Minor optimization for set object, swap bodies #142659
Description
Based on this observation:
set_swap_bodies is only called in two locations in this file. For both cases the second argument b is a newly created temporary which is discarded afterwards. So we could replace part (all?) of these branches with asserts. Part of the other work (e.g. copying back to b) might not be needed for the same reason.
We should investigate this idea and see if it's worth it.