[modular] User-driven validation

[epage]

Issue by epage Tuesday Nov 09, 2021 at 14:38 GMT Originally opened as https://github.com/clap-rs/clap/issues/3008

---

Please complete the following tasks

• [X] I have searched the discussions
• [X] I have searched the existing issues

Clap Version

master

Describe your use case

• We want to get clap smaller (#1365) and right now validation adds bulk regardless of what parts people use
  • We could put parts behind a feature flag but that could end up getting messy
• In https://github.com/clap-rs/clap/discussions/3005, a user requested specialized validation logic

Describe the solution you'd like

Define one or more App Validation trait

• Validate: Takes a built App and ArgMatches as an input, reports errors
• Requires: Takes a built App, ArgMatches, and args as input, reports which args are required
• (maybe) Default value: Takes a built App, ArgMatches, and args as input, reports default values for them

Steps

1. Refactor all requires, conflicts, etc APIs to be implemented in terms of this
   • Try to overcome weirdnesses in the default_value_if API with its use of Option by clarifying intent through builders
2. Expose the trait and structs to the user in a clap::validate
3. Deprecate the old Arg functions so now only those using this API pay the cost due to dead code elimination

Alternatives, if applicable

No response

Additional Context

See also https://github.com/clap-rs/clap/discussions/2832

Risks are usability and performance. The most basic APIs, like required, we probably need to keep baked in. Its the special inter-arg interactions that we should be focusing on.