docs: public static RexNode fromFieldAccess(
        final RowSignature rowSignature,
        final Project project,
        final int fieldNumber
    )
    {
      if (project == null) {
        // I don't think the factory impl matters here.
        return RexInputRef.of(fieldNumber, rowSignature.getRelDataType(new JavaTypeFactoryImpl()));
      } else {
        return project.getChildExps().get(fieldNumber);
      }
    }