--- a/python/mozbuild/mozbuild/backend/recursivemake.py
+++ b/python/mozbuild/mozbuild/backend/recursivemake.py
@@ -199,18 +199,16 @@ class BackendMakeFile(object):
     def __init__(self, srcdir, objdir, environment, topsrcdir, topobjdir):
         self.topsrcdir = topsrcdir
         self.srcdir = srcdir
         self.objdir = objdir
         self.relobjdir = mozpath.relpath(objdir, topobjdir)
         self.environment = environment
         self.name = mozpath.join(objdir, 'backend.mk')
 
-        # XPIDLFiles attached to this file.
-        self.idls = []
         self.xpt_name = None
 
         self.fh = FileAvoidWrite(self.name, capture_diff=True)
         self.fh.write('# THIS FILE WAS AUTOMATICALLY GENERATED. DO NOT EDIT.\n')
         self.fh.write('\n')
 
     def write(self, buf):
         self.fh.write(buf)
@@ -428,17 +426,16 @@ class RecursiveMakeBackend(CommonBackend
 
         backend_file = self._get_backend_file_for(obj)
 
         consumed = CommonBackend.consume_object(self, obj)
 
         # CommonBackend handles XPIDLFile and TestManifest, but we want to do
         # some extra things for them.
         if isinstance(obj, XPIDLFile):
-            backend_file.idls.append(obj)
             backend_file.xpt_name = '%s.xpt' % obj.module
             self._idl_dirs.add(obj.relobjdir)
 
         elif isinstance(obj, TestManifest):
             self._process_test_manifest(obj, backend_file)
 
         # If CommonBackend acknowledged the object, we're done with it.
         if consumed: