Support NonisolatedNonsendingByDefault

[0xTim]

Motivation

If you enabled the upcoming feature flag NonisolatedNonsendingByDefault in your package and use Swift Testing with a Suite Trait, you'll probably hit upon the issue that provideScope(for:testCase:performing:) doesn't conform to the protocol, even though from the error message they're the same. The fix is to annotate the closure with @concurrent but this should be done in the framework

Proposed solution

Support NonisolatedNonsendingByDefault by default

Alternatives considered

Make users fix it, a workaround but not ideal

Additional information

No response

[grynspan]

Good tech-debt work for 6.4. Not planning to touch 6.3 at this late stage.

[grynspan]

This tends to fall over because we have a textual interface and the compiler isn't correctly inferring our library's concurrency settings. Unclear if we should pursue a compiler fix rather than changes on our end.

[stmontgomery]

We can and should do this, so thank you for the issue.

That said, the main place this has come up is when test authors implement TestScoping, and the auto-completed version of provideScope(for:testCase:performing:) does not include the @concurrent attribute on the function parameter. But I believe this has been fixed in the compiler in swiftlang/swift#85187. My hope is that greatly alleviates the problem, even before we make this change.

[grynspan]

I tried it locally and explicitly adding @concurrent to the parameter does not affect the textual interface (i.e. it doesn't fix the problem).