As an easy start, how about letting build scripts read /usr, read and write a temporary build directory, have some /tmp scratch space, and be allowed to write its final output artifact. No network and otherwise isolated from the rest of the system.

I would argue that, if a build script doesn’t work in the setting, then it doesn’t deserve to be installable by a default cargo command.

So let each build script define its own level of sandboxing and then users can determine whether they are okay with that level or not, e.g. `cargo build --sandbox-level=...`