error: invalid `?` in type
  --> $DIR/infinite-assoc.rs:12:39
   |
LL | struct A((A, <S as Trait>::T<NOT_EXIST?>));
   |                                       ^ `?` is only allowed on expressions, not types
   |
help: if you meant to express that the type might not contain a value, use the `Option` wrapper type
   |
LL | struct A((A, <S as Trait>::T<Option<NOT_EXIST>>));
   |                              +++++++         ~

error[E0072]: recursive type `A` has infinite size
  --> $DIR/infinite-assoc.rs:12:1
   |
LL | struct A((A, <S as Trait>::T<NOT_EXIST?>));
   | ^^^^^^^^  - recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
LL | struct A((Box<A>, <S as Trait>::T<NOT_EXIST?>));
   |           ++++ +

error: aborting due to 2 previous errors

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