bug: There is no way to get current queue / current queue label under Linux

To aid in debugging of multi threaded / multi queue code it is at times useful to obtain a label of the current dispatch queue where a piece of code is executing.

This has been possible using a hacky solution documented by Quinn here:

https://developer.apple.com/forums/thread/701313

And although this works nicely under macOS and iOS, it does not compile under Linux.

import Dispatch

let queue = DispatchQueue(label: "hello-cruel-world")

func main() {
    queue.async {
        print("DispatchQueue.label: \(queue.label)")
        print("dqgl: \(String(cString: __dispatch_queue_get_label(nil)))")
        exit(0)
    }
    dispatchMain()
}

main()

q.swift:8:40: error: cannot find '__dispatch_queue_get_label' in scope
        print("dqgl: \(String(cString: __dispatch_queue_get_label(nil)))")
                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~
q.swift:8:67: error: 'nil' requires a contextual type
        print("dqgl: \(String(cString: __dispatch_queue_get_label(nil)))")
                                                                  ^

Alternatively, or yet better, it may be good to offer an API to get current queue where a piece of code is executing, as from the current queue we can get the label easily.

Please accept my apology if I am missing an obvious way how to achieve this.

Start with the Linux Dispatch import and the unavailable __dispatch_queue_get_label(nil) entry point, comparing it with the macOS and iOS behavior described in Quinn's linked reference. Determine whether Linux should expose the existing label lookup or provide a current-queue API, then verify that the reported sample compiles and returns the queue label.