errcode Go generate command learning. References https://pkg.go.dev/golang.org/x/tools https://blog.golang.org/generate https://docs.google.com/document/d/1V03LUfjSADDooDMhe-_K59EgpTEm3V8uvQRuNMAEnjg/edit# https://darjun.github.io/2019/08/21/golang-generate/