I'm not really a Nim expert, but I did a quick test on the scenario you laid out with these results: iamsafe could not call imodifyglobals, but could call a pro

I'm not really a Nim expert, but I did a quick test on the scenario you laid out with these results: iamsafe could not call imodifyglobals, but could call a proc with no side effects. I'd guess that a func can call out to any proc that the compiler knows is side effect free, with the compiler erring on the side of caution.