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

    /// 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>>(iter: I) -> Self {
    ///         let mut c = MyCollection::new();
    ///
    ///         for i in iter {
    ///             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>>(iter: 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. When
    /// extending a collection with an already existing key, that entry is updated
    /// or, in the case of collections that permit multiple entries with equal
    /// keys, that entry is inserted.
    ///
    /// # 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, iter: T) {
    ///
    ///         // The implementation is very straightforward: loop through the
    ///         // iterator, and add() each element to ourselves.
    ///         for elem in iter {
    ///             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, iter: 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, 4, 5, 6];
    ///
    /// let mut iter = numbers.iter();
    ///
    /// assert_eq!(Some(&1), iter.next());
    /// assert_eq!(Some(&6), iter.next_back());
    /// assert_eq!(Some(&5), iter.next_back());
    /// assert_eq!(Some(&2), iter.next());
    /// assert_eq!(Some(&3), iter.next());
    /// assert_eq!(Some(&4), iter.next());
    /// assert_eq!(None, iter.next());
    /// assert_eq!(None, iter.next_back());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait DoubleEndedIterator: Iterator {
        /// Removes and returns an element from the end of the iterator.
        ///
        /// Returns `None` when there are no more elements.
        ///
        /// The [trait-level] docs contain more details.
        ///
        /// [trait-level]: trait.DoubleEndedIterator.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let numbers = vec![1, 2, 3, 4, 5, 6];
        ///
        /// let mut iter = numbers.iter();
        ///
        /// assert_eq!(Some(&1), iter.next());
        /// assert_eq!(Some(&6), iter.next_back());
        /// assert_eq!(Some(&5), iter.next_back());
        /// assert_eq!(Some(&2), iter.next());
        /// assert_eq!(Some(&3), iter.next());
        /// assert_eq!(Some(&4), iter.next());
        /// 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 {
        /// 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());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        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
        }

        /// Returns whether the iterator is empty.
        ///
        /// This method has a default implementation using `self.len()`, so you
        /// don't need to implement it yourself.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(exact_size_is_empty)]
        ///
        /// let mut one_element = 0..1;
        /// assert!(!one_element.is_empty());
        ///
        /// assert_eq!(one_element.next(), Some(0));
        /// assert!(one_element.is_empty());
        ///
        /// assert_eq!(one_element.next(), None);
        /// ```
        #[inline]
        #[unstable(feature = "exact_size_is_empty", issue = "35428")]
        fn is_empty(&self) -> bool {
            self.len() == 0
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, I: ExactSizeIterator + ?Sized> ExactSizeIterator for &'a mut I {
        fn len(&self) -> usize {
            (**self).len()
        }
        fn is_empty(&self) -> bool {
            (**self).is_empty()
        }
    }

    /// Trait to represent types that can be created by summing up an iterator.
    ///
    /// This trait is used to implement the [`sum()`] method on iterators. Types which
    /// implement the trait can be generated by the [`sum()`] method. Like
    /// [`FromIterator`] this trait should rarely be called directly and instead
    /// interacted with through [`Iterator::sum()`].
    ///
    /// [`sum()`]: ../../std/iter/trait.Sum.html#tymethod.sum
    /// [`FromIterator`]: ../../std/iter/trait.FromIterator.html
    /// [`Iterator::sum()`]: ../../std/iter/trait.Iterator.html#method.sum
    #[stable(feature = "iter_arith_traits", since = "1.12.0")]
    pub trait Sum<A = Self>: Sized {
        /// Method which takes an iterator and generates `Self` from the elements by
        /// "summing up" the items.
        #[stable(feature = "iter_arith_traits", since = "1.12.0")]
        fn sum<I: Iterator<Item=A>>(iter: I) -> Self;
    }

