Rollup merge of #63285 - Mark-Simulacrum:rm-await-origin, r=Centril

[rust.git] / src / libcore / iter / adapters / mod.rs

use crate::cmp;
use crate::fmt;
use crate::ops::Try;
use crate::usize;
use crate::intrinsics;

use super::{Iterator, DoubleEndedIterator, ExactSizeIterator, FusedIterator, TrustedLen};
use super::LoopState;

mod chain;
mod flatten;
mod zip;

pub use self::chain::Chain;
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::flatten::{FlatMap, Flatten};
pub use self::zip::Zip;
pub(crate) use self::zip::TrustedRandomAccess;

/// A double-ended iterator with the direction inverted.
///
/// This `struct` is created by the [`rev`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`rev`]: trait.Iterator.html#method.rev
/// [`Iterator`]: trait.Iterator.html
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Rev<T> {
    iter: T
}
impl<T> Rev<T> {
    pub(super) fn new(iter: T) -> Rev<T> {
        Rev { iter }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Rev<I> where I: DoubleEndedIterator {
    type Item = <I as Iterator>::Item;

    #[inline]
    fn next(&mut self) -> Option<<I as Iterator>::Item> { self.iter.next_back() }
    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<<I as Iterator>::Item> { self.iter.nth_back(n) }

    fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R where
        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
    {
        self.iter.try_rfold(init, f)
    }

    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
        where F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.rfold(init, f)
    }

    #[inline]
    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
        where P: FnMut(&Self::Item) -> bool
    {
        self.iter.rfind(predicate)
    }

    #[inline]
    fn rposition<P>(&mut self, predicate: P) -> Option<usize> where
        P: FnMut(Self::Item) -> bool
    {
        self.iter.position(predicate)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> DoubleEndedIterator for Rev<I> where I: DoubleEndedIterator {
    #[inline]
    fn next_back(&mut self) -> Option<<I as Iterator>::Item> { self.iter.next() }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> { self.iter.nth(n) }

    fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R where
        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
    {
        self.iter.try_fold(init, f)
    }

    fn rfold<Acc, F>(self, init: Acc, f: F) -> Acc
        where F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.fold(init, f)
    }

    fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
        where P: FnMut(&Self::Item) -> bool
    {
        self.iter.find(predicate)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Rev<I>
    where I: ExactSizeIterator + DoubleEndedIterator
{
    fn len(&self) -> usize {
        self.iter.len()
    }

    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Rev<I>
    where I: FusedIterator + DoubleEndedIterator {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<I> TrustedLen for Rev<I>
    where I: TrustedLen + DoubleEndedIterator {}

/// An iterator that copies the elements of an underlying iterator.
///
/// This `struct` is created by the [`copied`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`copied`]: trait.Iterator.html#method.copied
/// [`Iterator`]: trait.Iterator.html
#[stable(feature = "iter_copied", since = "1.36.0")]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[derive(Clone, Debug)]
pub struct Copied<I> {
    it: I,
}

impl<I> Copied<I> {
    pub(super) fn new(it: I) -> Copied<I> {
        Copied { it }
    }
}

#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> Iterator for Copied<I>
    where I: Iterator<Item=&'a T>, T: Copy
{
    type Item = T;

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

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

    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R where
        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
    {
        self.it.try_fold(init, move |acc, &elt| f(acc, elt))
    }

    fn fold<Acc, F>(self, init: Acc, mut f: F) -> Acc
        where F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.it.fold(init, move |acc, &elt| f(acc, elt))
    }
}

#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> DoubleEndedIterator for Copied<I>
    where I: DoubleEndedIterator<Item=&'a T>, T: Copy
{
    fn next_back(&mut self) -> Option<T> {
        self.it.next_back().copied()
    }

    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R where
        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
    {
        self.it.try_rfold(init, move |acc, &elt| f(acc, elt))
    }

    fn rfold<Acc, F>(self, init: Acc, mut f: F) -> Acc
        where F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.it.rfold(init, move |acc, &elt| f(acc, elt))
    }
}

#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> ExactSizeIterator for Copied<I>
    where I: ExactSizeIterator<Item=&'a T>, T: Copy
{
    fn len(&self) -> usize {
        self.it.len()
    }

    fn is_empty(&self) -> bool {
        self.it.is_empty()
    }
}

#[stable(feature = "iter_copied", since = "1.36.0")]
impl<'a, I, T: 'a> FusedIterator for Copied<I>
    where I: FusedIterator<Item=&'a T>, T: Copy
{}

#[doc(hidden)]
unsafe impl<'a, I, T: 'a> TrustedRandomAccess for Copied<I>
    where I: TrustedRandomAccess<Item=&'a T>, T: Copy
{
    unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item {
        *self.it.get_unchecked(i)
    }

    #[inline]
    fn may_have_side_effect() -> bool {
        I::may_have_side_effect()
    }
}

#[stable(feature = "iter_copied", since = "1.36.0")]
unsafe impl<'a, I, T: 'a> TrustedLen for Copied<I>
    where I: TrustedLen<Item=&'a T>,
          T: Copy
{}

/// An iterator that clones the elements of an underlying iterator.
///
/// This `struct` is created by the [`cloned`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`cloned`]: trait.Iterator.html#method.cloned
/// [`Iterator`]: trait.Iterator.html
#[stable(feature = "iter_cloned", since = "1.1.0")]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[derive(Clone, Debug)]
pub struct Cloned<I> {
    it: I,
}
impl<I> Cloned<I> {
    pub(super) fn new(it: I) -> Cloned<I> {
        Cloned { it }
    }
}

#[stable(feature = "iter_cloned", since = "1.1.0")]
impl<'a, I, T: 'a> Iterator for Cloned<I>
    where I: Iterator<Item=&'a T>, T: Clone
{
    type Item = T;

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

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

    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R where
        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
    {
        self.it.try_fold(init, move |acc, elt| f(acc, elt.clone()))
    }

    fn fold<Acc, F>(self, init: Acc, mut f: F) -> Acc
        where F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.it.fold(init, move |acc, elt| f(acc, elt.clone()))
    }
}

