@atpfm The Derived Data folder stuff makes me think filesystems should have weak references. Just treat the space as empty, but don't delete it unless you need some space.

@boxed The stuff in there is important/necessary for building your app, so you don't want it disappearing mid-build because the OS needs space. (Also, Apple's OSes have the concept of "purgeable space" which is more like what you're asking for.)