struct MyFun(Box<Fn(&usize)>);

impl MyFun {
    fn new() -> Self {
        Self(Box::new(&a_fun))
    }
}

trait MyTrait<'a> {
    fn foo(arg: &'a str) -> usize;
}

fn a_fun<'a, A>(arg: &'a usize) -> A where A: MyTrait<'a> {
    unimplemented!()
}