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    use crate::fmt;
    use crate::marker;
    use crate::usize;

    use super::{FusedIterator, TrustedLen};

    /// An iterator that repeats an element endlessly.
    ///
    /// This `struct` is created by the [`repeat`] function. See its documentation for more.
    ///
    /// [`repeat`]: fn.repeat.html
    #[derive(Clone, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Repeat<A> {
        element: A,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Clone> Iterator for Repeat<A> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            Some(self.element.clone())
        }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            (usize::MAX, None)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Clone> DoubleEndedIterator for Repeat<A> {
        #[inline]
        fn next_back(&mut self) -> Option<A> {
            Some(self.element.clone())
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<A: Clone> FusedIterator for Repeat<A> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A: Clone> TrustedLen for Repeat<A> {}

    /// Creates a new iterator that endlessly repeats a single element.
    ///
    /// The `repeat()` function repeats a single value over and over again.
    ///
    /// Infinite iterators like `repeat()` are often used with adapters like
    /// [`take`], in order to make them finite.
    ///
    /// [`take`]: trait.Iterator.html#method.take
    ///
    /// If the element type of the iterator you need does not implement `Clone`,
    /// or if you do not want to keep the repeated element in memory, you can
    /// instead use the [`repeat_with`] function.
    ///
    /// [`repeat_with`]: fn.repeat_with.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // the number four 4ever:
    /// let mut fours = iter::repeat(4);
    ///
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    /// assert_eq!(Some(4), fours.next());
    ///
    /// // yup, still four
    /// assert_eq!(Some(4), fours.next());
    /// ```
    ///
    /// Going finite with [`take`]:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // that last example was too many fours. Let's only have four fours.
    /// let mut four_fours = iter::repeat(4).take(4);
    ///
    /// assert_eq!(Some(4), four_fours.next());
    /// assert_eq!(Some(4), four_fours.next());
    /// assert_eq!(Some(4), four_fours.next());
    /// assert_eq!(Some(4), four_fours.next());
    ///
    /// // ... and now we're done
    /// assert_eq!(None, four_fours.next());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn repeat<T: Clone>(elt: T) -> Repeat<T> {
        Repeat { element: elt }
    }

    /// 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.
    ///
    /// [`repeat_with`]: fn.repeat_with.html
    #[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> {}

    /// 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
    /// [`take`], in order to make them finite.
    ///
    /// [`take`]: trait.Iterator.html#method.take
    ///
    /// 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.
    ///
    /// [`repeat`]: fn.repeat.html
    ///
    /// 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.
    ///
    /// # 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 yields nothing.
    ///
    /// This `struct` is created by the [`empty`] function. See its documentation for more.
    ///
    /// [`empty`]: fn.empty.html
    #[stable(feature = "iter_empty", since = "1.2.0")]
    pub struct Empty<T>(marker::PhantomData<T>);

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<T> fmt::Debug for Empty<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("Empty")
        }
    }

    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> Iterator for Empty<T> {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            None
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (0, Some(0))
        }
    }

    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> DoubleEndedIterator for Empty<T> {
        fn next_back(&mut self) -> Option<T> {
            None
        }
    }

    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> ExactSizeIterator for Empty<T> {
        fn len(&self) -> usize {
            0
        }
    }

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<T> TrustedLen for Empty<T> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for Empty<T> {}

    // not #[derive] because that adds a Clone bound on T,
    // which isn't necessary.
    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> Clone for Empty<T> {
        fn clone(&self) -> Empty<T> {
            Empty(marker::PhantomData)
        }
    }

    // not #[derive] because that adds a Default bound on T,
    // which isn't necessary.
    #[stable(feature = "iter_empty", since = "1.2.0")]
    impl<T> Default for Empty<T> {
        fn default() -> Empty<T> {
            Empty(marker::PhantomData)
        }
    }

    /// Creates an iterator that yields nothing.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // this could have been an iterator over i32, but alas, it's just not.
    /// let mut nope = iter::empty::<i32>();
    ///
    /// assert_eq!(None, nope.next());
    /// ```
    #[stable(feature = "iter_empty", since = "1.2.0")]
    #[cfg_attr(not(bootstrap), rustc_const_stable(feature = "const_iter_empty", since = "1.32.0"))]
    pub const fn empty<T>() -> Empty<T> {
        Empty(marker::PhantomData)
    }

    /// An iterator that yields an element exactly once.
    ///
    /// This `struct` is created by the [`once`] function. See its documentation for more.
    ///
    /// [`once`]: fn.once.html
    #[derive(Clone, Debug)]
    #[stable(feature = "iter_once", since = "1.2.0")]
    pub struct Once<T> {
        inner: crate::option::IntoIter<T>,
    }

    #[stable(feature = "iter_once", since = "1.2.0")]
    impl<T> Iterator for Once<T> {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            self.inner.next()
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.inner.size_hint()
        }
    }

    #[stable(feature = "iter_once", since = "1.2.0")]
    impl<T> DoubleEndedIterator for Once<T> {
        fn next_back(&mut self) -> Option<T> {
            self.inner.next_back()
        }
    }

    #[stable(feature = "iter_once", since = "1.2.0")]
    impl<T> ExactSizeIterator for Once<T> {
        fn len(&self) -> usize {
            self.inner.len()
        }
    }

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<T> TrustedLen for Once<T> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for Once<T> {}

    /// Creates an iterator that yields an element exactly once.
    ///
    /// This is commonly used to adapt a single value into a [`chain`] of other
    /// kinds of iteration. Maybe you have an iterator that covers almost
    /// everything, but you need an extra special case. Maybe you have a function
    /// which works on iterators, but you only need to process one value.
    ///
    /// [`chain`]: trait.Iterator.html#method.chain
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::iter;
    ///
    /// // one is the loneliest number
    /// let mut one = iter::once(1);
    ///
    /// assert_eq!(Some(1), one.next());
    ///
    /// // just one, that's all we get
    /// assert_eq!(None, one.next());
    /// ```
    ///
    /// Chaining together with another iterator. Let's say that we want to iterate
    /// over each file of the `.foo` directory, but also a configuration file,
    /// `.foorc`:
    ///
    /// ```no_run
    /// use std::iter;
    /// use std::fs;
    /// use std::path::PathBuf;
    ///
    /// let dirs = fs::read_dir(".foo").unwrap();
    ///
    /// // we need to convert from an iterator of DirEntry-s to an iterator of
    /// // PathBufs, so we use map
    /// let dirs = dirs.map(|file| file.unwrap().path());
    ///
    /// // now, our iterator just for our config file
    /// let config = iter::once(PathBuf::from(".foorc"));
    ///
    /// // chain the two iterators together into one big iterator
    /// let files = dirs.chain(config);
    ///
    /// // this will give us all of the files in .foo as well as .foorc
    /// for f in files {
    ///     println!("{:?}", f);
    /// }
    /// ```
    #[stable(feature = "iter_once", since = "1.2.0")]
    pub fn once<T>(value: T) -> Once<T> {
        Once { inner: Some(value).into_iter() }
    }

    /// An iterator that yields a single element of type `A` by
    /// applying the provided closure `F: FnOnce() -> A`.
    ///
    /// This `struct` is created by the [`once_with`] function.
    /// See its documentation for more.
    ///
    /// [`once_with`]: fn.once_with.html
    #[derive(Copy, Clone, Debug)]
    #[unstable(feature = "iter_once_with", issue = "57581")]
    pub struct OnceWith<F> {
        gen: Option<F>,
    }

    #[unstable(feature = "iter_once_with", issue = "57581")]
    impl<A, F: FnOnce() -> A> Iterator for OnceWith<F> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            let f = self.gen.take()?;
            Some(f())
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.gen.iter().size_hint()
        }
    }

