In reply to @rnapier, here's how to reduce the implicit optionals in your code: factor them out.

dispatch-sync-safe.swift

func result() -> A {
    var result : A = dispatch_sync(resultQueue, {
        return self.result
    })
    return result
}

func dispatch_sync<R> (queue: dispatch_queue_t, block: Void -> R ) -> R {
    var result: R!
    dispatch_sync(queue) {
        result = block()
    }
    return result
}