Macro hello_egui_utils::fn_def

macro_rules! fn_def {
    (
        $(#[$docs:meta])*
        // Block that contains the callback function body
        $body:block,
        // The name of the callback fn
        $name:ident,
        // The parameters of the callback fn
        $($arg:ident: $typ:ty,)*
        // The generics of the callback fn
        ($($gen:tt)*),
        // The return type
        ($($return_type:tt)*),
        // The self declaration
        ($($callback_body_self:tt)*),
    ) => { ... };
}

This macro generates the callback fn