Return `Msg` or `()` in handlers. | Runtime panics | Stable Rust

handler_msg_or_unit_panics.rs

use std::any::{Any, TypeId};

#[derive(Debug)]
enum Msg {
    Increment,
}

fn main() {
    let handlers: Vec<Handler<Msg>> = vec![
        Handler::new(|| Msg::Increment),
        Handler::new(|| println!("Look, no msg!")),
        //Handler::new(|| 123),
    ];
    
    for handler in handlers {
        let msg = (handler.callback)();
        println!("Msg: {:?}", msg);
    }
}


struct Handler<Ms> {
    callback: Box<dyn Fn() -> Option<Ms>>,
}

impl<Ms: 'static> Handler<Ms> {
    fn new<T: 'static>(callback: impl FnOnce() -> T + Clone + 'static) -> Self {
        let t_type = TypeId::of::<T>();
        if t_type != TypeId::of::<Ms>() && t_type != TypeId::of::<()>() {
            panic!("Handler can return only Msg or ()!");            
        }
        
        let callback = move || {
          let output = &mut Some(callback.clone()()) as &mut dyn Any;
          output
            .downcast_mut::<Option<Ms>>()
            .and_then(Option::take)
        };
        Self {
            callback: Box::new(callback),
        }
    }
}

Can be run in https://play.rust-lang.org/