Auto merge of #31038 - gereeter:btree-panic, r=Gankro

…ving elements into it.

Fixes #31029.

Author: bors

src/libcollections/btree/node.rs

@@ -1027,6 +1027,8 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
             }
             self.node.as_leaf_mut().len -= 1;
 
+            left_node.as_leaf_mut().len += right_len as u16 + 1;
+
             if self.node.height > 1 {
                 ptr::copy_nonoverlapping(
                     right_node.cast_unchecked().as_internal().edges.as_ptr(),
@@ -1058,8 +1060,6 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
                 );
             }
 
-            left_node.as_leaf_mut().len += right_len as u16 + 1;
-
             Handle::new_edge(self.node, self.idx)
         }
     }