fix impl

mcp_codebase.py

@@ -1986,13 +1986,15 @@ def build_indexes():
 
             graph.add_node(node_id, **base_attrs, type=node_type)
 
-            # Try to find the target struct/trait that this impl implements
+            # Guard against empty lists that can be returned by the Rust parser
             target_name = None
-            if "implements_traits" in m:
-                target_name = m["implements_traits"][0]  # assume first trait
-            elif "implements" in m:
+            if "implements_traits" in m and isinstance(m["implements_traits"], list) and m["implements_traits"]:
+                # assume first trait
+                target_name = m["implements_traits"][0]
+            elif "implements" in m and isinstance(m["implements"], list) and m["implements"]:
                 target_name = m["implements"][0]
 
+
             if target_name:
                 target_node_id = f"{lang}::struct::{fpath}::{target_name}"
                 if target_node_id not in graph.graph: