error[E0072]: recursive type `Baz` has infinite size
  --> $DIR/issue-17431-2.rs:1:1
   |
LL | struct Baz { q: Option<Foo> }
   | ^^^^^^^^^^   -------------- recursive without indirection
   | |
   | recursive type has infinite size
   |
   = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `Baz` representable

error[E0072]: recursive type `Foo` has infinite size
  --> $DIR/issue-17431-2.rs:4:1
   |
LL | struct Foo { q: Option<Baz> }
   | ^^^^^^^^^^   -------------- recursive without indirection
   | |
   | recursive type has infinite size
   |
   = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `Foo` representable

error: aborting due to 2 previous errors

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