error[E0072]: recursive types `A` and `B` have infinite size
  --> $DIR/mutual-struct-recursion.rs:1:1
   |
LL | struct A<T> {
   | ^^^^^^^^^^^
...
LL |     y: B<T>,
   |        ---- recursive without indirection
...
LL | struct B<T> {
   | ^^^^^^^^^^^
LL |     z: A<T>
   |        ---- recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
LL ~     y: Box<B<T>>,
LL | }
LL | 
LL | struct B<T> {
LL ~     z: Box<A<T>>
   |

error[E0072]: recursive types `C` and `D` have infinite size
  --> $DIR/mutual-struct-recursion.rs:11:1
   |
LL | struct C<T> {
   | ^^^^^^^^^^^
...
LL |     y: Option<Option<D<T>>>,
   |                      ---- recursive without indirection
...
LL | struct D<T> {
   | ^^^^^^^^^^^
LL |     z: Option<Option<C<T>>>,
   |                      ---- recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
LL ~     y: Option<Option<Box<D<T>>>>,
LL | }
LL | 
LL | struct D<T> {
LL ~     z: Option<Option<Box<C<T>>>>,
   |

error: aborting due to 2 previous errors

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