What is the difference between?
struct Test();
struct Test(());
I understand that a struct can have tuples (unnamed fields). However, I am not sure what does (()) mean in such a case? That it's a struct with one element which is an empty tuple?
it's a struct with one element which is an empty tuple?
That's exactly right.
Technically these are different types, but they carry exactly the same amount of information as each other; none.
There is really no purpose in having that argument, and you generally wouldn't create a type like struct Test(()) except in a generic context. For example,
struct Test<T>(T);
where T ends up being () due to some other requirements.