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    use crate::iter::{FusedIterator, TrustedLen};

    /// Creates a new iterator that repeats elements of type `A` endlessly by
    /// applying the provided closure, the repeater, `F: FnMut() -> A`.
    ///
    /// The `repeat_with()` function calls the repeater over and over again.
    ///
    /// Infinite iterators like `repeat_with()` are often used with adapters like
    /// [`Iterator::take()`], in order to make them finite.
    ///
    /// If the element type of the iterator you need implements [`Clone`], and
    /// it is OK to keep the source element in memory, you should instead use
    /// the [`repeat()`] function.
    ///
    /// An iterator produced by `repeat_with()` is not a [`DoubleEndedIterator`].
    /// If you need `repeat_with()` to return a [`DoubleEndedIterator`],
    /// please open a GitHub issue explaining your use case.
    ///
    /// [`repeat()`]: crate::iter::repeat
    /// [`DoubleEndedIterator`]: crate::iter::DoubleEndedIterator
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // let's assume we have some value of a type that is not `Clone`
    /// // or which don't want to have in memory just yet because it is expensive:
    /// #[derive(PartialEq, Debug)]
    /// struct Expensive;
    ///
    /// // a particular value forever:
    /// let mut things = iter::repeat_with(|| Expensive);
    ///
    /// assert_eq!(Some(Expensive), things.next());
    /// assert_eq!(Some(Expensive), things.next());
    /// assert_eq!(Some(Expensive), things.next());
    /// assert_eq!(Some(Expensive), things.next());
    /// assert_eq!(Some(Expensive), things.next());
    /// ```
    ///
    /// Using mutation and going finite:
    ///
    /// ```rust
    /// use std::iter;
    ///
    /// // From the zeroth to the third power of two:
    /// let mut curr = 1;
    /// let mut pow2 = iter::repeat_with(|| { let tmp = curr; curr *= 2; tmp })
    ///                     .take(4);
    ///
    /// assert_eq!(Some(1), pow2.next());
    /// assert_eq!(Some(2), pow2.next());
    /// assert_eq!(Some(4), pow2.next());
    /// assert_eq!(Some(8), pow2.next());
    ///
    /// // ... and now we're done
    /// assert_eq!(None, pow2.next());
    /// ```
    #[inline]
    #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
    pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> {
        RepeatWith { repeater }
    }

    /// An iterator that repeats elements of type `A` endlessly by
    /// applying the provided closure `F: FnMut() -> A`.
    ///
    /// This `struct` is created by the [`repeat_with()`] function.
    /// See its documentation for more.
    #[derive(Copy, Clone, Debug)]
    #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
    pub struct RepeatWith<F> {
        repeater: F,
    }

    #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
    impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            Some((self.repeater)())
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            (usize::MAX, None)
        }
    }

    #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
    impl<A, F: FnMut() -> A> FusedIterator for RepeatWith<F> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A, F: FnMut() -> A> TrustedLen for RepeatWith<F> {}