use std::ptr;
use std::pin::Pin;
use std::mem::ManuallyDrop;

struct IntrusiveList {
    prev: *mut IntrusiveList,
    next: *mut IntrusiveList,
}

impl IntrusiveList {
    fn new() -> IntrusiveList {
        IntrusiveList {
            prev: ptr::null_mut(),
            next: ptr::null_mut(),
        }
    }

    fn append(mut self: Pin<&mut IntrusiveList>, mut other: Pin<&mut IntrusiveList>) {
        self.next = &mut *other;
        other.prev = &mut *self;
    }

    fn pin(&mut self) -> Pin<&mut IntrusiveList> {
        unsafe {
            Pin::new_unchecked(self)
        }
    }
}

impl Drop for IntrusiveList {
    fn drop(&mut self) {
        unsafe {
            if let Some(prev) = self.prev.as_mut() {
                prev.next = self.next;
            }
            if let Some(next) = self.next.as_mut() {
                next.prev = self.prev;
            }
        }
    }
}

fn foo(pinned_a: Pin<&mut IntrusiveList>) {
    let mut b = IntrusiveList::new();
    // Uncomment this line and run in Miri
    // let mut b = ManuallyDrop::new(b);
    let pinned_b = b.pin();
    pinned_a.append(pinned_b)
}

fn main() {
    let mut a = IntrusiveList::new();
    let pinned_a = a.pin();
    foo(pinned_a);
}