error[E0072]: recursive type `Foo` has infinite size
  --> $DIR/issue-17431-4.rs:3:1
   |
LL | struct Foo<T> { foo: Option<Option<Foo<T>>>, marker: marker::PhantomData<T> }
   | ^^^^^^^^^^^^^                      ------ recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
LL | struct Foo<T> { foo: Option<Option<Box<Foo<T>>>>, marker: marker::PhantomData<T> }
   |                                    ++++      +

error: aborting due to 1 previous error

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