Apology appreciated, but unnecessary.
I don’t want to derail a useful tool. It’s worth going a bit beyond “hope” as a strategy, however, and thinking about if (how) this might be exploited.
I doubt anyone will be mining crypto in your sandbox. But perhaps you should think about detection; might it be possible to mask a malicious crate with a second that attempts to detect sandboxed compilation, for instance?
In any case, I think this still looks exceedingly interesting in the typical case, which is of detecting the impact of bugs from non-malicious actors.
Given the widespread existence of wasm sandboxing, rustc itself might want to think about alternative strategies for running compiler plugins. I suspect there’d be a performance hit with such an approach, but wasm tooling is getting really good; perhaps it is minor.