To do list for Validation

[dmdashenkov]

In order to catch up with the existing validation mechanisms, this new validation library implementation needs to:

• Cover all the edge cases with integration tests.
• Add support for custom error messages defined via the options. This also includes changing the docs for options. See the error message format in rule.proto. Maybe, as a part of this change, we should move validation options from base into this repo.
• Implement the (is_required) constraint;
• Implement the (when) constraint;
• Add support for custom validators written in Java or Kotlin and associated with a proto type on runtime.
• Stop supporting (constraint_for) feature. Use custom validators instead.
• Implement a Gradle plugin that would configure the default validation behaviour for Java projects;

Post v2.0.0-M1

• Support JS and Dart.

[alexander-yevsyukov]

@armiol, let's discuss this issue and our actions on Monday. It's dated, but still relevant in several items.

[armiol]

As discussed vocally, the list of our actions will presumably be as follows:

1. Deprecate and remove the 'external" validation as a feature.
2. Review the state of the custom error messages once the respective works by @alexander-yevsyukov @yevhenii-nadtochii are done.
3. Postpone JS/TS/Dart validation until we are feature-complete with JVM.
4. Close this issue.

[alexander-yevsyukov]

Stop supporting (constraint_for) feature. Use custom validators instead.

The (constraint_for) option is deprecated in options.proto. Message validators have been recently implemented.