error[E0599]: no method named `func` found for struct `Foo` in the current scope
 --> examples/misc_3.rs:7:9
  |
2 | struct Foo {
  | ---------- method `func` not found for this struct
...
7 |     foo.func();
  |         ^^^^ field, not a method
  |
help: to call the function pointer stored in `func`, surround the field access with parentheses
  |
7 |     (foo.func)();
  |     +        +

For more information about this error, try `rustc --explain E0599`.
error: could not compile `code` (example "misc_3") due to 1 previous error

There is a syntactic difference between a method call and a normal call. expr.identifier() is always a method call and Foo does not have a method called func. To call the function stored in a field you need to add parenthesis:

fn bar(foo: Foo) {
    (foo.func)();
}

Note that the same problem does not apply to tuples and tuple structs, because you can't name a method with an integer identifier. The following example compiles and prints "ferrisUwu" when run:

struct Foo(fn());

fn print_heheh() {
    println!("ferrisUwu")
}

fn main() {
    let foo = Foo(print_heheh);
    foo.0();
}