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    use super::super::noop::*;
    use super::super::plumbing::*;
    use std::ptr;
    use std::slice;
    use std::sync::atomic::{AtomicUsize, Ordering};

    pub(super) struct CollectConsumer<'c, T: Send + 'c> {
        /// Tracks how many items we successfully wrote. Used to guarantee
        /// safety in the face of panics or buggy parallel iterators.
        ///
        /// In theory we could just produce this as a `CollectConsumer::Result`,
        /// folding local counts and reducing by addition, but that requires a
        /// certain amount of trust that the producer driving this will behave
        /// itself. Since this count is important to the safety of marking the
        /// memory initialized (`Vec::set_len`), we choose to keep it internal.
        writes: &'c AtomicUsize,

        /// A slice covering the target memory, not yet initialized!
        target: &'c mut [T],
    }

    pub(super) struct CollectFolder<'c, T: Send + 'c> {
        global_writes: &'c AtomicUsize,
        local_writes: usize,

        /// An iterator over the *uninitialized* target memory.
        target: slice::IterMut<'c, T>,
    }

    impl<'c, T: Send + 'c> CollectConsumer<'c, T> {
        /// The target memory is considered uninitialized, and will be
        /// overwritten without dropping anything.
        pub(super) fn new(writes: &'c AtomicUsize, target: &'c mut [T]) -> Self {
            CollectConsumer { writes, target }
        }
    }

    impl<'c, T: Send + 'c> Consumer<T> for CollectConsumer<'c, T> {
        type Folder = CollectFolder<'c, T>;
        type Reducer = NoopReducer;
        type Result = ();

        fn split_at(self, index: usize) -> (Self, Self, NoopReducer) {
            // instances Read in the fields from `self` and then
            // forget `self`, since it has been legitimately consumed
            // (and not dropped during unwinding).
            let CollectConsumer { writes, target } = self;

            // Produce new consumers. Normal slicing ensures that the
            // memory range given to each consumer is disjoint.
            let (left, right) = target.split_at_mut(index);
            (
                CollectConsumer::new(writes, left),
                CollectConsumer::new(writes, right),
                NoopReducer,
            )
        }

        fn into_folder(self) -> CollectFolder<'c, T> {
            CollectFolder {
                global_writes: self.writes,
                local_writes: 0,
                target: self.target.iter_mut(),
            }
        }

        fn full(&self) -> bool {
            false
        }
    }

    impl<'c, T: Send + 'c> Folder<T> for CollectFolder<'c, T> {
        type Result = ();

        fn consume(mut self, item: T) -> CollectFolder<'c, T> {
            // Compute target pointer and write to it. Safe because the iterator
            // does all the bounds checking; we're only avoiding the target drop.
            let head = self
                .target
                .next()
                .expect("too many values pushed to consumer");
            unsafe {
                ptr::write(head, item);
            }

            self.local_writes += 1;
            self
        }

        fn complete(self) {
            // NB: We don't explicitly check that the local writes were complete,
            // but `Collect::complete()` will assert the global write count.

            // track total values written
            self.global_writes
                .fetch_add(self.local_writes, Ordering::Relaxed);
        }

        fn full(&self) -> bool {
            false
        }
    }

    /// Pretend to be unindexed for `special_collect_into_vec`,
    /// but we should never actually get used that way...
    impl<'c, T: Send + 'c> UnindexedConsumer<T> for CollectConsumer<'c, T> {
        fn split_off_left(&self) -> Self {
            unreachable!("CollectConsumer must be indexed!")
        }
        fn to_reducer(&self) -> Self::Reducer {
            NoopReducer
        }
    }

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