Design Document: First Step Towards Modularity

[epage]

This is more of a note dumping ground so I don't forget this in the lead up to 5.0.

• Create sealed UsageWriter trait
  • Returns usage lines, without header or indentation
  • Create a NoUsage, FixedUsage, AutoUsage, AdaptiveUsage
• Create sealed HelpWriter trait
  • Create a NoHelp, FixedHelp, HelpTemplate, and AutoHelp
  • Generic over usage
• Create a Storage trait with sealed methods, making ArgMatches generic over it
  • Create new derive methods so this can be used implicitly to reduce constraints on derive and improve performance
• Create ArgParser that is generic over the above
• Deprecate all Command::...get_matches... functions except the most basic, pushing people to ArgParser
• Deprecate all Command help/usage writing functions (maybe)

End result

• Reduce binary size without the use of feature flags
• Reduce requirements on derive users
• Allow moving some Command state to ArgParser
• Open door for more experimentation with custom usage/help