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    // Copyright 2013-2014 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.

    //! Composable external iteration.
    //!
    //! If you've found yourself with a collection of some kind, and needed to
    //! perform an operation on the elements of said collection, you'll quickly run
    //! into 'iterators'. Iterators are heavily used in idiomatic Rust code, so
    //! it's worth becoming familiar with them.
    //!
    //! Before explaining more, let's talk about how this module is structured:
    //!
    //! # Organization
    //!
    //! This module is largely organized by type:
    //!
    //! * [Traits] are the core portion: these traits define what kind of iterators
    //!   exist and what you can do with them. The methods of these traits are worth
    //!   putting some extra study time into.
    //! * [Functions] provide some helpful ways to create some basic iterators.
    //! * [Structs] are often the return types of the various methods on this
    //!   module's traits. You'll usually want to look at the method that creates
    //!   the `struct`, rather than the `struct` itself. For more detail about why,
    //!   see '[Implementing Iterator](#implementing-iterator)'.
    //!
    //! [Traits]: #traits
    //! [Functions]: #functions
    //! [Structs]: #structs
    //!
    //! That's it! Let's dig into iterators.
    //!
    //! # Iterator
    //!
    //! The heart and soul of this module is the [`Iterator`] trait. The core of
    //! [`Iterator`] looks like this:
    //!
    //! ```
    //! trait Iterator {
    //!     type Item;
    //!     fn next(&mut self) -> Option<Self::Item>;
    //! }
    //! ```
    //!
    //! An iterator has a method, [`next()`], which when called, returns an
    //! [`Option`]`<Item>`. [`next()`] will return `Some(Item)` as long as there
    //! are elements, and once they've all been exhausted, will return `None` to
    //! indicate that iteration is finished. Individual iterators may choose to
    //! resume iteration, and so calling [`next()`] again may or may not eventually
    //! start returning `Some(Item)` again at some point.
    //!
    //! [`Iterator`]'s full definition includes a number of other methods as well,
    //! but they are default methods, built on top of [`next()`], and so you get
    //! them for free.
    //!
    //! Iterators are also composable, and it's common to chain them together to do
    //! more complex forms of processing. See the [Adapters](#adapters) section
    //! below for more details.
    //!
    //! [`Iterator`]: trait.Iterator.html
    //! [`next()`]: trait.Iterator.html#tymethod.next
    //! [`Option`]: ../../std/option/enum.Option.html
    //!
    //! # The three forms of iteration
    //!
    //! There are three common methods which can create iterators from a collection:
    //!
    //! * `iter()`, which iterates over `&T`.
    //! * `iter_mut()`, which iterates over `&mut T`.
    //! * `into_iter()`, which iterates over `T`.
    //!
    //! Various things in the standard library may implement one or more of the
    //! three, where appropriate.
    //!
    //! # Implementing Iterator
    //!
    //! Creating an iterator of your own involves two steps: creating a `struct` to
    //! hold the iterator's state, and then `impl`ementing [`Iterator`] for that
    //! `struct`. This is why there are so many `struct`s in this module: there is
    //! one for each iterator and iterator adapter.
    //!
    //! Let's make an iterator named `Counter` which counts from `1` to `5`:
    //!
    //! ```
    //! // First, the struct:
    //!
    //! /// An iterator which counts from one to five
    //! struct Counter {
    //!     count: usize,
    //! }
    //!
    //! // we want our count to start at one, so let's add a new() method to help.
    //! // This isn't strictly necessary, but is convenient. Note that we start
    //! // `count` at zero, we'll see why in `next()`'s implementation below.
    //! impl Counter {
    //!     fn new() -> Counter {
    //!         Counter { count: 0 }
    //!     }
    //! }
    //!
    //! // Then, we implement `Iterator` for our `Counter`:
    //!
    //! impl Iterator for Counter {
    //!     // we will be counting with usize
    //!     type Item = usize;
    //!
    //!     // next() is the only required method
    //!     fn next(&mut self) -> Option<usize> {
    //!         // increment our count. This is why we started at zero.
    //!         self.count += 1;
    //!
    //!         // check to see if we've finished counting or not.
    //!         if self.count < 6 {
    //!             Some(self.count)
    //!         } else {
    //!             None
    //!         }
    //!     }
    //! }
    //!
    //! // And now we can use it!
    //!
    //! let mut counter = Counter::new();
    //!
    //! let x = counter.next().unwrap();
    //! println!("{}", x);
    //!
    //! let x = counter.next().unwrap();
    //! println!("{}", x);
    //!
    //! let x = counter.next().unwrap();
    //! println!("{}", x);
    //!
    //! let x = counter.next().unwrap();
    //! println!("{}", x);
    //!
    //! let x = counter.next().unwrap();
    //! println!("{}", x);
    //! ```
    //!
    //! This will print `1` through `5`, each on their own line.
    //!
    //! Calling `next()` this way gets repetitive. Rust has a construct which can
    //! call `next()` on your iterator, until it reaches `None`. Let's go over that
    //! next.
    //!
    //! # for Loops and IntoIterator
    //!
    //! Rust's `for` loop syntax is actually sugar for iterators. Here's a basic
    //! example of `for`:
    //!
    //! ```
    //! let values = vec![1, 2, 3, 4, 5];
    //!
    //! for x in values {
    //!     println!("{}", x);
    //! }
    //! ```
    //!
    //! This will print the numbers one through five, each on their own line. But
    //! you'll notice something here: we never called anything on our vector to
    //! produce an iterator. What gives?
    //!
    //! There's a trait in the standard library for converting something into an
    //! iterator: [`IntoIterator`]. This trait has one method, [`into_iter()`],
    //! which converts the thing implementing [`IntoIterator`] into an iterator.
    //! Let's take a look at that `for` loop again, and what the compiler converts
    //! it into:
    //!
    //! [`IntoIterator`]: trait.IntoIterator.html
    //! [`into_iter()`]: trait.IntoIterator.html#tymethod.into_iter
    //!
    //! ```
    //! let values = vec![1, 2, 3, 4, 5];
    //!
    //! for x in values {
    //!     println!("{}", x);
    //! }
    //! ```
    //!
    //! Rust de-sugars this into:
    //!
    //! ```
    //! let values = vec![1, 2, 3, 4, 5];
    //! {
    //!     let result = match IntoIterator::into_iter(values) {
    //!         mut iter => loop {
    //!             match iter.next() {
    //!                 Some(x) => { println!("{}", x); },
    //!                 None => break,
    //!             }
    //!         },
    //!     };
    //!     result
    //! }
    //! ```
    //!
    //! First, we call `into_iter()` on the value. Then, we match on the iterator
    //! that returns, calling [`next()`] over and over until we see a `None`. At
    //! that point, we `break` out of the loop, and we're done iterating.
    //!
    //! There's one more subtle bit here: the standard library contains an
    //! interesting implementation of [`IntoIterator`]:
    //!
    //! ```ignore
    //! impl<I: Iterator> IntoIterator for I
    //! ```
    //!
    //! In other words, all [`Iterator`]s implement [`IntoIterator`], by just
    //! returning themselves. This means two things:
    //!
    //! 1. If you're writing an [`Iterator`], you can use it with a `for` loop.
    //! 2. If you're creating a collection, implementing [`IntoIterator`] for it
    //!    will allow your collection to be used with the `for` loop.
    //!
    //! # Adapters
    //!
    //! Functions which take an [`Iterator`] and return another [`Iterator`] are
    //! often called 'iterator adapters', as they're a form of the 'adapter
    //! pattern'.
    //!
    //! Common iterator adapters include [`map()`], [`take()`], and [`collect()`].
    //! For more, see their documentation.
    //!
    //! [`map()`]: trait.Iterator.html#method.map
    //! [`take()`]: trait.Iterator.html#method.take
    //! [`collect()`]: trait.Iterator.html#method.collect
    //!
    //! # Laziness
    //!
    //! Iterators (and iterator [adapters](#adapters)) are *lazy*. This means that
    //! just creating an iterator doesn't _do_ a whole lot. Nothing really happens
    //! until you call [`next()`]. This is sometimes a source of confusion when
    //! creating an iterator solely for its side effects. For example, the [`map()`]
    //! method calls a closure on each element it iterates over:
    //!
    //! ```
    //! # #![allow(unused_must_use)]
    //! let v = vec![1, 2, 3, 4, 5];
    //! v.iter().map(|x| println!("{}", x));
    //! ```
    //!
    //! This will not print any values, as we only created an iterator, rather than
    //! using it. The compiler will warn us about this kind of behavior:
    //!
    //! ```text
    //! warning: unused result which must be used: iterator adaptors are lazy and
    //! do nothing unless consumed
    //! ```
    //!
    //! The idiomatic way to write a [`map()`] for its side effects is to use a
    //! `for` loop instead:
    //!
    //! ```
    //! let v = vec![1, 2, 3, 4, 5];
    //!
    //! for x in &v {
    //!     println!("{}", x);
    //! }
    //! ```
    //!
    //! [`map()`]: trait.Iterator.html#method.map
    //!
    //! The two most common ways to evaluate an iterator are to use a `for` loop
    //! like this, or using the [`collect()`] adapter to produce a new collection.
    //!
    //! [`collect()`]: trait.Iterator.html#method.collect
    //!
    //! # Infinity
    //!
    //! Iterators do not have to be finite. As an example, an open-ended range is
    //! an infinite iterator:
    //!
    //! ```
    //! let numbers = 0..;
    //! ```
    //!
    //! It is common to use the [`take()`] iterator adapter to turn an infinite
    //! iterator into a finite one:
    //!
    //! ```
    //! let numbers = 0..;
    //! let five_numbers = numbers.take(5);
    //!
    //! for number in five_numbers {
    //!     println!("{}", number);
    //! }
    //! ```
    //!
    //! This will print the numbers `0` through `4`, each on their own line.
    //!
    //! [`take()`]: trait.Iterator.html#method.take

