# Trait std::iter::DoubleEndedIterator

1.0.0 · source ·

    pub trait DoubleEndedIterator: Iterator {
        // Required method
        fn next_back(&mut self) -> Option<Self::Item>;

        // Provided methods
        fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> { ... }
        fn nth_back(&mut self, n: usize) -> Option<Self::Item> { ... }
        fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> R
           where Self: Sized,
                 F: FnMut(B, Self::Item) -> R,
                 R: Try<Output = B> { ... }
        fn rfold<B, F>(self, init: B, f: F) -> B
           where Self: Sized,
                 F: FnMut(B, Self::Item) -> B { ... }
        fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
           where Self: Sized,
                 P: FnMut(&Self::Item) -> bool { ... }
    }

Expand description

An iterator able to yield elements from both ends.

Something that implements `DoubleEndedIterator` has one extra capability over something that implements `Iterator`: the ability to also take `Item`s from the back, as well as the front.

It is important to note that both back and forth work on the same range, and do not cross: iteration is over when they meet in the middle.

In a similar fashion to the `Iterator` protocol, once a `DoubleEndedIterator` returns `None` from a `next_back()`, calling it again may or may not ever return `Some` again. `next()` and `next_back()` are interchangeable for this purpose.

## Examples

Basic usage:

    let numbers = vec![1, 2, 3, 4, 5, 6];

    let mut iter = numbers.iter();

    assert_eq!(Some(&1), iter.next());
    assert_eq!(Some(&6), iter.next_back());
    assert_eq!(Some(&5), iter.next_back());
    assert_eq!(Some(&2), iter.next());
    assert_eq!(Some(&3), iter.next());
    assert_eq!(Some(&4), iter.next());
    assert_eq!(None, iter.next());
    assert_eq!(None, iter.next_back());

Run

## Required Methods§

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#### fn next_back(&mut self) -> Option<Self::Item>

Removes and returns an element from the end of the iterator.

Returns `None` when there are no more elements.

The trait-level docs contain more details.

##### Examples

Basic usage:

    let numbers = vec![1, 2, 3, 4, 5, 6];

    let mut iter = numbers.iter();

    assert_eq!(Some(&1), iter.next());
    assert_eq!(Some(&6), iter.next_back());
    assert_eq!(Some(&5), iter.next_back());
    assert_eq!(Some(&2), iter.next());
    assert_eq!(Some(&3), iter.next());
    assert_eq!(Some(&4), iter.next());
    assert_eq!(None, iter.next());
    assert_eq!(None, iter.next_back());

Run

##### Remarks

The elements yielded by `DoubleEndedIterator`’s methods may differ from the ones yielded by `Iterator`’s methods:

    let vec = vec![(1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b')];
    let uniq_by_fst_comp = || {
        let mut seen = std::collections::HashSet::new();
        vec.iter().copied().filter(move |x| seen.insert(x.0))
    };

    assert_eq!(uniq_by_fst_comp().last(), Some((2, 'a')));
    assert_eq!(uniq_by_fst_comp().next_back(), Some((2, 'b')));

    assert_eq!(
        uniq_by_fst_comp().fold(vec![], |mut v, x| {v.push(x); v}),
        vec![(1, 'a'), (2, 'a')]
    );
    assert_eq!(
        uniq_by_fst_comp().rfold(vec![], |mut v, x| {v.push(x); v}),
        vec![(2, 'b'), (1, 'c')]
    );

Run

## Provided Methods§

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#### fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize>

🔬This is a nightly-only experimental API. (`iter_advance_by` #77404)

Advances the iterator from the back by `n` elements.

`advance_back_by` is the reverse version of `advance_by`. This method will eagerly skip `n` elements starting from the back by calling `next_back` up to `n` times until `None` is encountered.

`advance_back_by(n)` will return `Ok(())` if the iterator successfully advances by `n` elements, or a `Err(NonZeroUsize)` with value `k` if `None` is encountered, where `k` is remaining number of steps that could not be advanced because the iterator ran out. If `self` is empty and `n` is non-zero, then this returns `Err(n)`. Otherwise, `k` is always less than `n`.

Calling `advance_back_by(0)` can do meaningful work, for example `Flatten` can advance its outer iterator until it finds an inner iterator that is not empty, which then often allows it to return a more accurate `size_hint()` than in its initial state.

