pub trait ExactSizeIterator: Iterator {
    // Provided methods
    fn len(&self) -> usize { ... }
    fn is_empty(&self) -> bool { ... }
}
Expand description

An iterator that knows its exact length.

Many Iterators don’t know how many times they will iterate, but some do. If an iterator knows how many times it can iterate, providing access to that information can be useful. For example, if you want to iterate backwards, a good start is to know where the end is.

When implementing an ExactSizeIterator, you must also implement Iterator. When doing so, the implementation of Iterator::size_hint must return the exact size of the iterator.

The len method has a default implementation, so you usually shouldn’t implement it. However, you may be able to provide a more performant implementation than the default, so overriding it in this case makes sense.

Note that this trait is a safe trait and as such does not and cannot guarantee that the returned length is correct. This means that unsafe code must not rely on the correctness of Iterator::size_hint. The unstable and unsafe TrustedLen trait gives this additional guarantee.

§When shouldn’t an adapter be ExactSizeIterator?

If an adapter makes an iterator longer, then it’s usually incorrect for that adapter to implement ExactSizeIterator. The inner exact-sized iterator might already be usize::MAX-long, and thus the length of the longer adapted iterator would no longer be exactly representable in usize.

This is why Chain<A, B> isn’t ExactSizeIterator, even when A and B are both ExactSizeIterator.

§Examples

Basic usage:

// a finite range knows exactly how many times it will iterate
let five = 0..5;

assert_eq!(5, five.len());

In the module-level docs, we implemented an Iterator, Counter. Let’s implement ExactSizeIterator for it as well:

impl ExactSizeIterator for Counter {
    // We can easily calculate the remaining number of iterations.
    fn len(&self) -> usize {
        5 - self.count
    }
}

// And now we can use it!

let mut counter = Counter::new();

assert_eq!(5, counter.len());
let _ = counter.next();
assert_eq!(4, counter.len());

Provided Methods§

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fn len(&self) -> usize

Returns the exact remaining length of the iterator.

The implementation ensures that the iterator will return exactly len() more times a Some(T) value, before returning None. This method has a default implementation, so you usually should not implement it directly. However, if you can provide a more efficient implementation, you can do so. See the trait-level docs for an example.

This function has the same safety guarantees as the Iterator::size_hint function.

§Examples

Basic usage:

// a finite range knows exactly how many times it will iterate
let mut range = 0..5;

assert_eq!(5, range.len());
let _ = range.next();
assert_eq!(4, range.len());
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fn is_empty(&self) -> bool

🔬This is a nightly-only experimental API. (exact_size_is_empty)

Returns true if the iterator is empty.

This method has a default implementation using ExactSizeIterator::len(), so you don’t need to implement it yourself.

§Examples

Basic usage:

#![feature(exact_size_is_empty)]

let mut one_element = std::iter::once(0);
assert!(!one_element.is_empty());

assert_eq!(one_element.next(), Some(0));
assert!(one_element.is_empty());

assert_eq!(one_element.next(), None);

Implementors§

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impl ExactSizeIterator for IndexVecIntoIter

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impl ExactSizeIterator for AttachedModuleIdIter

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impl ExactSizeIterator for ModuleIdIter

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impl ExactSizeIterator for Range<i8>

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impl ExactSizeIterator for Range<i16>

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impl ExactSizeIterator for Range<i32>

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impl ExactSizeIterator for Range<isize>

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impl ExactSizeIterator for Range<u8>

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impl ExactSizeIterator for Range<u16>

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impl ExactSizeIterator for Range<u32>

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impl ExactSizeIterator for Range<usize>

1.26.0 · source§

impl ExactSizeIterator for RangeInclusive<i8>

1.26.0 · source§

impl ExactSizeIterator for RangeInclusive<i16>

1.26.0 · source§

impl ExactSizeIterator for RangeInclusive<u8>

1.26.0 · source§

impl ExactSizeIterator for RangeInclusive<u16>

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impl ExactSizeIterator for Bytes<'_>

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impl ExactSizeIterator for core::ascii::EscapeDefault

1.20.0 · source§

impl ExactSizeIterator for EscapeDebug

1.11.0 · source§

impl ExactSizeIterator for core::char::EscapeDefault

1.11.0 · source§

impl ExactSizeIterator for EscapeUnicode

1.35.0 · source§

impl ExactSizeIterator for ToLowercase

1.35.0 · source§

impl ExactSizeIterator for ToUppercase

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impl ExactSizeIterator for Args

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impl ExactSizeIterator for ArgsOs

