[swift-evolution] Pitch: Cross-module inlining and specialization

[Slava Pestov]

> On Oct 3, 2017, at 11:05 PM, Jonas B <bobergj at gmail.com> wrote:
> 
> 
>> On 4 Oct 2017, at 14:33, Slava Pestov <spestov at apple.com <mailto:spestov at apple.com>> wrote:
>> 
>> @_versioned makes a symbol visible externally without making it visible from the language. There is no requirement that a @_versioned thing is @inlinable. It is used when you want to reference an internal function from an inlinable function. Eg,
>> 
>> internal func myImplDetail() { … }
>> 
>> @inlinable public func myPublicFunction() { myImplDetail() } // error!
>> 
>> —
>> 
>> @_versioned internal func myImplDetail() { … }
>> 
>> @inlinable public func myPublicFunction() { myImplDetail() } // OK
>> 
>> Slava
> 
> 
> From my language user point of view it would be more understandable if that was written with a single keyword, eg:
> @nonABI internal func myImplDetail() { }
> @nonABI public func myPublicFunction() { myImplDetail() }  // OK

The two attributes are different though. myImplDetail() is _not_ inlined into client code here, but myPublicFunction() is. So they need different attributes to express this intent.

> 
> Anyway, for my use case mentioned earlier (shipping a release version of my app bundle), that doesn’t really matter. I’d just like a compiler switch that made the whole module not having an ABI, essentially making all all methods and types @inlinable and @_versioned, using the terminology in your example.

Yes, that would be a nice feature to have but it is outside the scope of this proposal. First, we want to tackle shipping resilient modules — then we can discuss generalizing the notion of a “resilience domain” to encompass multiple modules. I realize the latter is more useful to third party developers though, and I apologize that in this particular case our priorities are at odds…

> 
> My other observation is that no matter how great the ergonomics, and no matter the naming of these attributes, very few people outside the compiler team is going to be able to successfully ship a versioned library without the “Checking Binary Compatibility” tool mentioned in https://github.com/apple/swift/blob/master/docs/LibraryEvolution.rst#checking-binary-compatibility <https://github.com/apple/swift/blob/master/docs/LibraryEvolution.rst#checking-binary-compatibility>.

I agree — even for members of the compiler team this tool would be very useful, both as a way of encoding our assumptions for sanity-checking, and making sure we don’t make a silly mistake with a future update to the standard library.

Slava

> 
> /Jonas
> 

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