The cycle warning occurs because your Expr enum definition creates a recursive type. In Rust, using Box<Expr> inside the Expr enum type causes a recursive relationship because Box<Expr> directly references Expr. This is why Rust gives a cycle warning.

Even though you mention phantom types, it seems that you're still providing a concrete type Expr for the Box instead of a phantom generic type that wouldn't require a real instance. Consider if you wanted to use a phantom type, you would use a generic parameter in the enum that doesn't need to exist at runtime, something like Box<PhantomData<T>> if T is a generic parameter not Expr itself.