Fix up the tests

tests/assert_macro.rs

@@ -10,21 +10,33 @@ fn math() {
     assert_eq!(x, 4);
 }
 
+// Because the `exit_protected` guard is never run, the compiler
+// warns us of unreachable code
+#[allow(unreachable_code)]
 fn early_return() -> usize {
     assert_no_alloc!(return 8)
 }
 
-fn into_box() -> Box<usize> {
-    Box::new(early_return())
-}
-
 #[test]
 #[should_panic]
 fn early_return_boxing() {
+    // The release-mode compiler is able to optimize through the Box
     if cfg!(debug_assertions) {
-        // The release-mode compiler is able to optimize through the Box
-        into_box();
+        Box::new(early_return());
     } else {
         panic!("Intentional")
     }
 }
+
+#[test]
+#[should_panic]
+fn list_push() {
+    let mut x = Vec::with_capacity(1);
+    x.push(1);
+
+    if cfg!(debug_assertions) {
+        assert_no_alloc!(x.push(12))
+    } else {
+        panic!("Intentional")
+    }
+}