##### Examples

Basic usage:

    #![feature(iter_advance_by)]

    use std::num::NonZeroUsize;
    let a = [3, 4, 5, 6];
    let mut iter = a.iter();

    assert_eq!(iter.advance_back_by(2), Ok(()));
    assert_eq!(iter.next_back(), Some(&4));
    assert_eq!(iter.advance_back_by(0), Ok(()));
    assert_eq!(iter.advance_back_by(100), Err(NonZeroUsize::new(99).unwrap())); // only `&3` was skipped

Run
1.37.0 · source

#### fn nth_back(&mut self, n: usize) -> Option<Self::Item>

Returns the `n`th element from the end of the iterator.

This is essentially the reversed version of `Iterator::nth()`. Although like most indexing operations, the count starts from zero, so `nth_back(0)` returns the first value from the end, `nth_back(1)` the second, and so on.

Note that all elements between the end and the returned element will be consumed, including the returned element. This also means that calling `nth_back(0)` multiple times on the same iterator will return different elements.

`nth_back()` will return `None` if `n` is greater than or equal to the length of the iterator.

##### Examples

Basic usage:

    let a = [1, 2, 3];
    assert_eq!(a.iter().nth_back(2), Some(&1));

Run

Calling `nth_back()` multiple times doesn’t rewind the iterator:

    let a = [1, 2, 3];

    let mut iter = a.iter();

    assert_eq!(iter.nth_back(1), Some(&2));
    assert_eq!(iter.nth_back(1), None);

Run

Returning `None` if there are less than `n + 1` elements:

    let a = [1, 2, 3];
    assert_eq!(a.iter().nth_back(10), None);

Run
1.27.0 · source

#### fn try_rfold<B, F, R>(&mut self, init: B, f: F) -> Rwhere Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Output = B>,

This is the reverse version of `Iterator::try_fold()`: it takes elements starting from the back of the iterator.

##### Examples

Basic usage:

    let a = ["1", "2", "3"];
    let sum = a.iter()
        .map(|&s| s.parse::<i32>())
        .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
    assert_eq!(sum, Ok(6));

Run

Short-circuiting:

    let a = ["1", "rust", "3"];
    let mut it = a.iter();
    let sum = it
        .by_ref()
        .map(|&s| s.parse::<i32>())
        .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
    assert!(sum.is_err());

    // Because it short-circuited, the remaining elements are still
    // available through the iterator.
    assert_eq!(it.next_back(), Some(&"1"));

Run
1.27.0 · source

#### fn rfold<B, F>(self, init: B, f: F) -> Bwhere Self: Sized, F: FnMut(B, Self::Item) -> B,

An iterator method that reduces the iterator’s elements to a single, final value, starting from the back.

This is the reverse version of `Iterator::fold()`: it takes elements starting from the back of the iterator.

`rfold()` takes two arguments: an initial value, and a closure with two arguments: an ‘accumulator’, and an element. The closure returns the value that the accumulator should have for the next iteration.

The initial value is the value the accumulator will have on the first call.

After applying this closure to every element of the iterator, `rfold()` returns the accumulator.

This operation is sometimes called ‘reduce’ or ‘inject’.

Folding is useful whenever you have a collection of something, and want to produce a single value from it.

Note: `rfold()` combines elements in a right-associative fashion. For associative operators like `+`, the order the elements are combined in is not important, but for non-associative operators like `-` the order will affect the final result. For a left-associative version of `rfold()`, see `Iterator::fold()`.

##### Examples

Basic usage:

    let a = [1, 2, 3];

    // the sum of all of the elements of a
    let sum = a.iter()
               .rfold(0, |acc, &x| acc + x);

    assert_eq!(sum, 6);

Run

This example demonstrates the right-associative nature of `rfold()`: it builds a string, starting with an initial value and continuing with each element from the back until the front:

    let numbers = [1, 2, 3, 4, 5];

    let zero = "0".to_string();

    let result = numbers.iter().rfold(zero, |acc, &x| {
        format!("({x} + {acc})")
    });

    assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");

Run
1.27.0 · source

#### fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>where Self: Sized, P: FnMut(&Self::Item) -> bool,

Searches for an element of an iterator from the back that satisfies a predicate.

`rfind()` takes a closure that returns `true` or `false`. It applies this closure to each element of the iterator, starting at the end, and if any of them return `true`, then `rfind()` returns `Some(element)`. If they all return `false`, it returns `None`.

`rfind()` is short-circuiting; in other words, it will stop processing as soon as the closure returns `true`.

Because `rfind()` takes a reference, and many iterators iterate over references, this leads to a possibly confusing situation where the argument is a double reference. You can see this effect in the examples below, with `&&x`.

