Add new section on trait bounds.

exercises/04_traits/13_drop/src/lib.rs

@@ -0,0 +1,23 @@
+// TODO: implement a so-called "Drop bomb": a type that panics when dropped
+//  unless a certain operation has been performed on it.
+//  You can see the expected API in the tests below.
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[should_panic]
+    fn test_drop_bomb() {
+        let bomb = DropBomb::new();
+        // The bomb should panic when dropped
+    }
+
+    #[test]
+    fn test_defused_drop_bomb() {
+        let mut bomb = DropBomb::new();
+        bomb.defuse();
+        // The bomb should not panic when dropped
+        // since it has been defused
+    }
+}