    #[unstable(feature = "iter_once_with", issue = "57581")]
    impl<A, F: FnOnce() -> A> DoubleEndedIterator for OnceWith<F> {
        fn next_back(&mut self) -> Option<A> {
            self.next()
        }
    }

    #[unstable(feature = "iter_once_with", issue = "57581")]
    impl<A, F: FnOnce() -> A> ExactSizeIterator for OnceWith<F> {
        fn len(&self) -> usize {
            self.gen.iter().len()
        }
    }

    #[unstable(feature = "iter_once_with", issue = "57581")]
    impl<A, F: FnOnce() -> A> FusedIterator for OnceWith<F> {}

    #[unstable(feature = "iter_once_with", issue = "57581")]
    unsafe impl<A, F: FnOnce() -> A> TrustedLen for OnceWith<F> {}

    /// Creates an iterator that lazily generates a value exactly once by invoking
    /// the provided closure.
    ///
    /// This is commonly used to adapt a single value generator into a [`chain`] of
    /// other kinds of iteration. Maybe you have an iterator that covers almost
    /// everything, but you need an extra special case. Maybe you have a function
    /// which works on iterators, but you only need to process one value.
    ///
    /// Unlike [`once`], this function will lazily generate the value on request.
    ///
    /// [`once`]: fn.once.html
    /// [`chain`]: trait.Iterator.html#method.chain
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(iter_once_with)]
    ///
    /// use std::iter;
    ///
    /// // one is the loneliest number
    /// let mut one = iter::once_with(|| 1);
    ///
    /// assert_eq!(Some(1), one.next());
    ///
    /// // just one, that's all we get
    /// assert_eq!(None, one.next());
    /// ```
    ///
    /// Chaining together with another iterator. Let's say that we want to iterate
    /// over each file of the `.foo` directory, but also a configuration file,
    /// `.foorc`:
    ///
    /// ```no_run
    /// #![feature(iter_once_with)]
    ///
    /// use std::iter;
    /// use std::fs;
    /// use std::path::PathBuf;
    ///
    /// let dirs = fs::read_dir(".foo").unwrap();
    ///
    /// // we need to convert from an iterator of DirEntry-s to an iterator of
    /// // PathBufs, so we use map
    /// let dirs = dirs.map(|file| file.unwrap().path());
    ///
    /// // now, our iterator just for our config file
    /// let config = iter::once_with(|| PathBuf::from(".foorc"));
    ///
    /// // chain the two iterators together into one big iterator
    /// let files = dirs.chain(config);
    ///
    /// // this will give us all of the files in .foo as well as .foorc
    /// for f in files {
    ///     println!("{:?}", f);
    /// }
    /// ```
    #[inline]
    #[unstable(feature = "iter_once_with", issue = "57581")]
    pub fn once_with<A, F: FnOnce() -> A>(gen: F) -> OnceWith<F> {
        OnceWith { gen: Some(gen) }
    }

    /// Creates a new iterator where each iteration calls the provided closure
    /// `F: FnMut() -> Option<T>`.
    ///
    /// This allows creating a custom iterator with any behavior
    /// without using the more verbose syntax of creating a dedicated type
    /// and implementing the `Iterator` trait for it.
    ///
    /// Note that the `FromFn` iterator doesn’t make assumptions about the behavior of the closure,
    /// and therefore conservatively does not implement [`FusedIterator`],
    /// or override [`Iterator::size_hint`] from its default `(0, None)`.
    ///
    /// [`FusedIterator`]: trait.FusedIterator.html
    /// [`Iterator::size_hint`]: trait.Iterator.html#method.size_hint
    ///
    /// The closure can use captures and its environment to track state across iterations. Depending on
    /// how the iterator is used, this may require specifying the `move` keyword on the closure.
    ///
    /// # Examples
    ///
    /// Let’s re-implement the counter iterator from [module-level documentation]:
    ///
    /// [module-level documentation]: index.html
    ///
    /// ```
    /// let mut count = 0;
    /// let counter = std::iter::from_fn(move || {
    ///     // Increment our count. This is why we started at zero.
    ///     count += 1;
    ///
    ///     // Check to see if we've finished counting or not.
    ///     if count < 6 {
    ///         Some(count)
    ///     } else {
    ///         None
    ///     }
    /// });
    /// assert_eq!(counter.collect::<Vec<_>>(), &[1, 2, 3, 4, 5]);
    /// ```
    #[inline]
    #[stable(feature = "iter_from_fn", since = "1.34.0")]
    pub fn from_fn<T, F>(f: F) -> FromFn<F>
    where
        F: FnMut() -> Option<T>,
    {
        FromFn(f)
    }