##### Examples

Basic usage:

    let a = [1, 2, 3];

    assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));

    assert_eq!(a.iter().rfind(|&&x| x == 5), None);

Run

Stopping at the first `true`:

    let a = [1, 2, 3];

    let mut iter = a.iter();

    assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));

    // we can still use `iter`, as there are more elements.
    assert_eq!(iter.next_back(), Some(&1));

Run

## Implementors§

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### impl DoubleEndedIterator for EscapeDefault

1.59.0 · source§

### impl DoubleEndedIterator for ToLowercase

1.59.0 · source§

### impl DoubleEndedIterator for ToUppercase

1.12.0 · source§

### impl DoubleEndedIterator for Args

1.12.0 · source§

### impl DoubleEndedIterator for ArgsOs

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### impl DoubleEndedIterator for Bytes<'_>

1.6.0 · source§

### impl DoubleEndedIterator for std::string::Drain<'_>

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### impl<'a> DoubleEndedIterator for Components<'a>

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### impl<'a> DoubleEndedIterator for std::path::Iter<'a>

1.60.0 · source§

### impl<'a> DoubleEndedIterator for EscapeAscii<'a>

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### impl<'a> DoubleEndedIterator for CharIndices<'a>

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### impl<'a> DoubleEndedIterator for Chars<'a>

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### impl<'a> DoubleEndedIterator for Lines<'a>

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### impl<'a> DoubleEndedIterator for LinesAny<'a>

1.34.0 · source§

### impl<'a> DoubleEndedIterator for SplitAsciiWhitespace<'a>

1.1.0 · source§

### impl<'a> DoubleEndedIterator for SplitWhitespace<'a>

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### impl<'a, A> DoubleEndedIterator for std::option::Iter<'a, A>

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### impl<'a, A> DoubleEndedIterator for std::option::IterMut<'a, A>

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### impl<'a, I> DoubleEndedIterator for &'a mut I where I: DoubleEndedIterator + ?Sized,

1.1.0 · source§

### impl<'a, I, T> DoubleEndedIterator for Cloned<I>where T: 'a + Clone, I: DoubleEndedIterator<Item = &'a T>,

1.36.0 · source§

### impl<'a, I, T> DoubleEndedIterator for Copied<I>where T: 'a + Copy, I: DoubleEndedIterator<Item = &'a T>,

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### impl<'a, K, V> DoubleEndedIterator for std::collections::btree_map::Iter<'a, K, V>where K: 'a, V: 'a,

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### impl<'a, K, V> DoubleEndedIterator for std::collections::btree_map::IterMut<'a, K, V>

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### impl<'a, K, V> DoubleEndedIterator for Keys<'a, K, V>

1.17.0 · source§

### impl<'a, K, V> DoubleEndedIterator for std::collections::btree_map::Range<'a, K, V>

1.17.0 · source§

### impl<'a, K, V> DoubleEndedIterator for RangeMut<'a, K, V>

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### impl<'a, K, V> DoubleEndedIterator for Values<'a, K, V>

1.10.0 · source§

### impl<'a, K, V> DoubleEndedIterator for ValuesMut<'a, K, V>

1.5.0 · source§

### impl<'a, P> DoubleEndedIterator for MatchIndices<'a, P>where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: DoubleEndedSearcher<'a>,

1.2.0 · source§

### impl<'a, P> DoubleEndedIterator for Matches<'a, P>where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: DoubleEndedSearcher<'a>,

1.5.0 · source§

### impl<'a, P> DoubleEndedIterator for RMatchIndices<'a, P>where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: DoubleEndedSearcher<'a>,

1.2.0 · source§

### impl<'a, P> DoubleEndedIterator for RMatches<'a, P>where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: DoubleEndedSearcher<'a>,

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### impl<'a, P> DoubleEndedIterator for std::str::RSplit<'a, P>where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: DoubleEndedSearcher<'a>,

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### impl<'a, P> DoubleEndedIterator for RSplitTerminator<'a, P>where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: DoubleEndedSearcher<'a>,

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### impl<'a, P> DoubleEndedIterator for std::str::Split<'a, P>where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: DoubleEndedSearcher<'a>,

1.51.0 · source§

### impl<'a, P> DoubleEndedIterator for std::str::SplitInclusive<'a, P>where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: DoubleEndedSearcher<'a>,

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### impl<'a, P> DoubleEndedIterator for SplitTerminator<'a, P>where P: Pattern<'a>, <P as Pattern<'a>>::Searcher: DoubleEndedSearcher<'a>,