#[stable(feature = "iter_cloned", since = "1.1.0")]
impl<'a, I, T: 'a> DoubleEndedIterator for Cloned<I>
    where I: DoubleEndedIterator<Item=&'a T>, T: Clone
{
    fn next_back(&mut self) -> Option<T> {
        self.it.next_back().cloned()
    }

    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R where
        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
    {
        self.it.try_rfold(init, move |acc, elt| f(acc, elt.clone()))
    }

    fn rfold<Acc, F>(self, init: Acc, mut f: F) -> Acc
        where F: FnMut(Acc, Self::Item) -> Acc,
    {
        self.it.rfold(init, move |acc, elt| f(acc, elt.clone()))
    }
}

#[stable(feature = "iter_cloned", since = "1.1.0")]
impl<'a, I, T: 'a> ExactSizeIterator for Cloned<I>
    where I: ExactSizeIterator<Item=&'a T>, T: Clone
{
    fn len(&self) -> usize {
        self.it.len()
    }

    fn is_empty(&self) -> bool {
        self.it.is_empty()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<'a, I, T: 'a> FusedIterator for Cloned<I>
    where I: FusedIterator<Item=&'a T>, T: Clone
{}

#[doc(hidden)]
unsafe impl<'a, I, T: 'a> TrustedRandomAccess for Cloned<I>
    where I: TrustedRandomAccess<Item=&'a T>, T: Clone
{
    default unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item {
        self.it.get_unchecked(i).clone()
    }

    #[inline]
    default fn may_have_side_effect() -> bool { true }
}

#[doc(hidden)]
unsafe impl<'a, I, T: 'a> TrustedRandomAccess for Cloned<I>
    where I: TrustedRandomAccess<Item=&'a T>, T: Copy
{
    unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item {
        *self.it.get_unchecked(i)
    }

    #[inline]
    fn may_have_side_effect() -> bool {
        I::may_have_side_effect()
    }
}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<'a, I, T: 'a> TrustedLen for Cloned<I>
    where I: TrustedLen<Item=&'a T>,
          T: Clone
{}

/// An iterator that repeats endlessly.
///
/// This `struct` is created by the [`cycle`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`cycle`]: trait.Iterator.html#method.cycle
/// [`Iterator`]: trait.Iterator.html
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Cycle<I> {
    orig: I,
    iter: I,
}
impl<I: Clone> Cycle<I> {
    pub(super) fn new(iter: I) -> Cycle<I> {
        Cycle { orig: iter.clone(), iter }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Cycle<I> where I: Clone + Iterator {
    type Item = <I as Iterator>::Item;

    #[inline]
    fn next(&mut self) -> Option<<I as Iterator>::Item> {
        match self.iter.next() {
            None => { self.iter = self.orig.clone(); self.iter.next() }
            y => y
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        // the cycle iterator is either empty or infinite
        match self.orig.size_hint() {
            sz @ (0, Some(0)) => sz,
            (0, _) => (0, None),
            _ => (usize::MAX, None)
        }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Cycle<I> where I: Clone + Iterator {}

/// An iterator for stepping iterators by a custom amount.
///
/// This `struct` is created by the [`step_by`] method on [`Iterator`]. See
/// its documentation for more.
///
/// [`step_by`]: trait.Iterator.html#method.step_by
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "iterator_step_by", since = "1.28.0")]
#[derive(Clone, Debug)]
pub struct StepBy<I> {
    iter: I,
    step: usize,
    first_take: bool,
}
impl<I> StepBy<I> {
    pub(super) fn new(iter: I, step: usize) -> StepBy<I> {
        assert!(step != 0);
        StepBy { iter, step: step - 1, first_take: true }
    }
}

#[stable(feature = "iterator_step_by", since = "1.28.0")]
impl<I> Iterator for StepBy<I> where I: Iterator {
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        if self.first_take {
            self.first_take = false;
            self.iter.next()
        } else {
            self.iter.nth(self.step)
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let inner_hint = self.iter.size_hint();

        if self.first_take {
            let f = |n| if n == 0 { 0 } else { 1 + (n-1)/(self.step+1) };
            (f(inner_hint.0), inner_hint.1.map(f))
        } else {
            let f = |n| n / (self.step+1);
            (f(inner_hint.0), inner_hint.1.map(f))
        }
    }

    #[inline]
    fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
        if self.first_take {
            self.first_take = false;
            let first = self.iter.next();
            if n == 0 {
                return first;
            }
            n -= 1;
        }
        // n and self.step are indices, we need to add 1 to get the amount of elements
        // When calling `.nth`, we need to subtract 1 again to convert back to an index
        // step + 1 can't overflow because `.step_by` sets `self.step` to `step - 1`
        let mut step = self.step + 1;
        // n + 1 could overflow
        // thus, if n is usize::MAX, instead of adding one, we call .nth(step)
        if n == usize::MAX {
            self.iter.nth(step - 1);
        } else {
            n += 1;
        }

        // overflow handling
        loop {
            let mul = n.checked_mul(step);
            if unsafe { intrinsics::likely(mul.is_some()) } {
                return self.iter.nth(mul.unwrap() - 1);
            }
            let div_n = usize::MAX / n;
            let div_step = usize::MAX / step;
            let nth_n = div_n * n;
            let nth_step = div_step * step;
            let nth = if nth_n > nth_step {
                step -= div_n;
                nth_n
            } else {
                n -= div_step;
                nth_step
            };
            self.iter.nth(nth - 1);
        }
    }
}

impl<I> StepBy<I> where I: ExactSizeIterator {
    // The zero-based index starting from the end of the iterator of the
    // last element. Used in the `DoubleEndedIterator` implementation.
    fn next_back_index(&self) -> usize {
        let rem = self.iter.len() % (self.step + 1);
        if self.first_take {
            if rem == 0 { self.step } else { rem - 1 }
        } else {
            rem
        }
    }
}

