[test] Mark _Differentiable tests as unsupported in Swift-in-the-OS configurations

[lorentey]

The new _Differentiable module is not available in any shipping OS release, but its public API currently doesn’t have availability, either.

Temporarily disable tests that import it when we’re testing with OS-provided libraries.

The typecheck test test/AutoDiff/stdlib/differentiable_protocol.swift imports _Differentable but still somehow succeeds in these configs, so leave that one enabled.

rdar://57975086

[lorentey]

@swift-ci smoke test

[dan-zheng]

Hi @lorentey,

I have a few questions, if you don't mind.

1. Is the robust fix for this issue to add @available attributes to public APIs in the _Differentiation module? If yes, what arguments should we specify in the @available attributes?

2. In the meantime, should we add the following:

// We currently lack availability information (rdar://57975086)
// UNSUPPORTED: use_os_stdlib

To all test/AutoDiff tests that import the _Differentiation module?

3. Is there a @swift-ci command we can run to prevent breakages due to missing UNSUPPORTED: use_os_stdlib?

Thanks! cc @rxwei @marcrasi

[lorentey]

Hey @dan-zheng! 👋

In general, we need to describe availability for anything that ships in ABI-stable OS releases. For in-development Swift versions, this is usually done by adding an @available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) incantation to all ABI-affecting declarations.

However, _Differentiable is a special case since (1) it's a brand new, standalone module and (2) it's underscored which makes it non-public and potentially not ready for ABI stability. We'll need to figure out what's the right way to handle it; but for now I believe it is okay to keep things as they are!

This PR was just a quick emergency fix to unblock our CI. We're still investigating and we will likely need to adjust or (preferably) revert this soon. (Especially since the back-deployment tests aren't failing on ci.swift.org... 🤔)

[lorentey]

Update: Yep, the issue was a misconfiguration entirely on our side; I'll revert this in a moment.