Suppress warnings for elevator exercise (#2668)

src/user-defined-types/exercise.md

@@ -41,3 +41,13 @@ out of these structures.
 
 {{#include exercise.rs:main}}
 ```
+
+<details>
+
+- If students ask about `#![allow(dead_code)]` at the top of the exercise, it's
+  necessary because the only thing we do with the `Event` type is print it out.
+  Due to a nuance of how the compiler checks for dead code this causes it to
+  think that the code is unused. They can ignore it for the purpose of this
+  exercise.
+
+</details>

src/user-defined-types/exercise.rs

@@ -12,10 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#![allow(dead_code)]
-
 // ANCHOR: solution
 // ANCHOR: event
+#![allow(dead_code)]
+
 #[derive(Debug)]
 /// An event in the elevator system that the controller must react to.
 enum Event {