    #![stable(feature = "rust1", since = "1.0.0")]

    use clone::Clone;
    use cmp;
    use cmp::{Ord, PartialOrd, PartialEq, Ordering};
    use default::Default;
    use fmt;
    use marker;
    use mem;
    use num::{Zero, One};
    use ops::{self, Add, Sub, FnMut, Mul};
    use option::Option::{self, Some, None};
    use marker::Sized;
    use usize;

    fn _assert_is_object_safe(_: &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 = "`{Self}` is not an iterator; maybe try calling \
                                `.iter()` or a similar method"]
    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.
        ///
        /// # 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.
        ///
        /// # 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 one 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
        /// let iter = 0..;
        ///
        /// assert_eq!((0, 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()`]: #method.next
        ///
        /// # 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.
        ///
        /// # 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]
        #[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.
        ///
        /// # 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
        }

        /// Consumes the `n` first elements of the iterator, then returns the
        /// `next()` one.
        ///
        /// This method will evaluate the iterator `n` times, discarding those elements.
        /// After it does so, it will call [`next()`] and return its value.
        ///
        /// [`next()`]: #method.next
        ///
        /// Like most indexing operations, the count starts from zero, so `nth(0)`
        /// returns the first value, `nth(1)` the second, and so on.
        ///
        /// `nth()` will return `None` if `n` is larger than the length of the
        /// iterator.
        ///
        /// # 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` 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> where Self: Sized {
            for x in self {
                if n == 0 { return Some(x) }
                n -= 1;
            }
            None
        }

        /// 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.
        ///
        /// When either iterator returns `None`, all further calls to `next()`
        /// will return `None`.
        ///
        /// # 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
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter> where
            Self: Sized, U: IntoIterator
        {
            Zip{a: self, b: 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/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}
        }

        /// 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)`, 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`.
        ///
        /// [`Option<T>`]: ../../std/option/enum.Option.html
        ///
        /// Why `filter_map()` and not just [`filter()`].[`map()`]? The key is in this
        /// part:
        ///
        /// [`filter()`]: #method.filter
        /// [`map()`]: #method.map
        ///
        /// > If the closure returns `Some(element)`, 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", "2", "lol"];
        ///
        /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
        ///
        /// assert_eq!(iter.next(), Some(1));
        /// assert_eq!(iter.next(), Some(2));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// Here's the same example, but with [`filter()`] and [`map()`]:
        ///
        /// ```
        /// let a = ["1", "2", "lol"];
        ///
        /// let mut iter = a.iter()
        ///                 .map(|s| s.parse().ok())
        ///                 .filter(|s| s.is_some());
        ///
        /// assert_eq!(iter.next(), Some(Some(1)));
        /// assert_eq!(iter.next(), Some(Some(2)));
        /// assert_eq!(iter.next(), None);
        /// ```
        ///
        /// There's an extra layer of `Some` in there.
        #[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.
        ///
        /// [`usize`]: ../../std/primitive.usize.html
        /// [`zip()`]: #method.zip
        ///
        /// # 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.
        ///
        /// [`usize::MAX`]: ../../std/usize/constant.MAX.html
        ///
        /// # Panics
        ///
        /// The returned iterator might panic if the to-be-returned index would
        /// overflow a `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// let mut iter = a.iter().enumerate();
        ///
        /// assert_eq!(iter.next(), Some((0, &1)));
        /// assert_eq!(iter.next(), Some((1, &2)));
        /// assert_eq!(iter.next(), Some((2, &3)));
        /// 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 look at the `next()` element without
        /// consuming it.
        ///
        /// Adds a [`peek()`] method to an iterator. See its documentation for
        /// more information.
        ///
        /// [`peek()`]: struct.Peekable.html#method.peek
        ///
        /// # 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;
        ///
        ///     // the value passed on to the next iteration
        ///     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.
        ///
        /// [`map()`]: #method.map
        ///
        /// 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.
        ///
        /// # 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{iter: self, f: f, frontiter: None, backiter: None }
        }

        /// 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.
        ///
        /// # 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 much more common for `inspect()` to be used as a debugging tool
        /// than to exist in your final code, but never say never.
        ///
        /// # 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
        /// 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
        /// ```
        #[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>`] can
        /// be thought of as single `Result<Collection<T>, E>`. See the examples
        /// below for more.
        ///
        /// [`String`]: ../../std/string/struct.String.html
        /// [`Result<T, E>`]: ../../std/result/enum.Result.html
        /// [`char`]: ../../std/primitive.char.html
        ///
        /// 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()` 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>`]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);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        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 adaptor 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.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// // the sum of all of the elements of a
        /// 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:
        ///
        /// ```
        /// 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>(self, init: B, mut f: F) -> B where
            Self: Sized, F: FnMut(B, Self::Item) -> B,
        {
            let mut accum = init;
            for x in self {
                accum = f(accum, x);
            }
            accum
        }

        /// 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
        {
            for x in self {
                if !f(x) {
                    return false;
                }
            }
            true
        }

        /// 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
        {
            for x in self {
                if f(x) {
                    return true;
                }
            }
            false
        }

        /// 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`.
        ///
        /// # 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,
        {
            for x in self {
                if predicate(&x) { return Some(x) }
            }
            None
        }

        /// 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.
        ///
        /// # 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];
        ///
        /// 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));
        /// ```
        #[inline]
        #[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,
        {
            // `enumerate` might overflow.
            for (i, x) in self.enumerate() {
                if predicate(x) {
                    return Some(i);
                }
            }
            None
        }

        /// 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`.
        ///
        /// # 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
        {
            let mut i = self.len();

            while let Some(v) = self.next_back() {
                if predicate(v) {
                    return Some(i - 1);
                }
                // No need for an overflow check here, because `ExactSizeIterator`
                // implies that the number of elements fits into a `usize`.
                i -= 1;
            }
            None
        }

        /// Returns the maximum element of an iterator.
        ///
        /// If the two elements are equally maximum, the latest element is
        /// returned.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// assert_eq!(a.iter().max(), Some(&3));
        /// ```
        #[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 the two elements are equally minimum, the first element is
        /// returned.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = [1, 2, 3];
        ///
        /// assert_eq!(a.iter().min(), Some(&1));
        /// ```
        #[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.
        ///
        /// Returns the rightmost element if the comparison determines two elements
        /// to be equally maximum.
        ///
        /// # 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 minimum value from the
        /// specified function.
        ///
        /// Returns the latest element if the comparison determines two elements
        /// to be equally minimum.
        ///
        /// # 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)
        }

        /// 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)>,
        {
            struct SizeHint<A>(usize, Option<usize>, marker::PhantomData<A>);
            impl<A> Iterator for SizeHint<A> {
                type Item = A;

                fn next(&mut self) -> Option<A> { None }
                fn size_hint(&self) -> (usize, Option<usize>) {
                    (self.0, self.1)
                }
            }

            let (lo, hi) = self.size_hint();
            let mut ts: FromA = Default::default();
            let mut us: FromB = Default::default();

            ts.extend(SizeHint(lo, hi, marker::PhantomData));
            us.extend(SizeHint(lo, hi, marker::PhantomData));

            for (t, u) in self {
                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`.
        ///
        /// # 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.
        ///
        /// # 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.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(iter_arith)]
        ///
        /// let a = [1, 2, 3];
        /// let sum: i32 = a.iter().sum();
        ///
        /// assert_eq!(sum, 6);
        /// ```
        #[unstable(feature = "iter_arith", reason = "bounds recently changed",
                   issue = "27739")]
        fn sum<S>(self) -> S where
            S: Add<Self::Item, Output=S> + Zero,
            Self: Sized,
        {
            self.fold(Zero::zero(), |s, e| s + e)
        }

        /// Iterates over the entire iterator, multiplying all the elements
        ///
        /// An empty iterator returns the one value of the type.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(iter_arith)]
        ///
        /// 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);
        /// ```
        #[unstable(feature="iter_arith", reason = "bounds recently changed",
                   issue = "27739")]
        fn product<P>(self) -> P where
            P: Mul<Self::Item, Output=P> + One,
            Self: Sized,
        {
            self.fold(One::one(), |p, e| p * e)
        }

        /// 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 {
                match (self.next(), other.next()) {
                    (None, None) => return Ordering::Equal,
                    (None, _   ) => return Ordering::Less,
                    (_   , None) => return Ordering::Greater,
                    (Some(x), Some(y)) => 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 {
                match (self.next(), other.next()) {
                    (None, None) => return Some(Ordering::Equal),
                    (None, _   ) => return Some(Ordering::Less),
                    (_   , None) => return Some(Ordering::Greater),
                    (Some(x), Some(y)) => 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 {
                match (self.next(), other.next()) {
                    (None, None) => return true,
                    (None, _) | (_, None) => return false,
                    (Some(x), Some(y)) => 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 {
                match (self.next(), other.next()) {
                    (None, None) => return false,
                    (None, _) | (_, None) => return true,
                    (Some(x), Some(y)) => if x.ne(&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 {
                match (self.next(), other.next()) {
                    (None, None) => return false,
                    (None, _   ) => return true,
                    (_   , None) => return false,
                    (Some(x), Some(y)) => {
                        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 {
                match (self.next(), other.next()) {
                    (None, None) => return true,
                    (None, _   ) => return true,
                    (_   , None) => return false,
                    (Some(x), Some(y)) => {
                        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 {
                match (self.next(), other.next()) {
                    (None, None) => return false,
                    (None, _   ) => return false,
                    (_   , None) => return true,
                    (Some(x), Some(y)) => {
                        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 {
                match (self.next(), other.next()) {
                    (None, None) => return true,
                    (None, _   ) => return false,
                    (_   , None) => return true,
                    (Some(x), Some(y)) => {
                        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(|mut sel| {
            let mut sel_p = f_proj(&sel);

            for x in it {
                let x_p = f_proj(&x);
                if f_cmp(&sel_p,  &sel, &x_p, &x) {
                    sel = x;
                    sel_p = x_p;
                }
            }
            (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() }
    }

