[5.5][Async Refactoring] Wrap code in a continuation if conversion doesn't yield reasonable results

[ahoppen]

• Explanation: If we are requested to convert a function to async, but the call in the function’s body that eventually calls the completion handler doesn’t have an async alternative, we are currently copying the call as-is, replacing any calls to the completion handler by placeholders. This is seldom what the developer wants and it would be better to wrap the function in withChecked(Throwing)Continuation and replace all calls to the completion handlers by calls to continuation.resume(throwing/returning:).
• Scope: Async refactoring where the completion handler is called from a function that doesn’t have an async alternative
• Risk: Low
• Testing: Added regression test
• Issue: rdar://79304583
• Reviewer: @bnbarham (Ben Barham), @hamishknight (Hamish Knight), @ktoso (Konrad Malawski) on original PR [Async Refactoring] Wrap code in a continuation if conversion doesn't yield reasonable results #38193

Example 1

Base Code

func testDispatch(completionHandler: @escaping (Int) -> Void) {
  DispatchQueue.global.async {
     completionHandler(longSyncFunc())
  }
}

Old Refactoring Result

func testDispatch() async -> Int  {
  DispatchQueue.global.async {
     <#completionHandler#>(longSyncFunc())
  }
}

New Refactoring Result

func testDispatch() async -> Int {
  return await withCheckedContinuation { continuation in
    DispatchQueue.global.async {
      continuation.resume(returning: longSyncFunc())
    }
  }
}

Example 2

Base Code

func testUrlSession(completionHandler: @escaping (Data) -> Void) {
  let task = URLSession.shared.dataTask(with: request) { data, response, error in
    completion(data!)
  }
  task.resume()
}

Old Refactoring Result

func testUrlSession() async -> Data {
  let task = URLSession.shared.dataTask(with: request) { data, response, error in
    <#completion#>(data!)
  }
  task.resume()
}

New Refactoring Result

func testDataTask() async -> Int?
  return await withCheckedContinuation { continuation in
    let task = URLSession.shared.dataTask { data, response, error in
      continuation.resume(returning: data!)
    }
    task.resume()
  }
}

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test