Rust actually supports both 1:1 and M:N threading models, depending on which one you want. It's true that it used to be M:N by default and is now 1:1 by default, though.

> Rust disallows shared-memory concurrency (with a few exceptions),

To elaborate: Rust eliminates mutable state being passed across a concurrency boundary at compile time. Sometimes, you really need shared mutable state, though, so you're able to use 'unsafe' to implement the safety the compiler can't infer for you. The obvious ones (only allowing access via a mutex and reference counting (both atomic and non)) are already in the standard library.