Fix unused macro rule in no-default-features mode
Fix unused macro rule in no-default-features mode

    warning: 1st rule of macro `ast_struct` is never used
     --> src/macros.rs:2:5
      |
    2 | /     (
    3 | |         [$($attrs_pub:tt)*]
    4 | |         struct $name:ident #full $($rest:tt)*
    5 | |     ) => {
      | |_____^
      |
      = note: `#[warn(unused_macro_rules)]` on by default

dtolnay/syn