Matches involving uninhabited types

I have a generic type Expr<A, U, B> that represents a formal expression over the set of atoms A, with unary operators U and binary operators B. In a certain place in my program I have expressions that only use binary operations, so I created a type enum Empty {} to represent the set of unary operators for those expressions. Unfortunately I cannot ignore the Expr::Unary(op, expr) arm of match expressions over such expressions, because the compiler demands I handle a case that cannot exist.
Is there a way to convince the compiler that such values are impossible? I'm trying to keep my usage of panicking macros such as unreachable!() to a minimum, so grepping for them actually produces meaningful results.

I believe this is one of the things being worked out as part of the ! stuff. As far as I know, you need to do something with an uninhabited value (like match x {}) to explicitly make the code path divergent.