My crate (docs, source) provides versions of Flatten and FlatMap that take advantage of their intermediate yielded items implementing the also provided ConstSizeIntoIterator to accurately gauge their remaining size. In a previous version ConstSizeFlatten and ConstSizeFlatMap implemented ExactSizeIterator, but I was made aware of the fact that even if the component iterators implement ExactSizeIterator correctly, the resulting length could be above usize::MAX meaning that len() could panic.
I believe this functionality should be in core’s Flatten and FlatMap, because it actually already is—for arrays. The problem is that core implements this rather obtusely and non-extensibly via a private trait and specialization. It seems to me that once specialization is stabilized a new feature could be built on top of it, trait-conditional control flow, wherein a function can be functionally different for types that implement a trait and ones that don’t. This would allow a much more sensible implementation than core currently uses that could be made public while providing the improved accuracy my crate provides.
What core does is instead of having a trait ConstSizeIntoIterator: IntoIterator { const SIZE: usize; }, it has a trait ConstSizeIntoIterator: IntoIterator { const SIZE: Option<usize>; } with a blanket impl for all compatible types, and specializes this implementation for {,&{,mut} }[T; N]. I believe that at least some traits could automatically be converted into this form in the background allowing for an easy minimal implementation of trait-conditional control flow.
My crate’s ConstSizeIntoIterator currently requires implementor’s IntoIter implements ExactSizeIterator, but I am looking into relaxing this restriction.