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    use crate::iter::{InPlaceIterable, Iterator};
    use crate::ops::{ControlFlow, Try};

    mod chain;
    mod cloned;
    mod copied;
    mod cycle;
    mod enumerate;
    mod filter;
    mod filter_map;
    mod flatten;
    mod fuse;
    mod inspect;
    mod map;
    mod map_while;
    mod peekable;
    mod rev;
    mod scan;
    mod skip;
    mod skip_while;
    mod step_by;
    mod take;
    mod take_while;
    mod zip;

    pub use self::{
        chain::Chain, cycle::Cycle, enumerate::Enumerate, filter::Filter, filter_map::FilterMap,
        flatten::FlatMap, fuse::Fuse, inspect::Inspect, map::Map, peekable::Peekable, rev::Rev,
        scan::Scan, skip::Skip, skip_while::SkipWhile, take::Take, take_while::TakeWhile, zip::Zip,
    };

    #[stable(feature = "iter_cloned", since = "1.1.0")]
    pub use self::cloned::Cloned;

    #[stable(feature = "iterator_step_by", since = "1.28.0")]
    pub use self::step_by::StepBy;

    #[stable(feature = "iterator_flatten", since = "1.29.0")]
    pub use self::flatten::Flatten;

    #[stable(feature = "iter_copied", since = "1.36.0")]
    pub use self::copied::Copied;

    #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
    pub use self::map_while::MapWhile;

    #[unstable(feature = "trusted_random_access", issue = "none")]
    pub use self::zip::TrustedRandomAccess;

    /// This trait provides transitive access to source-stage in an interator-adapter pipeline
    /// under the conditions that
    /// * the iterator source `S` itself implements `SourceIter<Source = S>`
    /// * there is a delegating implementation of this trait for each adapter in the pipeline between
    ///   the source and the pipeline consumer.
    ///
    /// When the source is an owning iterator struct (commonly called `IntoIter`) then
    /// this can be useful for specializing [`FromIterator`] implementations or recovering the
    /// remaining elements after an iterator has been partially exhausted.
    ///
    /// Note that implementations do not necessarily have to provide access to the inner-most
    /// source of a pipeline. A stateful intermediate adapter might eagerly evaluate a part
    /// of the pipeline and expose its internal storage as source.
    ///
    /// The trait is unsafe because implementers must uphold additional safety properties.
    /// See [`as_inner`] for details.
    ///
    /// # Examples
    ///
    /// Retrieving a partially consumed source:
    ///
    /// ```
    /// # #![feature(inplace_iteration)]
    /// # use std::iter::SourceIter;
    ///
    /// let mut iter = vec![9, 9, 9].into_iter().map(|i| i * i);
    /// let _ = iter.next();
    /// let mut remainder = std::mem::replace(unsafe { iter.as_inner() }, Vec::new().into_iter());
    /// println!("n = {} elements remaining", remainder.len());
    /// ```
    ///
    /// [`FromIterator`]: crate::iter::FromIterator
    /// [`as_inner`]: SourceIter::as_inner
    #[unstable(issue = "none", feature = "inplace_iteration")]
    pub unsafe trait SourceIter {
        /// A source stage in an iterator pipeline.
        type Source: Iterator;

        /// Retrieve the source of an iterator pipeline.
        ///
        /// # Safety
        ///
        /// Implementations of must return the same mutable reference for their lifetime, unless
        /// replaced by a caller.
        /// Callers may only replace the reference when they stopped iteration and drop the
        /// iterator pipeline after extracting the source.
        ///
        /// This means iterator adapters can rely on the source not changing during
        /// iteration but they cannot rely on it in their Drop implementations.
        ///
        /// Implementing this method means adapters relinquish private-only access to their
        /// source and can only rely on guarantees made based on method receiver types.
        /// The lack of restricted access also requires that adapters must uphold the source's
        /// public API even when they have access to its internals.
        ///
        /// Callers in turn must expect the source to be in any state that is consistent with
        /// its public API since adapters sitting between it and the source have the same
        /// access. In particular an adapter may have consumed more elements than strictly necessary.
        ///
        /// The overall goal of these requirements is to let the consumer of a pipeline use
        /// * whatever remains in the source after iteration has stopped
        /// * the memory that has become unused by advancing a consuming iterator
        ///
        /// [`next()`]: Iterator::next()
        unsafe fn as_inner(&mut self) -> &mut Self::Source;
    }

    /// 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.
    pub(crate) struct ResultShunt<'a, I, E> {
        iter: I,
        error: &'a mut Result<(), 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(crate) fn process_results<I, T, E, F, U>(iter: I, mut f: F) -> Result<U, E>
    where
        I: Iterator<Item = Result<T, E>>,
        for<'a> F: FnMut(ResultShunt<'a, I, E>) -> U,
    {
        let mut error = Ok(());
        let shunt = ResultShunt { iter, error: &mut error };
        let value = f(shunt);
        error.map(|()| value)
    }

    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> {
            self.find(|_| true)
        }

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

        fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
        where
            F: FnMut(B, Self::Item) -> R,
            R: Try<Ok = B>,
        {
            let error = &mut *self.error;
            self.iter
                .try_fold(init, |acc, x| match x {
                    Ok(x) => ControlFlow::from_try(f(acc, x)),
                    Err(e) => {
                        *error = Err(e);
                        ControlFlow::Break(try { acc })
                    }
                })
                .into_try()
        }

        fn fold<B, F>(mut self, init: B, fold: F) -> B
        where
            Self: Sized,
            F: FnMut(B, Self::Item) -> B,
        {
            #[inline]
            fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
                move |acc, x| Ok(f(acc, x))
            }

            self.try_fold(init, ok(fold)).unwrap()
        }
    }