    /// Trait to represent types that can be created by multiplying elements of an
    /// iterator.
    ///
    /// This trait is used to implement the [`product()`] method on iterators. Types
    /// which implement the trait can be generated by the [`product()`] method. Like
    /// [`FromIterator`] this trait should rarely be called directly and instead
    /// interacted with through [`Iterator::product()`].
    ///
    /// [`product()`]: ../../std/iter/trait.Product.html#tymethod.product
    /// [`FromIterator`]: ../../std/iter/trait.FromIterator.html
    /// [`Iterator::product()`]: ../../std/iter/trait.Iterator.html#method.product
    #[stable(feature = "iter_arith_traits", since = "1.12.0")]
    pub trait Product<A = Self>: Sized {
        /// Method which takes an iterator and generates `Self` from the elements by
        /// multiplying the items.
        #[stable(feature = "iter_arith_traits", since = "1.12.0")]
        fn product<I: Iterator<Item=A>>(iter: I) -> Self;
    }

    // NB: explicitly use Add and Mul here to inherit overflow checks
    macro_rules! integer_sum_product {
        (@impls $zero:expr, $one:expr, $($a:ty)*) => ($(
            #[stable(feature = "iter_arith_traits", since = "1.12.0")]
            impl Sum for $a {
                fn sum<I: Iterator<Item=$a>>(iter: I) -> $a {
                    iter.fold($zero, Add::add)
                }
            }

            #[stable(feature = "iter_arith_traits", since = "1.12.0")]
            impl Product for $a {
                fn product<I: Iterator<Item=$a>>(iter: I) -> $a {
                    iter.fold($one, Mul::mul)
                }
            }

            #[stable(feature = "iter_arith_traits", since = "1.12.0")]
            impl<'a> Sum<&'a $a> for $a {
                fn sum<I: Iterator<Item=&'a $a>>(iter: I) -> $a {
                    iter.fold($zero, Add::add)
                }
            }

            #[stable(feature = "iter_arith_traits", since = "1.12.0")]
            impl<'a> Product<&'a $a> for $a {
                fn product<I: Iterator<Item=&'a $a>>(iter: I) -> $a {
                    iter.fold($one, Mul::mul)
                }
            }
        )*);
        ($($a:ty)*) => (
            integer_sum_product!(@impls 0, 1, $($a)+);
            integer_sum_product!(@impls Wrapping(0), Wrapping(1), $(Wrapping<$a>)+);
        );
    }

    macro_rules! float_sum_product {
        ($($a:ident)*) => ($(
            #[stable(feature = "iter_arith_traits", since = "1.12.0")]
            impl Sum for $a {
                fn sum<I: Iterator<Item=$a>>(iter: I) -> $a {
                    iter.fold(0.0, |a, b| a + b)
                }
            }

            #[stable(feature = "iter_arith_traits", since = "1.12.0")]
            impl Product for $a {
                fn product<I: Iterator<Item=$a>>(iter: I) -> $a {
                    iter.fold(1.0, |a, b| a * b)
                }
            }

            #[stable(feature = "iter_arith_traits", since = "1.12.0")]
            impl<'a> Sum<&'a $a> for $a {
                fn sum<I: Iterator<Item=&'a $a>>(iter: I) -> $a {
                    iter.fold(0.0, |a, b| a + *b)
                }
            }

            #[stable(feature = "iter_arith_traits", since = "1.12.0")]
            impl<'a> Product<&'a $a> for $a {
                fn product<I: Iterator<Item=&'a $a>>(iter: I) -> $a {
                    iter.fold(1.0, |a, b| a * *b)
                }
            }
        )*)
    }

    integer_sum_product! { i8 i16 i32 i64 isize u8 u16 u32 u64 usize }
    float_sum_product! { f32 f64 }

