Trait std::iter::TrustedLen
source · pub unsafe trait TrustedLen: Iterator { }trusted_len #37572)Expand description
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.
When shouldn’t an adapter be TrustedLen?
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.
Safety
This trait must only be implemented when the contract is upheld. Consumers
of this trait must inspect Iterator::size_hint()’s upper bound.