error[E0072]: recursive type `Foo` has infinite size
  --> $DIR/issue-17431-1.rs:1:1
   |
LL | struct Foo { foo: Option<Option<Foo>> }
   | ^^^^^^^^^^                      --- recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
LL | struct Foo { foo: Option<Option<Box<Foo>>> }
   |                                 ++++   +

error: aborting due to previous error

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