GeezerVison

While working on something else, I noticed that type inference for ActionState was broken in some cases. The problem is that, when given a chance, TypeScript will simplify an ActionState<Fs> type as much as possible, which results in a concrete object type that doesn't mention Fs. That causes problems for functions which accept ActionState<Fs> values, but need to know the feature keys Fs such as the callee function from testing-utils.ts.

Assuming that we have a function test(action: ActionState<["Logger", "Env"]): boolean, then before this change the result of callee(test) would be a value of type CallableEnvBuilder<[], boolean, readonly (keyof FeatureState)[]>. That's no good since readonly (keyof FeatureState)[] satisfies any StateFeature[] constraint and doesn't tell us which features the function actually needs.

The fix here is a bit complex, but fundamentally focuses around adding a property to the result of ActionState that tracks Fs. That's straight-forward, but we also need to ensure that ActionState values which provide a superset of the required properties can be passed to functions which don't require all of it. That is accomplished by making the extra property a function and tracking the features Fs in its parameter to exploit the contravariance of function parameters.

Finally, we also need to account for Env implying ReadOnlyEnv. That's addressed by expanding "Env" to "Env" | "ReadOnlyEnv" in the tracked features.

Extending ActionState objects is also tricky because in e.g. the existing { ...action, features } constructions, features: FeatureEnablement and so the "Features" key is not mentioned. The extendActionState function works around that by using a reverse lookup (implemented in FeatureNameFor) to lookup the feature key for a given object.

Risk assessment

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• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

Environments:

• Testing/None - This change does not impact any CodeQL workflows in production.

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• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).
• End-to-end tests - I am depending on PR checks (i.e. tests in pr-checks).

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Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.