cyclic type reference in rust

cyclic_type_reference.rs

use std::fmt;

trait Listener where Self: Sized + fmt::Debug {
    fn on_event(&self, obj: &TestObject<Self>, args: &str);
}

#[derive(Debug)]
struct TestObject<F> where F:Listener {
    listener: F
}

#[derive(Debug)]
struct TestListener;
impl Listener for TestListener {
    fn on_event(&self, obj: &TestObject<Self>, args: &str) {
        println!("on_event {:?} {}", obj, args);
    }
}

fn main() {
    let listener = TestListener{};
    let obj = TestObject{listener: listener};
    obj.listener.on_event(&obj, "hello world");
}

error[E0277]: the size for values of type `Self` cannot be known at compilation time
 --> test.rs:2:5
  |
2 |     fn on_event(obj: &TestObject<Self>, args: &str);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `Self`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = help: consider adding a `where Self: std::marker::Sized` bound
note: required by `TestObject`
 --> test.rs:5:1
  |
5 | struct TestObject<F> where F: Listener {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The most recent version is fixed by using trait object.

Tried with trait object again, it also works.