Await.For_testingval with_never :
'r. f:(t @ local -> 'r @ local unique once) @ local once ->
'r @ local unique oncewith_never ~f calls f with an implementation of awaiting that should never be used. If await is called with the implementation, it will raise.
This is useful in tests, for testing operations which otherwise might conditionally block in a single-threaded manner that never needs to block.
Bear in mind that proper implementations of await do not usually raise and are not documented to potentially raise. This means that abstractions built on await may e.g. leave the program in an invalid state when using with_never.