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### impl<'a, T> DoubleEndedIterator for std::collections::binary_heap::Iter<'a, T>

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### impl<'a, T> DoubleEndedIterator for std::collections::btree_set::Iter<'a, T>

1.17.0 · source§

### impl<'a, T> DoubleEndedIterator for std::collections::btree_set::Range<'a, T>

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### impl<'a, T> DoubleEndedIterator for std::collections::linked_list::Iter<'a, T>

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### impl<'a, T> DoubleEndedIterator for std::collections::linked_list::IterMut<'a, T>

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### impl<'a, T> DoubleEndedIterator for std::collections::vec_deque::Iter<'a, T>

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### impl<'a, T> DoubleEndedIterator for std::collections::vec_deque::IterMut<'a, T>

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### impl<'a, T> DoubleEndedIterator for std::result::Iter<'a, T>

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### impl<'a, T> DoubleEndedIterator for std::result::IterMut<'a, T>

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### impl<'a, T> DoubleEndedIterator for Chunks<'a, T>

1.31.0 · source§

### impl<'a, T> DoubleEndedIterator for ChunksExact<'a, T>

1.31.0 · source§

### impl<'a, T> DoubleEndedIterator for ChunksExactMut<'a, T>

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### impl<'a, T> DoubleEndedIterator for ChunksMut<'a, T>

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### impl<'a, T> DoubleEndedIterator for std::slice::Iter<'a, T>

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### impl<'a, T> DoubleEndedIterator for std::slice::IterMut<'a, T>

1.31.0 · source§

### impl<'a, T> DoubleEndedIterator for RChunks<'a, T>

1.31.0 · source§

### impl<'a, T> DoubleEndedIterator for RChunksExact<'a, T>

1.31.0 · source§

### impl<'a, T> DoubleEndedIterator for RChunksExactMut<'a, T>

1.31.0 · source§

### impl<'a, T> DoubleEndedIterator for RChunksMut<'a, T>

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### impl<'a, T> DoubleEndedIterator for Windows<'a, T>

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### impl<'a, T, P> DoubleEndedIterator for GroupBy<'a, T, P>where T: 'a, P: FnMut(&T, &T) -> bool,

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### impl<'a, T, P> DoubleEndedIterator for GroupByMut<'a, T, P>where T: 'a, P: FnMut(&T, &T) -> bool,

1.27.0 · source§

### impl<'a, T, P> DoubleEndedIterator for std::slice::RSplit<'a, T, P>where P: FnMut(&T) -> bool,

1.27.0 · source§

### impl<'a, T, P> DoubleEndedIterator for RSplitMut<'a, T, P>where P: FnMut(&T) -> bool,

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### impl<'a, T, P> DoubleEndedIterator for std::slice::Split<'a, T, P>where P: FnMut(&T) -> bool,

1.51.0 · source§

### impl<'a, T, P> DoubleEndedIterator for std::slice::SplitInclusive<'a, T, P>where P: FnMut(&T) -> bool,

1.51.0 · source§

### impl<'a, T, P> DoubleEndedIterator for SplitInclusiveMut<'a, T, P>where P: FnMut(&T) -> bool,

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### impl<'a, T, P> DoubleEndedIterator for SplitMut<'a, T, P>where P: FnMut(&T) -> bool,

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### impl<'a, T, const N: usize> DoubleEndedIterator for std::slice::ArrayChunks<'a, T, N>

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### impl<'a, T, const N: usize> DoubleEndedIterator for ArrayChunksMut<'a, T, N>

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### impl<'a, T, const N: usize> DoubleEndedIterator for ArrayWindows<'a, T, N>

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### impl<A> DoubleEndedIterator for std::ops::Range<A>where A: Step,

1.26.0 · source§

### impl<A> DoubleEndedIterator for RangeInclusive<A>where A: Step,

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### impl<A> DoubleEndedIterator for std::option::IntoIter<A>

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### impl<A> DoubleEndedIterator for Repeat<A>where A: Clone,

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### impl<A, B> DoubleEndedIterator for Chain<A, B>where A: DoubleEndedIterator, B: DoubleEndedIterator<Item = <A as Iterator>::Item>,

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### impl<A, B> DoubleEndedIterator for Zip<A, B>where A: DoubleEndedIterator + ExactSizeIterator, B: DoubleEndedIterator + ExactSizeIterator,

1.43.0 · source§

### impl<A, F> DoubleEndedIterator for OnceWith<F>where F: FnOnce() -> A,

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### impl<B, I, F> DoubleEndedIterator for FilterMap<I, F>where I: DoubleEndedIterator, F: FnMut(<I as Iterator>::Item) -> Option<B>,

