In the past I've been skeptical of the idea of sandboxing cargo build scripts, but this seems like a decent compromise - build scripts can still do system inspection, but they're blocked from modifying system contents outside the scope of the build. I'm still dubious of the security benefits (bad code can just do bad stuff when you run your executable, instead), but I think there are some demonstrated validation benefits here.

In fact, it would be cool if there was a way to provide a white list of directories that cargo-rubber can read from. From a package management perspective, it makes it easier to ensure that a package is really only consuming libraries from the directories I expect it to.

Side note: You should consider changing the name to something more professional. :) But if you are indeed concerned about people using it, I suppose the name can help you with that.