Fix crash when running swift test for swift-build

[rconnell9]

Fix crash when running swift test for swift-build.

Was consistently able to cause the following crash prior to this change:
Thread 1 Crashed: 0 libswift_Concurrency.dylib 0x286d730b4 swift_task_alloc + 92 1 SwiftBuildPackageTests 0x1136b9c78 withExtendedLifetime<A, B, C>(_:_:) + 256 (Misc+Async.swift:124) 2 SwiftBuildPackageTests 0x113715c39 static NSTask.getOutput(url:arguments:currentDirectoryURL:environment:interruptible:) + 1 (Process.swift:86)

[rconnell9]

@swift-ci test

[ktoso]

Lgtm!

[jakepetroules]

Ahh, nice catch. I think it's a compiler bug that withExtendedLifetime can contain async code despite its closure parameter not being annotated async?

Do we have a bug on the compiler to diagnose this as a compile-time error instead?

[jakepetroules]

@swift-ci test

[jakepetroules]

Ahh, nice catch. I think it's a compiler bug that withExtendedLifetime can contain async code despite its closure parameter not being annotated async?

Do we have a bug on the compiler to diagnose this as a compile-time error instead?

I mis-remembered, I was thinking of withoutActuallyEscaping. The async withExtendedLifetime is something coming from Swift Build's own codebase, in Misc+Async.swift:

@_alwaysEmitIntoClient
public func withExtendedLifetime<T: ~Copyable, E: Error, Result: ~Copyable>(
    _ x: borrowing T,
    _ body: (borrowing T) async throws(E) -> Result
) async throws(E) -> Result {
    defer { withExtendedLifetime(x) { } }
    return try await body(x)
}

(making this crash even more puzzling, since this patch is effectively just inlining the code)

[ktoso]

Yeah it's a bit puzzling but it seems something about this pattern was upsetting 🤔
Might be worth a follow up investigation in Swift itself