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### impl<B, I, F> DoubleEndedIterator for Map<I, F>where I: DoubleEndedIterator, F: FnMut(<I as Iterator>::Item) -> B,

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### impl<I> DoubleEndedIterator for ByRefSized<'_, I>where I: DoubleEndedIterator,

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### impl<I> DoubleEndedIterator for Enumerate<I>where I: ExactSizeIterator + DoubleEndedIterator,

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### impl<I> DoubleEndedIterator for Fuse<I>where I: DoubleEndedIterator,

1.38.0 · source§

### impl<I> DoubleEndedIterator for Peekable<I>where I: DoubleEndedIterator,

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### impl<I> DoubleEndedIterator for Rev<I>where I: DoubleEndedIterator,

1.9.0 · source§

### impl<I> DoubleEndedIterator for Skip<I>where I: DoubleEndedIterator + ExactSizeIterator,

1.38.0 · source§

### impl<I> DoubleEndedIterator for StepBy<I>where I: DoubleEndedIterator + ExactSizeIterator,

1.38.0 · source§

### impl<I> DoubleEndedIterator for Take<I>where I: DoubleEndedIterator + ExactSizeIterator,

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### impl<I, A> DoubleEndedIterator for Box<I, A>where I: DoubleEndedIterator + ?Sized, A: Allocator,

1.21.0 · source§

### impl<I, A> DoubleEndedIterator for Splice<'_, I, A>where I: Iterator, A: Allocator,

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### impl<I, F> DoubleEndedIterator for Inspect<I, F>where I: DoubleEndedIterator, F: FnMut(&<I as Iterator>::Item),

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### impl<I, P> DoubleEndedIterator for Filter<I, P>where I: DoubleEndedIterator, P: FnMut(&<I as Iterator>::Item) -> bool,

1.29.0 · source§

### impl<I, U> DoubleEndedIterator for Flatten<I>where I: DoubleEndedIterator, <I as Iterator>::Item: IntoIterator<IntoIter = U, Item = <U as Iterator>::Item>, U: DoubleEndedIterator,

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### impl<I, U, F> DoubleEndedIterator for FlatMap<I, U, F>where I: DoubleEndedIterator, F: FnMut(<I as Iterator>::Item) -> U, U: IntoIterator, <U as IntoIterator>::IntoIter: DoubleEndedIterator,

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### impl<I, const N: usize> DoubleEndedIterator for std::iter::ArrayChunks<I, N>where I: DoubleEndedIterator + ExactSizeIterator,

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### impl<K, V, A> DoubleEndedIterator for std::collections::btree_map::IntoIter<K, V, A>where A: Allocator + Clone,

1.54.0 · source§

### impl<K, V, A> DoubleEndedIterator for IntoKeys<K, V, A>where A: Allocator + Clone,

1.54.0 · source§

### impl<K, V, A> DoubleEndedIterator for IntoValues<K, V, A>where A: Allocator + Clone,

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### impl<T> DoubleEndedIterator for std::result::IntoIter<T>

1.2.0 · source§

### impl<T> DoubleEndedIterator for Empty<T>

1.2.0 · source§

### impl<T> DoubleEndedIterator for Once<T>

1.6.0 · source§

### impl<T, A> DoubleEndedIterator for std::collections::binary_heap::Drain<'_, T, A>where A: Allocator,

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### impl<T, A> DoubleEndedIterator for std::collections::binary_heap::IntoIter<T, A>where A: Allocator,

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### impl<T, A> DoubleEndedIterator for std::collections::btree_set::IntoIter<T, A>where A: Allocator + Clone,

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### impl<T, A> DoubleEndedIterator for std::collections::linked_list::IntoIter<T, A>where A: Allocator,

1.6.0 · source§

### impl<T, A> DoubleEndedIterator for std::collections::vec_deque::Drain<'_, T, A>where A: Allocator,

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### impl<T, A> DoubleEndedIterator for std::collections::vec_deque::IntoIter<T, A>where A: Allocator,

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### impl<T, A> DoubleEndedIterator for std::vec::IntoIter<T, A>where A: Allocator,

1.6.0 · source§

### impl<T, A, FH> DoubleEndedIterator for std::vec::Drain<'_, T, A, FH>where A: Allocator, FH: FailureHandling,

1.40.0 · source§

### impl<T, const N: usize> DoubleEndedIterator for std::array::IntoIter<T, N>