Investigating reducing compile time impact of Reflection.hpp

[gennaroprota]

Alan suggested investigating whether compile times can be reduced for translation units that include Reflection.hpp, which currently requires parsing all metadata-related includes due to BOOST_DESCRIBE_xxx() invocations.

Background

This issue originated from a discussion in PR #1130. Currently:

• Struct and enum definitions live in public include files.
• BOOST_DESCRIBE_xxx() invocations live in Reflection.hpp (private, under src/lib/).
• Only .cpp files in src/lib/ include Reflection.hpp (currently ~4 files).

While this design already provides reasonable separation, there may be opportunities to further reduce compile time overhead.

Questions to investigate

• Can we forward-declare the internal entities that BOOST_DESCRIBE_xxx() generates?
• Could we create a macro that forward-declares whatever entities the describe macros would create?
• Are there other techniques to encapsulate the describe metadata without requiring full definitions?

Additional considerations

Personally, I'm not fond of a solution that requires knowing the implementation details of Boost.Describe, especially since we are not having an issue with compile times.