add testcase for double-drop during Vec in-place collection

(cherry picked from commit fa89c0f)

Author: the8472

library/alloc/tests/vec.rs

@@ -1027,7 +1027,7 @@ fn test_from_iter_specialization_head_tail_drop() {
 }
 
 #[test]
-fn test_from_iter_specialization_panic_drop() {
+fn test_from_iter_specialization_panic_during_iteration_drops() {
     let drop_count: Vec<_> = (0..=2).map(|_| Rc::new(())).collect();
     let src: Vec<_> = drop_count.iter().cloned().collect();
     let iter = src.into_iter();
@@ -1050,6 +1050,42 @@ fn test_from_iter_specialization_panic_drop() {
     );
 }
 
+#[test]
+fn test_from_iter_specialization_panic_during_drop_leaks() {
+    static mut DROP_COUNTER: usize = 0;
+
+    #[derive(Debug)]
+    enum Droppable {
+        DroppedTwice(Box<i32>),
+        PanicOnDrop,
+    }
+
+    impl Drop for Droppable {
+        fn drop(&mut self) {
+            match self {
+                Droppable::DroppedTwice(_) => {
+                    unsafe {
+                        DROP_COUNTER += 1;
+                    }
+                    println!("Dropping!")
+                }
+                Droppable::PanicOnDrop => {
+                    if !std::thread::panicking() {
+                        panic!();
+                    }
+                }
+            }
+        }
+    }
+
+    let _ = std::panic::catch_unwind(AssertUnwindSafe(|| {
+        let v = vec![Droppable::DroppedTwice(Box::new(123)), Droppable::PanicOnDrop];
+        let _ = v.into_iter().take(0).collect::<Vec<_>>();
+    }));
+
+    assert_eq!(unsafe { DROP_COUNTER }, 1);
+}
+
 #[test]
 fn test_cow_from() {
     let borrowed: &[_] = &["borrowed", "(slice)"];