    /// Conversion from an `Iterator`.
    ///
    /// By implementing `FromIterator` for a type, you define how it will be
    /// created from an iterator. This is common for types which describe a
    /// collection of some kind.
    ///
    /// `FromIterator`'s [`from_iter()`] is rarely called explicitly, and is instead
    /// used through [`Iterator`]'s [`collect()`] method. See [`collect()`]'s
    /// documentation for more examples.
    ///
    /// [`from_iter()`]: #tymethod.from_iter
    /// [`Iterator`]: trait.Iterator.html
    /// [`collect()`]: trait.Iterator.html#method.collect
    ///
    /// See also: [`IntoIterator`].
    ///
    /// [`IntoIterator`]: trait.IntoIterator.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter::FromIterator;
    ///
    /// let five_fives = std::iter::repeat(5).take(5);
    ///
    /// let v = Vec::from_iter(five_fives);
    ///
    /// assert_eq!(v, vec![5, 5, 5, 5, 5]);
    /// ```
    ///
    /// Using [`collect()`] to implicitly use `FromIterator`:
    ///
    /// ```
    /// let five_fives = std::iter::repeat(5).take(5);
    ///
    /// let v: Vec<i32> = five_fives.collect();
    ///
    /// assert_eq!(v, vec![5, 5, 5, 5, 5]);
    /// ```
    ///
    /// Implementing `FromIterator` for your type:
    ///
    /// ```
    /// use std::iter::FromIterator;
    ///
    /// // A sample collection, that's just a wrapper over Vec<T>
    /// #[derive(Debug)]
    /// struct MyCollection(Vec<i32>);
    ///
    /// // Let's give it some methods so we can create one and add things
    /// // to it.
    /// impl MyCollection {
    ///     fn new() -> MyCollection {
    ///         MyCollection(Vec::new())
    ///     }
    ///
    ///     fn add(&mut self, elem: i32) {
    ///         self.0.push(elem);
    ///     }
    /// }
    ///
    /// // and we'll implement FromIterator
    /// impl FromIterator<i32> for MyCollection {
    ///     fn from_iter<I: IntoIterator<Item=i32>>(iterator: I) -> Self {
    ///         let mut c = MyCollection::new();
    ///
    ///         for i in iterator {
    ///             c.add(i);
    ///         }
    ///
    ///         c
    ///     }
    /// }
    ///
    /// // Now we can make a new iterator...
    /// let iter = (0..5).into_iter();
    ///
    /// // ... and make a MyCollection out of it
    /// let c = MyCollection::from_iter(iter);
    ///
    /// assert_eq!(c.0, vec![0, 1, 2, 3, 4]);
    ///
    /// // collect works too!
    ///
    /// let iter = (0..5).into_iter();
    /// let c: MyCollection = iter.collect();
    ///
    /// assert_eq!(c.0, vec![0, 1, 2, 3, 4]);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_on_unimplemented="a collection of type `{Self}` cannot be \
                              built from an iterator over elements of type `{A}`"]
    pub trait FromIterator<A>: Sized {
        /// Creates a value from an iterator.
        ///
        /// See the [module-level documentation] for more.
        ///
        /// [module-level documentation]: trait.FromIterator.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::iter::FromIterator;
        ///
        /// let five_fives = std::iter::repeat(5).take(5);
        ///
        /// let v = Vec::from_iter(five_fives);
        ///
        /// assert_eq!(v, vec![5, 5, 5, 5, 5]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn from_iter<T: IntoIterator<Item=A>>(iterator: T) -> Self;
    }

    /// Conversion into an `Iterator`.
    ///
    /// By implementing `IntoIterator` for a type, you define how it will be
    /// converted to an iterator. This is common for types which describe a
    /// collection of some kind.
    ///
    /// One benefit of implementing `IntoIterator` is that your type will [work
    /// with Rust's `for` loop syntax](index.html#for-loops-and-intoiterator).
    ///
    /// See also: [`FromIterator`].
    ///
    /// [`FromIterator`]: trait.FromIterator.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let v = vec![1, 2, 3];
    ///
    /// let mut iter = v.into_iter();
    ///
    /// let n = iter.next();
    /// assert_eq!(Some(1), n);
    ///
    /// let n = iter.next();
    /// assert_eq!(Some(2), n);
    ///
    /// let n = iter.next();
    /// assert_eq!(Some(3), n);
    ///
    /// let n = iter.next();
    /// assert_eq!(None, n);
    /// ```
    ///
    /// Implementing `IntoIterator` for your type:
    ///
    /// ```
    /// // A sample collection, that's just a wrapper over Vec<T>
    /// #[derive(Debug)]
    /// struct MyCollection(Vec<i32>);
    ///
    /// // Let's give it some methods so we can create one and add things
    /// // to it.
    /// impl MyCollection {
    ///     fn new() -> MyCollection {
    ///         MyCollection(Vec::new())
    ///     }
    ///
    ///     fn add(&mut self, elem: i32) {
    ///         self.0.push(elem);
    ///     }
    /// }
    ///
    /// // and we'll implement IntoIterator
    /// impl IntoIterator for MyCollection {
    ///     type Item = i32;
    ///     type IntoIter = ::std::vec::IntoIter<i32>;
    ///
    ///     fn into_iter(self) -> Self::IntoIter {
    ///         self.0.into_iter()
    ///     }
    /// }
    ///
    /// // Now we can make a new collection...
    /// let mut c = MyCollection::new();
    ///
    /// // ... add some stuff to it ...
    /// c.add(0);
    /// c.add(1);
    /// c.add(2);
    ///
    /// // ... and then turn it into an Iterator:
    /// for (i, n) in c.into_iter().enumerate() {
    ///     assert_eq!(i as i32, n);
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait IntoIterator {
        /// The type of the elements being iterated over.
        #[stable(feature = "rust1", since = "1.0.0")]
        type Item;

        /// Which kind of iterator are we turning this into?
        #[stable(feature = "rust1", since = "1.0.0")]
        type IntoIter: Iterator<Item=Self::Item>;

        /// Creates an iterator from a value.
        ///
        /// See the [module-level documentation] for more.
        ///
        /// [module-level documentation]: trait.IntoIterator.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let v = vec![1, 2, 3];
        ///
        /// let mut iter = v.into_iter();
        ///
        /// let n = iter.next();
        /// assert_eq!(Some(1), n);
        ///
        /// let n = iter.next();
        /// assert_eq!(Some(2), n);
        ///
        /// let n = iter.next();
        /// assert_eq!(Some(3), n);
        ///
        /// let n = iter.next();
        /// assert_eq!(None, n);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn into_iter(self) -> Self::IntoIter;
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator> IntoIterator for I {
        type Item = I::Item;
        type IntoIter = I;

        fn into_iter(self) -> I {
            self
        }
    }

    /// Extend a collection with the contents of an iterator.
    ///
    /// Iterators produce a series of values, and collections can also be thought
    /// of as a series of values. The `Extend` trait bridges this gap, allowing you
    /// to extend a collection by including the contents of that iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// // You can extend a String with some chars:
    /// let mut message = String::from("The first three letters are: ");
    ///
    /// message.extend(&['a', 'b', 'c']);
    ///
    /// assert_eq!("abc", &message[29..32]);
    /// ```
    ///
    /// Implementing `Extend`:
    ///
    /// ```
    /// // A sample collection, that's just a wrapper over Vec<T>
    /// #[derive(Debug)]
    /// struct MyCollection(Vec<i32>);
    ///
    /// // Let's give it some methods so we can create one and add things
    /// // to it.
    /// impl MyCollection {
    ///     fn new() -> MyCollection {
    ///         MyCollection(Vec::new())
    ///     }
    ///
    ///     fn add(&mut self, elem: i32) {
    ///         self.0.push(elem);
    ///     }
    /// }
    ///
    /// // since MyCollection has a list of i32s, we implement Extend for i32
    /// impl Extend<i32> for MyCollection {
    ///
    ///     // This is a bit simpler with the concrete type signature: we can call
    ///     // extend on anything which can be turned into an Iterator which gives
    ///     // us i32s. Because we need i32s to put into MyCollection.
    ///     fn extend<T: IntoIterator<Item=i32>>(&mut self, iterable: T) {
    ///
    ///         // The implementation is very straightforward: loop through the
    ///         // iterator, and add() each element to ourselves.
    ///         for elem in iterable {
    ///             self.add(elem);
    ///         }
    ///     }
    /// }
    ///
    /// let mut c = MyCollection::new();
    ///
    /// c.add(5);
    /// c.add(6);
    /// c.add(7);
    ///
    /// // let's extend our collection with three more numbers
    /// c.extend(vec![1, 2, 3]);
    ///
    /// // we've added these elements onto the end
    /// assert_eq!("MyCollection([5, 6, 7, 1, 2, 3])", format!("{:?}", c));
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Extend<A> {
        /// Extends a collection with the contents of an iterator.
        ///
        /// As this is the only method for this trait, the [trait-level] docs
        /// contain more details.
        ///
        /// [trait-level]: trait.Extend.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // You can extend a String with some chars:
        /// let mut message = String::from("abc");
        ///
        /// message.extend(['d', 'e', 'f'].iter());
        ///
        /// assert_eq!("abcdef", &message);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn extend<T: IntoIterator<Item=A>>(&mut self, iterable: T);
    }

    /// An iterator able to yield elements from both ends.
    ///
    /// Something that implements `DoubleEndedIterator` has one extra capability
    /// over something that implements [`Iterator`]: the ability to also take
    /// `Item`s from the back, as well as the front.
    ///
    /// It is important to note that both back and forth work on the same range,
    /// and do not cross: iteration is over when they meet in the middle.
    ///
    /// In a similar fashion to the [`Iterator`] protocol, once a
    /// `DoubleEndedIterator` returns `None` from a `next_back()`, calling it again
    /// may or may not ever return `Some` again. `next()` and `next_back()` are
    /// interchangable for this purpose.
    ///
    /// [`Iterator`]: trait.Iterator.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let numbers = vec![1, 2, 3];
    ///
    /// let mut iter = numbers.iter();
    ///
    /// assert_eq!(Some(&1), iter.next());
    /// assert_eq!(Some(&3), iter.next_back());
    /// assert_eq!(Some(&2), iter.next_back());
    /// assert_eq!(None, iter.next());
    /// assert_eq!(None, iter.next_back());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait DoubleEndedIterator: Iterator {
        /// An iterator able to yield elements from both ends.
        ///
        /// As this is the only method for this trait, the [trait-level] docs
        /// contain more details.
        ///
        /// [trait-level]: trait.DoubleEndedIterator.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let numbers = vec![1, 2, 3];
        ///
        /// let mut iter = numbers.iter();
        ///
        /// assert_eq!(Some(&1), iter.next());
        /// assert_eq!(Some(&3), iter.next_back());
        /// assert_eq!(Some(&2), iter.next_back());
        /// assert_eq!(None, iter.next());
        /// assert_eq!(None, iter.next_back());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn next_back(&mut self) -> Option<Self::Item>;
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
        fn next_back(&mut self) -> Option<I::Item> { (**self).next_back() }
    }

