Why do I have to use PhantomData? What is it doing in this case?

September 2, 2023

The following code compiles only if I include the line marked with "???". If I remove the line I get lots of error message about parameter "S" never being used. I stumbled upon this "problem" in a larger scenario and had a hard time pinning it down. As far as I can tell, it happens due to the circular references between the types.

struct Question<S> {
    x: Vec<AnswerRef<S>>,
    _y: PhantomData<S>  // ???
}

struct QuestionRef<S>(Rc<Question<S>>);

struct Answer<S> {
    x: Option<QuestionRef<S>>
}

struct AnswerRef<S>(Rc<Answer<S>>);

Adding the PhantomData solves the problem, but I don’t understand why. I don’t see why it should make a difference in this case. Can somebody explain it to me?

>Solution :

The Rust compiler is quite clever here, it detects the cyclic usage of S and determines that you can remove it completely to get the same data structures. You can use PhantomData to "use" type parameters that are unused otherwise as they are here.