More magics on Option<T>?

language design
16

Another case that &mut Option<dyn Trait> might be useful is to handle the Box<FnOnce> issue.

fn main() {
    let c = ||{};
    let b :Box<dyn FnOnce()> = Box::new(c);
    b()
}

Today the above results in E0161: cannot move out of box. But we can image to have compiler support for desugaring it to:

fn main() {
    let c = ||{};
    let b:Box<dyn FnOnce()> = box c;
    //this only copies the vtable, and move the destructor from b to the place holder
    let _place_holder:&mut Option<dyn FnOnce()> = unbox_to_some(b);
    //fn call_once_derived(self: &mut Option<dyn FnOnce()>) 
    //{ (self.take().unsafe_unwrap())() }
    _place_holder.call_once_derived()
}