    /// An iterator that knows its exact length.
    ///
    /// Many [`Iterator`]s don't know how many times they will iterate, but some do.
    /// If an iterator knows how many times it can iterate, providing access to
    /// that information can be useful. For example, if you want to iterate
    /// backwards, a good start is to know where the end is.
    ///
    /// When implementing an `ExactSizeIterator`, You must also implement
    /// [`Iterator`]. When doing so, the implementation of [`size_hint()`] *must*
    /// return the exact size of the iterator.
    ///
    /// [`Iterator`]: trait.Iterator.html
    /// [`size_hint()`]: trait.Iterator.html#method.size_hint
    ///
    /// The [`len()`] method has a default implementation, so you usually shouldn't
    /// implement it. However, you may be able to provide a more performant
    /// implementation than the default, so overriding it in this case makes sense.
    ///
    /// [`len()`]: #method.len
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// // a finite range knows exactly how many times it will iterate
    /// let five = 0..5;
    ///
    /// assert_eq!(5, five.len());
    /// ```
    ///
    /// In the [module level docs][moddocs], we implemented an [`Iterator`],
    /// `Counter`. Let's implement `ExactSizeIterator` for it as well:
    ///
    /// [moddocs]: index.html
    ///
    /// ```
    /// # struct Counter {
    /// #     count: usize,
    /// # }
    /// # impl Counter {
    /// #     fn new() -> Counter {
    /// #         Counter { count: 0 }
    /// #     }
    /// # }
    /// # impl Iterator for Counter {
    /// #     type Item = usize;
    /// #     fn next(&mut self) -> Option<usize> {
    /// #         self.count += 1;
    /// #         if self.count < 6 {
    /// #             Some(self.count)
    /// #         } else {
    /// #             None
    /// #         }
    /// #     }
    /// # }
    /// impl ExactSizeIterator for Counter {
    ///     // We already have the number of iterations, so we can use it directly.
    ///     fn len(&self) -> usize {
    ///         self.count
    ///     }
    /// }
    ///
    /// // And now we can use it!
    ///
    /// let counter = Counter::new();
    ///
    /// assert_eq!(0, counter.len());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait ExactSizeIterator: Iterator {
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        /// Returns the exact number of times the iterator will iterate.
        ///
        /// This method has a default implementation, so you usually should not
        /// implement it directly. However, if you can provide a more efficient
        /// implementation, you can do so. See the [trait-level] docs for an
        /// example.
        ///
        /// This function has the same safety guarantees as the [`size_hint()`]
        /// function.
        ///
        /// [trait-level]: trait.ExactSizeIterator.html
        /// [`size_hint()`]: trait.Iterator.html#method.size_hint
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // a finite range knows exactly how many times it will iterate
        /// let five = 0..5;
        ///
        /// assert_eq!(5, five.len());
        /// ```
        fn len(&self) -> usize {
            let (lower, upper) = self.size_hint();
            // Note: This assertion is overly defensive, but it checks the invariant
            // guaranteed by the trait. If this trait were rust-internal,
            // we could use debug_assert!; assert_eq! will check all Rust user
            // implementations too.
            assert_eq!(upper, Some(lower));
            lower
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, I: ExactSizeIterator + ?Sized> ExactSizeIterator for &'a mut I {}

    // All adaptors that preserve the size of the wrapped iterator are fine
    // Adaptors that may overflow in `size_hint` are not, i.e. `Chain`.
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> ExactSizeIterator for Enumerate<I> where I: ExactSizeIterator {}
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: ExactSizeIterator, F> ExactSizeIterator for Inspect<I, F> where
        F: FnMut(&I::Item),
    {}
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> ExactSizeIterator for Rev<I>
        where I: ExactSizeIterator + DoubleEndedIterator {}
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F> where
        F: FnMut(I::Item) -> B,
    {}
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> ExactSizeIterator for Zip<A, B>
        where A: ExactSizeIterator, B: ExactSizeIterator {}

    /// An double-ended iterator with the direction inverted.
    ///
    /// This `struct` is created by the [`rev()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`rev()`]: trait.Iterator.html#method.rev
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Rev<T> {
        iter: T
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> Iterator for Rev<I> where I: DoubleEndedIterator {
        type Item = <I as Iterator>::Item;

        #[inline]
        fn next(&mut self) -> Option<<I as Iterator>::Item> { self.iter.next_back() }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> DoubleEndedIterator for Rev<I> where I: DoubleEndedIterator {
        #[inline]
        fn next_back(&mut self) -> Option<<I as Iterator>::Item> { self.iter.next() }
    }

    /// An iterator that clones the elements of an underlying iterator.
    ///
    /// This `struct` is created by the [`cloned()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`cloned()`]: trait.Iterator.html#method.cloned
    /// [`Iterator`]: trait.Iterator.html
    #[stable(feature = "iter_cloned", since = "1.1.0")]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[derive(Clone, Debug)]
    pub struct Cloned<I> {
        it: I,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, I, T: 'a> Iterator for Cloned<I>
        where I: Iterator<Item=&'a T>, T: Clone
    {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            self.it.next().cloned()
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.it.size_hint()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, I, T: 'a> DoubleEndedIterator for Cloned<I>
        where I: DoubleEndedIterator<Item=&'a T>, T: Clone
    {
        fn next_back(&mut self) -> Option<T> {
            self.it.next_back().cloned()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, I, T: 'a> ExactSizeIterator for Cloned<I>
        where I: ExactSizeIterator<Item=&'a T>, T: Clone
    {}

    /// An iterator that repeats endlessly.
    ///
    /// This `struct` is created by the [`cycle()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`cycle()`]: trait.Iterator.html#method.cycle
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Cycle<I> {
        orig: I,
        iter: I,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> Iterator for Cycle<I> where I: Clone + Iterator {
        type Item = <I as Iterator>::Item;

        #[inline]
        fn next(&mut self) -> Option<<I as Iterator>::Item> {
            match self.iter.next() {
                None => { self.iter = self.orig.clone(); self.iter.next() }
                y => y
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            // the cycle iterator is either empty or infinite
            match self.orig.size_hint() {
                sz @ (0, Some(0)) => sz,
                (0, _) => (0, None),
                _ => (usize::MAX, None)
            }
        }
    }

    /// An iterator that strings two iterators together.
    ///
    /// This `struct` is created by the [`chain()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`chain()`]: trait.Iterator.html#method.chain
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Chain<A, B> {
        a: A,
        b: B,
        state: ChainState,
    }

    // The iterator protocol specifies that iteration ends with the return value
    // `None` from `.next()` (or `.next_back()`) and it is unspecified what
    // further calls return. The chain adaptor must account for this since it uses
    // two subiterators.
    //
    //  It uses three states:
    //
    //  - Both: `a` and `b` are remaining
    //  - Front: `a` remaining
    //  - Back: `b` remaining
    //
    //  The fourth state (neither iterator is remaining) only occurs after Chain has
    //  returned None once, so we don't need to store this state.
    #[derive(Clone, Debug)]
    enum ChainState {
        // both front and back iterator are remaining
        Both,
        // only front is remaining
        Front,
        // only back is remaining
        Back,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> Iterator for Chain<A, B> where
        A: Iterator,
        B: Iterator<Item = A::Item>
    {
        type Item = A::Item;

        #[inline]
        fn next(&mut self) -> Option<A::Item> {
            match self.state {
                ChainState::Both => match self.a.next() {
                    elt @ Some(..) => elt,
                    None => {
                        self.state = ChainState::Back;
                        self.b.next()
                    }
                },
                ChainState::Front => self.a.next(),
                ChainState::Back => self.b.next(),
            }
        }

        #[inline]
        fn count(self) -> usize {
            match self.state {
                ChainState::Both => self.a.count() + self.b.count(),
                ChainState::Front => self.a.count(),
                ChainState::Back => self.b.count(),
            }
        }

        #[inline]
        fn nth(&mut self, mut n: usize) -> Option<A::Item> {
            match self.state {
                ChainState::Both | ChainState::Front => {
                    for x in self.a.by_ref() {
                        if n == 0 {
                            return Some(x)
                        }
                        n -= 1;
                    }
                    if let ChainState::Both = self.state {
                        self.state = ChainState::Back;
                    }
                }
                ChainState::Back => {}
            }
            if let ChainState::Back = self.state {
                self.b.nth(n)
            } else {
                None
            }
        }

        #[inline]
        fn last(self) -> Option<A::Item> {
            match self.state {
                ChainState::Both => {
                    // Must exhaust a before b.
                    let a_last = self.a.last();
                    let b_last = self.b.last();
                    b_last.or(a_last)
                },
                ChainState::Front => self.a.last(),
                ChainState::Back => self.b.last()
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (a_lower, a_upper) = self.a.size_hint();
            let (b_lower, b_upper) = self.b.size_hint();

            let lower = a_lower.saturating_add(b_lower);

            let upper = match (a_upper, b_upper) {
                (Some(x), Some(y)) => x.checked_add(y),
                _ => None
            };

            (lower, upper)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> DoubleEndedIterator for Chain<A, B> where
        A: DoubleEndedIterator,
        B: DoubleEndedIterator<Item=A::Item>,
    {
        #[inline]
        fn next_back(&mut self) -> Option<A::Item> {
            match self.state {
                ChainState::Both => match self.b.next_back() {
                    elt @ Some(..) => elt,
                    None => {
                        self.state = ChainState::Front;
                        self.a.next_back()
                    }
                },
                ChainState::Front => self.a.next_back(),
                ChainState::Back => self.b.next_back(),
            }
        }
    }

