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    //! Composable external iteration.
    //!
    //! If you've found yourself with a collection of some kind, and needed to
    //! perform an operation on the elements of said collection, you'll quickly run
    //! into 'iterators'. Iterators are heavily used in idiomatic Rust code, so
    //! it's worth becoming familiar with them.
    //!
    //! Before explaining more, let's talk about how this module is structured:
    //!
    //! # Organization
    //!
    //! This module is largely organized by type:
    //!
    //! * [Traits] are the core portion: these traits define what kind of iterators
    //!   exist and what you can do with them. The methods of these traits are worth
    //!   putting some extra study time into.
    //! * [Functions] provide some helpful ways to create some basic iterators.
    //! * [Structs] are often the return types of the various methods on this
    //!   module's traits. You'll usually want to look at the method that creates
    //!   the `struct`, rather than the `struct` itself. For more detail about why,
    //!   see '[Implementing Iterator](#implementing-iterator)'.
    //!
    //! [Traits]: #traits
    //! [Functions]: #functions
    //! [Structs]: #structs
    //!
    //! That's it! Let's dig into iterators.
    //!
    //! # Iterator
    //!
    //! The heart and soul of this module is the [`Iterator`] trait. The core of
    //! [`Iterator`] looks like this:
    //!
    //! ```
    //! trait Iterator {
    //!     type Item;
    //!     fn next(&mut self) -> Option<Self::Item>;
    //! }
    //! ```
    //!
    //! An iterator has a method, [`next`], which when called, returns an
    //! [`Option`]`<Item>`. [`next`] will return `Some(Item)` as long as there
    //! are elements, and once they've all been exhausted, will return `None` to
    //! indicate that iteration is finished. Individual iterators may choose to
    //! resume iteration, and so calling [`next`] again may or may not eventually
    //! start returning `Some(Item)` again at some point.
    //!
    //! [`Iterator`]'s full definition includes a number of other methods as well,
    //! but they are default methods, built on top of [`next`], and so you get
    //! them for free.
    //!
    //! Iterators are also composable, and it's common to chain them together to do
    //! more complex forms of processing. See the [Adapters](#adapters) section
    //! below for more details.
    //!
    //! [`Iterator`]: trait.Iterator.html
    //! [`next`]: trait.Iterator.html#tymethod.next
    //! [`Option`]: ../../std/option/enum.Option.html
    //!
    //! # The three forms of iteration
    //!
    //! There are three common methods which can create iterators from a collection:
    //!
    //! * `iter()`, which iterates over `&T`.
    //! * `iter_mut()`, which iterates over `&mut T`.
    //! * `into_iter()`, which iterates over `T`.
    //!
    //! Various things in the standard library may implement one or more of the
    //! three, where appropriate.
    //!
    //! # Implementing Iterator
    //!
    //! Creating an iterator of your own involves two steps: creating a `struct` to
    //! hold the iterator's state, and then `impl`ementing [`Iterator`] for that
    //! `struct`. This is why there are so many `struct`s in this module: there is
    //! one for each iterator and iterator adapter.
    //!
    //! Let's make an iterator named `Counter` which counts from `1` to `5`:
    //!
    //! ```
    //! // First, the struct:
    //!
    //! /// An iterator which counts from one to five
    //! struct Counter {
    //!     count: usize,
    //! }
    //!
    //! // we want our count to start at one, so let's add a new() method to help.
    //! // This isn't strictly necessary, but is convenient. Note that we start
    //! // `count` at zero, we'll see why in `next()`'s implementation below.
    //! impl Counter {
    //!     fn new() -> Counter {
    //!         Counter { count: 0 }
    //!     }
    //! }
    //!
    //! // Then, we implement `Iterator` for our `Counter`:
    //!
    //! impl Iterator for Counter {
    //!     // we will be counting with usize
    //!     type Item = usize;
    //!
    //!     // next() is the only required method
    //!     fn next(&mut self) -> Option<Self::Item> {
    //!         // Increment our count. This is why we started at zero.
    //!         self.count += 1;
    //!
    //!         // Check to see if we've finished counting or not.
    //!         if self.count < 6 {
    //!             Some(self.count)
    //!         } else {
    //!             None
    //!         }
    //!     }
    //! }
    //!
    //! // And now we can use it!
    //!
    //! let mut counter = Counter::new();
    //!
    //! let x = counter.next().unwrap();
    //! println!("{}", x);
    //!
    //! let x = counter.next().unwrap();
    //! println!("{}", x);
    //!
    //! let x = counter.next().unwrap();
    //! println!("{}", x);
    //!
    //! let x = counter.next().unwrap();
    //! println!("{}", x);
    //!
    //! let x = counter.next().unwrap();
    //! println!("{}", x);
    //! ```
    //!
    //! This will print `1` through `5`, each on their own line.
    //!
    //! Calling `next()` this way gets repetitive. Rust has a construct which can
    //! call `next()` on your iterator, until it reaches `None`. Let's go over that
    //! next.
    //!
    //! Also note that `Iterator` provides a default implementation of methods such as `nth` and `fold`
    //! which call `next` internally. However, it is also possible to write a custom implementation of
    //! methods like `nth` and `fold` if an iterator can compute them more efficiently without calling
    //! `next`.
    //!
    //! # for Loops and IntoIterator
    //!
