Hello Rust

Sadly, simply having a .c file in your project isn't enough to make it work. Maybe that'll be a future Cargo feature!

We need to tell Rust that the C function exists. In src/main.rs, add the following:

#![allow(unused)]
fn main() {
extern "C" {
    fn say_hello();
}
}

There's a few things here to remember:

• extern means that the function is defined elsewhere.
• "C" is the calling convention. This is the default for C, and is typically the only one you'll use.
• fn say_hello(); is the function signature. For now, it's up to you to match void say_hello() to fn say_hello() (easy enough - just wait...).

Next up, we need to call the function. In src/main.rs, add the following:

/// Safety: C is inherently unsafe!
fn main() {
    // Let's say "hello world" from C.
    unsafe {
        say_hello();
    }
}

There's a bit to mention here, too:

• We have a Safety comment! If you are using unsafe code, Clippy really wants you to have one of these.
• We're wrapping say_hello in unsafe.

For all the eager beavers who ran cargo run - it won't work yet! We haven't actually compiled the C code.