fn main() {
    let mut d = D;
    visit_clobber(&mut d, |_| panic!());
}


pub fn visit_clobber<T, F>(t: &mut T, f: F) where F: FnOnce(T) -> T {
    unsafe { std::ptr::write(t, f(std::ptr::read(t))); }
}


struct D;
impl Drop for D {
    fn drop(&mut self) {
        println!("drop");
    }
}