After a long list of discussion about the performance problem from using varargs and the reason that we can't find a great pair for it, it would be best to remove support for it for now. It will come back if we can find a good pair for it. For now, Bon Voyage, `#among?`.
There're a lot of places in Rails source code which make a lot of sense to switching to Object#in? or Object#either? instead of using .include?.