.Option.html#variant.Some
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
        ///
        /// assert_eq!(a.iter().find(|&&x| x == 5), None);
        /// ```
        ///
        /// Stopping at the first `true`:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter();
        ///
        /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
        ///
        /// // we can still use `iter`, as there are more elements.
        /// assert_eq!(iter.next(), Some(&3));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item> where
            Self: Sized,
            P: FnMut(&Self::Item) -> bool,
        {
            self.try_for_each(move |x| {
                if predicate(&x) { LoopState::Break(x) }
                else { LoopState::Continue(()) }
            }).break_value()
        }

        /// Applies function to the elements of iterator and returns
        /// the first non-none result.
        ///
        /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
        ///
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(iterator_find_map)]
        /// let a = ["lol", "NaN", "2", "5"];
        ///
        /// let mut first_number = a.iter().find_map(|s| s.parse().ok());
        ///
        /// assert_eq!(first_number, Some(2));
        /// ```
        #[inline]
        #[unstable(feature = "iterator_find_map",
                   reason = "unstable new API",
                   issue = "49602")]
        fn find_map<B, F>(&mut self, mut f: F) -> Option<B> where
            Self: Sized,
            F: FnMut(Self::Item) -> Option<B>,
        {
            self.try_for_each(move |x| {
                match f(x) {
                    Some(x) => LoopState::Break(x),
                    None => LoopState::Continue(()),
                }
            }).break_value()
        }

        /// Searches for an element in an iterator, returning its index.
        ///
        /// `position()` takes a closure that returns `true` or `false`. It applies
        /// this closure to each element of the iterator, and if one of them
        /// returns `true`, then `position()` returns [`Some(index)`]. If all of
        /// them return `false`, it returns [`None`].
        ///
        /// `position()` is short-circuiting; in other words, it will stop
        /// processing as soon as it finds a `true`.
        ///
        /// # Overflow Behavior
        ///
        /// The method does no guarding against overflows, so if there are more
        /// than [`usize::MAX`] non-matching elements, it either produces the wrong
        /// result or panics. If debug assertions are enabled, a panic is
        /// guaranteed.
        ///
        /// # Panics
        ///
        /// This function might panic if the iterator has more than `usize::MAX`
        /// non-matching elements.
        ///
        /// [`Some(index)`]: ../../std/option/enum.Option.html#variant.Some
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        /// [`usize::MAX`]: ../../std/usize/constant.MAX.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
        ///
        /// assert_eq!(a.iter().position(|&x| x == 5), None);
        /// ```
        ///
        /// Stopping at the first `true`:
        ///
        /// ```
        /// let a = [1, 2, 3, 4];
        ///
        /// let mut iter = a.iter();
        ///
        /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
        ///
        /// // we can still use `iter`, as there are more elements.
        /// assert_eq!(iter.next(), Some(&3));
        ///
        /// // The returned index depends on iterator state
        /// assert_eq!(iter.position(|&x| x == 4), Some(0));
        ///
        /// ```
        #[inline]
        #[rustc_inherit_overflow_checks]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn position<P>(&mut self, mut predicate: P) -> Option<usize> where
            Self: Sized,
            P: FnMut(Self::Item) -> bool,
        {
            // The addition might panic on overflow
            self.try_fold(0, move |i, x| {
                if predicate(x) { LoopState::Break(i) }
                else { LoopState::Continue(i + 1) }
            }).break_value()
        }

        /// Searches for an element in an iterator from the right, returning its
        /// index.
        ///
        /// `rposition()` takes a closure that returns `true` or `false`. It applies
        /// this closure to each element of the iterator, starting from the end,
        /// and if one of them returns `true`, then `rposition()` returns
        /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
        ///
        /// `rposition()` is short-circuiting; in other words, it will stop
        /// processing as soon as it finds a `true`.
        ///
        /// [`Some(index)`]: ../../std/option/enum.Option.html#variant.Some
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
        ///
        /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
        /// ```
        ///
        /// Stopping at the first `true`:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter();
        ///
        /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
        ///
        /// // we can still use `iter`, as there are more elements.
        /// assert_eq!(iter.next(), Some(&1));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn rposition<P>(&mut self, mut predicate: P) -> Option<usize> where
            P: FnMut(Self::Item) -> bool,
            Self: Sized + ExactSizeIterator + DoubleEndedIterator
        {
            // No need for an overflow check here, because `ExactSizeIterator`
            // implies that the number of elements fits into a `usize`.
            let n = self.len();
            self.try_rfold(n, move |i, x| {
                let i = i - 1;
                if predicate(x) { LoopState::Break(i) }
                else { LoopState::Continue(i) }
            }).break_value()
        }

