Macro make_map_property

macro_rules! make_map_property {
    (
        // currently unused
        property: $prop_name:expr;
        // currently unused
        marker: $marker_name:ident;
        value: $value_ty:path;
        keyed_data_marker: $keyed_data_marker:ty;
        func:
        $(#[$doc:meta])*
        $vis2:vis const $constname:ident => $singleton:ident;
        $vis:vis fn $name:ident();
    ) => { ... };
}