In Rust, what is `fn() -> ()`?

Question

I have a grasp of the Fn (capital-F) traits: Fn, FnMut, FnOnce. I understand that they are traits and work like traits.

But wha

Answer 1

It is a function pointer type.

It refers only to a function, not a closure, since it contains just the address of the function not the captured environment a closure needs.

A Fn trait (capital F) can refer either to a closure or a function.