I think the proposal is over specialized. Why should the array elements
neccessarily become keys?

Nobu, I agree except 1) key,value in Enumerable? 2) other clearly
polymorphic solution:

module Enumerable
  def to_h
    hash = {}
    each_with_index {|value,index| hash[i] = value}
    hash
  end
end

Hence hash key <-> array index. And it is by definition "Enumerable".

T.