How to write attribute macros

You write attribute macros by defining a procedural macro in a separate crate with the proc-macro crate type and using the #[proc_macro_attribute] function attribute to process the attribute and generate code.

// In a crate with `proc-macro = true` in Cargo.toml
use proc_macro::TokenStream;
use quote::quote;

#[proc_macro_attribute]
pub fn my_attr(_attr: TokenStream, item: TokenStream) -> TokenStream {
    let mut ast = syn::parse_macro_input!(item as syn::ItemFn);
    ast.sig.ident = syn::Ident::new(&format!("{}_wrapper", ast.sig.ident), ast.sig.ident.span());
    TokenStream::from(quote! { #ast })
}

Apply it to your code like this:

use my_macro_crate::my_attr;

#[my_attr]
fn hello() {
    println!("Hello!");
}

The macro renames hello to hello_wrapper at compile time.

An attribute macro is a special tool that lets you write code that automatically modifies other code when you compile it. You use it to add custom behavior or validation to functions, structs, or modules by placing a special tag above them. Think of it like a smart label on a box that tells the factory to automatically pack the contents in a specific way before shipping.