Isn't that exactly what a thread is? Well, threads generally have some management by the OS to allow more threads than CPU/share tasks evenly, but at their core

Isn't that exactly what a thread is? Well, threads generally have some management by the OS to allow more threads than CPU/share tasks evenly, but at their core, they allow you to give each core some code to run on. (And yes, Rust's threading safety applies equally well to that. It doesn't care about the details of how code is running concurrently/in parallel, just that it could be.)