    /// An iterator that iterates two other iterators simultaneously.
    ///
    /// This `struct` is created by the [`zip()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`zip()`]: trait.Iterator.html#method.zip
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Zip<A, B> {
        a: A,
        b: B
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> Iterator for Zip<A, B> where A: Iterator, B: Iterator
    {
        type Item = (A::Item, B::Item);

        #[inline]
        fn next(&mut self) -> Option<(A::Item, B::Item)> {
            self.a.next().and_then(|x| {
                self.b.next().and_then(|y| {
                    Some((x, y))
                })
            })
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (a_lower, a_upper) = self.a.size_hint();
            let (b_lower, b_upper) = self.b.size_hint();

            let lower = cmp::min(a_lower, b_lower);

            let upper = match (a_upper, b_upper) {
                (Some(x), Some(y)) => Some(cmp::min(x,y)),
                (Some(x), None) => Some(x),
                (None, Some(y)) => Some(y),
                (None, None) => None
            };

            (lower, upper)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> DoubleEndedIterator for Zip<A, B> where
        A: DoubleEndedIterator + ExactSizeIterator,
        B: DoubleEndedIterator + ExactSizeIterator,
    {
        #[inline]
        fn next_back(&mut self) -> Option<(A::Item, B::Item)> {
            let a_sz = self.a.len();
            let b_sz = self.b.len();
            if a_sz != b_sz {
                // Adjust a, b to equal length
                if a_sz > b_sz {
                    for _ in 0..a_sz - b_sz { self.a.next_back(); }
                } else {
                    for _ in 0..b_sz - a_sz { self.b.next_back(); }
                }
            }
            match (self.a.next_back(), self.b.next_back()) {
                (Some(x), Some(y)) => Some((x, y)),
                (None, None) => None,
                _ => unreachable!(),
            }
        }
    }

    /// An iterator that maps the values of `iter` with `f`.
    ///
    /// This `struct` is created by the [`map()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`map()`]: trait.Iterator.html#method.map
    /// [`Iterator`]: trait.Iterator.html
    ///
    /// # Notes about side effects
    ///
    /// The [`map()`] iterator implements [`DoubleEndedIterator`], meaning that
    /// you can also [`map()`] backwards:
    ///
    /// ```rust
    /// let v: Vec<i32> = vec![1, 2, 3].into_iter().rev().map(|x| x + 1).collect();
    ///
    /// assert_eq!(v, [4, 3, 2]);
    /// ```
    ///
    /// [`DoubleEndedIterator`]: trait.DoubleEndedIterator.html
    ///
    /// But if your closure has state, iterating backwards may act in a way you do
    /// not expect. Let's go through an example. First, in the forward direction:
    ///
    /// ```rust
    /// let mut c = 0;
    ///
    /// for pair in vec!['a', 'b', 'c'].into_iter()
    ///                                .map(|letter| { c += 1; (letter, c) }) {
    ///     println!("{:?}", pair);
    /// }
    /// ```
    ///
    /// This will print "('a', 1), ('b', 2), ('c', 3)".
    ///
    /// Now consider this twist where we add a call to `rev`. This version will
    /// print `('c', 1), ('b', 2), ('a', 3)`. Note that the letters are reversed,
    /// but the values of the counter still go in order. This is because `map()` is
    /// still being called lazilly on each item, but we are popping items off the
    /// back of the vector now, instead of shifting them from the front.
    ///
    /// ```rust
    /// let mut c = 0;
    ///
    /// for pair in vec!['a', 'b', 'c'].into_iter()
    ///                                .map(|letter| { c += 1; (letter, c) })
    ///                                .rev() {
    ///     println!("{:?}", pair);
    /// }
    /// ```
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct Map<I, F> {
        iter: I,
        f: F,
    }

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<I: fmt::Debug, F> fmt::Debug for Map<I, F> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Map")
                .field("iter", &self.iter)
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B, I: Iterator, F> Iterator for Map<I, F> where F: FnMut(I::Item) -> B {
        type Item = B;

        #[inline]
        fn next(&mut self) -> Option<B> {
            self.iter.next().map(&mut self.f)
        }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for Map<I, F> where
        F: FnMut(I::Item) -> B,
    {
        #[inline]
        fn next_back(&mut self) -> Option<B> {
            self.iter.next_back().map(&mut self.f)
        }
    }

    /// An iterator that filters the elements of `iter` with `predicate`.
    ///
    /// This `struct` is created by the [`filter()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`filter()`]: trait.Iterator.html#method.filter
    /// [`Iterator`]: trait.Iterator.html
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct Filter<I, P> {
        iter: I,
        predicate: P,
    }

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<I: fmt::Debug, P> fmt::Debug for Filter<I, P> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Filter")
                .field("iter", &self.iter)
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator, P> Iterator for Filter<I, P> where P: FnMut(&I::Item) -> bool {
        type Item = I::Item;

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            for x in self.iter.by_ref() {
                if (self.predicate)(&x) {
                    return Some(x);
                }
            }
            None
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, upper) = self.iter.size_hint();
            (0, upper) // can't know a lower bound, due to the predicate
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: DoubleEndedIterator, P> DoubleEndedIterator for Filter<I, P>
        where P: FnMut(&I::Item) -> bool,
    {
        #[inline]
        fn next_back(&mut self) -> Option<I::Item> {
            for x in self.iter.by_ref().rev() {
                if (self.predicate)(&x) {
                    return Some(x);
                }
            }
            None
        }
    }

    /// An iterator that uses `f` to both filter and map elements from `iter`.
    ///
    /// This `struct` is created by the [`filter_map()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`filter_map()`]: trait.Iterator.html#method.filter_map
    /// [`Iterator`]: trait.Iterator.html
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct FilterMap<I, F> {
        iter: I,
        f: F,
    }

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<I: fmt::Debug, F> fmt::Debug for FilterMap<I, F> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("FilterMap")
                .field("iter", &self.iter)
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B, I: Iterator, F> Iterator for FilterMap<I, F>
        where F: FnMut(I::Item) -> Option<B>,
    {
        type Item = B;

        #[inline]
        fn next(&mut self) -> Option<B> {
            for x in self.iter.by_ref() {
                if let Some(y) = (self.f)(x) {
                    return Some(y);
                }
            }
            None
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, upper) = self.iter.size_hint();
            (0, upper) // can't know a lower bound, due to the predicate
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for FilterMap<I, F>
        where F: FnMut(I::Item) -> Option<B>,
    {
        #[inline]
        fn next_back(&mut self) -> Option<B> {
            for x in self.iter.by_ref().rev() {
                if let Some(y) = (self.f)(x) {
                    return Some(y);
                }
            }
            None
        }
    }

    /// An iterator that yields the current count and the element during iteration.
    ///
    /// This `struct` is created by the [`enumerate()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`enumerate()`]: trait.Iterator.html#method.enumerate
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Enumerate<I> {
        iter: I,
        count: usize,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> Iterator for Enumerate<I> where I: Iterator {
        type Item = (usize, <I as Iterator>::Item);

        /// # 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
        ///
        /// Might panic if the index of the element overflows a `usize`.
        #[inline]
        fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
            self.iter.next().map(|a| {
                let ret = (self.count, a);
                // Possible undefined overflow.
                self.count += 1;
                ret
            })
        }

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

        #[inline]
        fn nth(&mut self, n: usize) -> Option<(usize, I::Item)> {
            self.iter.nth(n).map(|a| {
                let i = self.count + n;
                self.count = i + 1;
                (i, a)
            })
        }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> DoubleEndedIterator for Enumerate<I> where
        I: ExactSizeIterator + DoubleEndedIterator
    {
        #[inline]
        fn next_back(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
            self.iter.next_back().map(|a| {
                let len = self.iter.len();
                // Can safely add, `ExactSizeIterator` promises that the number of
                // elements fits into a `usize`.
                (self.count + len, a)
            })
        }
    }

    /// An iterator with a `peek()` that returns an optional reference to the next
    /// element.
    ///
    /// This `struct` is created by the [`peekable()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`peekable()`]: trait.Iterator.html#method.peekable
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Peekable<I: Iterator> {
        iter: I,
        peeked: Option<I::Item>,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator> Iterator for Peekable<I> {
        type Item = I::Item;

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            match self.peeked {
                Some(_) => self.peeked.take(),
                None => self.iter.next(),
            }
        }

        #[inline]
        fn count(self) -> usize {
            (if self.peeked.is_some() { 1 } else { 0 }) + self.iter.count()
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<I::Item> {
            match self.peeked {
                Some(_) if n == 0 => self.peeked.take(),
                Some(_) => {
                    self.peeked = None;
                    self.iter.nth(n-1)
                },
                None => self.iter.nth(n)
            }
        }

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

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (lo, hi) = self.iter.size_hint();
            if self.peeked.is_some() {
                let lo = lo.saturating_add(1);
                let hi = hi.and_then(|x| x.checked_add(1));
                (lo, hi)
            } else {
                (lo, hi)
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I> {}

    impl<I: Iterator> Peekable<I> {
        /// Returns a reference to the next() value without advancing the iterator.
        ///
        /// The `peek()` method will return the value that a call to [`next()`] would
        /// return, but does not advance the iterator. Like [`next()`], if there is
        /// a value, it's wrapped in a `Some(T)`, but if the iterator is over, it
        /// will return `None`.
        ///
        /// [`next()`]: trait.Iterator.html#tymethod.next
        ///
        /// Because `peek()` returns reference, and many iterators iterate over
        /// references, this leads to a possibly confusing situation where the
        /// return value is a double reference. You can see this effect in the
        /// examples below, with `&&i32`.
        ///
        /// # 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")]
        pub fn peek(&mut self) -> Option<&I::Item> {
            if self.peeked.is_none() {
                self.peeked = self.iter.next();
            }
            match self.peeked {
                Some(ref value) => Some(value),
                None => None,
            }
        }

        /// Checks if the iterator has finished iterating.
        ///
        /// Returns `true` if there are no more elements in the iterator, and
        /// `false` if there are.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(peekable_is_empty)]
        ///
        /// let xs = [1, 2, 3];
        ///
        /// let mut iter = xs.iter().peekable();
        ///
        /// // there are still elements to iterate over
        /// assert_eq!(iter.is_empty(), false);
        ///
        /// // let's consume the iterator
        /// iter.next();
        /// iter.next();
        /// iter.next();
        ///
        /// assert_eq!(iter.is_empty(), true);
        /// ```
        #[unstable(feature = "peekable_is_empty", issue = "32111")]
        #[inline]
        pub fn is_empty(&mut self) -> bool {
            self.peek().is_none()
        }
    }

