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

error: aborting due to previous error

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