#[stable(feature = "double_ended_step_by_iterator", since = "1.38.0")]
impl<I> DoubleEndedIterator for StepBy<I> where I: DoubleEndedIterator + ExactSizeIterator {
    #[inline]
    fn next_back(&mut self) -> Option<Self::Item> {
        self.iter.nth_back(self.next_back_index())
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        // `self.iter.nth_back(usize::MAX)` does the right thing here when `n`
        // is out of bounds because the length of `self.iter` does not exceed
        // `usize::MAX` (because `I: ExactSizeIterator`) and `nth_back` is
        // zero-indexed
        let n = n
            .saturating_mul(self.step + 1)
            .saturating_add(self.next_back_index());
        self.iter.nth_back(n)
    }
}

// StepBy can only make the iterator shorter, so the len will still fit.
#[stable(feature = "iterator_step_by", since = "1.28.0")]
impl<I> ExactSizeIterator for StepBy<I> where I: ExactSizeIterator {}

/// An iterator that maps the values of `iter` with `f`.
///
/// This `struct` is created by the [`map`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`map`]: trait.Iterator.html#method.map
/// [`Iterator`]: trait.Iterator.html
///
/// # Notes about side effects
///
/// The [`map`] iterator implements [`DoubleEndedIterator`], meaning that
/// you can also [`map`] backwards:
///
/// ```rust
/// let v: Vec<i32> = vec![1, 2, 3].into_iter().map(|x| x + 1).rev().collect();
///
/// assert_eq!(v, [4, 3, 2]);
/// ```
///
/// [`DoubleEndedIterator`]: trait.DoubleEndedIterator.html
///
/// But if your closure has state, iterating backwards may act in a way you do
/// not expect. Let's go through an example. First, in the forward direction:
///
/// ```rust
/// let mut c = 0;
///
/// for pair in vec!['a', 'b', 'c'].into_iter()
///                                .map(|letter| { c += 1; (letter, c) }) {
///     println!("{:?}", pair);
/// }
/// ```
///
/// This will print "('a', 1), ('b', 2), ('c', 3)".
///
/// Now consider this twist where we add a call to `rev`. This version will
/// print `('c', 1), ('b', 2), ('a', 3)`. Note that the letters are reversed,
/// but the values of the counter still go in order. This is because `map()` is
/// still being called lazily on each item, but we are popping items off the
/// back of the vector now, instead of shifting them from the front.
///
/// ```rust
/// let mut c = 0;
///
/// for pair in vec!['a', 'b', 'c'].into_iter()
///                                .map(|letter| { c += 1; (letter, c) })
///                                .rev() {
///     println!("{:?}", pair);
/// }
/// ```
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct Map<I, F> {
    iter: I,
    f: F,
}
impl<I, F> Map<I, F> {
    pub(super) fn new(iter: I, f: F) -> Map<I, F> {
        Map { iter, f }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, F> fmt::Debug for Map<I, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Map")
            .field("iter", &self.iter)
            .finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: Iterator, F> Iterator for Map<I, F> where F: FnMut(I::Item) -> B {
    type Item = B;

    #[inline]
    fn next(&mut self) -> Option<B> {
        self.iter.next().map(&mut self.f)
    }

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

    fn try_fold<Acc, G, R>(&mut self, init: Acc, mut g: G) -> R where
        Self: Sized, G: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let f = &mut self.f;
        self.iter.try_fold(init, move |acc, elt| g(acc, f(elt)))
    }

    fn fold<Acc, G>(self, init: Acc, mut g: G) -> Acc
        where G: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut f = self.f;
        self.iter.fold(init, move |acc, elt| g(acc, f(elt)))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for Map<I, F> where
    F: FnMut(I::Item) -> B,
{
    #[inline]
    fn next_back(&mut self) -> Option<B> {
        self.iter.next_back().map(&mut self.f)
    }

    fn try_rfold<Acc, G, R>(&mut self, init: Acc, mut g: G) -> R where
        Self: Sized, G: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let f = &mut self.f;
        self.iter.try_rfold(init, move |acc, elt| g(acc, f(elt)))
    }

    fn rfold<Acc, G>(self, init: Acc, mut g: G) -> Acc
        where G: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut f = self.f;
        self.iter.rfold(init, move |acc, elt| g(acc, f(elt)))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F>
    where F: FnMut(I::Item) -> B
{
    fn len(&self) -> usize {
        self.iter.len()
    }

    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<B, I: FusedIterator, F> FusedIterator for Map<I, F>
    where F: FnMut(I::Item) -> B {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<B, I, F> TrustedLen for Map<I, F>
    where I: TrustedLen,
          F: FnMut(I::Item) -> B {}

#[doc(hidden)]
unsafe impl<B, I, F> TrustedRandomAccess for Map<I, F>
    where I: TrustedRandomAccess,
          F: FnMut(I::Item) -> B,
{
    unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item {
        (self.f)(self.iter.get_unchecked(i))
    }
    #[inline]
    fn may_have_side_effect() -> bool { true }
}

/// An iterator that filters the elements of `iter` with `predicate`.
///
/// This `struct` is created by the [`filter`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`filter`]: trait.Iterator.html#method.filter
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct Filter<I, P> {
    iter: I,
    predicate: P,
}
impl<I, P> Filter<I, P> {
    pub(super) fn new(iter: I, predicate: P) -> Filter<I, P> {
        Filter { iter, predicate }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, P> fmt::Debug for Filter<I, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Filter")
            .field("iter", &self.iter)
            .finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, P> Iterator for Filter<I, P> where P: FnMut(&I::Item) -> bool {
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        self.try_for_each(Err).err()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // can't know a lower bound, due to the predicate
    }

    // this special case allows the compiler to make `.filter(_).count()`
    // branchless. Barring perfect branch prediction (which is unattainable in
    // the general case), this will be much faster in >90% of cases (containing
    // virtually all real workloads) and only a tiny bit slower in the rest.
    //
    // Having this specialization thus allows us to write `.filter(p).count()`
    // where we would otherwise write `.map(|x| p(x) as usize).sum()`, which is
    // less readable and also less backwards-compatible to Rust before 1.10.
    //
    // Using the branchless version will also simplify the LLVM byte code, thus
    // leaving more budget for LLVM optimizations.
    #[inline]
    fn count(self) -> usize {
        let mut predicate = self.predicate;
        self.iter.map(|x| predicate(&x) as usize).sum()
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let predicate = &mut self.predicate;
        self.iter.try_fold(init, move |acc, item| if predicate(&item) {
            fold(acc, item)
        } else {
            Try::from_ok(acc)
        })
    }

