swift 6.0.1 crash when running tests swift testing with @MainActor annotation only on linux

[doozMen]

Describe the bug
Only happens on linux

1. clone https://github.com/doozMen/swift-testing-main-bug
2. run unit tests with visual studio in debug

You will see it crashes. The docs state that Thread.isMainThread is not available. Weirdly it is on macOS just not on linux.

To Reproduce
run the test project on linux and maybe add code

    #expect(Thread.isMainThread == true)

This will work on mac and not on linux

Expected behavior

Not creating deadlock because of doing dispach on main in sync from main

Screenshots

Environment

• swift-snapshot-testing version [1.17..5]
• Swift [6.0.1]
• OS: aarch64-unknown-linux-gnu

Additional context
Maybe also a bug in swift but not sure as when you the check is unavailable in async context according to the docs. So you should not rely on it?

Maybe have a asser option that is marked with @mainactor might be better?

[stephencelis]

@doozMen Thanks for the report. We're running SnapshotTesting on Linux for pointfree.co, but are on a particular branch that uses this logic to dispatch to the main queue instead:

swift-snapshot-testing/Sources/SnapshotTesting/AssertSnapshot.swift

Lines 352 to 359 in 7de6fa5

	static func registerIfNeeded() {
	DispatchQueue.mainSync {
	if !registered {
	registered = true
	XCTestObservationCenter.shared.addTestObserver(CleanCounterBetweenTestCases())
	}
	}
	}

Would you be open to exploring if this fix would address the problem for you when applied to the main branch?