Macro hello_egui_utils::async_fn_def

macro_rules! async_fn_def {
    (
        $(#[$docs:meta])*
        // Block that contains the async logic
        $body:block,
        // The name of the async fn
        $name:ident,
        // The path to the callback fn (e.g. Self::callback)
        $callback_fn_path:path,
        // The parameters of the async fn. The semicolon in front of the mutt is a hack to circumvent ambiguity
        ($($(;$mutt:ident)? $arg:ident: $typ:ty,)*)
        // The parameters of the call to the callback fn
        ($($call_args:ident,)*)
        // The generics of the async fn
        ($($gen:tt)*),
        // Return type
        ($($return_type:tt)*),
        // Self reference
        ($($callback_body_self:tt)*),
    ) => { ... };
}

This macro generates the async fn