def _verify_forward_declarations_used(self, forward_declarations,
                                          decl_uses, file_uses):
        """Find all the forward declarations that are not used."""
        for cls in forward_declarations:
            if cls in file_uses:
                if not decl_uses[cls] & USES_DECLARATION:
                    node = forward_declarations[cls]
                    msg = ("'{}' forward declared, "
                           'but needs to be #included'.format(cls))
                    self._add_warning(msg, node)
            else:
                if decl_uses[cls] == UNUSED:
                    node = forward_declarations[cls]
                    msg = "'{}' not used".format(cls)
                    self._add_warning(msg, node)
Find all the forward declarations that are not used.