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    use crate::ops::{Mul, Add};
    use crate::num::Wrapping;

    /// 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;
    }

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

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

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

            #[$attr]
            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,
                    #[stable(feature = "iter_arith_traits", since = "1.12.0")],
                    $($a)*);
            integer_sum_product!(@impls Wrapping(0), Wrapping(1),
                    #[stable(feature = "wrapping_iter_arith", since = "1.14.0")],
                    $(Wrapping<$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 i128 isize u8 u16 u32 u64 u128 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,
                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,
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            if self.error.is_some() {
                (0, Some(0))
            } else {
                let (_, upper) = self.iter.size_hint();
                (0, upper)
            }
        }
    }

    #[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>,
    {
        /// Takes each element in the `Iterator`: if it is an `Err`, no further
        /// elements are taken, and the `Err` is returned. Should no `Err` occur,
        /// the sum of all elements is returned.
        ///
        /// # Examples
        ///
        /// This sums up every integer in a vector, rejecting the sum if a negative
        /// element is encountered:
        ///
        /// ```
        /// let v = vec![1, 2];
        /// let res: Result<i32, &'static str> = v.iter().map(|&x: &i32|
        ///     if x < 0 { Err("Negative element found") }
        ///     else { Ok(x) }
        /// ).sum();
        /// assert_eq!(res, Ok(3));
        /// ```
        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>,
    {
        /// Takes each element in the `Iterator`: if it is an `Err`, no further
        /// elements are taken, and the `Err` is returned. Should no `Err` occur,
        /// the product of all elements is returned.
        fn product<I>(iter: I) -> Result<T, E>
            where I: Iterator<Item = Result<U, E>>,
        {
            ResultShunt::process(iter, |i| i.product())
        }
    }

    /// An iterator adapter that produces output as long as the underlying
    /// iterator produces `Option::Some` values.
    struct OptionShunt<I> {
        iter: I,
        exited_early: bool,
    }

    impl<I, T> OptionShunt<I>
    where
        I: Iterator<Item = Option<T>>,
    {
        /// Process the given iterator as if it yielded a `T` instead of a
        /// `Option<T>`. Any `None` value will stop the inner iterator and
        /// the overall result will be a `None`.
        pub fn process<F, U>(iter: I, mut f: F) -> Option<U>
        where
            F: FnMut(&mut Self) -> U,
        {
            let mut shunt = OptionShunt::new(iter);
            let value = f(shunt.by_ref());
            shunt.reconstruct(value)
        }

        fn new(iter: I) -> Self {
            OptionShunt {
                iter,
                exited_early: false,
            }
        }

        /// Consume the adapter and rebuild a `Option` value.
        fn reconstruct<U>(self, val: U) -> Option<U> {
            if self.exited_early {
                None
            } else {
                Some(val)
            }
        }
    }

    impl<I, T> Iterator for OptionShunt<I>
    where
        I: Iterator<Item = Option<T>>,
    {
        type Item = T;

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

        fn size_hint(&self) -> (usize, Option<usize>) {
            if self.exited_early {
                (0, Some(0))
            } else {
                let (_, upper) = self.iter.size_hint();
                (0, upper)
            }
        }
    }

    #[stable(feature = "iter_arith_traits_option", since = "1.37.0")]
    impl<T, U> Sum<Option<U>> for Option<T>
    where
        T: Sum<U>,
    {
        /// Takes each element in the `Iterator`: if it is a `None`, no further
        /// elements are taken, and the `None` is returned. Should no `None` occur,
        /// the sum of all elements is returned.
        ///
        /// # Examples
        ///
        /// This sums up the position of the character 'a' in a vector of strings,
        /// if a word did not have the character 'a' the operation returns `None`:
        ///
        /// ```
        /// let words = vec!["have", "a", "great", "day"];
        /// let total: Option<usize> = words.iter().map(|w| w.find('a')).sum();
        /// assert_eq!(total, Some(5));
        /// ```
        fn sum<I>(iter: I) -> Option<T>
        where
            I: Iterator<Item = Option<U>>,
        {
            OptionShunt::process(iter, |i| i.sum())
        }
    }

    #[stable(feature = "iter_arith_traits_option", since = "1.37.0")]
    impl<T, U> Product<Option<U>> for Option<T>
    where
        T: Product<U>,
    {
        /// Takes each element in the `Iterator`: if it is a `None`, no further
        /// elements are taken, and the `None` is returned. Should no `None` occur,
        /// the product of all elements is returned.
        fn product<I>(iter: I) -> Option<T>
        where
            I: Iterator<Item = Option<U>>,
        {
            OptionShunt::process(iter, |i| i.product())
        }
    }

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