error[E0072]: recursive type `U` has infinite size
  --> $DIR/union-nonrepresentable.rs:3:1
   |
LL | union U {
   | ^^^^^^^ recursive type has infinite size
LL |     a: u8,
LL |     b: U,
   |     ---- recursive without indirection
   |
   = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `U` representable

error: aborting due to previous error

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