The second one scares me. It implies some_state is mutated (or not, we may just be logging something) by the do_stuff function while the first makes it very clear that some_state is in charge of doing stuff and that the implementation is aware of how some_state is implemented itself.

OTOH, the second one would be much better (and imply immutability) if it were written as

  new_state = do_stuff(some_state)

But it'd also allocate a new state.