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 some indirection (e.g., a `Box`, `Rc`, or `&`) to make `Baz` representable
   |
LL | struct Baz { q: Option<Box<Foo>> }
   |                        ++++   +

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 some indirection (e.g., a `Box`, `Rc`, or `&`) to make `Foo` representable
   |
LL | struct Foo { q: Option<Box<Baz>> }
   |                        ++++   +

error: aborting due to 2 previous errors

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