// check-pass 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); }