Clarify completion condition for elevator exercise (#1574)

Addresses the first part of #1565.

src/user-defined-types/exercise.md

@@ -8,7 +8,17 @@ We will create a data structure to represent an event in an elevator control
 system. It is up to you to define the types and functions to construct various
 events. Use `#[derive(Debug)]` to allow the types to be formatted with `{:?}`.
 
-```rust,compile_fail
+This exercise only requires creating and populating data structures so that
+`main` runs without errors. The next part of the course will cover getting data
+out of these structures.
+
+```rust,should_panic
+{{#include exercise.rs:event}}
+    // TODO: add required variants
+}
+
+{{#include exercise.rs:direction}}
+
 {{#include exercise.rs:car_arrived}}
     todo!()
 }
@@ -31,6 +41,3 @@ events. Use `#[derive(Debug)]` to allow the types to be formatted with `{:?}`.
 
 {{#include exercise.rs:main}}
 ```
-
-This exercise only requires creating data structures. The next part of the
-course will cover getting data out of these structures.

src/user-defined-types/exercise.rs

@@ -15,9 +15,11 @@
 #![allow(dead_code)]
 
 // ANCHOR: solution
+// ANCHOR: event
 #[derive(Debug)]
 /// An event in the elevator system that the controller must react to.
 enum Event {
+    // ANCHOR_END: event
     /// A button was pressed.
     ButtonPressed(Button),
 
@@ -34,12 +36,14 @@ enum Event {
 /// A floor is represented as an integer.
 type Floor = i32;
 
+// ANCHOR: direction
 /// A direction of travel.
 #[derive(Debug)]
 enum Direction {
     Up,
     Down,
 }
+// ANCHOR_END: direction
 
 /// A user-accessible button.
 #[derive(Debug)]