include/grpc/impl/codegenThis directory exists so that generated C++ code can include selected files upon which it depends without having to depend on the entire gRPC C++ library. This directory thus exists to support include/grpcpp/impl/codegen. This constraint 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. C++ 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 the gRPC C++ library no longer has a need for an impl/codegen directory of its own).