        /// Returns the maximum element of an iterator.
        ///
        /// If several elements are equally maximum, the last element is
        /// returned. If the iterator is empty, [`None`] is returned.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        /// let b: Vec<u32> = Vec::new();
        ///
        /// assert_eq!(a.iter().max(), Some(&3));
        /// assert_eq!(b.iter().max(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn max(self) -> Option<Self::Item> where Self: Sized, Self::Item: Ord
        {
            select_fold1(self,
                         |_| (),
                         // switch to y even if it is only equal, to preserve
                         // stability.
                         |_, x, _, y| *x <= *y)
                .map(|(_, x)| x)
        }

        /// Returns the minimum element of an iterator.
        ///
        /// If several elements are equally minimum, the first element is
        /// returned. If the iterator is empty, [`None`] is returned.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        /// let b: Vec<u32> = Vec::new();
        ///
        /// assert_eq!(a.iter().min(), Some(&1));
        /// assert_eq!(b.iter().min(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn min(self) -> Option<Self::Item> where Self: Sized, Self::Item: Ord
        {
            select_fold1(self,
                         |_| (),
                         // only switch to y if it is strictly smaller, to
                         // preserve stability.
                         |_, x, _, y| *x > *y)
                .map(|(_, x)| x)
        }

        /// Returns the element that gives the maximum value from the
        /// specified function.
        ///
        /// If several elements are equally maximum, the last element is
        /// returned. If the iterator is empty, [`None`] is returned.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let a = [-3_i32, 0, 1, 5, -10];
        /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
        /// ```
        #[inline]
        #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
        fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
            where Self: Sized, F: FnMut(&Self::Item) -> B,
        {
            select_fold1(self,
                         f,
                         // switch to y even if it is only equal, to preserve
                         // stability.
                         |x_p, _, y_p, _| x_p <= y_p)
                .map(|(_, x)| x)
        }

        /// Returns the element that gives the maximum value with respect to the
        /// specified comparison function.
        ///
        /// If several elements are equally maximum, the last element is
        /// returned. If the iterator is empty, [`None`] is returned.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let a = [-3_i32, 0, 1, 5, -10];
        /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
        /// ```
        #[inline]
        #[stable(feature = "iter_max_by", since = "1.15.0")]
        fn max_by<F>(self, mut compare: F) -> Option<Self::Item>
            where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering,
        {
            select_fold1(self,
                         |_| (),
                         // switch to y even if it is only equal, to preserve
                         // stability.
                         |_, x, _, y| Ordering::Greater != compare(x, y))
                .map(|(_, x)| x)
        }

        /// Returns the element that gives the minimum value from the
        /// specified function.
        ///
        /// If several elements are equally minimum, the first element is
        /// returned. If the iterator is empty, [`None`] is returned.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let a = [-3_i32, 0, 1, 5, -10];
        /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
        /// ```
        #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
        fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
            where Self: Sized, F: FnMut(&Self::Item) -> B,
        {
            select_fold1(self,
                         f,
                         // only switch to y if it is strictly smaller, to
                         // preserve stability.
                         |x_p, _, y_p, _| x_p > y_p)
                .map(|(_, x)| x)
        }

        /// Returns the element that gives the minimum value with respect to the
        /// specified comparison function.
        ///
        /// If several elements are equally minimum, the first element is
        /// returned. If the iterator is empty, [`None`] is returned.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let a = [-3_i32, 0, 1, 5, -10];
        /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
        /// ```
        #[inline]
        #[stable(feature = "iter_min_by", since = "1.15.0")]
        fn min_by<F>(self, mut compare: F) -> Option<Self::Item>
            where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering,
        {
            select_fold1(self,
                         |_| (),
                         // switch to y even if it is strictly smaller, to
                         // preserve stability.
                         |_, x, _, y| Ordering::Greater == compare(x, y))
                .map(|(_, x)| x)
        }

