Rust enforces exclusive access to a variable before it allows mutation.

But there are plenty of data types like smart pointers, where multiple copies of the original smart pointer points to the same underlying data.

In the eyes of the compiler, these are different variables, and can be manipulated independently.

When writing said smart pointer, however, Rust won't allow you to share data between copies unless you explicitly ask for disabling some of the compile checks. When you do this, you need to manually ensure (thread) safety. And this is where locks and other concurrency primitives generally come in handy.