Here's a suggestion, do you think it could be this simple? If you write a package-manager, don't execute any of the downloaded code in an automated fashion. No hooks, no build-time metaprograms. If your language _needs_ metaprogramming to function, it's a huge secondary issue that I don't know how to solve. You can try to make a meta-program annotation which disables side-effects, but the metaprogram ultimately must write memory which expands the amount of code generated. All risk is introduced when the person downloading cannot preview the content in a safe place.