        /// Reverses an iterator's direction.
        ///
        /// Usually, iterators iterate from left to right. After using `rev()`,
        /// an iterator will instead iterate from right to left.
        ///
        /// This is only possible if the iterator has an end, so `rev()` only
        /// works on [`DoubleEndedIterator`]s.
        ///
        /// [`DoubleEndedIterator`]: trait.DoubleEndedIterator.html
        ///
        /// # Examples
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter().rev();
        ///
        /// assert_eq!(iter.next(), Some(&3));
        /// assert_eq!(iter.next(), Some(&2));
        /// assert_eq!(iter.next(), Some(&1));
        ///
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn rev(self) -> Rev<Self> where Self: Sized + DoubleEndedIterator {
            Rev{iter: self}
        }

        /// Converts an iterator of pairs into a pair of containers.
        ///
        /// `unzip()` consumes an entire iterator of pairs, producing two
        /// collections: one from the left elements of the pairs, and one
        /// from the right elements.
        ///
        /// This function is, in some sense, the opposite of [`zip`].
        ///
        /// [`zip`]: #method.zip
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [(1, 2), (3, 4)];
        ///
        /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
        ///
        /// assert_eq!(left, [1, 3]);
        /// assert_eq!(right, [2, 4]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB) where
            FromA: Default + Extend<A>,
            FromB: Default + Extend<B>,
            Self: Sized + Iterator<Item=(A, B)>,
        {
            let mut ts: FromA = Default::default();
            let mut us: FromB = Default::default();

            self.for_each(|(t, u)| {
                ts.extend(Some(t));
                us.extend(Some(u));
            });

            (ts, us)
        }

        /// Creates an iterator which [`clone`]s all of its elements.
        ///
        /// This is useful when you have an iterator over `&T`, but you need an
        /// iterator over `T`.
        ///
        /// [`clone`]: ../../std/clone/trait.Clone.html#tymethod.clone
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let v_cloned: Vec<_> = a.iter().cloned().collect();
        ///
        /// // cloned is the same as .map(|&x| x), for integers
        /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
        ///
        /// assert_eq!(v_cloned, vec![1, 2, 3]);
        /// assert_eq!(v_map, vec![1, 2, 3]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn cloned<'a, T: 'a>(self) -> Cloned<Self>
            where Self: Sized + Iterator<Item=&'a T>, T: Clone
        {
            Cloned { it: self }
        }

        /// Repeats an iterator endlessly.
        ///
        /// Instead of stopping at [`None`], the iterator will instead start again,
        /// from the beginning. After iterating again, it will start at the
        /// beginning again. And again. And again. Forever.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut it = a.iter().cycle();
        ///
        /// assert_eq!(it.next(), Some(&1));
        /// assert_eq!(it.next(), Some(&2));
        /// assert_eq!(it.next(), Some(&3));
        /// assert_eq!(it.next(), Some(&1));
        /// assert_eq!(it.next(), Some(&2));
        /// assert_eq!(it.next(), Some(&3));
        /// assert_eq!(it.next(), Some(&1));
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        fn cycle(self) -> Cycle<Self> where Self: Sized + Clone {
            Cycle{orig: self.clone(), iter: self}
        }

        /// Sums the elements of an iterator.
        ///
        /// Takes each element, adds them together, and returns the result.
        ///
        /// An empty iterator returns the zero value of the type.
        ///
        /// # Panics
        ///
        /// When calling `sum()` and a primitive integer type is being returned, this
        /// method will panic if the computation overflows and debug assertions are
        /// enabled.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        /// let sum: i32 = a.iter().sum();
        ///
        /// assert_eq!(sum, 6);
        /// ```
        #[stable(feature = "iter_arith", since = "1.11.0")]
        fn sum<S>(self) -> S
            where Self: Sized,
                  S: Sum<Self::Item>,
        {
            Sum::sum(self)
        }

