[swift-evolution] Proposal: Change rules for implicit captures due to nested closure captures

[Jordan Rose]

Good idea, Kevin. Something like this seems reasonable (though I admit I just skimmed the message). In the mean time, maybe we can add a warning for implicit captures with a different strength?

(I'd also be happy to treat "unowned" + "weak" as "error, be explicit" rather than "strong".)

There is one case where an explicit capture behaves differently from an implicit one: capturing local variables (as opposed to local constants). There's an example of this in the Swift Programming Language book, in the reference section: "Capture Lists <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/doc/uid/TP40014097-CH32-ID544>".

Jordan

> On Dec 8, 2015, at 12:44, Kevin Ballard via swift-evolution <swift-evolution at swift.org> wrote:
> 
> In Swift code today, when using nested closures, if the inner closure weakly captures an object (e.g. `self`) that isn't otherwise captured by the outer closure, the outer closure implicitly strongly captures the object. This behavior is unlikely to be what the programmer intended, and results in unwanted object lifetime extension without making it obvious in the code.
>  
> In practice, you'll find this happening in code that looks like
>  
> class SomeViewController: UIViewController {
>     // ...
>     func foo(url: NSURL) {
>         let task = NSURLSession.sharedSession().dataTaskWithURL(url) { data, response, error in
>             let result = // process data
>             dispatch_async(dispatch_get_main_queue()) { [weak self] in
>                 self?.handleResult(result)
>             }
>         }
>         task.resume()
>     }
> }
>  
> In here, at first glance it looks like the view controller is being weakly referenced. But in actuality, the view controller is being retained for the duration of the background processing, and is then only converted to a weak reference at the moment where it tries to hop back on to the main thread. So it's basically the worst of both worlds; the view controller lives far longer than intended, but it goes away right at the moment where it could be useful again. It's even worse if the programmer expected the view controller's deinit() to cancel the network task, as that can never happen. The fix for this code is to move the `[weak self]` up to the outer closure, but since the outer closure never actually touches self directly, it's not immediately obvious that this is required.
>  
> My proposal is to change the rules so that whenever a closure captures an object only because a nested closure did so, then the outer closure should capture it using the same ownership semantics (this includes unowned(unsafe), unowned(safe), weak, and strong). If there are multiple nested closures that capture it, then we use the following rules:
>  
> * If all nested captures use the same ownership, then the outer capture uses that ownership.
> * If any nested capture is strong, the outer capture is strong.
> * If at least one nested capture is weak, and at least one capture is unowned or unowned(unsafe), the outer capture is strong. This is because there's no (safe) way to convert from weak -> unowned, or from unowned -> weak, and we should not crash upon the creation of the nested closure, so the outer capture must be strong.
> * If at least one nested capture is unowned(unsafe), and at least one nested capture is unowned(safe), then the outer capture is unowned(safe).
>  
> This can be visualized with the following diagram, where the outer closures uses the right-most node that covers all the children:
>  
>          .--- weak <-------------------------------.
>         /                                           \
> strong <                                             + no capture
>         \                                           /
>          '--- unowned(safe) <--- unowned(unsafe) --'
>  
> -Kevin Ballard
> 
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