Add official support of C++ annotation to specify ROOT IO attribute for data members.

[pcanal]

As seen in https://github.com/cms-sw/cmssw/pull/40435, ROOT I/O annotation can be moved from the comments to using C++ attributes (in particular because support for the comment is implemented internally through the same mechanics).

To quote the referred pull request:

The way dictionary information are propagated from the C++ code or XML dictionaries to reflex and cling is rather roundabout:

• a
• <field name="data_" comment="!"/> tags XML dictionaries are parsed by genreflex and injected into the LLVM AST of the corresponding C++ code as comments //!;
• C++ comments like //! or //[size_] are converted by genreflex/rootcling into LLVM AST annotations; cling parses the LLVM annotations and uses them to generate the desired behavior in the dictionaries.

This approach does not work well with macro-generated data members:

• macros cannot generate comments, so //! or //[size_] cannot be used directly;
• macros cannot easily be used to generate the class_def.xml file, requiring manual intervention for their implementation and maintenance.

However, it turns out that dictionaries can bypass the comments and use LLVM annotations directly within the C++ code. So

private:
  int size_;
  float* data_;       //[size_]
  float* transient_;  //!

can be also expressed as

private:
  int size_;
  float* data_ [[clang::annotate("[size_]")]];
  float* transient_ [[clang::annotate("!")]];

and annotations can be generated by macros.

In order to avoid spurious warnings when compiling the header, we should offer a (set of) macro(s), eg:

• ROOT_IO_TRANSIENT can be used to annotate transient data members, like `//!

• ROO_IO_SIZE(SIZE) can be used to annotate dynamic arrays, like //[SIZE] or

• ROOT_IO_ANNOTATE that can be used with ROOT_IO_ANNOATE("!") and ROOT_IO_ANNOTATE("[size]")

The advantage of the earlier case would be to (possibly) allow simplification of the internal parsing, by using:

  float* data_ [[rootio::size("size_")]];
  float* transient_ [[rootio::transient]];

[Axel-Naumann]

We should not use the internal names here, but dedicated ROOT I/O attribute names.

If that's not possible then we should at least allow for future cling attributes that are not I/O comments, i.e. some form of scoping of attribute names. (And yes I now see that you already stated that, sorry!)

[pcanal]

If/when we officially support the annotation as input we need to resolve/decide/do-something the potential conflict between the input from comments, selection xml and the annotation.

[fwyzard]

How do you deal now with conflicts between the comments and the xml selection ?

[pcanal]

Currently genreflex (mostly) wins. The comment tag has priority over the comment in the C++ code but the transient and persistent tag do not.

[pcanal]

And for completeness sake's, currently the clang::annotate wins over both.