Most objects are cloneable, but not all. For example you can’t dup nil: 

  nil.dup # => TypeError: can't dup NilClass

Classes may signal their instances are not duplicable removing dup/clone or raising exceptions from them. So, to dup an arbitrary object you normally use an optimistic approach and are ready to catch an exception, say: 

  arbitrary_object.dup rescue object

Rails dups objects in a few critical spots where they are not that arbitrary. That rescue is very expensive (like 40 times slower than a predicate), and it is often triggered.

That’s why we hardcode the following cases and check duplicable? instead of using that rescue idiom.

Aliases

  • __send__
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