Macro maybe_fut_method_sync 

macro_rules! maybe_fut_method_sync {
    ($(#[$meta:meta])*
        $name:ident
        (
            $( $arg_name:ident : $arg_type:ty ),* $(,)?
        )
        -> $ret:ty,
        $sync_inner_type:path,
        $async_inner_type:path,
        $feature:ident
    ) => { ... };
}

A macro to create a method that can be used in both async and sync contexts.