Illustrate the fact that nonsending asynchronous functions know their isolation.

IsolationTests.swift

import Testing

@Test func tests() async {
    await nonisolatedFunc()
    await mainActorIsolatedFunc()
    await SomeActor().someActorIsolatedFunc()
}

@concurrent nonisolated func nonisolatedFunc() async {
    await callee()
}

@MainActor func mainActorIsolatedFunc() async {
    await callee()
}

actor SomeActor {
    func someActorIsolatedFunc() async {
        await callee()
    }
}

nonisolated(nonsending) func callee(caller: String = #function) async {
    let isolation = #isolation
    print("static isolation of callee: \(isolation, default: "nonisolated") when called from: \(caller)")
}

// static isolation of callee: nonisolated when called from: nonisolatedFunc()
// static isolation of callee: Swift.MainActor when called from: mainActorIsolatedFunc()
// static isolation of callee: IsolationTests.SomeActor when called from: someActorIsolatedFunc()