    /// An iterator that rejects elements while `predicate` is true.
    ///
    /// This `struct` is created by the [`skip_while()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`skip_while()`]: trait.Iterator.html#method.skip_while
    /// [`Iterator`]: trait.Iterator.html
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct SkipWhile<I, P> {
        iter: I,
        flag: bool,
        predicate: P,
    }

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<I: fmt::Debug, P> fmt::Debug for SkipWhile<I, P> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("SkipWhile")
                .field("iter", &self.iter)
                .field("flag", &self.flag)
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator, P> Iterator for SkipWhile<I, P>
        where P: FnMut(&I::Item) -> bool
    {
        type Item = I::Item;

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            for x in self.iter.by_ref() {
                if self.flag || !(self.predicate)(&x) {
                    self.flag = true;
                    return Some(x);
                }
            }
            None
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, upper) = self.iter.size_hint();
            (0, upper) // can't know a lower bound, due to the predicate
        }
    }

    /// An iterator that only accepts elements while `predicate` is true.
    ///
    /// This `struct` is created by the [`take_while()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`take_while()`]: trait.Iterator.html#method.take_while
    /// [`Iterator`]: trait.Iterator.html
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct TakeWhile<I, P> {
        iter: I,
        flag: bool,
        predicate: P,
    }

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<I: fmt::Debug, P> fmt::Debug for TakeWhile<I, P> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("TakeWhile")
                .field("iter", &self.iter)
                .field("flag", &self.flag)
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator, P> Iterator for TakeWhile<I, P>
        where P: FnMut(&I::Item) -> bool
    {
        type Item = I::Item;

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            if self.flag {
                None
            } else {
                self.iter.next().and_then(|x| {
                    if (self.predicate)(&x) {
                        Some(x)
                    } else {
                        self.flag = true;
                        None
                    }
                })
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, upper) = self.iter.size_hint();
            (0, upper) // can't know a lower bound, due to the predicate
        }
    }

    /// An iterator that skips over `n` elements of `iter`.
    ///
    /// This `struct` is created by the [`skip()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`skip()`]: trait.Iterator.html#method.skip
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Skip<I> {
        iter: I,
        n: usize
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> Iterator for Skip<I> where I: Iterator {
        type Item = <I as Iterator>::Item;

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            if self.n == 0 {
                self.iter.next()
            } else {
                let old_n = self.n;
                self.n = 0;
                self.iter.nth(old_n)
            }
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<I::Item> {
            // Can't just add n + self.n due to overflow.
            if self.n == 0 {
                self.iter.nth(n)
            } else {
                let to_skip = self.n;
                self.n = 0;
                // nth(n) skips n+1
                if self.iter.nth(to_skip-1).is_none() {
                    return None;
                }
                self.iter.nth(n)
            }
        }

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

        #[inline]
        fn last(mut self) -> Option<I::Item> {
            if self.n == 0 {
                self.iter.last()
            } else {
                let next = self.next();
                if next.is_some() {
                    // recurse. n should be 0.
                    self.last().or(next)
                } else {
                    None
                }
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (lower, upper) = self.iter.size_hint();

            let lower = lower.saturating_sub(self.n);
            let upper = upper.map(|x| x.saturating_sub(self.n));

            (lower, upper)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator {}

    #[stable(feature = "double_ended_skip_iterator", since = "1.8.0")]
    impl<I> DoubleEndedIterator for Skip<I> where I: DoubleEndedIterator + ExactSizeIterator {
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.len() > 0 {
                self.iter.next_back()
            } else {
                None
            }
        }
    }

    /// An iterator that only iterates over the first `n` iterations of `iter`.
    ///
    /// This `struct` is created by the [`take()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`take()`]: trait.Iterator.html#method.take
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Take<I> {
        iter: I,
        n: usize
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> Iterator for Take<I> where I: Iterator{
        type Item = <I as Iterator>::Item;

        #[inline]
        fn next(&mut self) -> Option<<I as Iterator>::Item> {
            if self.n != 0 {
                self.n -= 1;
                self.iter.next()
            } else {
                None
            }
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<I::Item> {
            if self.n > n {
                self.n -= n + 1;
                self.iter.nth(n)
            } else {
                if self.n > 0 {
                    self.iter.nth(self.n - 1);
                    self.n = 0;
                }
                None
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (lower, upper) = self.iter.size_hint();

            let lower = cmp::min(lower, self.n);

            let upper = match upper {
                Some(x) if x < self.n => Some(x),
                _ => Some(self.n)
            };

            (lower, upper)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> ExactSizeIterator for Take<I> where I: ExactSizeIterator {}

    /// An iterator to maintain state while iterating another iterator.
    ///
    /// This `struct` is created by the [`scan()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`scan()`]: trait.Iterator.html#method.scan
    /// [`Iterator`]: trait.Iterator.html
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct Scan<I, St, F> {
        iter: I,
        f: F,
        state: St,
    }

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<I: fmt::Debug, St: fmt::Debug, F> fmt::Debug for Scan<I, St, F> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Scan")
                .field("iter", &self.iter)
                .field("state", &self.state)
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B, I, St, F> Iterator for Scan<I, St, F> where
        I: Iterator,
        F: FnMut(&mut St, I::Item) -> Option<B>,
    {
        type Item = B;

        #[inline]
        fn next(&mut self) -> Option<B> {
            self.iter.next().and_then(|a| (self.f)(&mut self.state, a))
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, upper) = self.iter.size_hint();
            (0, upper) // can't know a lower bound, due to the scan function
        }
    }

    /// An iterator that maps each element to an iterator, and yields the elements
    /// of the produced iterators.
    ///
    /// This `struct` is created by the [`flat_map()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`flat_map()`]: trait.Iterator.html#method.flat_map
    /// [`Iterator`]: trait.Iterator.html
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct FlatMap<I, U: IntoIterator, F> {
        iter: I,
        f: F,
        frontiter: Option<U::IntoIter>,
        backiter: Option<U::IntoIter>,
    }

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<I: fmt::Debug, U: IntoIterator, F> fmt::Debug for FlatMap<I, U, F>
        where U::IntoIter: fmt::Debug
    {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("FlatMap")
                .field("iter", &self.iter)
                .field("frontiter", &self.frontiter)
                .field("backiter", &self.backiter)
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator, U: IntoIterator, F> Iterator for FlatMap<I, U, F>
        where F: FnMut(I::Item) -> U,
    {
        type Item = U::Item;

        #[inline]
        fn next(&mut self) -> Option<U::Item> {
            loop {
                if let Some(ref mut inner) = self.frontiter {
                    if let Some(x) = inner.by_ref().next() {
                        return Some(x)
                    }
                }
                match self.iter.next().map(&mut self.f) {
                    None => return self.backiter.as_mut().and_then(|it| it.next()),
                    next => self.frontiter = next.map(IntoIterator::into_iter),
                }
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (flo, fhi) = self.frontiter.as_ref().map_or((0, Some(0)), |it| it.size_hint());
            let (blo, bhi) = self.backiter.as_ref().map_or((0, Some(0)), |it| it.size_hint());
            let lo = flo.saturating_add(blo);
            match (self.iter.size_hint(), fhi, bhi) {
                ((0, Some(0)), Some(a), Some(b)) => (lo, a.checked_add(b)),
                _ => (lo, None)
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: DoubleEndedIterator, U, F> DoubleEndedIterator for FlatMap<I, U, F> where
        F: FnMut(I::Item) -> U,
        U: IntoIterator,
        U::IntoIter: DoubleEndedIterator
    {
        #[inline]
        fn next_back(&mut self) -> Option<U::Item> {
            loop {
                if let Some(ref mut inner) = self.backiter {
                    if let Some(y) = inner.next_back() {
                        return Some(y)
                    }
                }
                match self.iter.next_back().map(&mut self.f) {
                    None => return self.frontiter.as_mut().and_then(|it| it.next_back()),
                    next => self.backiter = next.map(IntoIterator::into_iter),
                }
            }
        }
    }

    /// An iterator that yields `None` forever after the underlying iterator
    /// yields `None` once.
    ///
    /// This `struct` is created by the [`fuse()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`fuse()`]: trait.Iterator.html#method.fuse
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Fuse<I> {
        iter: I,
        done: bool
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> Iterator for Fuse<I> where I: Iterator {
        type Item = <I as Iterator>::Item;

        #[inline]
        fn next(&mut self) -> Option<<I as Iterator>::Item> {
            if self.done {
                None
            } else {
                let next = self.iter.next();
                self.done = next.is_none();
                next
            }
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<I::Item> {
            if self.done {
                None
            } else {
                let nth = self.iter.nth(n);
                self.done = nth.is_none();
                nth
            }
        }

        #[inline]
        fn last(self) -> Option<I::Item> {
            if self.done {
                None
            } else {
                self.iter.last()
            }
        }

        #[inline]
        fn count(self) -> usize {
            if self.done {
                0
            } else {
                self.iter.count()
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            if self.done {
                (0, Some(0))
            } else {
                self.iter.size_hint()
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> DoubleEndedIterator for Fuse<I> where I: DoubleEndedIterator {
        #[inline]
        fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
            if self.done {
                None
            } else {
                let next = self.iter.next_back();
                self.done = next.is_none();
                next
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> ExactSizeIterator for Fuse<I> where I: ExactSizeIterator {}

    /// An iterator that calls a function with a reference to each element before
    /// yielding it.
    ///
    /// This `struct` is created by the [`inspect()`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`inspect()`]: trait.Iterator.html#method.inspect
    /// [`Iterator`]: trait.Iterator.html
    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct Inspect<I, F> {
        iter: I,
        f: F,
    }

