The method Iterator::len() is surprising, as all the other uses of len() do not consume the value. len() would make more sense to be called count(), but that would collide with the current Iterator::count(|T| -> bool) -> unit method. That method, however, is a bit redundant, and can be easily replaced with iter.filter(|x| x < 5).count(). After this change, we could then define the len() method on iter::ExactSize.