{"ast_node_type":"variable","tokens":[{"end":94,"kind":"keyword","start":0,"text":"let"}]}
        let implicit_inputs = self.regioncx.universal_regions().defining_ty.implicit_inputs();
    ) -> Option<RegionName> {
        let argument_index = self.regioncx.get_argument_index_for_region(self.infcx.tcx, fr)?;