[swift-dev] Casting shadow protocols

[Dave Abrahams]

on Mon Nov 07 2016, Alexis <abeingessner-AT-apple.com> wrote:

>> On Nov 4, 2016, at 11:55 PM, Dave Abrahams via swift-dev <swift-dev at swift.org> wrote:
>> 
>> 
>> on Fri Nov 04 2016, Slava Pestov <swift-dev-AT-swift.org <http://swift-dev-at-swift.org/>> wrote:
>> 
>>> If the casts are always in one direction, can you make one protocol
>>> refine another?
>> 
>> Yeah, I am shocked if they don't do that already.
>
> They do; _NSFoo: _NSFooCore
>
> But the problem is that we have _NSFooCores and want _NSFoos.

Right.  Then you need type punning without any dynamic checks,
a.k.a. _unsafeReferenceCast

>
>> 
>>> Also note that @objc protocols are self-conforming as long as they
>>> don’t contain initializers or static methods, but I’m not sure if that
>>> helps.
>> 
>> Doesn't; we're not using these in a generic context; they're just
>> existentials.
>> 
>>> 
>>> 
>>>> On Nov 4, 2016, at 4:29 PM, Alexis via swift-dev <swift-dev at swift.org> wrote:
>>>> 
>>>> The swift standard library has this nasty little pattern/problem in it:
>>>> 
>>>> The types in the core library want to know about several types
>>>> defined in foundation: NSString, NSArray, NSDictionary, etc. But
>>>> core is imported by Foundation, so it can’t (circular references
>>>> between modules). Thankfully, everything in ObjC is pretty opaquely
>>>> defined and uniform, and Swift knows how to hook into that uniform
>>>> layout. So the core library defines Shadow Protocols which provide
>>>> whatever subset of that type’s API is considered interesting. These
>>>> protocols are then used in place of the ObjC types. There’s also
>>>> some magic compiler hooks so core lib types can subclass those
>>>> foundation types.
>>>> 
>>>> However there’s sometimes two Shadow Protocols: one that defines the
>>>> APIs the stdlib should provide (_NSFooCore), and one that extends
>>>> that with extra APIs the stdlib wants to consume (_NSFoo). This
>>>> leads to an awkward situation: as far as the runtime is concerned,
>>>> the stdlib’s _NSFooCores don’t conform to _NSFoo! We know they do
>>>> because it’s all just a big lie to hook into ObjC message passing
>>>> with a bit of type safety, but the runtime doesn’t. So if you try to
>>>> do a safe type cast, it will fail. This leads to a situation where
>>>> we sprinkle code with unsafeBitCast’s to _NSFoo which is a massive
>>>> refactoring hazard.
>>>> 
>>>> For instance, there was a struct-containing-a-class that was being
>>>> cast to _NSFoo in HashedCollections. This happened to work (but was
>>>> probably still a violation of strict aliasing?) because the struct’s
>>>> only field was the class. However the struct was later changed to a
>>>> class, which silently made the cast completely incorrect, banishing
>>>> the real _NSFoo to the shadow (protocol) realm.
>>>> 
>>>> Can we do anything better here? Note that there’s a few places where
>>>> we also cast an AnyObject into an _NSFoo, but there’s some chance
>>>> this is all legacy junk that can be updated to at least use
>>>> _NSFooCore, if not _NSFoo itself.
>>>> _______________________________________________
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>>>> swift-dev at swift.org
>>>> https://lists.swift.org/mailman/listinfo/swift-dev
>>> 
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>> 
>> -- 
>> -Dave
>> 
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-- 
-Dave