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

error: aborting due to previous error

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