You're not up to your elbows if somebody else has already done the work for you. If the input to your macro is a valid production in Rust, then all the parsing work that's incumbent on the macro author is to write

    let ast = parse_macro_input!(input as Foo);

Where foo is some type defined by the `syn` crate and there's one for every production in Rust's grammar. But neither are you limited to those. You can also extend or replace that grammar as you see fit, but in that case the added parsing is on you.