    //! Rust's `for` loop syntax is actually sugar for iterators. Here's a basic
    //! example of `for`:
    //!
    //! ```
    //! let values = vec![1, 2, 3, 4, 5];
    //!
    //! for x in values {
    //!     println!("{}", x);
    //! }
    //! ```
    //!
    //! This will print the numbers one through five, each on their own line. But
    //! you'll notice something here: we never called anything on our vector to
    //! produce an iterator. What gives?
    //!
    //! There's a trait in the standard library for converting something into an
    //! iterator: [`IntoIterator`]. This trait has one method, [`into_iter`],
    //! which converts the thing implementing [`IntoIterator`] into an iterator.
    //! Let's take a look at that `for` loop again, and what the compiler converts
    //! it into:
    //!
    //! [`IntoIterator`]: trait.IntoIterator.html
    //! [`into_iter`]: trait.IntoIterator.html#tymethod.into_iter
    //!
    //! ```
    //! let values = vec![1, 2, 3, 4, 5];
    //!
    //! for x in values {
    //!     println!("{}", x);
    //! }
    //! ```
    //!
    //! Rust de-sugars this into:
    //!
    //! ```
    //! let values = vec![1, 2, 3, 4, 5];
    //! {
    //!     let result = match IntoIterator::into_iter(values) {
    //!         mut iter => loop {
    //!             let next;
    //!             match iter.next() {
    //!                 Some(val) => next = val,
    //!                 None => break,
    //!             };
    //!             let x = next;
    //!             let () = { println!("{}", x); };
    //!         },
    //!     };
    //!     result
    //! }
    //! ```
    //!
    //! First, we call `into_iter()` on the value. Then, we match on the iterator
    //! that returns, calling [`next`] over and over until we see a `None`. At
    //! that point, we `break` out of the loop, and we're done iterating.
    //!
    //! There's one more subtle bit here: the standard library contains an
    //! interesting implementation of [`IntoIterator`]:
    //!
    //! ```ignore (only-for-syntax-highlight)
    //! impl<I: Iterator> IntoIterator for I
    //! ```
    //!
    //! In other words, all [`Iterator`]s implement [`IntoIterator`], by just
    //! returning themselves. This means two things:
    //!
    //! 1. If you're writing an [`Iterator`], you can use it with a `for` loop.
    //! 2. If you're creating a collection, implementing [`IntoIterator`] for it
    //!    will allow your collection to be used with the `for` loop.
    //!
    //! # Adapters
    //!
    //! Functions which take an [`Iterator`] and return another [`Iterator`] are
    //! often called 'iterator adapters', as they're a form of the 'adapter
    //! pattern'.
    //!
    //! Common iterator adapters include [`map`], [`take`], and [`filter`].
    //! For more, see their documentation.
    //!
    //! [`map`]: trait.Iterator.html#method.map
    //! [`take`]: trait.Iterator.html#method.take
    //! [`filter`]: trait.Iterator.html#method.filter
    //!
    //! # Laziness
    //!
    //! Iterators (and iterator [adapters](#adapters)) are *lazy*. This means that
    //! just creating an iterator doesn't _do_ a whole lot. Nothing really happens
    //! until you call [`next`]. This is sometimes a source of confusion when
    //! creating an iterator solely for its side effects. For example, the [`map`]
    //! method calls a closure on each element it iterates over:
    //!
    //! ```
    //! # #![allow(unused_must_use)]
    //! let v = vec![1, 2, 3, 4, 5];
    //! v.iter().map(|x| println!("{}", x));
    //! ```
    //!
    //! This will not print any values, as we only created an iterator, rather than
    //! using it. The compiler will warn us about this kind of behavior:
    //!
    //! ```text
    //! warning: unused result that must be used: iterators are lazy and
    //! do nothing unless consumed
    //! ```
    //!
    //! The idiomatic way to write a [`map`] for its side effects is to use a
    //! `for` loop instead:
    //!
    //! ```
    //! let v = vec![1, 2, 3, 4, 5];
    //!
    //! for x in &v {
    //!     println!("{}", x);
    //! }
    //! ```
    //!
    //! [`map`]: trait.Iterator.html#method.map
    //!
    //! The two most common ways to evaluate an iterator are to use a `for` loop
    //! like this, or using the [`collect`] method to produce a new collection.
    //!
    //! [`collect`]: trait.Iterator.html#method.collect
    //!
    //! # Infinity
    //!
    //! Iterators do not have to be finite. As an example, an open-ended range is
    //! an infinite iterator:
    //!
    //! ```
    //! let numbers = 0..;
    //! ```
    //!
    //! It is common to use the [`take`] iterator adapter to turn an infinite
    //! iterator into a finite one:
    //!
    //! ```
    //! let numbers = 0..;
    //! let five_numbers = numbers.take(5);
    //!
    //! for number in five_numbers {
    //!     println!("{}", number);
    //! }
    //! ```
    //!
    //! This will print the numbers `0` through `4`, each on their own line.
    //!
    //! Bear in mind that methods on infinite iterators, even those for which a
    //! result can be determined mathematically in finite time, may not terminate.
    //! Specifically, methods such as [`min`], which in the general case require
    //! traversing every element in the iterator, are likely not to return
    //! successfully for any infinite iterators.
    //!
    //! ```no_run
    //! let ones = std::iter::repeat(1);
    //! let least = ones.min().unwrap(); // Oh no! An infinite loop!
    //! // `ones.min()` causes an infinite loop, so we won't reach this point!
    //! println!("The smallest number one is {}.", least);
    //! ```
    //!
    //! [`take`]: trait.Iterator.html#method.take
    //! [`min`]: trait.Iterator.html#method.min

