[skybrian]

It's not magic; you have to work at it. (For example, make sure that zip doesn't put timestamps in the file.) But it's designed so that code generators should act as pure functions from input files to output files, and many generators actually are, especially the built in ones. If you do this then the build system will help you.

Writing generators to run this way is kind of a pain, actually, sort of like writing code to run in a sandbox. Also, the generators themselves must be checked in, and often built from source. But we consider the results worth it.