Not an Emacs user but for the life of me I don't understand what justification there is for auto-completion to allow any capabilities other than (a) memory allocation (to generate the code), and perhaps (b) reading the filesystem. Neither of which requires arbitrary code. Why in the world is anything else allowed? And what makes this so damn hard to implement after all these years? It would seem trivial to interpret a macro manually, or to just introduce an evaluation mode that can't do anything else, no?