Rust provides a powerful macro system that allows metaprogramming. As you've seen in previous chapters, macros look like functions, except that their name ends with a bang !, but instead of generating a function call, macros are expanded into source code that gets compiled with the rest of the program.

Macros are created using the macro_rules! macro.

// This is a simple macro named `say_hello`.
macro_rules! say_hello {
    // `()` indicates that the macro takes no argument.
    () => (
        // The macro will expand into the contents of this block.

fn main() {
    // This call will expand into `println!("Hello");`