How to Write Declarative Macros with macro_rules! in Rust

Use the macro_rules! construct to define a declarative macro that matches input patterns and expands into code at compile time.

#[macro_export]
macro_rules! vec {
    ( $( $x:expr ),* ) => {
        {
            let mut temp_vec = Vec::new();
            $(
                temp_vec.push($x);
            )*
            temp_vec
        }
    };
}

The #[macro_export] attribute makes the macro available when the crate is imported. The pattern ( $( $x:expr ),* ) matches a comma-separated list of expressions, and the body generates code to push each expression into a new Vec.

A declarative macro is a rule that tells the compiler how to replace a specific pattern of code with a different block of code before the program runs. Think of it like a template or a find-and-replace tool that works on the structure of your code rather than just data values. You use them when you need to generate repetitive code or handle a variable number of arguments that functions cannot support.