How to Use the syn and quote Crates for Proc Macros

Use syn to parse Rust code into an Abstract Syntax Tree (AST) and quote to generate new Rust code from that tree.

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

#[proc_macro_derive(HelloMacro)]
pub fn hello_macro_derive(input: TokenStream) -> TokenStream {
    let ast = syn::parse(input).unwrap();
    let name = &ast.ident;
    let generated = quote! {
        impl HelloMacro for #name {
            fn hello_macro() {
                println!("Hello, Macro! My name is {}!", stringify!(#name));
            }
        }
    };
    generated.into()
}

The syn crate reads Rust code and turns it into a structured map that a computer can understand. The quote crate takes that map and writes new Rust code based on it. Together, they let you write programs that automatically generate other programs, like a template engine for code.