{"ast_node_type":"other","tokens":[{"end":69,"kind":"keyword","start":0,"text":"//"}]}
            // but we do know it must implement `Future<Output = X>`.
            // If the expected trait is `AsyncFn(...) -> X`, we don't know what the return type is,
            self.extract_async_fn_sig_from_projection(projection_ty, projected_ty)