Macros Basics

10 examples to get you started with Macros - 7 basic and 3 intermediate.

Prerequisites

• Rust 1.97.0 (edition 2024)
• Understanding that macros expand at compile time

Basic Examples

1. Declarative vs Procedural

// declarative: macro_rules!
macro_rules! say_hello {
    () => { println!("Hello"); };
}
 
// procedural: #[derive(Serialize)] implemented by proc macro crate

• Declarative macros match token patterns
• Procedural macros are Rust functions on token streams
• Both run at compile time
• Prefer functions/generics when possible

Related: macro_rules!

2. macro_rules! Hello

macro_rules! greet {
    ($name:expr) => {
        println!("Hello, {}", $name);
    };
}
 
fn main() {
    greet!("Rust");
}

• $name:expr captures an expression
• Expands to plain Rust at call site
• No type checking inside matcher until expansion
• Use cargo expand to debug output

3. Macro Hygiene

Macros do not accidentally capture local variables from caller (hygienic for macro_rules!).

macro_rules! double {
    ($x:expr) => { $x * 2 };
}
 
fn demo() {
    let x = 10;
    assert_eq!(double!(5), 10); // uses 5, not local x
}

• Hygiene reduces surprise name collisions
• Proc macros can break hygiene if generating identifiers carelessly
• Still think about exported crate paths

Related: Debugging Macros

4. Built-in Macros

println!("debug = {:?}", dbg!(value));
todo!();
unreachable!();

• format!, vec!, panic! are macros
• include_str!, env! compile-time embed
• cfg! and concat! for conditional build

5. When Not to Use Macros

// Prefer generic function:
fn max<T: Ord>(a: T, b: T) -> T {
    if a > b { a } else { b }
}

• Functions have better errors and IDE support
• Macros for repetitive syntax DSLs
• Derive proc macros for traits

Related: Macros Best Practices

6. Fragment Specifiers

macro_rules! id {
    ($x:ident) => { stringify!($x) };
}

Common specifiers: expr, ident, ty, path, stmt, block, tt.

7. Repetition

macro_rules! vec_str {
    ($($x:expr),*) => {
        vec![$($x.to_string()),*]
    };
}

• $()* repeats pattern
• $(,)* optional comma separation
• Powers vec! and similar APIs

Related: macro_rules!

Intermediate Examples

8. Derive Macros

#[derive(Debug, Clone, serde::Serialize)]
struct User { id: u64 }

• Implemented as proc macros in external crates
• syn + quote parse and generate code

Related: Derive Macros

9. Attribute Macros

#[tokio::main]
async fn main() { }

• Attribute proc macros rewrite the item below
• Used by frameworks for boilerplate reduction

Related: Attribute Macros

10. Debugging Expansion

cargo install cargo-expand
cargo expand -p mycrate my_fn

• Always expand when macro errors confuse
• Pair with trace_macros! (unstable) sparingly

Related: Debugging Macros

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+. Y29kZWd1aWRlcy5pb3xjZ2lvMjA1fDIwMjYwNw==