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include/grpc/impl/codegen/grpc_types.h

@@ -558,6 +558,9 @@ typedef struct grpc_op {
       grpc_metadata_array* trailing_metadata;
       grpc_status_code* status;
       grpc_slice* status_details;
+      /** If this is not nullptr, it will be populated with the full fidelity
+       * error string for debugging purposes. The application is responsible
+       * for freeing the data. */
       const char** error_string;
     } recv_status_on_client;
     struct grpc_op_recv_close_on_server {