The different types of mutability are also quite important for the concept of "borrowing" in Rust, which is simply taking a reference. The compiler ensures that

The different types of mutability are also quite important for the concept of "borrowing" in Rust, which is simply taking a reference. The compiler ensures that ownership of the variable isn't transferred while any such reference is active, and only one mutable reference (of a mutable type) can exist at any one time. This means that bugs like that shown for Swift are impossible in Rust, because the language ensures no two things can ever modify it at the same time.