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impl ExactSizeIterator for Chain<'_>

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impl ExactSizeIterator for U32Digits<'_>

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impl ExactSizeIterator for U64Digits<'_>

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impl ExactSizeIterator for ExportsIter<'_>

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impl ExactSizeIterator for IntoIter

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impl<'a> ExactSizeIterator for IndexVecIter<'a>

1.57.0 · source§

impl<'a> ExactSizeIterator for CommandArgs<'a>

1.57.0 · source§

impl<'a> ExactSizeIterator for CommandEnvs<'a>

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impl<'a> ExactSizeIterator for CapturesPatternIter<'a>

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impl<'a> ExactSizeIterator for GroupInfoPatternNames<'a>

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impl<'a> ExactSizeIterator for ModuleImportsIter<'a>

1.1.0 · source§

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

1.36.0 · source§

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

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impl<'a, Idx, T> ExactSizeIterator for Iter<'a, Idx, T>
where Idx: ArenaIndex,

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impl<'a, Idx, T> ExactSizeIterator for IterMut<'a, Idx, T>
where Idx: ArenaIndex,

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impl<'a, K> ExactSizeIterator for Iter<'a, K>

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impl<'a, K> ExactSizeIterator for Iter<'a, K>

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impl<'a, K, V> ExactSizeIterator for Iter<'a, K, V>

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impl<'a, K, V> ExactSizeIterator for IterMut<'a, K, V>

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impl<'a, K, V> ExactSizeIterator for Values<'a, K, V>

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impl<'a, K, V> ExactSizeIterator for ValuesMut<'a, K, V>

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impl<'a, S, T> ExactSizeIterator for SliceChooseIter<'a, S, T>
where S: Index<usize, Output = T> + 'a + ?Sized, T: 'a,

1.31.0 · source§

impl<'a, T> ExactSizeIterator for RChunksExact<'a, T>

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impl<'a, T> ExactSizeIterator for syn::punctuated::Iter<'a, T>

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impl<'a, T> ExactSizeIterator for syn::punctuated::IterMut<'a, T>

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impl<'a, T> ExactSizeIterator for Drain<'a, T>
where T: Array,

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impl<'a, T> ExactSizeIterator for Iter<'a, T>

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impl<'a, T> ExactSizeIterator for SectionLimitedIntoIter<'a, T>
where T: FromReader<'a>,

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impl<'a, T> ExactSizeIterator for SectionLimitedIntoIterWithOffsets<'a, T>
where T: FromReader<'a>,

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impl<'a, T, P> ExactSizeIterator for Pairs<'a, T, P>

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impl<'a, T, P> ExactSizeIterator for PairsMut<'a, T, P>

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impl<'c, 'h> ExactSizeIterator for SubCaptureMatches<'c, 'h>

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impl<'c, 'h> ExactSizeIterator for SubCaptureMatches<'c, 'h>

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impl<'r> ExactSizeIterator for CaptureNames<'r>

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impl<'r> ExactSizeIterator for CaptureNames<'r>

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impl<A> ExactSizeIterator for RepeatN<A>
where A: Clone,

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impl<A> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::rust::option::IntoIter<A>

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impl<A> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::rust::option::Iter<'_, A>

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impl<A> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::rust::option::IterMut<'_, A>

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impl<A> ExactSizeIterator for IntoIter<A>
where A: Array,

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impl<A, B> ExactSizeIterator for Zip<A, B>

1.43.0 · source§

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

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

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impl<I> ExactSizeIterator for &mut I

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impl<I> ExactSizeIterator for Enumerate<I>

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impl<I> ExactSizeIterator for Fuse<I>

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impl<I> ExactSizeIterator for Peekable<I>

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impl<I> ExactSizeIterator for Rev<I>

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impl<I> ExactSizeIterator for Skip<I>

1.28.0 · source§

impl<I> ExactSizeIterator for StepBy<I>

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impl<I> ExactSizeIterator for Take<I>

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impl<I, A> ExactSizeIterator for Box<I, A>

1.21.0 · source§

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

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

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impl<I, F, R, const N: usize> ExactSizeIterator for MapWindows<I, F, N>
where I: ExactSizeIterator, F: FnMut(&[<I as Iterator>::Item; N]) -> R,

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impl<I, const N: usize> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::rust::iter::ArrayChunks<I, N>

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impl<K> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_set::Drain<'_, K>

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impl<K> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_set::IntoIter<K>

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impl<K> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_set::Iter<'_, K>

