Trait object passed as generic argument by using ?Sized as relaxation.

trait_object_in_generic_argument.rs

use std::fmt;

trait Handler: fmt::Debug  {
    fn handle(&mut self);
}

#[derive(Debug)]
struct Value(i32);
#[derive(Debug)]
struct Data {
    value: Value,
}
impl Default for Data  {
    fn default() -> Self {
        Data { value: Value(0) }
    }
}
impl Handler for Data {
    fn handle(&mut self) {
        self.value.0 += 1;
        println!("- value: {:?}", self);
    }
}

fn proc<H: Handler + ?Sized>(h: &mut H) {
    h.handle();
}

// Equivalent
// fn proc(h: &mut dyn Handler) {
//     h.handle();
// }

fn main() {
    let mut d: Box<dyn Handler> = Box::new(Data::default());
    d.handle();
    proc(d.as_mut());
    
    println!("{:?}", d);
}