        /// Iterates over the entire iterator, multiplying all the elements
        ///
        /// An empty iterator returns the one value of the type.
        ///
        /// # Panics
        ///
        /// When calling `product()` and a primitive integer type is being returned,
        /// method will panic if the computation overflows and debug assertions are
        /// enabled.
        ///
        /// # Examples
        ///
        /// ```
        /// fn factorial(n: u32) -> u32 {
        ///     (1..).take_while(|&i| i <= n).product()
        /// }
        /// assert_eq!(factorial(0), 1);
        /// assert_eq!(factorial(1), 1);
        /// assert_eq!(factorial(5), 120);
        /// ```
        #[stable(feature = "iter_arith", since = "1.11.0")]
        fn product<P>(self) -> P
            where Self: Sized,
                  P: Product<Self::Item>,
        {
            Product::product(self)
        }

        /// Lexicographically compares the elements of this `Iterator` with those
        /// of another.
        #[stable(feature = "iter_order", since = "1.5.0")]
        fn cmp<I>(mut self, other: I) -> Ordering where
            I: IntoIterator<Item = Self::Item>,
            Self::Item: Ord,
            Self: Sized,
        {
            let mut other = other.into_iter();

            loop {
                let x = match self.next() {
                    None => if other.next().is_none() {
                        return Ordering::Equal
                    } else {
                        return Ordering::Less
                    },
                    Some(val) => val,
                };

                let y = match other.next() {
                    None => return Ordering::Greater,
                    Some(val) => val,
                };

                match x.cmp(&y) {
                    Ordering::Equal => (),
                    non_eq => return non_eq,
                }
            }
        }

        /// Lexicographically compares the elements of this `Iterator` with those
        /// of another.
        #[stable(feature = "iter_order", since = "1.5.0")]
        fn partial_cmp<I>(mut self, other: I) -> Option<Ordering> where
            I: IntoIterator,
            Self::Item: PartialOrd<I::Item>,
            Self: Sized,
        {
            let mut other = other.into_iter();

            loop {
                let x = match self.next() {
                    None => if other.next().is_none() {
                        return Some(Ordering::Equal)
                    } else {
                        return Some(Ordering::Less)
                    },
                    Some(val) => val,
                };

                let y = match other.next() {
                    None => return Some(Ordering::Greater),
                    Some(val) => val,
                };

                match x.partial_cmp(&y) {
                    Some(Ordering::Equal) => (),
                    non_eq => return non_eq,
                }
            }
        }

        /// Determines if the elements of this `Iterator` are equal to those of
        /// another.
        #[stable(feature = "iter_order", since = "1.5.0")]
        fn eq<I>(mut self, other: I) -> bool where
            I: IntoIterator,
            Self::Item: PartialEq<I::Item>,
            Self: Sized,
        {
            let mut other = other.into_iter();

            loop {
                let x = match self.next() {
                    None => return other.next().is_none(),
                    Some(val) => val,
                };

                let y = match other.next() {
                    None => return false,
                    Some(val) => val,
                };

                if x != y { return false }
            }
        }

        /// Determines if the elements of this `Iterator` are unequal to those of
        /// another.
        #[stable(feature = "iter_order", since = "1.5.0")]
        fn ne<I>(mut self, other: I) -> bool where
            I: IntoIterator,
            Self::Item: PartialEq<I::Item>,
            Self: Sized,
        {
            let mut other = other.into_iter();

            loop {
                let x = match self.next() {
                    None => return other.next().is_some(),
                    Some(val) => val,
                };

                let y = match other.next() {
                    None => return true,
                    Some(val) => val,
                };

                if x != y { return true }
            }
        }

        /// Determines if the elements of this `Iterator` are lexicographically
        /// less than those of another.
        #[stable(feature = "iter_order", since = "1.5.0")]
        fn lt<I>(mut self, other: I) -> bool where
            I: IntoIterator,
            Self::Item: PartialOrd<I::Item>,
            Self: Sized,
        {
            let mut other = other.into_iter();

            loop {
                let x = match self.next() {
                    None => return other.next().is_some(),
                    Some(val) => val,
                };

                let y = match other.next() {
                    None => return false,
                    Some(val) => val,
                };

                match x.partial_cmp(&y) {
                    Some(Ordering::Less) => return true,
                    Some(Ordering::Equal) => (),
                    Some(Ordering::Greater) => return false,
                    None => return false,
                }
            }
        }

