Macro define_callbacks 

macro_rules! define_callbacks {
    (
        // You might expect the key to be `$K:ty`, but it needs to be `$($K:tt)*` so that
        // `query_helper_param_ty!` can match on specific type names.
        $(
            $(#[$attr:meta])*
            [$($modifiers:tt)*]
            fn $name:ident($($K:tt)*) -> $V:ty,
        )*
    ) => { ... };
}