Module language 

The language a policy’s predicate is written in — an open union, the same shape bot.did.registration’s actor union already uses and the same shape this crate gave SubjectKind: Known plus Unknown.

The owner’s framing: “an open union of known types. This includes at least like… regex, and cedar? And then people who want something different need to patch the binary and write a new policy type.” This is that union. It names nothing about how a wrapper record spells its language — plan/policy.md leaves the wrapper’s shape unsettled — only which languages a build can turn into a working Evaluator.

What a build does with a language it does not have

It holds the policy and refuses what it covers; it does not fail to parse the record. A loader resolving a wrapper’s declared language to a Predicate turns Language::Unknown into Predicate::Unusable — that variant’s doc comment names an unknown language as its motivating case — never into a parse failure for the record as a whole. The rest of the record (its applicability) is still known-good data even when its language is not, and Predicate::Unusable is what lets the tree deny exactly that applicability instead of everything or nothing.

Enums

KnownLanguage

The evaluator languages this workspace names. Naming a member here is naming a sibling crate that implements Evaluator for it — a decision for whoever maintains this workspace, not something a policy record can request into existence — but naming one ahead of the crate that implements it is exactly what an open union is for: it makes that crate’s later arrival an addition to this list, not a change to its shape.

Language

A policy language, known to this build or not.