We want to run the same tests between Clojure and ClojureScript. However some of our CLJS tests are asynchronous and require the use of the async macro. Clojure doesn't have the async macro, which makes test portability difficult. If we were to add the async macro, there are at least two possible routes to go down: imitating the async behaviour of cljs.test, or copying the implementation. If we were to imitate the behaviour, we could use the following async macro. It creates a promise, runs the body, blocks on the promise, and done delivers the promise, allowing the test to continue.
[done & body]
`(let [p# (promise)
~done #(deliver p# nil)]
This has the advantage that it integrates with the current way tests are run in Clojure, and doesn't require any of the surrounding tooling to be aware of the async testing.
Imitating the implementation would be much more invasive. It would probably mean changing the behaviour of run-tests to return nil like ClojureScript does, and a host of other changes. This means it is likely a non-starter.
I lean heavily towards the first option.
To answer the question "Why do we need this at all when it is a no-op?", this is useful so we can write the same tests in both Clojure and ClojureScript. It would be much more tricky to write without it.