    #![stable(feature = "rust1", since = "1.0.0")]

    use crate::ops::Try;

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::traits::Iterator;

    #[unstable(feature = "step_trait",
               reason = "likely to be replaced by finer-grained traits",
               issue = "42168")]
    pub use self::range::Step;

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::sources::{Repeat, repeat};
    #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
    pub use self::sources::{RepeatWith, repeat_with};
    #[stable(feature = "iter_empty", since = "1.2.0")]
    pub use self::sources::{Empty, empty};
    #[stable(feature = "iter_once", since = "1.2.0")]
    pub use self::sources::{Once, once};
    #[unstable(feature = "iter_once_with", issue = "57581")]
    pub use self::sources::{OnceWith, once_with};
    #[stable(feature = "iter_from_fn", since = "1.34.0")]
    pub use self::sources::{FromFn, from_fn};
    #[stable(feature = "iter_successors", since = "1.34.0")]
    pub use self::sources::{Successors, successors};

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::traits::{FromIterator, IntoIterator, DoubleEndedIterator, Extend};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::traits::{ExactSizeIterator, Sum, Product};
    #[stable(feature = "fused", since = "1.26.0")]
    pub use self::traits::FusedIterator;
    #[unstable(feature = "trusted_len", issue = "37572")]
    pub use self::traits::TrustedLen;

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::adapters::{Rev, Cycle, Chain, Zip, Map, Filter, FilterMap, Enumerate};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::adapters::{Peekable, SkipWhile, TakeWhile, Skip, Take, Scan, FlatMap};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::adapters::{Fuse, Inspect};
    #[stable(feature = "iter_cloned", since = "1.1.0")]
    pub use self::adapters::Cloned;
    #[stable(feature = "iterator_step_by", since = "1.28.0")]
    pub use self::adapters::StepBy;
    #[stable(feature = "iterator_flatten", since = "1.29.0")]
    pub use self::adapters::Flatten;
    #[stable(feature = "iter_copied", since = "1.36.0")]
    pub use self::adapters::Copied;

    pub(crate) use self::adapters::{TrustedRandomAccess, process_results};

    mod range;
    mod sources;
    mod traits;
    mod adapters;

    /// Used to make try_fold closures more like normal loops
    #[derive(PartialEq)]
    enum LoopState<C, B> {
        Continue(C),
        Break(B),
    }

    impl<C, B> Try for LoopState<C, B> {
        type Ok = C;
        type Error = B;
        #[inline]
        fn into_result(self) -> Result<Self::Ok, Self::Error> {
            match self {
                LoopState::Continue(y) => Ok(y),
                LoopState::Break(x) => Err(x),
            }
        }
        #[inline]
        fn from_error(v: Self::Error) -> Self { LoopState::Break(v) }
        #[inline]
        fn from_ok(v: Self::Ok) -> Self { LoopState::Continue(v) }
    }

    impl<C, B> LoopState<C, B> {
        #[inline]
        fn break_value(self) -> Option<B> {
            match self {
                LoopState::Continue(..) => None,
                LoopState::Break(x) => Some(x),
            }
        }
    }

    impl<R: Try> LoopState<R::Ok, R> {
        #[inline]
        fn from_try(r: R) -> Self {
            match Try::into_result(r) {
                Ok(v) => LoopState::Continue(v),
                Err(v) => LoopState::Break(Try::from_error(v)),
            }
        }
        #[inline]
        fn into_try(self) -> R {
            match self {
                LoopState::Continue(v) => Try::from_ok(v),
                LoopState::Break(v) => v,
            }
        }
    }

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