PhantomData<fn(U)> confuse me

[Yandros]

Looking for a deeper understanding of PhantomData

What is PhantomData<T> The two defining properties of PhantomData<T> are the following: At runtime, PhantomData<T> is exactly like (), i.e., nothing. This gives the peace of mind of it being (a) zero-cost (abstraction) For compile-time analysis, PhantomData<T> is exactly like T The usefulness of such a thing may not be obvious at first glance, but it turns out that types have properties (or more precisely, they can lack properties), which in turn leads to the containing struct trans…