Macro extern_impl 

macro_rules! extern_impl {
    {
        $(#[espy(
            $(debug = $description:literal)?
        )])?
        $vis:vis fn $this:ident <$host:lifetime> (&self, $argument:ident)
            $body:tt
    } => { ... };
    {
        $(#[espy(
            // Allows the user to set adebug string to be shown used by the Extern trait.
            //
            // This is a format string, and the `self` parameter may be bound
            // by using square brackets after "debug".
            // Using `self` directly will not work because it is not considered in scope of the macro.
            $(debug$([$debug_name:ident])? = $description:literal)?
        )])?
        $vis:vis struct $this:ident {
            $($name:ident: $value:expr),* $(,)?
        }
    } => { ... };
}