{"ast_node_type":"other","tokens":[{"end":33,"kind":"keyword","start":0,"text":")"}]}
    ) -> Option<&hir::Ty<'tcx>> {
        argument_index: usize,
        let fn_decl = self.infcx.tcx.hir_fn_decl_by_hir_id(self.mir_hir_id())?;