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    // Copyright 2013-2016 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use fmt;
    use marker;
    use 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()) }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<A: Clone> FusedIterator for Repeat<A> {}

    /// Creates a new iterator that endlessly repeats a single element.
    ///
    /// The `repeat()` function repeats a single value over and over and over and
    /// over and over and 🔁.
    ///
    /// Infinite iterators like `repeat()` are often used with adapters like
    /// [`take()`], in order to make them finite.
    ///
    /// [`take()`]: trait.Iterator.html#method.take
    ///
    /// # 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 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> {}

    #[unstable(feature = "fused", issue = "35602")]
    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")]
    pub 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: ::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> {}

    #[unstable(feature = "fused", issue = "35602")]
    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() }
    }

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