pub type ConstValue = ConstValue<Interner>;
enum ConstValue { BoundVar(BoundVar), InferenceVar(InferenceVar), Placeholder(PlaceholderIndex), Concrete(ConcreteConst<Interner>), }
Bound var (e.g. a parameter).
Constant whose value is being inferred.
Lifetime on some yet-unknown placeholder.
Concrete constant value.