add another example

Missing.md

@@ -5,3 +5,7 @@ Missing APIs from the types we cover (APIs have been added after this formalizat
 * Structural conversion for slices.  The matching operations in our model would be
   `&mut Cell<(A, B)>` -> `&mut (Cell<A>, Cell<B>)` and
   `&Cell<(A, B)>` -> `&(Cell<A>, Cell<B>)` (both being NOPs).
+
+# ZST
+
+* Something like the example from <https://github.com/rust-lang/unsafe-code-guidelines/issues/168#issuecomment-512528361>