Add test showing broken behavior of BinaryHeap::retain

Author: dtolnay

library/alloc/src/collections/binary_heap/tests.rs

@@ -474,6 +474,23 @@ fn test_retain() {
     assert!(a.is_empty());
 }
 
+#[test]
+fn test_retain_catch_unwind() {
+    let mut heap = BinaryHeap::from(vec![3, 1, 2]);
+
+    // Removes the 3, then unwinds out of retain.
+    let _ = catch_unwind(AssertUnwindSafe(|| {
+        heap.retain(|e| {
+            if *e == 1 {
+                panic!();
+            }
+            false
+        });
+    }));
+
+    assert_eq!(heap.into_vec(), [1, 2]); // BAD!!
+}
+
 // old binaryheap failed this test
 //
 // Integrity means that all elements are present after a comparison panics,