How to Write an Attribute Macro in Rust

An attribute macro in Rust is a procedural macro that transforms an item based on a custom attribute, defined using the proc_macro crate and the #[proc_macro_attribute] function.

use proc_macro::TokenStream;
use quote::quote;

#[proc_macro_attribute]
pub fn my_attr(_attr: TokenStream, item: TokenStream) -> TokenStream {
    let mut item_tokens = item.clone();
    // Transform logic here
    item_tokens
}

1. Create a new crate with proc-macro as the crate type in Cargo.toml.
2. Add proc-macro = true to the [lib] section of Cargo.toml.
3. Import TokenStream from proc_macro and quote from the quote crate.
4. Define a function decorated with #[proc_macro_attribute] that takes the attribute tokens and the item tokens.
5. Implement the transformation logic inside the function to modify the item tokens.
6. Return the modified TokenStream from the function.

An attribute macro is a tool that lets you write code that automatically modifies other code when you add a special tag to it. It works like a spell that changes a sentence when you put a specific sticker on it. You use this when you want to generate repetitive code or enforce rules on your functions and structs automatically.