It does! But you still have to actually "use" it, meaning that it appears as a field in the definition, and you have pass a PhantomData value whenever you create new instances of your data type.

In Haskell, you can omit these fields entirely, and achieve the same thing just by annotating the function.

For example, in Haskell, we can have

  data Const a b = Const a

whereas in Rust, it would be:

  struct Const<A, B> {
      konst: A, 
      // does not exist at run-time
      discard: PhantomData<B>
  }