Is there any reason to not implement the primitive? At the very least, we should probably add it and just return null so it falls back to the Ruby code.

@nirvdrum where are these primitives added? I'll add the null case.

Working with the array methods so far, I think there be a more general purpose primitive that we could create that could replace copy_from. copy_from for example doesn't allow growing or shrinking the array.

I think it would be something like this but in java:

def splice_arrays( array, other, a, b, m, n)
  array[a,n] = other[m,n]
end

We have the primitive node classes in org.jruby.truffle.nodes.rubinius, but surprisingly, we don't seem to have an Array one yet. That's where I'd advocate putting it.

The primitive name itself forms the actual namespace, so it doesn't need to be tied to a particular class . . . that's just for organization. As an example, Rubinius::Character is a subclass of String, much like the Tuple and Array relationship. But I recently added a "character" primitive to the StringPrimitiveNodes class.