The async refactorings ignore whether a completion handler had
`@escaping` or not. In preparation of fixing this, fix up all functions
to have `@escaping` for their completion handler parameter.
Also some small miscellaneous fixes in order to reduce the number of
warnings output on test failures and also the addition of `REQUIRES:
concurrency` on all tests.
Previously, when a completion handler call had arguments for both the success and error parameters, we were always interpreting it as an error call. In case the error argument was an `Optional`, this could cause us to generate code that didn't compile, because we `throw`ed the `Error?` or passed it to `continuation.resume(throwing)`, both of which didn't work.
We now generate an `if let` statement that checks if the error is `nil`. If it is, the error is thrown. Otherwise, we interpret the call as a success call and return the result.
- Example 1 (convert to continuation)
-- Base Code
```swift
func test(completionHandler: (Int?, Error?) -> Void) {
withoutAsyncAlternativeThrowing { (theValue, theError) in
completionHandler(theValue, theError)
}
}
```
-- Old Refactoring Result
```swift
func test() async throws -> Int {
return try await withCheckedThrowingContinuation { continuation in
withoutAsyncAlternativeThrowing { (theValue, theError) in
continuation.resume(throwing: theError) // error: Argument type 'Error?' does not conform to expected type 'Error'
}
}
}
-- New Refactoring Result
```swift
func testThrowingContinuationRelayingErrorAndResult() async throws -> Int {
return try await withCheckedThrowingContinuation { continuation in
withoutAsyncAlternativeThrowing { (theValue, theError) in
if let error = theError {
continuation.resume(throwing: error)
} else {
guard let theValue = theValue else {
fatalError("Expected non-nil success argument 'theValue' for nil error")
}
continuation.resume(returning: theValue)
}
}
}
}
```
- Example 2 (convert to async/await)
-- Base Code
```swift
func test(completion: (String?, Error?) -> Void) {
simpleErr() { (res, err) in
completion(res, err)
}
}
```
-- Old Refactoring Result
```swift
func test() async throws -> String {
let res = try await simpleErr()
throw <#err#>
}
```
-- New Refactoring Result
```swift
func test() async throws -> String {
let res = try await simpleErr()
return res
}
```
