Add state.trace call when matching "must_rewrite" modules

_pytest/assertion/rewrite.py

@@ -161,6 +161,7 @@ def _should_rewrite(self, name, fn_pypath, state):
 
         for marked in self._must_rewrite:
             if marked.startswith(name):
+                state.trace("matched marked file %r (from %r)" % (name, marked))
                 return True
 
         return False