include/grpcpp/impl/codegenThis directory exists so that generated code can include selected files upon which it depends without having to depend on the entire gRPC C++ library. This is particularly relevant for users of bazel, particularly if they use the multi-lingual proto_library target type. Generated code that uses this target only depends on the gRPC C++ targets associated with these header files, not the entire gRPC C++ codebase since that would make the build time of these types of targets excessively large (particularly when they are not even C++ specific).
User code should not include anything from this directory. Only generated code and gRPC library code should include contents from this directory. User code should instead include contents from the main grpcpp directory or its accessible subcomponents like grpcpp/support. It is possible that we may remove this directory altogether if the motivations for its existence are no longer strong enough (e.g., if most users migrate away from the proto_library target type or if the additional overhead of depending on gRPC C++ is not high).