    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut predicate = self.predicate;
        self.iter.fold(init, move |acc, item| if predicate(&item) {
            fold(acc, item)
        } else {
            acc
        })
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: DoubleEndedIterator, P> DoubleEndedIterator for Filter<I, P>
    where P: FnMut(&I::Item) -> bool,
{
    #[inline]
    fn next_back(&mut self) -> Option<I::Item> {
        self.try_rfold((), |_, x| Err(x)).err()
    }

    #[inline]
    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let predicate = &mut self.predicate;
        self.iter.try_rfold(init, move |acc, item| if predicate(&item) {
            fold(acc, item)
        } else {
            Try::from_ok(acc)
        })
    }

    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut predicate = self.predicate;
        self.iter.rfold(init, move |acc, item| if predicate(&item) {
            fold(acc, item)
        } else {
            acc
        })
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I: FusedIterator, P> FusedIterator for Filter<I, P>
    where P: FnMut(&I::Item) -> bool {}

/// An iterator that uses `f` to both filter and map elements from `iter`.
///
/// This `struct` is created by the [`filter_map`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`filter_map`]: trait.Iterator.html#method.filter_map
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct FilterMap<I, F> {
    iter: I,
    f: F,
}
impl<I, F> FilterMap<I, F> {
    pub(super) fn new(iter: I, f: F) -> FilterMap<I, F> {
        FilterMap { iter, f }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, F> fmt::Debug for FilterMap<I, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("FilterMap")
            .field("iter", &self.iter)
            .finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: Iterator, F> Iterator for FilterMap<I, F>
    where F: FnMut(I::Item) -> Option<B>,
{
    type Item = B;

    #[inline]
    fn next(&mut self) -> Option<B> {
        self.try_for_each(Err).err()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // can't know a lower bound, due to the predicate
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let f = &mut self.f;
        self.iter.try_fold(init, move |acc, item| match f(item) {
            Some(x) => fold(acc, x),
            None => Try::from_ok(acc),
        })
    }

    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut f = self.f;
        self.iter.fold(init, move |acc, item| match f(item) {
            Some(x) => fold(acc, x),
            None => acc,
        })
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for FilterMap<I, F>
    where F: FnMut(I::Item) -> Option<B>,
{
    #[inline]
    fn next_back(&mut self) -> Option<B> {
        self.try_rfold((), |_, x| Err(x)).err()
    }

    #[inline]
    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let f = &mut self.f;
        self.iter.try_rfold(init, move |acc, item| match f(item) {
            Some(x) => fold(acc, x),
            None => Try::from_ok(acc),
        })
    }

    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut f = self.f;
        self.iter.rfold(init, move |acc, item| match f(item) {
            Some(x) => fold(acc, x),
            None => acc,
        })
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<B, I: FusedIterator, F> FusedIterator for FilterMap<I, F>
    where F: FnMut(I::Item) -> Option<B> {}

/// An iterator that yields the current count and the element during iteration.
///
/// This `struct` is created by the [`enumerate`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`enumerate`]: trait.Iterator.html#method.enumerate
/// [`Iterator`]: trait.Iterator.html
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Enumerate<I> {
    iter: I,
    count: usize,
}
impl<I> Enumerate<I> {
    pub(super) fn new(iter: I) -> Enumerate<I> {
        Enumerate { iter, count: 0 }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Enumerate<I> where I: Iterator {
    type Item = (usize, <I as Iterator>::Item);

    /// # Overflow Behavior
    ///
    /// The method does no guarding against overflows, so enumerating more than
    /// `usize::MAX` elements either produces the wrong result or panics. If
    /// debug assertions are enabled, a panic is guaranteed.
    ///
    /// # Panics
    ///
    /// Might panic if the index of the element overflows a `usize`.
    #[inline]
    #[rustc_inherit_overflow_checks]
    fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
        self.iter.next().map(|a| {
            let ret = (self.count, a);
            // Possible undefined overflow.
            self.count += 1;
            ret
        })
    }

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

    #[inline]
    #[rustc_inherit_overflow_checks]
    fn nth(&mut self, n: usize) -> Option<(usize, I::Item)> {
        self.iter.nth(n).map(|a| {
            let i = self.count + n;
            self.count = i + 1;
            (i, a)
        })
    }

    #[inline]
    fn count(self) -> usize {
        self.iter.count()
    }

    #[inline]
    #[rustc_inherit_overflow_checks]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let count = &mut self.count;
        self.iter.try_fold(init, move |acc, item| {
            let acc = fold(acc, (*count, item));
            *count += 1;
            acc
        })
    }

    #[inline]
    #[rustc_inherit_overflow_checks]
    fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut count = self.count;
        self.iter.fold(init, move |acc, item| {
            let acc = fold(acc, (count, item));
            count += 1;
            acc
        })
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> DoubleEndedIterator for Enumerate<I> where
    I: ExactSizeIterator + DoubleEndedIterator
{
    #[inline]
    fn next_back(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
        self.iter.next_back().map(|a| {
            let len = self.iter.len();
            // Can safely add, `ExactSizeIterator` promises that the number of
            // elements fits into a `usize`.
            (self.count + len, a)
        })
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<(usize, <I as Iterator>::Item)> {
        self.iter.nth_back(n).map(|a| {
            let len = self.iter.len();
            // Can safely add, `ExactSizeIterator` promises that the number of
            // elements fits into a `usize`.
            (self.count + len, a)
        })
    }

    #[inline]
    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        // Can safely add and subtract the count, as `ExactSizeIterator` promises
        // that the number of elements fits into a `usize`.
        let mut count = self.count + self.iter.len();
        self.iter.try_rfold(init, move |acc, item| {
            count -= 1;
            fold(acc, (count, item))
        })
    }

