What Is the Difference Between Declarative and Procedural Macros?

Declarative macros use macro_rules! to match patterns and expand code, while procedural macros are Rust functions that programmatically generate code for custom derives, attributes, or function-like calls. Declarative macros are defined with macro_rules! and work like pattern matching, whereas procedural macros require a separate crate and the proc_macro crate to manipulate tokens directly.

// Declarative macro using macro_rules!
macro_rules! say_hello {
    () => {
        println!("Hello!");
    };
}

fn main() {
    say_hello!();
}

// Procedural macro requires a separate crate with #[proc_macro_derive]
// and the proc_macro crate to define custom derive logic.

Declarative macros are like templates that fill in blanks based on rules you write, while procedural macros are small programs that write code for you. You use declarative macros for simple text replacements and procedural macros when you need complex logic to generate new types or attributes. Think of declarative macros as a mail-merge and procedural macros as a custom software tool.