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    use iter::plumbing::*;
    use iter::*;

    /// Creates a parallel iterator that produces an element exactly once.
    ///
    /// This admits no parallelism on its own, but it could be chained to existing
    /// parallel iterators to extend their contents, or otherwise used for any code
    /// that deals with generic parallel iterators.
    ///
    /// # Examples
    ///
    /// ```
    /// use rayon::prelude::*;
    /// use rayon::iter::once;
    ///
    /// let pi = (0..1234).into_par_iter()
    ///     .chain(once(-1))
    ///     .chain(1234..10_000);
    ///
    /// assert_eq!(pi.clone().count(), 10_001);
    /// assert_eq!(pi.clone().filter(|&x| x < 0).count(), 1);
    /// assert_eq!(pi.position_any(|x| x < 0), Some(1234));
    /// ```
    pub fn once<T: Send>(item: T) -> Once<T> {
        Once { item }
    }

    /// Iterator adaptor for [the `once()` function](fn.once.html).
    #[derive(Clone, Debug)]
    pub struct Once<T: Send> {
        item: T,
    }

    impl<T: Send> ParallelIterator for Once<T> {
        type Item = T;

        fn drive_unindexed<C>(self, consumer: C) -> C::Result
        where
            C: UnindexedConsumer<Self::Item>,
        {
            self.drive(consumer)
        }

        fn opt_len(&self) -> Option<usize> {
            Some(1)
        }
    }

    impl<T: Send> IndexedParallelIterator for Once<T> {
        fn drive<C>(self, consumer: C) -> C::Result
        where
            C: Consumer<Self::Item>,
        {
            consumer.into_folder().consume(self.item).complete()
        }

        fn len(&self) -> usize {
            1
        }

        fn with_producer<CB>(self, callback: CB) -> CB::Output
        where
            CB: ProducerCallback<Self::Item>,
        {
            // Let `OptionProducer` handle it.
            Some(self.item).into_par_iter().with_producer(callback)
        }
    }

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