Add more tests for UB

src/tests.rs

@@ -1023,3 +1023,29 @@ fn drain_keep_rest() {
 
     assert_eq!(a, SmallVec::<[i32; 3]>::from_slice(&[1i32, 3, 5, 6, 7, 8]));
 }
+
+/// This assortment of tests, in combination with miri, verifies we handle UB on fishy arguments
+/// given to SmallVec. Draining and extending the allocation are fairly well-tested earlier, but
+/// `smallvec.insert(usize::MAX, val)` once slipped by!
+///
+/// All code that indexes into SmallVecs should be tested with such "trivially wrong" args.
+#[test]
+fn max_dont_panic() {
+    let mut sv: SmallVec<[i32; 2]> = smallvec![0];
+    let _ = sv.get(usize::MAX);
+    sv.truncate(usize::MAX);
+}
+
+#[test]
+#[should_panic]
+fn max_remove() {
+    let mut sv: SmallVec<[i32; 2]> = smallvec![0];
+    sv.remove(usize::MAX);
+}
+
+#[test]
+#[should_panic]
+fn max_swap_remove() {
+    let mut sv: SmallVec<[i32; 2]> = smallvec![0];
+    sv.swap_remove(usize::MAX);
+}