error[E0072]: recursive type `ListNode` has infinite size
  --> $DIR/E0072.rs:11:1
   |
11 | struct ListNode { //~ ERROR has infinite size
   | ^^^^^^^^^^^^^^^ recursive type has infinite size
12 |     head: u8,
13 |     tail: Option<ListNode>,
   |     ---------------------- recursive without indirection
   |
   = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `ListNode` representable

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0072"