problem with generation of defaults, cannot compile

[KasperJanssens]

When having a protocol buffer message like this:

message Bla { optional string bla = 1; } the defaults generator will generate a default_string function that is not used in the default record. By default it will make this declaration recursive, so 'let rec default_string ... and ...'. However, this rec flag is not necessary because the record doesn't use it and the compiler will throw a warning. As we prefer not to switch off compiler warnings this is an issue for us.