    /// An iterator adapter that produces output as long as the underlying
    /// iterator produces `Result::Ok` values.
    ///
    /// If an error is encountered, the iterator stops and the error is
    /// stored. The error may be recovered later via `reconstruct`.
    struct ResultShunt<I, E> {
        iter: I,
        error: Option<E>,
    }

    impl<I, T, E> ResultShunt<I, E>
        where I: Iterator<Item = Result<T, E>>
    {
        /// Process the given iterator as if it yielded a `T` instead of a
        /// `Result<T, _>`. Any errors will stop the inner iterator and
        /// the overall result will be an error.
        pub fn process<F, U>(iter: I, mut f: F) -> Result<U, E>
            where F: FnMut(&mut Self) -> U
        {
            let mut shunt = ResultShunt::new(iter);
            let value = f(shunt.by_ref());
            shunt.reconstruct(value)
        }

        fn new(iter: I) -> Self {
            ResultShunt {
                iter: iter,
                error: None,
            }
        }

        /// Consume the adapter and rebuild a `Result` value. This should
        /// *always* be called, otherwise any potential error would be
        /// lost.
        fn reconstruct<U>(self, val: U) -> Result<U, E> {
            match self.error {
                None => Ok(val),
                Some(e) => Err(e),
            }
        }
    }

    impl<I, T, E> Iterator for ResultShunt<I, E>
        where I: Iterator<Item = Result<T, E>>
    {
        type Item = T;

        fn next(&mut self) -> Option<Self::Item> {
            match self.iter.next() {
                Some(Ok(v)) => Some(v),
                Some(Err(e)) => {
                    self.error = Some(e);
                    None
                }
                None => None,
            }
        }
    }

    #[stable(feature = "iter_arith_traits_result", since="1.16.0")]
    impl<T, U, E> Sum<Result<U, E>> for Result<T, E>
        where T: Sum<U>,
    {
        fn sum<I>(iter: I) -> Result<T, E>
            where I: Iterator<Item = Result<U, E>>,
        {
            ResultShunt::process(iter, |i| i.sum())
        }
    }

    #[stable(feature = "iter_arith_traits_result", since="1.16.0")]
    impl<T, U, E> Product<Result<U, E>> for Result<T, E>
        where T: Product<U>,
    {
        fn product<I>(iter: I) -> Result<T, E>
            where I: Iterator<Item = Result<U, E>>,
        {
            ResultShunt::process(iter, |i| i.product())
        }
    }

    /// An iterator that always continues to yield `None` when exhausted.
    ///
    /// Calling next on a fused iterator that has returned `None` once is guaranteed
    /// to return [`None`] again. This trait is should be implemented by all iterators
    /// that behave this way because it allows for some significant optimizations.
    ///
    /// Note: In general, you should not use `FusedIterator` in generic bounds if
    /// you need a fused iterator. Instead, you should just call [`Iterator::fuse()`]
    /// on the iterator. If the iterator is already fused, the additional [`Fuse`]
    /// wrapper will be a no-op with no performance penalty.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    /// [`Iterator::fuse()`]: ../../std/iter/trait.Iterator.html#method.fuse
    /// [`Fuse`]: ../../std/iter/struct.Fuse.html
    #[unstable(feature = "fused", issue = "35602")]
    pub trait FusedIterator: Iterator {}

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, I: FusedIterator + ?Sized> FusedIterator for &'a mut I {}

    /// An iterator that reports an accurate length using size_hint.
    ///
    /// The iterator reports a size hint where it is either exact
    /// (lower bound is equal to upper bound), or the upper bound is [`None`].
    /// The upper bound must only be [`None`] if the actual iterator length is
    /// larger than [`usize::MAX`].
    ///
    /// The iterator must produce exactly the number of elements it reported.
    ///
    /// # Safety
    ///
    /// This trait must only be implemented when the contract is upheld.
    /// Consumers of this trait must inspect [`.size_hint()`]’s upper bound.
    ///
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    /// [`usize::MAX`]: ../../std/usize/constant.MAX.html
    /// [`.size_hint()`]: ../../std/iter/trait.Iterator.html#method.size_hint
    #[unstable(feature = "trusted_len", issue = "37572")]
    pub unsafe trait TrustedLen : Iterator {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<'a, I: TrustedLen + ?Sized> TrustedLen for &'a mut I {}

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