Document the JSON format used for DRT

[khieta]

Category

DRT infrastructure

Describe the feature you'd like to request

PR #124 makes it somewhat easier to customize DRT by abstracting away the details of our Dafny/Java implementation. This allows users to (with minimal work) provide their own implementation of Cedar to differentially test against the cedar-policy crate. However, users must figure out for themselves how to translate cedar-policy types like Request and Entities into appropriate format for their implementation.

In practice, we expect users to use JSON to pass data between Rust and their custom implementation, which is what we currently do for our Dafny/Java implementation. In particular, we rely on the generated serde_json JSON representation of the cedar-policy types. We should document this somewhere, and perhaps change the interfaces introduced in PR #124 to use JSON inputs/outputs rather than structured cedar-policy types.

Describe alternatives you've considered

No response

Additional context

No response

Is this something that you'd be interested in working on?

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

[cdisselkoen]

A note that the code in #124 actually depends on the cedar-policy-core types like Request and Entities, not the cedar-policy ones. (In fact, the Request in cedar-policy doesn't implement Serialize or Deserialize, and that was intentional on our part.) This means that the JSON format used for DRT is not stable, and can change with internal implementation changes in Core. #75 proposes to change this, and should probably be done before we tackle this issue here, to avoid wasting work.