Not sure about this. We also have Array#insert, but it's not insert!. I'm not sure there's a good use case for swapping elements in an array for a copy...

I am not sure either, is there a common scenario where you would use a "swap & copy"? Usually when swapping elements we want to do it in-place. And if we only have one operation then #swap is fine.

The bang (!) does not mean "destructive" nor lack of it mean non destructive either. The bang sign means "the bang version is more dangerous than its non bang counterpart; handle with care".
https://www.ruby-forum.com/topic/176830#773946

Fair enough. I don't know that there's common scenario where swapping in a copy is needed, but I ran into one and was surprised by Array#swap. If the consensus is that not every destructive method needs a non-destructive sibling, I'll close this.

Yes, not every destructive methods needs a non-destructive method. And in particular, swap is often used to efficiently swap elements, and duping the array kind of defeats that purpose :-)

Thank you in any case for brining up this discussion.