error[E0072]: recursive type `Pong` has infinite size
  --> $DIR/issue-2718-a.rs:8:5
   |
LL |     pub struct Pong(SendPacket<Ping>);
   |     ^^^^^^^^^^^^^^^ ---------------- recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
LL |     pub struct Pong(Box<SendPacket<Ping>>);
   |                     ++++                +

error: aborting due to previous error

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