error[E0072]: recursive type `Foo` has infinite size
  --> $DIR/issue-17431-6.rs:3:1
   |
LL | enum Foo { X(Mutex<Option<Foo>>) }
   | ^^^^^^^^                  --- recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
LL | enum Foo { X(Mutex<Option<Box<Foo>>>) }
   |                           ++++   +

error: aborting due to previous error

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