This program compiles but leaks memory. Why?
use std::rc::Rc;
use std::cell::RefCell;
struct Node {
next: RefCell<Option<Rc<Node>>>,
}
fn main() {
let a = Rc::new(Node { next: RefCell::new(None) });
let b = Rc::new(Node { next: RefCell::new(None) });
*a.next.borrow_mut() = Some(Rc::clone(&b));
*b.next.borrow_mut() = Some(Rc::clone(&a));
}