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