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impl<K, A> ExactSizeIterator for Drain<'_, K, A>
where A: Allocator + Clone,

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impl<K, A> ExactSizeIterator for Drain<'_, K, A>
where A: Allocator + Clone,

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impl<K, A> ExactSizeIterator for IntoIter<K, A>
where A: Allocator + Clone,

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impl<K, A> ExactSizeIterator for IntoIter<K, A>
where A: Allocator + Clone,

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::btree_map::Iter<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::btree_map::IterMut<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::btree_map::Keys<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::btree_map::Values<'_, K, V>

1.10.0 · source§

impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::btree_map::ValuesMut<'_, K, V>

1.6.0 · source§

impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_map::Drain<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_map::IntoIter<K, V>

1.54.0 · source§

impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_map::IntoKeys<K, V>

1.54.0 · source§

impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_map::IntoValues<K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_map::Iter<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_map::IterMut<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_map::Keys<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_map::Values<'_, K, V>

1.10.0 · source§

impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::hash_map::ValuesMut<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::map::Drain<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::map::IntoIter<K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::map::IntoKeys<K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::map::IntoValues<K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::map::Iter<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::map::IterMut<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::map::Keys<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::map::Values<'_, K, V>

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impl<K, V> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::map::ValuesMut<'_, K, V>

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impl<K, V> ExactSizeIterator for indexmap::map::Drain<'_, K, V>

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impl<K, V> ExactSizeIterator for indexmap::map::IntoIter<K, V>

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impl<K, V> ExactSizeIterator for indexmap::map::IntoKeys<K, V>

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impl<K, V> ExactSizeIterator for indexmap::map::IntoValues<K, V>

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impl<K, V> ExactSizeIterator for indexmap::map::Iter<'_, K, V>

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impl<K, V> ExactSizeIterator for indexmap::map::IterMut<'_, K, V>

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impl<K, V> ExactSizeIterator for indexmap::map::Keys<'_, K, V>

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impl<K, V> ExactSizeIterator for indexmap::map::Values<'_, K, V>

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impl<K, V> ExactSizeIterator for indexmap::map::ValuesMut<'_, K, V>

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impl<K, V> ExactSizeIterator for IntoIter<K, V>

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impl<K, V> ExactSizeIterator for Iter<'_, K, V>

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impl<K, V> ExactSizeIterator for Iter<'_, K, V>

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impl<K, V> ExactSizeIterator for IterMut<'_, K, V>

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impl<K, V> ExactSizeIterator for IterMut<'_, K, V>

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impl<K, V> ExactSizeIterator for Keys<'_, K, V>

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impl<K, V> ExactSizeIterator for Keys<'_, K, V>

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impl<K, V> ExactSizeIterator for Values<'_, K, V>

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impl<K, V> ExactSizeIterator for Values<'_, K, V>

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impl<K, V> ExactSizeIterator for ValuesMut<'_, K, V>

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impl<K, V> ExactSizeIterator for ValuesMut<'_, K, V>

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impl<K, V, A> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::btree_map::IntoIter<K, V, A>
where A: Allocator + Clone,

1.54.0 · source§

impl<K, V, A> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::btree_map::IntoKeys<K, V, A>
where A: Allocator + Clone,

1.54.0 · source§

impl<K, V, A> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::btree_map::IntoValues<K, V, A>
where A: Allocator + Clone,

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impl<K, V, A> ExactSizeIterator for Drain<'_, K, V, A>
where A: Allocator + Clone,

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impl<K, V, A> ExactSizeIterator for Drain<'_, K, V, A>
where A: Allocator + Clone,

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impl<K, V, A> ExactSizeIterator for IntoIter<K, V, A>
where A: Allocator + Clone,

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impl<K, V, A> ExactSizeIterator for IntoIter<K, V, A>
where A: Allocator + Clone,

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

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

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

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

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impl<T> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::btree_set::Iter<'_, T>

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impl<T> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::set::Drain<'_, T>

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impl<T> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::set::IntoIter<T>

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impl<T> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::indexmap::set::Iter<'_, T>

1.2.0 · source§

impl<T> ExactSizeIterator for Empty<T>

1.2.0 · source§

impl<T> ExactSizeIterator for Once<T>

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impl<T> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::rust::result::IntoIter<T>

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impl<T> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::rust::result::Iter<'_, T>

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impl<T> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::rust::result::IterMut<'_, T>

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impl<T> ExactSizeIterator for Chunks<'_, T>

