The costs of threads vs processes are identical if you're prepared to put some stuff in virtual memory (except on Windows, and I don't know if that has improved

The costs of threads vs processes are identical if you're prepared to put some stuff in virtual memory (except on Windows, and I don't know if that has improved). Processes default to full isolation, threads are the reverse, but Rust has enough compile-time guarantees that it can do its own isolation for most purposes. I imagine they could go back and forth on this one without breaking source compatibility.