    /// An iterator where each iteration calls the provided closure `F: FnMut() -> Option<T>`.
    ///
    /// This `struct` is created by the [`iter::from_fn`] function.
    /// See its documentation for more.
    ///
    /// [`iter::from_fn`]: fn.from_fn.html
    #[derive(Clone)]
    #[stable(feature = "iter_from_fn", since = "1.34.0")]
    pub struct FromFn<F>(F);

    #[stable(feature = "iter_from_fn", since = "1.34.0")]
    impl<T, F> Iterator for FromFn<F>
    where
        F: FnMut() -> Option<T>,
    {
        type Item = T;

        #[inline]
        fn next(&mut self) -> Option<Self::Item> {
            (self.0)()
        }
    }

    #[stable(feature = "iter_from_fn", since = "1.34.0")]
    impl<F> fmt::Debug for FromFn<F> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("FromFn").finish()
        }
    }

    /// Creates a new iterator where each successive item is computed based on the preceding one.
    ///
    /// The iterator starts with the given first item (if any)
    /// and calls the given `FnMut(&T) -> Option<T>` closure to compute each item’s successor.
    ///
    /// ```
    /// use std::iter::successors;
    ///
    /// let powers_of_10 = successors(Some(1_u16), |n| n.checked_mul(10));
    /// assert_eq!(powers_of_10.collect::<Vec<_>>(), &[1, 10, 100, 1_000, 10_000]);
    /// ```
    #[stable(feature = "iter_successors", since = "1.34.0")]
    pub fn successors<T, F>(first: Option<T>, succ: F) -> Successors<T, F>
    where
        F: FnMut(&T) -> Option<T>,
    {
        // If this function returned `impl Iterator<Item=T>`
        // it could be based on `unfold` and not need a dedicated type.
        // However having a named `Successors<T, F>` type allows it to be `Clone` when `T` and `F` are.
        Successors { next: first, succ }
    }

    /// An new iterator where each successive item is computed based on the preceding one.
    ///
    /// This `struct` is created by the [`successors`] function.
    /// See its documentation for more.
    ///
    /// [`successors`]: fn.successors.html
    #[derive(Clone)]
    #[stable(feature = "iter_successors", since = "1.34.0")]
    pub struct Successors<T, F> {
        next: Option<T>,
        succ: F,
    }

    #[stable(feature = "iter_successors", since = "1.34.0")]
    impl<T, F> Iterator for Successors<T, F>
    where
        F: FnMut(&T) -> Option<T>,
    {
        type Item = T;

        #[inline]
        fn next(&mut self) -> Option<Self::Item> {
            let item = self.next.take()?;
            self.next = (self.succ)(&item);
            Some(item)
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            if self.next.is_some() { (1, None) } else { (0, Some(0)) }
        }
    }

    #[stable(feature = "iter_successors", since = "1.34.0")]
    impl<T, F> FusedIterator for Successors<T, F> where F: FnMut(&T) -> Option<T> {}

    #[stable(feature = "iter_successors", since = "1.34.0")]
    impl<T: fmt::Debug, F> fmt::Debug for Successors<T, F> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Successors").field("next", &self.next).finish()
        }
    }