Macro function 

macro_rules! function {
    {
        $vis:vis async fn $fn_name:ident(
            $nvim:ident: $nvim_ty:ty,
            $($arg:tt: $type:ty),*)
            -> $result_ty:ident<() $(, $err_ty:tt)?>;
    } => { ... };
    {
        $vis:vis async fn $fn_name:ident(
            $nvim:ident: $nvim_ty:ty,
            $($arg:tt: $type:tt),*)
            -> $result_ty:ident<$ok_ty:tt $(, $err_ty:tt)?>;
    } => { ... };
    {
        $(
            $vis:vis async fn $fn_name:ident($($stuff:tt)*) -> $result_ty:ident<$ok_ty:tt $(, $err_ty:tt)?>;
        )*
    } => { ... };
}