Always implement `ExactSizeIterator` for `std::iter::Take`

[pitdicker]

Currently std::iter::Take only implements ExactSizeIterator if its parent iterator implements that. But Take knows how many elements it will return at most, so I think it makes sense if it would always implement ExactSizeIterator.

It would not be correct if the parent iterator produces less items, but this trait is only a best effort to help optimizations, right?

[sfackler]

It would not be correct if the parent iterator produces less items, but this trait is only a best effort to help optimizations, right?

No, it states that you know the exact size of the iterator. If you don't you shouldn't implement it.

[Enet4]

Take already takes advantage of the given upper bound in size_hint. However, implementing ExactSizeIterator requires the adaptor to know exactly which elements it contains, and not just an estimation.

[pitdicker]

Both thanks for the replies, I now understand the role of ExactSizeIterator a bit better!