> By itself knows the exact size of the iterator as part of the iterator type…

std::array::IntoIter<T, N> already satisfies that requirement. The result of mapping over a std::array::IntoIter<T, N> should be std::iter::Map<array::IntoIter<T, N>, F>, which also has the number of elements at the type level and even inherits ExactSizeIterator from array::IntoIter. The problem, I think, is that ExactSizeIterator only says that the size is known at run time; there is no "ConstSizeIterator<N>" trait which would witness that the exact size is known at compile time. If there were then you could potentially collect the items from a ConstSizeIterator<N> (which could be implemented by array::IntoIter<T, N> and inherited by iter::Map<array::IntoIter<T, N>, F>) into an array of N elements without any runtime checks or allocation.