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Showing content from https://doc.rust-lang.org/nightly/std/iter/trait.TrustedLen.html below:

TrustedLen in std::iter - Rust

pub unsafe trait TrustedLen: Iterator { }

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 an Iterator::size_hint() of (usize::MAX, None).

The iterator must produce exactly the number of elements it reported or diverge before reaching the end.

If an adapter makes an iterator shorter by a given amount, then it’s usually incorrect for that adapter to implement TrustedLen. The inner iterator might return more than usize::MAX items, but there’s no way to know what k elements less than that will be, since the size_hint from the inner iterator has already saturated and lost that information.

This is why Skip<I> isn’t TrustedLen, even when I implements TrustedLen.

This trait must only be implemented when the contract is upheld. Consumers of this trait must inspect Iterator::size_hint()’s upper bound.

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