Macro maybe_fut_constructor 

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

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