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<I: fmt::Debug, F> fmt::Debug for Inspect<I, F> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Inspect")
                .field("iter", &self.iter)
                .finish()
        }
    }

    impl<I: Iterator, F> Inspect<I, F> where F: FnMut(&I::Item) {
        #[inline]
        fn do_inspect(&mut self, elt: Option<I::Item>) -> Option<I::Item> {
            if let Some(ref a) = elt {
                (self.f)(a);
            }

            elt
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator, F> Iterator for Inspect<I, F> where F: FnMut(&I::Item) {
        type Item = I::Item;

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            let next = self.iter.next();
            self.do_inspect(next)
        }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: DoubleEndedIterator, F> DoubleEndedIterator for Inspect<I, F>
        where F: FnMut(&I::Item),
    {
        #[inline]
        fn next_back(&mut self) -> Option<I::Item> {
            let next = self.iter.next_back();
            self.do_inspect(next)
        }
    }

    /// Objects that can be stepped over in both directions.
    ///
    /// The `steps_between` function provides a way to efficiently compare
    /// two `Step` objects.
    #[unstable(feature = "step_trait",
               reason = "likely to be replaced by finer-grained traits",
               issue = "27741")]
    pub trait Step: PartialOrd + Sized {
        /// Steps `self` if possible.
        fn step(&self, by: &Self) -> Option<Self>;

        /// Returns the number of steps between two step objects. The count is
        /// inclusive of `start` and exclusive of `end`.
        ///
        /// Returns `None` if it is not possible to calculate `steps_between`
        /// without overflow.
        fn steps_between(start: &Self, end: &Self, by: &Self) -> Option<usize>;
    }

    macro_rules! step_impl_unsigned {
        ($($t:ty)*) => ($(
            #[unstable(feature = "step_trait",
                       reason = "likely to be replaced by finer-grained traits",
                       issue = "27741")]
            impl Step for $t {
                #[inline]
                fn step(&self, by: &$t) -> Option<$t> {
                    (*self).checked_add(*by)
                }
                #[inline]
                #[allow(trivial_numeric_casts)]
                fn steps_between(start: &$t, end: &$t, by: &$t) -> Option<usize> {
                    if *by == 0 { return None; }
                    if *start < *end {
                        // Note: We assume $t <= usize here
                        let diff = (*end - *start) as usize;
                        let by = *by as usize;
                        if diff % by > 0 {
                            Some(diff / by + 1)
                        } else {
                            Some(diff / by)
                        }
                    } else {
                        Some(0)
                    }
                }
            }
        )*)
    }
    macro_rules! step_impl_signed {
        ($($t:ty)*) => ($(
            #[unstable(feature = "step_trait",
                       reason = "likely to be replaced by finer-grained traits",
                       issue = "27741")]
            impl Step for $t {
                #[inline]
                fn step(&self, by: &$t) -> Option<$t> {
                    (*self).checked_add(*by)
                }
                #[inline]
                #[allow(trivial_numeric_casts)]
                fn steps_between(start: &$t, end: &$t, by: &$t) -> Option<usize> {
                    if *by == 0 { return None; }
                    let diff: usize;
                    let by_u: usize;
                    if *by > 0 {
                        if *start >= *end {
                            return Some(0);
                        }
                        // Note: We assume $t <= isize here
                        // Use .wrapping_sub and cast to usize to compute the
                        // difference that may not fit inside the range of isize.
                        diff = (*end as isize).wrapping_sub(*start as isize) as usize;
                        by_u = *by as usize;
                    } else {
                        if *start <= *end {
                            return Some(0);
                        }
                        diff = (*start as isize).wrapping_sub(*end as isize) as usize;
                        by_u = (*by as isize).wrapping_mul(-1) as usize;
                    }
                    if diff % by_u > 0 {
                        Some(diff / by_u + 1)
                    } else {
                        Some(diff / by_u)
                    }
                }
            }
        )*)
    }

    macro_rules! step_impl_no_between {
        ($($t:ty)*) => ($(
            #[unstable(feature = "step_trait",
                       reason = "likely to be replaced by finer-grained traits",
                       issue = "27741")]
            impl Step for $t {
                #[inline]
                fn step(&self, by: &$t) -> Option<$t> {
                    (*self).checked_add(*by)
                }
                #[inline]
                fn steps_between(_a: &$t, _b: &$t, _by: &$t) -> Option<usize> {
                    None
                }
            }
        )*)
    }

    step_impl_unsigned!(usize u8 u16 u32);
    step_impl_signed!(isize i8 i16 i32);
    #[cfg(target_pointer_width = "64")]
    step_impl_unsigned!(u64);
    #[cfg(target_pointer_width = "64")]
    step_impl_signed!(i64);
    // If the target pointer width is not 64-bits, we
    // assume here that it is less than 64-bits.
    #[cfg(not(target_pointer_width = "64"))]
    step_impl_no_between!(u64 i64);

    /// An adapter for stepping range iterators by a custom amount.
    ///
    /// The resulting iterator handles overflow by stopping. The `A`
    /// parameter is the type being iterated over, while `R` is the range
    /// type (usually one of `std::ops::{Range, RangeFrom, RangeInclusive}`.
    #[derive(Clone, Debug)]
    #[unstable(feature = "step_by", reason = "recent addition",
               issue = "27741")]
    pub struct StepBy<A, R> {
        step_by: A,
        range: R,
    }

    impl<A: Step> ops::RangeFrom<A> {
        /// Creates an iterator starting at the same point, but stepping by
        /// the given amount at each iteration.
        ///
        /// # Examples
        ///
        /// ```
        /// # #![feature(step_by)]
        ///
        /// for i in (0u8..).step_by(2).take(10) {
        ///     println!("{}", i);
        /// }
        /// ```
        ///
        /// This prints the first ten even natural integers (0 to 18).
        #[unstable(feature = "step_by", reason = "recent addition",
                   issue = "27741")]
        pub fn step_by(self, by: A) -> StepBy<A, Self> {
            StepBy {
                step_by: by,
                range: self
            }
        }
    }

    impl<A: Step> ops::Range<A> {
        /// Creates an iterator with the same range, but stepping by the
        /// given amount at each iteration.
        ///
        /// The resulting iterator handles overflow by stopping.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(step_by)]
        ///
        /// for i in (0..10).step_by(2) {
        ///     println!("{}", i);
        /// }
        /// ```
        ///
        /// This prints:
        ///
        /// ```text
        /// 0
        /// 2
        /// 4
        /// 6
        /// 8
        /// ```
        #[unstable(feature = "step_by", reason = "recent addition",
                   issue = "27741")]
        pub fn step_by(self, by: A) -> StepBy<A, Self> {
            StepBy {
                step_by: by,
                range: self
            }
        }
    }

    impl<A: Step> ops::RangeInclusive<A> {
        /// Creates an iterator with the same range, but stepping by the
        /// given amount at each iteration.
        ///
        /// The resulting iterator handles overflow by stopping.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(step_by, inclusive_range_syntax)]
        ///
        /// for i in (0...10).step_by(2) {
        ///     println!("{}", i);
        /// }
        /// ```
        ///
        /// This prints:
        ///
        /// ```text
        /// 0
        /// 2
        /// 4
        /// 6
        /// 8
        /// 10
        /// ```
        #[unstable(feature = "step_by", reason = "recent addition",
                   issue = "27741")]
        pub fn step_by(self, by: A) -> StepBy<A, Self> {
            StepBy {
                step_by: by,
                range: self
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A> Iterator for StepBy<A, ops::RangeFrom<A>> where
        A: Clone,
        for<'a> &'a A: Add<&'a A, Output = A>
    {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            let mut n = &self.range.start + &self.step_by;
            mem::swap(&mut n, &mut self.range.start);
            Some(n)
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            (usize::MAX, None) // Too bad we can't specify an infinite lower bound
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Step + Zero + Clone> Iterator for StepBy<A, ops::Range<A>> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            let rev = self.step_by < A::zero();
            if (rev && self.range.start > self.range.end) ||
               (!rev && self.range.start < self.range.end)
            {
                match self.range.start.step(&self.step_by) {
                    Some(mut n) => {
                        mem::swap(&mut self.range.start, &mut n);
                        Some(n)
                    },
                    None => {
                        let mut n = self.range.end.clone();
                        mem::swap(&mut self.range.start, &mut n);
                        Some(n)
                    }
                }
            } else {
                None
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            match Step::steps_between(&self.range.start,
                                      &self.range.end,
                                      &self.step_by) {
                Some(hint) => (hint, Some(hint)),
                None       => (0, None)
            }
        }
    }

    #[unstable(feature = "inclusive_range",
               reason = "recently added, follows RFC",
               issue = "28237")]
    impl<A: Step + Zero + Clone> Iterator for StepBy<A, ops::RangeInclusive<A>> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            use ops::RangeInclusive::*;

            // this function has a sort of odd structure due to borrowck issues
            // we may need to replace self.range, so borrows of start and end need to end early

            let (finishing, n) = match self.range {
                Empty { .. } => return None, // empty iterators yield no values

                NonEmpty { ref mut start, ref mut end } => {
                    let zero = A::zero();
                    let rev = self.step_by < zero;

                    // march start towards (maybe past!) end and yield the old value
                    if (rev && start >= end) ||
                       (!rev && start <= end)
                    {
                        match start.step(&self.step_by) {
                            Some(mut n) => {
                                mem::swap(start, &mut n);
                                (None, Some(n)) // yield old value, remain non-empty
                            },
                            None => {
                                let mut n = end.clone();
                                mem::swap(start, &mut n);
                                (None, Some(n)) // yield old value, remain non-empty
                            }
                        }
                    } else {
                        // found range in inconsistent state (start at or past end), so become empty
                        (Some(mem::replace(end, zero)), None)
                    }
                }
            };

            // turn into an empty iterator if we've reached the end
            if let Some(end) = finishing {
                self.range = Empty { at: end };
            }

            n
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            use ops::RangeInclusive::*;

            match self.range {
                Empty { .. } => (0, Some(0)),

                NonEmpty { ref start, ref end } =>
                    match Step::steps_between(start,
                                              end,
                                              &self.step_by) {
                        Some(hint) => (hint.saturating_add(1), hint.checked_add(1)),
                        None       => (0, None)
                    }
            }
        }
    }

    macro_rules! range_exact_iter_impl {
        ($($t:ty)*) => ($(
            #[stable(feature = "rust1", since = "1.0.0")]
            impl ExactSizeIterator for ops::Range<$t> { }

