technically yes, but it'll change the semantics in a not useful way. Because if you leave off the & on self then the method won't borrow self (take it by reference) but consume it (take it by value by 'moving' it), so you can't use it any more after calling clone(), which basically makes the method a no-op instead of copying the struct.

Thanks pornel and rcxdude, this makes sense to me now. I was thinking it shouldn't be dereferencing because when references were previously introduced (the print_number example), we could call methods on the reference without any special syntax. But I guess it makes sense that instance methods work that way, while other functions need to know if we have a reference or the real object, and in this regard it works more or less like C++.