Having written a data serialization library based on codegen, I would have to say that Go is relatively simple to codegen. One big annoyance is the import system of "if you don't need it, it's an error to import it.", which, while simple enough for manual code, means you have to do extra work while generating code to keep track of what you actually use (i.e. if none of the things are a time.Time, you don't need to import "time"). The other approach is to use stuff like "var _ = time.Now" kind of constructs, which do make the codegen easier, but I personally feel are far dirtier.

One very nice feature of go is that the formatter is available as a library (https://golang.org/pkg/go/format/#Source), so you can gen "raw" code and then throw it through the formatter to get nice pretty output.