[swift] UnavailableFunction Swift rule

[tomidelucca]

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• The PR is submitted against master. The PMD team will merge back to support branches as needed.
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PR Description:

We are adding a second Java rule for Swift. This rule checks that an availability modifier was added to any function that's unimplemented.

Here is an example:

class ViewController: UIViewController {
  public required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }
}

This is a perfectly valid piece of Swift code, but if you call the init?(coder:) initializer of the ViewController class the app will crash due to the fatalError(). This initializer needs to be implemented to extend UIViewController, so removing the method will not work.

This rule checks that you add the @available(*, unavailable) attribute to the function and initializer declarations. With this attribute present, the function will not be able to be called, but the compiler will see it as implemented so there won't be any compile errors.

The expected non-triggering version of the code snippet looks like this:

class ViewController: UIViewController {
  @available(*, unavailable)
  public required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }
}

[pmd-test]

	1 Message
📖	No java rules are changed!

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[jsotuyod]

@tomidelucca please rebase, #1877 has now been merged

[tomidelucca]

@jsotuyod done!