1.31.0 · source§

impl<T> ExactSizeIterator for ChunksExact<'_, T>

1.31.0 · source§

impl<T> ExactSizeIterator for ChunksExactMut<'_, T>

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impl<T> ExactSizeIterator for ChunksMut<'_, T>

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impl<T> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::rust::slice::Iter<'_, T>

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impl<T> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::rust::slice::IterMut<'_, T>

1.31.0 · source§

impl<T> ExactSizeIterator for RChunks<'_, T>

1.31.0 · source§

impl<T> ExactSizeIterator for RChunksExactMut<'_, T>

1.31.0 · source§

impl<T> ExactSizeIterator for RChunksMut<'_, T>

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impl<T> ExactSizeIterator for Windows<'_, T>

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impl<T> ExactSizeIterator for alloc::collections::binary_heap::Iter<'_, T>

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impl<T> ExactSizeIterator for alloc::collections::linked_list::Iter<'_, T>

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impl<T> ExactSizeIterator for alloc::collections::linked_list::IterMut<'_, T>

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impl<T> ExactSizeIterator for alloc::collections::vec_deque::iter::Iter<'_, T>

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impl<T> ExactSizeIterator for alloc::collections::vec_deque::iter_mut::IterMut<'_, T>

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impl<T> ExactSizeIterator for indexmap::set::Drain<'_, T>

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impl<T> ExactSizeIterator for indexmap::set::IntoIter<T>

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impl<T> ExactSizeIterator for indexmap::set::Iter<'_, T>

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impl<T> ExactSizeIterator for syn::punctuated::IntoIter<T>

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impl<T> ExactSizeIterator for IntoIter<T>

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impl<T> ExactSizeIterator for RawIter<T>

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impl<T> ExactSizeIterator for RawIter<T>

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impl<T> ExactSizeIterator for WasmFuncTypeInputs<'_, T>
where T: WasmFuncType,

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impl<T> ExactSizeIterator for WasmFuncTypeInputs<'_, T>
where T: WasmFuncType,

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impl<T> ExactSizeIterator for WasmFuncTypeOutputs<'_, T>
where T: WasmFuncType,

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impl<T> ExactSizeIterator for WasmFuncTypeOutputs<'_, T>
where T: WasmFuncType,

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impl<T, A> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::btree_set::IntoIter<T, A>
where A: Allocator + Clone,

1.6.0 · source§

impl<T, A> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::vec::Drain<'_, T, A>
where A: Allocator,

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impl<T, A> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::vec::IntoIter<T, A>
where A: Allocator,

1.6.0 · source§

impl<T, A> ExactSizeIterator for alloc::collections::binary_heap::Drain<'_, T, A>
where A: Allocator,

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impl<T, A> ExactSizeIterator for DrainSorted<'_, T, A>
where T: Ord, A: Allocator,

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impl<T, A> ExactSizeIterator for alloc::collections::binary_heap::IntoIter<T, A>
where A: Allocator,

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impl<T, A> ExactSizeIterator for IntoIterSorted<T, A>
where T: Ord, A: Allocator,

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impl<T, A> ExactSizeIterator for alloc::collections::linked_list::IntoIter<T, A>
where A: Allocator,

1.6.0 · source§

impl<T, A> ExactSizeIterator for alloc::collections::vec_deque::drain::Drain<'_, T, A>
where A: Allocator,

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impl<T, A> ExactSizeIterator for alloc::collections::vec_deque::into_iter::IntoIter<T, A>
where A: Allocator,

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impl<T, A> ExactSizeIterator for RawDrain<'_, T, A>
where A: Allocator + Clone,

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impl<T, A> ExactSizeIterator for RawDrain<'_, T, A>
where A: Allocator + Clone,

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impl<T, A> ExactSizeIterator for RawIntoIter<T, A>
where A: Allocator + Clone,

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impl<T, A> ExactSizeIterator for RawIntoIter<T, A>
where A: Allocator + Clone,

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impl<T, N> ExactSizeIterator for GenericArrayIter<T, N>
where N: ArrayLength<T>,

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impl<T, P> ExactSizeIterator for IntoPairs<T, P>

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impl<T, const N: usize> ExactSizeIterator for radix_engine::types::radix_engine_common::prelude::rust::slice::ArrayChunks<'_, T, N>

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impl<T, const N: usize> ExactSizeIterator for ArrayChunksMut<'_, T, N>

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impl<T, const N: usize> ExactSizeIterator for ArrayWindows<'_, T, N>

1.40.0 · source§

impl<T, const N: usize> ExactSizeIterator for core::array::iter::IntoIter<T, N>