Attempted to call something which isn't a function nor a method.
Erroneous code examples:
```compile_fail,E0618
enum X {
Entry,
}
X::Entry(); // error: expected function, tuple struct or tuple variant,
// found `X::Entry`
// Or even simpler:
let x = 0i32;
x(); // error: expected function, tuple struct or tuple variant, found `i32`
```
Only functions and methods can be called using `()`. Example:
```
// We declare a function:
fn i_am_a_function() {}
// And we call it:
i_am_a_function();
```