    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        // Can safely add and subtract the count, as `ExactSizeIterator` promises
        // that the number of elements fits into a `usize`.
        let mut count = self.count + self.iter.len();
        self.iter.rfold(init, move |acc, item| {
            count -= 1;
            fold(acc, (count, item))
        })
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Enumerate<I> where I: ExactSizeIterator {
    fn len(&self) -> usize {
        self.iter.len()
    }

    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}

#[doc(hidden)]
unsafe impl<I> TrustedRandomAccess for Enumerate<I>
    where I: TrustedRandomAccess
{
    unsafe fn get_unchecked(&mut self, i: usize) -> (usize, I::Item) {
        (self.count + i, self.iter.get_unchecked(i))
    }

    fn may_have_side_effect() -> bool {
        I::may_have_side_effect()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Enumerate<I> where I: FusedIterator {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<I> TrustedLen for Enumerate<I>
    where I: TrustedLen,
{}


/// An iterator with a `peek()` that returns an optional reference to the next
/// element.
///
/// This `struct` is created by the [`peekable`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`peekable`]: trait.Iterator.html#method.peekable
/// [`Iterator`]: trait.Iterator.html
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Peekable<I: Iterator> {
    iter: I,
    /// Remember a peeked value, even if it was None.
    peeked: Option<Option<I::Item>>,
}
impl<I: Iterator> Peekable<I> {
    pub(super) fn new(iter: I) -> Peekable<I> {
        Peekable { iter, peeked: None }
    }
}

// Peekable must remember if a None has been seen in the `.peek()` method.
// It ensures that `.peek(); .peek();` or `.peek(); .next();` only advances the
// underlying iterator at most once. This does not by itself make the iterator
// fused.
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator> Iterator for Peekable<I> {
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        match self.peeked.take() {
            Some(v) => v,
            None => self.iter.next(),
        }
    }

    #[inline]
    #[rustc_inherit_overflow_checks]
    fn count(mut self) -> usize {
        match self.peeked.take() {
            Some(None) => 0,
            Some(Some(_)) => 1 + self.iter.count(),
            None => self.iter.count(),
        }
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        match self.peeked.take() {
            Some(None) => None,
            Some(v @ Some(_)) if n == 0 => v,
            Some(Some(_)) => self.iter.nth(n - 1),
            None => self.iter.nth(n),
        }
    }

    #[inline]
    fn last(mut self) -> Option<I::Item> {
        let peek_opt = match self.peeked.take() {
            Some(None) => return None,
            Some(v) => v,
            None => None,
        };
        self.iter.last().or(peek_opt)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let peek_len = match self.peeked {
            Some(None) => return (0, Some(0)),
            Some(Some(_)) => 1,
            None => 0,
        };
        let (lo, hi) = self.iter.size_hint();
        let lo = lo.saturating_add(peek_len);
        let hi = hi.and_then(|x| x.checked_add(peek_len));
        (lo, hi)
    }

    #[inline]
    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R where
        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
    {
        let acc = match self.peeked.take() {
            Some(None) => return Try::from_ok(init),
            Some(Some(v)) => f(init, v)?,
            None => init,
        };
        self.iter.try_fold(acc, f)
    }

    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        let acc = match self.peeked {
            Some(None) => return init,
            Some(Some(v)) => fold(init, v),
            None => init,
        };
        self.iter.fold(acc, fold)
    }
}

#[stable(feature = "double_ended_peek_iterator", since = "1.38.0")]
impl<I> DoubleEndedIterator for Peekable<I> where I: DoubleEndedIterator {
    #[inline]
    fn next_back(&mut self) -> Option<Self::Item> {
        self.iter.next_back().or_else(|| self.peeked.take().and_then(|x| x))
    }

    #[inline]
    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R where
        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
    {
        match self.peeked.take() {
            Some(None) => return Try::from_ok(init),
            Some(Some(v)) => match self.iter.try_rfold(init, &mut f).into_result() {
                Ok(acc) => f(acc, v),
                Err(e) => {
                    self.peeked = Some(Some(v));
                    Try::from_error(e)
                }
            },
            None => self.iter.try_rfold(init, f),
        }
    }

    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        match self.peeked {
            Some(None) => return init,
            Some(Some(v)) => {
                let acc = self.iter.rfold(init, &mut fold);
                fold(acc, v)
            }
            None => self.iter.rfold(init, fold),
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I> {}

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

impl<I: Iterator> Peekable<I> {
    /// Returns a reference to the next() value without advancing the iterator.
    ///
    /// Like [`next`], if there is a value, it is wrapped in a `Some(T)`.
    /// But if the iteration is over, `None` is returned.
    ///
    /// [`next`]: trait.Iterator.html#tymethod.next
    ///
    /// Because `peek()` returns a reference, and many iterators iterate over
    /// references, there can be a possibly confusing situation where the
    /// return value is a double reference. You can see this effect in the
    /// examples below.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let xs = [1, 2, 3];
    ///
    /// let mut iter = xs.iter().peekable();
    ///
    /// // peek() lets us see into the future
    /// assert_eq!(iter.peek(), Some(&&1));
    /// assert_eq!(iter.next(), Some(&1));
    ///
    /// assert_eq!(iter.next(), Some(&2));
    ///
    /// // The iterator does not advance even if we `peek` multiple times
    /// assert_eq!(iter.peek(), Some(&&3));
    /// assert_eq!(iter.peek(), Some(&&3));
    ///
    /// assert_eq!(iter.next(), Some(&3));
    ///
    /// // After the iterator is finished, so is `peek()`
    /// assert_eq!(iter.peek(), None);
    /// assert_eq!(iter.next(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn peek(&mut self) -> Option<&I::Item> {
        let iter = &mut self.iter;
        self.peeked.get_or_insert_with(|| iter.next()).as_ref()
    }
}