        /// Determines if the elements of this `Iterator` are lexicographically
        /// less or equal to those of another.
        #[stable(feature = "iter_order", since = "1.5.0")]
        fn le<I>(mut self, other: I) -> bool where
            I: IntoIterator,
            Self::Item: PartialOrd<I::Item>,
            Self: Sized,
        {
            let mut other = other.into_iter();

            loop {
                let x = match self.next() {
                    None => { other.next(); return true; },
                    Some(val) => val,
                };

                let y = match other.next() {
                    None => return false,
                    Some(val) => val,
                };

                match x.partial_cmp(&y) {
                    Some(Ordering::Less) => return true,
                    Some(Ordering::Equal) => (),
                    Some(Ordering::Greater) => return false,
                    None => return false,
                }
            }
        }

        /// Determines if the elements of this `Iterator` are lexicographically
        /// greater than those of another.
        #[stable(feature = "iter_order", since = "1.5.0")]
        fn gt<I>(mut self, other: I) -> bool where
            I: IntoIterator,
            Self::Item: PartialOrd<I::Item>,
            Self: Sized,
        {
            let mut other = other.into_iter();

            loop {
                let x = match self.next() {
                    None => { other.next(); return false; },
                    Some(val) => val,
                };

                let y = match other.next() {
                    None => return true,
                    Some(val) => val,
                };

                match x.partial_cmp(&y) {
                    Some(Ordering::Less) => return false,
                    Some(Ordering::Equal) => (),
                    Some(Ordering::Greater) => return true,
                    None => return false,
                }
            }
        }

        /// Determines if the elements of this `Iterator` are lexicographically
        /// greater than or equal to those of another.
        #[stable(feature = "iter_order", since = "1.5.0")]
        fn ge<I>(mut self, other: I) -> bool where
            I: IntoIterator,
            Self::Item: PartialOrd<I::Item>,
            Self: Sized,
        {
            let mut other = other.into_iter();

            loop {
                let x = match self.next() {
                    None => return other.next().is_none(),
                    Some(val) => val,
                };

                let y = match other.next() {
                    None => return true,
                    Some(val) => val,
                };

                match x.partial_cmp(&y) {
                    Some(Ordering::Less) => return false,
                    Some(Ordering::Equal) => (),
                    Some(Ordering::Greater) => return true,
                    None => return false,
                }
            }
        }
    }

    /// Select an element from an iterator based on the given "projection"
    /// and "comparison" function.
    ///
    /// This is an idiosyncratic helper to try to factor out the
    /// commonalities of {max,min}{,_by}. In particular, this avoids
    /// having to implement optimizations several times.
    #[inline]
    fn select_fold1<I, B, FProj, FCmp>(mut it: I,
                                       mut f_proj: FProj,
                                       mut f_cmp: FCmp) -> Option<(B, I::Item)>
        where I: Iterator,
              FProj: FnMut(&I::Item) -> B,
              FCmp: FnMut(&B, &I::Item, &B, &I::Item) -> bool
    {
        // start with the first element as our selection. This avoids
        // having to use `Option`s inside the loop, translating to a
        // sizeable performance gain (6x in one case).
        it.next().map(|first| {
            let first_p = f_proj(&first);

            it.fold((first_p, first), |(sel_p, sel), x| {
                let x_p = f_proj(&x);
                if f_cmp(&sel_p, &sel, &x_p, &x) {
                    (x_p, x)
                } else {
                    (sel_p, sel)
                }
            })
        })
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, I: Iterator + ?Sized> Iterator for &'a mut I {
        type Item = I::Item;
        fn next(&mut self) -> Option<I::Item> { (**self).next() }
        fn size_hint(&self) -> (usize, Option<usize>) { (**self).size_hint() }
        fn nth(&mut self, n: usize) -> Option<Self::Item> {
            (**self).nth(n)
        }
    }

# Help

## Keyboard Shortcuts

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  Show this help dialog
`S`
  Focus the search field
`↑`
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`↹`
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`⏎`
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`+`
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## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g. `str,u8` or `String,struct:Vec,test`)