Macro extern_ty 

macro_rules! extern_ty {
    ($(
        $(#[$attr:meta])*
        $vis:vis type $i:ident;
    )*) => { ... };
}

Create an uninhabited type that can’t be constructed. It implements Debug, Clone, and Copy, but these aren’t meaningful for extern types so they should eventually be removed.

Really what we want here is something that also can’t be named without indirection (in ADTs or function signatures), but this doesn’t exist.