Fix VecDeque::append capacity overflow for ZSTs

library/alloc/src/collections/vec_deque/mod.rs

@@ -1917,7 +1917,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     #[stable(feature = "append", since = "1.4.0")]
     pub fn append(&mut self, other: &mut Self) {
         if T::IS_ZST {
-            self.len += other.len;
+            self.len = self.len.checked_add(other.len).expect("capacity overflow");
             other.len = 0;
             other.head = 0;
             return;

library/alloc/tests/vec_deque.rs

@@ -1045,6 +1045,20 @@ fn test_append_double_drop() {
     assert_eq!(count_b, 1);
 }
 
+#[test]
+#[should_panic]
+fn test_append_zst_capacity_overflow() {
+    let mut v = Vec::with_capacity(usize::MAX);
+    // note: using resize instead of set_len here would
+    //       be *extremely* slow in unoptimized builds.
+    // SAFETY: `v` has capacity `usize::MAX`, and no initialization
+    //         is needed for empty tuples.
+    unsafe { v.set_len(usize::MAX) };
+    let mut v = VecDeque::from(v);
+    let mut w = vec![()].into();
+    v.append(&mut w);
+}
+
 #[test]
 fn test_retain() {
     let mut buf = VecDeque::new();