error[E0072]: recursive type `ListNode` has infinite size
  --> $DIR/E0072.rs:1:1
   |
LL | struct ListNode {
   | ^^^^^^^^^^^^^^^
LL |     head: u8,
LL |     tail: Option<ListNode>,
   |                  -------- recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
LL |     tail: Option<Box<ListNode>>,
   |                  ++++        +

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0072`.