/// An iterator that rejects elements while `predicate` returns `true`.
///
/// This `struct` is created by the [`skip_while`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`skip_while`]: trait.Iterator.html#method.skip_while
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct SkipWhile<I, P> {
    iter: I,
    flag: bool,
    predicate: P,
}
impl<I, P> SkipWhile<I, P> {
    pub(super) fn new(iter: I, predicate: P) -> SkipWhile<I, P> {
        SkipWhile { iter, flag: false, predicate }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, P> fmt::Debug for SkipWhile<I, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SkipWhile")
            .field("iter", &self.iter)
            .field("flag", &self.flag)
            .finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, P> Iterator for SkipWhile<I, P>
    where P: FnMut(&I::Item) -> bool
{
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        let flag = &mut self.flag;
        let pred = &mut self.predicate;
        self.iter.find(move |x| {
            if *flag || !pred(x) {
                *flag = true;
                true
            } else {
                false
            }
        })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // can't know a lower bound, due to the predicate
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        if !self.flag {
            match self.next() {
                Some(v) => init = fold(init, v)?,
                None => return Try::from_ok(init),
            }
        }
        self.iter.try_fold(init, fold)
    }

    #[inline]
    fn fold<Acc, Fold>(mut self, mut init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if !self.flag {
            match self.next() {
                Some(v) => init = fold(init, v),
                None => return init,
            }
        }
        self.iter.fold(init, fold)
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I, P> FusedIterator for SkipWhile<I, P>
    where I: FusedIterator, P: FnMut(&I::Item) -> bool {}

/// An iterator that only accepts elements while `predicate` returns `true`.
///
/// This `struct` is created by the [`take_while`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`take_while`]: trait.Iterator.html#method.take_while
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct TakeWhile<I, P> {
    iter: I,
    flag: bool,
    predicate: P,
}
impl<I, P> TakeWhile<I, P> {
    pub(super) fn new(iter: I, predicate: P) -> TakeWhile<I, P> {
        TakeWhile { iter, flag: false, predicate }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, P> fmt::Debug for TakeWhile<I, P> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TakeWhile")
            .field("iter", &self.iter)
            .field("flag", &self.flag)
            .finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, P> Iterator for TakeWhile<I, P>
    where P: FnMut(&I::Item) -> bool
{
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        if self.flag {
            None
        } else {
            self.iter.next().and_then(|x| {
                if (self.predicate)(&x) {
                    Some(x)
                } else {
                    self.flag = true;
                    None
                }
            })
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.flag {
            (0, Some(0))
        } else {
            let (_, upper) = self.iter.size_hint();
            (0, upper) // can't know a lower bound, due to the predicate
        }
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        if self.flag {
            Try::from_ok(init)
        } else {
            let flag = &mut self.flag;
            let p = &mut self.predicate;
            self.iter.try_fold(init, move |acc, x|{
                if p(&x) {
                    LoopState::from_try(fold(acc, x))
                } else {
                    *flag = true;
                    LoopState::Break(Try::from_ok(acc))
                }
            }).into_try()
        }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I, P> FusedIterator for TakeWhile<I, P>
    where I: FusedIterator, P: FnMut(&I::Item) -> bool {}

/// An iterator that skips over `n` elements of `iter`.
///
/// This `struct` is created by the [`skip`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`skip`]: trait.Iterator.html#method.skip
/// [`Iterator`]: trait.Iterator.html
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Skip<I> {
    iter: I,
    n: usize
}
impl<I> Skip<I> {
    pub(super) fn new(iter: I, n: usize) -> Skip<I> {
        Skip { iter, n }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Skip<I> where I: Iterator {
    type Item = <I as Iterator>::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        if self.n == 0 {
            self.iter.next()
        } else {
            let old_n = self.n;
            self.n = 0;
            self.iter.nth(old_n)
        }
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        // Can't just add n + self.n due to overflow.
        if self.n == 0 {
            self.iter.nth(n)
        } else {
            let to_skip = self.n;
            self.n = 0;
            // nth(n) skips n+1
            if self.iter.nth(to_skip-1).is_none() {
                return None;
            }
            self.iter.nth(n)
        }
    }

    #[inline]
    fn count(self) -> usize {
        self.iter.count().saturating_sub(self.n)
    }

    #[inline]
    fn last(mut self) -> Option<I::Item> {
        if self.n == 0 {
            self.iter.last()
        } else {
            let next = self.next();
            if next.is_some() {
                // recurse. n should be 0.
                self.last().or(next)
            } else {
                None
            }
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (lower, upper) = self.iter.size_hint();

        let lower = lower.saturating_sub(self.n);
        let upper = upper.map(|x| x.saturating_sub(self.n));

        (lower, upper)
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let n = self.n;
        self.n = 0;
        if n > 0 {
            // nth(n) skips n+1
            if self.iter.nth(n - 1).is_none() {
                return Try::from_ok(init);
            }
        }
        self.iter.try_fold(init, fold)
    }

    #[inline]
    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if self.n > 0 {
            // nth(n) skips n+1
            if self.iter.nth(self.n - 1).is_none() {
                return init;
            }
        }
        self.iter.fold(init, fold)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator {}

#[stable(feature = "double_ended_skip_iterator", since = "1.9.0")]
impl<I> DoubleEndedIterator for Skip<I> where I: DoubleEndedIterator + ExactSizeIterator {
    fn next_back(&mut self) -> Option<Self::Item> {
        if self.len() > 0 {
            self.iter.next_back()
        } else {
            None
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<I::Item> {
        let len = self.len();
        if n < len {
            self.iter.nth_back(n)
        } else {
            if len > 0 {
                // consume the original iterator
                self.iter.nth_back(len-1);
            }
            None
        }
    }

    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let mut n = self.len();
        if n == 0 {
            Try::from_ok(init)
        } else {
            self.iter.try_rfold(init, move |acc, x| {
                n -= 1;
                let r = fold(acc, x);
                if n == 0 { LoopState::Break(r) }
                else { LoopState::from_try(r) }
            }).into_try()
        }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Skip<I> where I: FusedIterator {}

/// An iterator that only iterates over the first `n` iterations of `iter`.
///
/// This `struct` is created by the [`take`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`take`]: trait.Iterator.html#method.take
/// [`Iterator`]: trait.Iterator.html
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Take<I> {
    pub(super) iter: I,
    pub(super) n: usize
}
impl<I> Take<I> {
    pub(super) fn new(iter: I, n: usize) -> Take<I> {
        Take { iter, n }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Take<I> where I: Iterator{
    type Item = <I as Iterator>::Item;

