Pop Quiz: Why is say_hello wrapped in unsafe?

1. C is inherently unsafe.
2. Unsafe improves performance.
3. Unsafe is required for FFI.

The answer is 3, although I really want it to be 1.

unsafe doesn't actually mean that a function is unsafe! It means that the function falls outside what Rust can guarantee is safe. FFI functions are inherently unsafe, because you are calling code outside of Rust's control.