[adastra22]

I think the whole point is to prevent build scripts from doing arbitrary things. The sandbox should give access to the source code being built, record changes to these files (and/or new files generated in the same directories), and that's about it.

[insanitybit]

That's a cool idea but it would be a major breaking change. For proc-macros it's less of a major breaking change, though it is absolutely still one.