pub trait Trait1 {
  type C;
}

struct T1;
impl Trait1 for T1 {
  type C = usize;
}
pub trait Callback<T: Trait1>: FnMut(<T as Trait1>::C) {}
impl<T: Trait1, F: FnMut(<T as Trait1>::C)> Callback<T> for F { }


pub struct State<T: Trait1> {
  callback: Option<Box<dyn Callback<T>>>,
}
impl<T: Trait1> State<T> {
  fn new() -> Self { Self { callback: None } }
  fn test_cb(&mut self, d: <T as Trait1>::C) {
    (self.callback.as_mut().unwrap())(d)
  }
}

fn main() {
  let mut s = State::<T1>::new();
  s.test_cb(1);
}