            #[unstable(feature = "inclusive_range",
                       reason = "recently added, follows RFC",
                       issue = "28237")]
            impl ExactSizeIterator for ops::RangeInclusive<$t> { }
        )*)
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Step + One> Iterator for ops::Range<A> where
        for<'a> &'a A: Add<&'a A, Output = A>
    {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            if self.start < self.end {
                let mut n = &self.start + &A::one();
                mem::swap(&mut n, &mut self.start);
                Some(n)
            } else {
                None
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            match Step::steps_between(&self.start, &self.end, &A::one()) {
                Some(hint) => (hint, Some(hint)),
                None => (0, None)
            }
        }
    }

    // Ranges of u64 and i64 are excluded because they cannot guarantee having
    // a length <= usize::MAX, which is required by ExactSizeIterator.
    range_exact_iter_impl!(usize u8 u16 u32 isize i8 i16 i32);

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Step + One + Clone> DoubleEndedIterator for ops::Range<A> where
        for<'a> &'a A: Add<&'a A, Output = A>,
        for<'a> &'a A: Sub<&'a A, Output = A>
    {
        #[inline]
        fn next_back(&mut self) -> Option<A> {
            if self.start < self.end {
                self.end = &self.end - &A::one();
                Some(self.end.clone())
            } else {
                None
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Step + One> Iterator for ops::RangeFrom<A> where
        for<'a> &'a A: Add<&'a A, Output = A>
    {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            let mut n = &self.start + &A::one();
            mem::swap(&mut n, &mut self.start);
            Some(n)
        }
    }

    #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
    impl<A: Step + One> Iterator for ops::RangeInclusive<A> where
        for<'a> &'a A: Add<&'a A, Output = A>
    {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            use ops::RangeInclusive::*;

            // this function has a sort of odd structure due to borrowck issues
            // we may need to replace self, so borrows of self.start and self.end need to end early

            let (finishing, n) = match *self {
                Empty { .. } => (None, None), // empty iterators yield no values

                NonEmpty { ref mut start, ref mut end } => {
                    if start == end {
                        (Some(mem::replace(end, A::one())), Some(mem::replace(start, A::one())))
                    } else if start < end {
                        let one = A::one();
                        let mut n = &*start + &one;
                        mem::swap(&mut n, start);

                        // if the iterator is done iterating, it will change from NonEmpty to Empty
                        // to avoid unnecessary drops or clones, we'll reuse either start or end
                        // (they are equal now, so it doesn't matter which)
                        // to pull out end, we need to swap something back in -- use the previously
                        // created A::one() as a dummy value

                        (if n == *end { Some(mem::replace(end, one)) } else { None },
                        // ^ are we done yet?
                        Some(n)) // < the value to output
                    } else {
                        (Some(mem::replace(start, A::one())), None)
                    }
                }
            };

            // turn into an empty iterator if this is the last value
            if let Some(end) = finishing {
                *self = Empty { at: end };
            }

            n
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            use ops::RangeInclusive::*;

            match *self {
                Empty { .. } => (0, Some(0)),

                NonEmpty { ref start, ref end } =>
                    match Step::steps_between(start, end, &A::one()) {
                        Some(hint) => (hint.saturating_add(1), hint.checked_add(1)),
                        None => (0, None),
                    }
            }
        }
    }

    #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
    impl<A: Step + One> DoubleEndedIterator for ops::RangeInclusive<A> where
        for<'a> &'a A: Add<&'a A, Output = A>,
        for<'a> &'a A: Sub<&'a A, Output = A>
    {
        #[inline]
        fn next_back(&mut self) -> Option<A> {
            use ops::RangeInclusive::*;

            // see Iterator::next for comments

            let (finishing, n) = match *self {
                Empty { .. } => return None,

                NonEmpty { ref mut start, ref mut end } => {
                    if start == end {
                        (Some(mem::replace(start, A::one())), Some(mem::replace(end, A::one())))
                    } else if start < end {
                        let one = A::one();
                        let mut n = &*end - &one;
                        mem::swap(&mut n, end);

                        (if n == *start { Some(mem::replace(start, one)) } else { None },
                         Some(n))
                    } else {
                        (Some(mem::replace(end, A::one())), None)
                    }
                }
            };

            if let Some(start) = finishing {
                *self = Empty { at: start };
            }

            n
        }
    }

    /// An iterator that repeats an element endlessly.
    ///
    /// This `struct` is created by the [`repeat()`] function. See its documentation for more.
    ///
    /// [`repeat()`]: fn.repeat.html
    #[derive(Clone, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Repeat<A> {
        element: A
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Clone> Iterator for Repeat<A> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> { Some(self.element.clone()) }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) { (usize::MAX, None) }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Clone> DoubleEndedIterator for Repeat<A> {
        #[inline]
        fn next_back(&mut self) -> Option<A> { Some(self.element.clone()) }
    }

    /// Creates a new iterator that endlessly repeats a single element.
    ///
    /// The `repeat()` function repeats a single value over and over and over and
    /// over and over and 🔁.
    ///
    /// Infinite iterators like `repeat()` are often used with adapters like
    /// [`take()`], in order to make them finite.
    ///
    /// [`take()`]: trait.Iterator.html#method.take
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // the number four 4ever:
    /// let mut fours = iter::repeat(4);
    ///
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    ///
    /// // yup, still four
    /// assert_eq!(Some(4), fours.next());
    /// ```
    ///
    /// Going finite with [`take()`]:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // that last example was too many fours. Let's only have four fours.
    /// let mut four_fours = iter::repeat(4).take(4);
    ///
    /// assert_eq!(Some(4), four_fours.next());
    /// assert_eq!(Some(4), four_fours.next());
    /// assert_eq!(Some(4), four_fours.next());
    /// assert_eq!(Some(4), four_fours.next());
    ///
    /// // ... and now we're done
    /// assert_eq!(None, four_fours.next());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn repeat<T: Clone>(elt: T) -> Repeat<T> {
        Repeat{element: elt}
    }

    /// An iterator that yields nothing.
    ///
    /// This `struct` is created by the [`empty()`] function. See its documentation for more.
    ///
    /// [`empty()`]: fn.empty.html
    #[stable(feature = "iter_empty", since = "1.2.0")]
    pub struct Empty<T>(marker::PhantomData<T>);

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<T> fmt::Debug for Empty<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("Empty")
        }
    }

    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> Iterator for Empty<T> {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            None
        }

        fn size_hint(&self) -> (usize, Option<usize>){
            (0, Some(0))
        }
    }

    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> DoubleEndedIterator for Empty<T> {
        fn next_back(&mut self) -> Option<T> {
            None
        }
    }

    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> ExactSizeIterator for Empty<T> {
        fn len(&self) -> usize {
            0
        }
    }

    // not #[derive] because that adds a Clone bound on T,
    // which isn't necessary.
    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> Clone for Empty<T> {
        fn clone(&self) -> Empty<T> {
            Empty(marker::PhantomData)
        }
    }

    // not #[derive] because that adds a Default bound on T,
    // which isn't necessary.
    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> Default for Empty<T> {
        fn default() -> Empty<T> {
            Empty(marker::PhantomData)
        }
    }

    /// Creates an iterator that yields nothing.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // this could have been an iterator over i32, but alas, it's just not.
    /// let mut nope = iter::empty::<i32>();
    ///
    /// assert_eq!(None, nope.next());
    /// ```
    #[stable(feature = "iter_empty", since = "1.2.0")]
    pub fn empty<T>() -> Empty<T> {
        Empty(marker::PhantomData)
    }

    /// An iterator that yields an element exactly once.
    ///
    /// This `struct` is created by the [`once()`] function. See its documentation for more.
    ///
    /// [`once()`]: fn.once.html
    #[derive(Clone, Debug)]
    #[stable(feature = "iter_once", since = "1.2.0")]
    pub struct Once<T> {
        inner: ::option::IntoIter<T>
    }

    #[stable(feature = "iter_once", since = "1.2.0")]
    impl<T> Iterator for Once<T> {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            self.inner.next()
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.inner.size_hint()
        }
    }

    #[stable(feature = "iter_once", since = "1.2.0")]
    impl<T> DoubleEndedIterator for Once<T> {
        fn next_back(&mut self) -> Option<T> {
            self.inner.next_back()
        }
    }

    #[stable(feature = "iter_once", since = "1.2.0")]
    impl<T> ExactSizeIterator for Once<T> {
        fn len(&self) -> usize {
            self.inner.len()
        }
    }

    /// Creates an iterator that yields an element exactly once.
    ///
    /// This is commonly used to adapt a single value into a [`chain()`] of other
    /// kinds of iteration. Maybe you have an iterator that covers almost
    /// everything, but you need an extra special case. Maybe you have a function
    /// which works on iterators, but you only need to process one value.
    ///
    /// [`chain()`]: trait.Iterator.html#method.chain
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // one is the loneliest number
    /// let mut one = iter::once(1);
    ///
    /// assert_eq!(Some(1), one.next());
    ///
    /// // just one, that's all we get
    /// assert_eq!(None, one.next());
    /// ```
    ///
    /// Chaining together with another iterator. Let's say that we want to iterate
    /// over each file of the `.foo` directory, but also a configuration file,
    /// `.foorc`:
    ///
    /// ```no_run
    /// use std::iter;
    /// use std::fs;
    /// use std::path::PathBuf;
    ///
    /// let dirs = fs::read_dir(".foo").unwrap();
    ///
    /// // we need to convert from an iterator of DirEntry-s to an iterator of
    /// // PathBufs, so we use map
    /// let dirs = dirs.map(|file| file.unwrap().path());
    ///
    /// // now, our iterator just for our config file
    /// let config = iter::once(PathBuf::from(".foorc"));
    ///
    /// // chain the two iterators together into one big iterator
    /// let files = dirs.chain(config);
    ///
    /// // this will give us all of the files in .foo as well as .foorc
    /// for f in files {
    ///     println!("{:?}", f);
    /// }
    /// ```
    #[stable(feature = "iter_once", since = "1.2.0")]
    pub fn once<T>(value: T) -> Once<T> {
        Once { inner: Some(value).into_iter() }
    }

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