    #[inline]
    fn next(&mut self) -> Option<<I as Iterator>::Item> {
        if self.n != 0 {
            self.n -= 1;
            self.iter.next()
        } else {
            None
        }
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        if self.n > n {
            self.n -= n + 1;
            self.iter.nth(n)
        } else {
            if self.n > 0 {
                self.iter.nth(self.n - 1);
                self.n = 0;
            }
            None
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.n == 0 {
            return (0, Some(0));
        }

        let (lower, upper) = self.iter.size_hint();

        let lower = cmp::min(lower, self.n);

        let upper = match upper {
            Some(x) if x < self.n => Some(x),
            _ => Some(self.n)
        };

        (lower, upper)
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        if self.n == 0 {
            Try::from_ok(init)
        } else {
            let n = &mut self.n;
            self.iter.try_fold(init, move |acc, x| {
                *n -= 1;
                let r = fold(acc, x);
                if *n == 0 { LoopState::Break(r) }
                else { LoopState::from_try(r) }
            }).into_try()
        }
    }
}

#[stable(feature = "double_ended_take_iterator", since = "1.38.0")]
impl<I> DoubleEndedIterator for Take<I> where I: DoubleEndedIterator + ExactSizeIterator {
    #[inline]
    fn next_back(&mut self) -> Option<Self::Item> {
        if self.n == 0 {
            None
        } else {
            let n = self.n;
            self.n -= 1;
            self.iter.nth_back(self.iter.len().saturating_sub(n))
        }
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
        let len = self.iter.len();
        if self.n > n {
            let m = len.saturating_sub(self.n) + n;
            self.n -= n + 1;
            self.iter.nth_back(m)
        } else {
            if len > 0 {
                self.iter.nth_back(len - 1);
            }
            None
        }
    }

    #[inline]
    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok = Acc>
    {
        if self.n == 0 {
            Try::from_ok(init)
        } else {
            let len = self.iter.len();
            if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() {
                Try::from_ok(init)
            } else {
                self.iter.try_rfold(init, fold)
            }
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Take<I> where I: ExactSizeIterator {}

#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Take<I> where I: FusedIterator {}

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

/// An iterator to maintain state while iterating another iterator.
///
/// This `struct` is created by the [`scan`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`scan`]: trait.Iterator.html#method.scan
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct Scan<I, St, F> {
    iter: I,
    f: F,
    state: St,
}
impl<I, St, F> Scan<I, St, F> {
    pub(super) fn new(iter: I, state: St, f: F) -> Scan<I, St, F> {
        Scan { iter, state, f }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, St: fmt::Debug, F> fmt::Debug for Scan<I, St, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Scan")
            .field("iter", &self.iter)
            .field("state", &self.state)
            .finish()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I, St, F> Iterator for Scan<I, St, F> where
    I: Iterator,
    F: FnMut(&mut St, I::Item) -> Option<B>,
{
    type Item = B;

    #[inline]
    fn next(&mut self) -> Option<B> {
        self.iter.next().and_then(|a| (self.f)(&mut self.state, a))
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.iter.size_hint();
        (0, upper) // can't know a lower bound, due to the scan function
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let state = &mut self.state;
        let f = &mut self.f;
        self.iter.try_fold(init, move |acc, x| {
            match f(state, x) {
                None => LoopState::Break(Try::from_ok(acc)),
                Some(x) => LoopState::from_try(fold(acc, x)),
            }
        }).into_try()
    }
}

/// An iterator that yields `None` forever after the underlying iterator
/// yields `None` once.
///
/// This `struct` is created by the [`fuse`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`fuse`]: trait.Iterator.html#method.fuse
/// [`Iterator`]: trait.Iterator.html
#[derive(Clone, Debug)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Fuse<I> {
    iter: I,
    done: bool
}
impl<I> Fuse<I> {
    pub(super) fn new(iter: I) -> Fuse<I> {
        Fuse { iter, done: false }
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I> FusedIterator for Fuse<I> where I: Iterator {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> Iterator for Fuse<I> where I: Iterator {
    type Item = <I as Iterator>::Item;

    #[inline]
    default fn next(&mut self) -> Option<<I as Iterator>::Item> {
        if self.done {
            None
        } else {
            let next = self.iter.next();
            self.done = next.is_none();
            next
        }
    }

    #[inline]
    default fn nth(&mut self, n: usize) -> Option<I::Item> {
        if self.done {
            None
        } else {
            let nth = self.iter.nth(n);
            self.done = nth.is_none();
            nth
        }
    }

    #[inline]
    default fn last(self) -> Option<I::Item> {
        if self.done {
            None
        } else {
            self.iter.last()
        }
    }

    #[inline]
    default fn count(self) -> usize {
        if self.done {
            0
        } else {
            self.iter.count()
        }
    }

    #[inline]
    default fn size_hint(&self) -> (usize, Option<usize>) {
        if self.done {
            (0, Some(0))
        } else {
            self.iter.size_hint()
        }
    }

    #[inline]
    default fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        if self.done {
            Try::from_ok(init)
        } else {
            let acc = self.iter.try_fold(init, fold)?;
            self.done = true;
            Try::from_ok(acc)
        }
    }

    #[inline]
    default fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if self.done {
            init
        } else {
            self.iter.fold(init, fold)
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> DoubleEndedIterator for Fuse<I> where I: DoubleEndedIterator {
    #[inline]
    default fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
        if self.done {
            None
        } else {
            let next = self.iter.next_back();
            self.done = next.is_none();
            next
        }
    }

    #[inline]
    default fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
        if self.done {
            None
        } else {
            let nth = self.iter.nth_back(n);
            self.done = nth.is_none();
            nth
        }
    }

    #[inline]
    default fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        if self.done {
            Try::from_ok(init)
        } else {
            let acc = self.iter.try_rfold(init, fold)?;
            self.done = true;
            Try::from_ok(acc)
        }
    }

