Macro maybe_fut_function 

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

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