If so I can see that working for very simple code, but what about wanting to use Tasks/Threads or do I/O. Is that converted to native system calls at compile time?
After a series of recent changes, Rust's runtime has been radically reduced. I/O goes through system calls, and the threading system is just native OS threads. There's a very small amount of support needed for the "Rust runtime", including things like stack guards, but it's much smaller than it was.
The RFC describing the changes is [^1] and the actual commit that finalized them is [^2].