    #[inline]
    default fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        if self.done {
            init
        } else {
            self.iter.rfold(init, fold)
        }
    }
}

unsafe impl<I> TrustedRandomAccess for Fuse<I>
    where I: TrustedRandomAccess,
{
    unsafe fn get_unchecked(&mut self, i: usize) -> I::Item {
        self.iter.get_unchecked(i)
    }

    fn may_have_side_effect() -> bool {
        I::may_have_side_effect()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I> Iterator for Fuse<I> where I: FusedIterator {
    #[inline]
    fn next(&mut self) -> Option<<I as Iterator>::Item> {
        self.iter.next()
    }

    #[inline]
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        self.iter.nth(n)
    }

    #[inline]
    fn last(self) -> Option<I::Item> {
        self.iter.last()
    }

    #[inline]
    fn count(self) -> usize {
        self.iter.count()
    }

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

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        self.iter.try_fold(init, fold)
    }

    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.fold(init, fold)
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I> DoubleEndedIterator for Fuse<I>
    where I: DoubleEndedIterator + FusedIterator
{
    #[inline]
    fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
        self.iter.next_back()
    }

    #[inline]
    fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
        self.iter.nth_back(n)
    }

    #[inline]
    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        self.iter.try_rfold(init, fold)
    }

    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.rfold(init, fold)
    }
}


#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ExactSizeIterator for Fuse<I> where I: ExactSizeIterator {
    fn len(&self) -> usize {
        self.iter.len()
    }

    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}

/// An iterator that calls a function with a reference to each element before
/// yielding it.
///
/// This `struct` is created by the [`inspect`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`inspect`]: trait.Iterator.html#method.inspect
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct Inspect<I, F> {
    iter: I,
    f: F,
}
impl<I, F> Inspect<I, F> {
    pub(super) fn new(iter: I, f: F) -> Inspect<I, F> {
        Inspect { iter, f }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, F> fmt::Debug for Inspect<I, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Inspect")
            .field("iter", &self.iter)
            .finish()
    }
}

impl<I: Iterator, F> Inspect<I, F> where F: FnMut(&I::Item) {
    #[inline]
    fn do_inspect(&mut self, elt: Option<I::Item>) -> Option<I::Item> {
        if let Some(ref a) = elt {
            (self.f)(a);
        }

        elt
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, F> Iterator for Inspect<I, F> where F: FnMut(&I::Item) {
    type Item = I::Item;

    #[inline]
    fn next(&mut self) -> Option<I::Item> {
        let next = self.iter.next();
        self.do_inspect(next)
    }

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

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let f = &mut self.f;
        self.iter.try_fold(init, move |acc, item| { f(&item); fold(acc, item) })
    }

    #[inline]
    fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut f = self.f;
        self.iter.fold(init, move |acc, item| { f(&item); fold(acc, item) })
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: DoubleEndedIterator, F> DoubleEndedIterator for Inspect<I, F>
    where F: FnMut(&I::Item),
{
    #[inline]
    fn next_back(&mut self) -> Option<I::Item> {
        let next = self.iter.next_back();
        self.do_inspect(next)
    }

    #[inline]
    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
    {
        let f = &mut self.f;
        self.iter.try_rfold(init, move |acc, item| { f(&item); fold(acc, item) })
    }

    #[inline]
    fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where Fold: FnMut(Acc, Self::Item) -> Acc,
    {
        let mut f = self.f;
        self.iter.rfold(init, move |acc, item| { f(&item); fold(acc, item) })
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: ExactSizeIterator, F> ExactSizeIterator for Inspect<I, F>
    where F: FnMut(&I::Item)
{
    fn len(&self) -> usize {
        self.iter.len()
    }

    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I: FusedIterator, F> FusedIterator for Inspect<I, F>
    where F: FnMut(&I::Item) {}

/// An iterator adapter that produces output as long as the underlying
/// iterator produces `Option::Some` values.
pub(crate) struct OptionShunt<I> {
    iter: I,
    exited_early: bool,
}

impl<I, T> OptionShunt<I>
where
    I: Iterator<Item = Option<T>>,
{
    /// Process the given iterator as if it yielded a `T` instead of a
    /// `Option<T>`. Any `None` value will stop the inner iterator and
    /// the overall result will be a `None`.
    pub fn process<F, U>(iter: I, mut f: F) -> Option<U>
    where
        F: FnMut(&mut Self) -> U,
    {
        let mut shunt = OptionShunt::new(iter);
        let value = f(shunt.by_ref());
        shunt.reconstruct(value)
    }

    fn new(iter: I) -> Self {
        OptionShunt {
            iter,
            exited_early: false,
        }
    }

    /// Consume the adapter and rebuild a `Option` value.
    fn reconstruct<U>(self, val: U) -> Option<U> {
        if self.exited_early {
            None
        } else {
            Some(val)
        }
    }
}

impl<I, T> Iterator for OptionShunt<I>
where
    I: Iterator<Item = Option<T>>,
{
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        match self.iter.next() {
            Some(Some(v)) => Some(v),
            Some(None) => {
                self.exited_early = true;
                None
            }
            None => None,
        }
    }

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

/// 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. The error may be recovered later via `reconstruct`.
pub(crate) struct ResultShunt<I, E> {
    iter: I,
    error: Option<E>,
}

impl<I, T, E> ResultShunt<I, E>
    where I: Iterator<Item = Result<T, 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 fn process<F, U>(iter: I, mut f: F) -> Result<U, E>
        where F: FnMut(&mut Self) -> U
    {
        let mut shunt = ResultShunt::new(iter);
        let value = f(shunt.by_ref());
        shunt.reconstruct(value)
    }

    fn new(iter: I) -> Self {
        ResultShunt {
            iter,
            error: None,
        }
    }

    /// Consume the adapter and rebuild a `Result` value. This should
    /// *always* be called, otherwise any potential error would be
    /// lost.
    fn reconstruct<U>(self, val: U) -> Result<U, E> {
        match self.error {
            None => Ok(val),
            Some(e) => Err(e),
        }
    }
}

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> {
        match self.iter.next() {
            Some(Ok(v)) => Some(v),
            Some(Err(e)) => {
                self.error = Some(e);
                None
            }
            None => None,
        }
    }

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