You generally don't have to enforce any invariants manually unless you're implementing your own abstractions from unsafe primitives like raw pointers.

Multi-threading in general is made safe by separate mechanisms which don't permit shared mutability, but there's little reason to restrict it on a single thread when it can be verified to be safe. You still cannot mutate a collection while iterating over it for example, but you can easily mutate disjoint parts of it which is something I commonly encounter in Rust.