docs: public static Variable upcastToGeneratedBuilder(
        SourceBuilder code, Datatype datatype, String builder) {
      return code.scope().computeIfAbsent(Declaration.UPCAST, () -> {
        Variable base = new Variable("base");
        code.addLine(UPCAST_COMMENT)
            .addLine("%s %s = %s;", datatype.getGeneratedBuilder(), base, builder);
        return base;
      });
    }