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/// An iterator that always continues to yield `None` when exhausted. /// /// Calling next on a fused iterator that has returned `None` once is guaranteed /// to return [`None`] again. This trait should be implemented by all iterators /// that behave this way because it allows optimizing [`Iterator::fuse`]. /// /// Note: In general, you should not use `FusedIterator` in generic bounds if /// you need a fused iterator. Instead, you should just call [`Iterator::fuse`] /// on the iterator. If the iterator is already fused, the additional [`Fuse`] /// wrapper will be a no-op with no performance penalty. /// /// [`None`]: ../../std/option/enum.Option.html#variant.None /// [`Iterator::fuse`]: ../../std/iter/trait.Iterator.html#method.fuse /// [`Fuse`]: ../../std/iter/struct.Fuse.html #[stable(feature = "fused", since = "1.26.0")] pub trait FusedIterator: Iterator {} #[stable(feature = "fused", since = "1.26.0")] impl<I: FusedIterator + ?Sized> FusedIterator for &mut I {} /// An iterator that reports an accurate length using size_hint. /// /// The iterator reports a size hint where it is either exact /// (lower bound is equal to upper bound), or the upper bound is [`None`]. /// The upper bound must only be [`None`] if the actual iterator length is /// larger than [`usize::MAX`]. In that case, the lower bound must be /// [`usize::MAX`], resulting in a [`.size_hint`] of `(usize::MAX, None)`. /// /// The iterator must produce exactly the number of elements it reported /// or diverge before reaching the end. /// /// # Safety /// /// This trait must only be implemented when the contract is upheld. /// Consumers of this trait must inspect [`.size_hint`]’s upper bound. /// /// [`None`]: ../../std/option/enum.Option.html#variant.None /// [`usize::MAX`]: ../../std/usize/constant.MAX.html /// [`.size_hint`]: ../../std/iter/trait.Iterator.html#method.size_hint #[unstable(feature = "trusted_len", issue = "37572")] pub unsafe trait TrustedLen : Iterator {} #[unstable(feature = "trusted_len", issue = "37572")] unsafe impl<I: TrustedLen + ?Sized> TrustedLen for &mut I {}