Automatically synthesizing help strings for enum valued flags

[jiawen]

The flags library has pretty good support for enum valued flags via the AbslParseFlag and AbslUnparseFlag extension points. In the course of maintaining a codebase that uses it, one thing that came up is divergence between the manually written help message and the set of enum values. I.e.,

ABSL_FLAG(my_ns::MyEnumType, my_flag,  my_ns::MyEnumType::kRed,
          "Permitted values: red, green, blue");

As you can imagine, if I add kYellow to my_ns::MyEnumType and corresponding support in AbslParseFlag and AbslUnparseFlag, the help message will not be updated. I have to grep the entire codebase for all uses of this enum as a flag and update the error messages.

I'd like to explore the design space for synthesizing the help message automatically. Ideally, it would take advantage of AbslUnparseFlag to iterate over all enum values (!) and return a string help message, potentially with a preamble and epilogue.

As I type this, I realize that the developer could of course manually do this as:

ABSL_FLAG(my_ns::MyEnumType, my_flag,  my_ns::MyEnumType::kRed,
          makeHelpMessage<my_ns::MyEnumType>("preamble", "epilogue"));  // With template magic to iterate over the enum values.

[derekmauro]

What you are asking for is enum reflection. Abseil won't be implementing that. https://github.com/Neargye/magic_enum is a library that I know of that does that.