Fix & test leak of some BTreeMap nodes on panic during into_iter

src/liballoc/collections/btree/map.rs

@@ -1477,6 +1477,14 @@ fn drop(&mut self) {
                 // Continue the same loop we perform below. This only runs when unwinding, so we
                 // don't have to care about panics this time (they'll abort).
                 while let Some(_) = self.0.next() {}
+
+                // No need to avoid the shared root, because the tree was definitely not empty.
+                unsafe {
+                    let mut node = ptr::read(&self.0.front).into_node().forget_type();
+                    while let Some(parent) = node.deallocate_and_ascend() {
+                        node = parent.into_node().forget_type();
+                    }
+                }
             }
         }
 
@@ -1491,7 +1499,8 @@ fn drop(&mut self) {
             if node.is_shared_root() {
                 return;
             }
-
+            // Most of the nodes have been deallocated while traversing
+            // but one pile from a leaf up to the root is left standing.
             while let Some(parent) = node.deallocate_and_ascend() {
                 node = parent.into_node().forget_type();
             }

src/liballoc/tests/btree/map.rs

@@ -1021,7 +1021,7 @@ fn test_split_off_large_random_sorted() {
 }
 
 #[test]
-fn test_into_iter_drop_leak() {
+fn test_into_iter_drop_leak_1() {
     static DROPS: AtomicU32 = AtomicU32::new(0);
 
     struct D;
@@ -1045,3 +1045,27 @@ fn drop(&mut self) {
 
     assert_eq!(DROPS.load(Ordering::SeqCst), 5);
 }
+
+#[test]
+fn test_into_iter_drop_leak_2() {
+    let size = 12; // to obtain tree with 2 levels (having edges to leaf nodes)
+    static DROPS: AtomicU32 = AtomicU32::new(0);
+    static PANIC_POINT: AtomicU32 = AtomicU32::new(0);
+
+    struct D;
+    impl Drop for D {
+        fn drop(&mut self) {
+            if DROPS.fetch_add(1, Ordering::SeqCst) == PANIC_POINT.load(Ordering::SeqCst) {
+                panic!("panic in `drop`");
+            }
+        }
+    }
+
+    for panic_point in vec![0, 1, size - 2, size - 1] {
+        DROPS.store(0, Ordering::SeqCst);
+        PANIC_POINT.store(panic_point, Ordering::SeqCst);
+        let map: BTreeMap<_, _> = (0..size).map(|i| (i, D)).collect();
+        catch_unwind(move || drop(map.into_iter())).ok();
+        assert_eq!(DROPS.load(Ordering::SeqCst), size);
+    }
+}