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.
        queries {
            $(
                $(#[$attr:meta])*
                fn $name:ident($($K:tt)*) -> $V:ty
                {
                    // Search for (QMODLIST) to find all occurrences of this query modifier list.
                    anon: $anon:literal,
                    arena_cache: $arena_cache:literal,
                    cache_on_disk: $cache_on_disk:literal,
                    cycle_error_handling: $cycle_error_handling:ident,
                    depth_limit: $depth_limit:literal,
                    eval_always: $eval_always:literal,
                    feedable: $feedable:literal,
                    no_hash: $no_hash:literal,
                    returns_error_guaranteed: $returns_error_guaranteed:literal,
                    separate_provide_extern: $separate_provide_extern:literal,
                }
            )*
        }
        // Non-queries are unused here.
        non_queries { $($_:tt)* }
    ) => { ... };
}