Added CVarArg support to NSString and String

[Molanda]

This adds CVarArg protocol support to NSString and String.

print(String(format: "Testing %@ %@ %@", "One", "Two", "Three"))

Testing One Two Three

NSString support was easy since %@ already exists for CFString in CoreFoundation.

String was a little bit more challenging, since withVaList doesn't provide an autorelease pool to retain the required NSString instance, which is needed for the CFString.

I solved this by using OperationQueue to schedule the release. I'm not sure if there are any other options at this point.

This resolves SR-957.

[Molanda]

I created Swift PR #32311 to implement a new _CVarArgObject protocol to retain the NSString.

This PR depends on that one to be approved and merged first.

[millenomi]

Please test with the following:
swiftlang/swift#32311

@swift-ci please test

[Molanda]

Adding a reminder that this PR may need to be updated to match the new ObjC constraint on the _CVarArgObject protocol.

[Molanda]

With Swift PR #32311 merged into master, we can now proceed on this one.

It is my understanding that this implementation of Foundation expects that there will never by any Objective-C runtime (at least that is what the introductory README states). However, I did find some cases of #if !_runtime(_ObjC) in the code.

I can think of a few options...

1. Leave this as it is currently, as my understanding is correct.
2. #if out the String extension when using ObjC runtime. My best guess is this would be to test the build for an Apple platform rather than actually making something usable, since Apple platforms have their own implementation of Foundation.
3. #if the String extension to provide an autorelease implementation when using ObjC runtime. Is this even possible?

[Molanda]

To play it safe, I picked #2. This allows it to build in Xcode using stock Swift without the _CVarArgObject protocol.

[Molanda]

@millenomi Would you mind running the tests when you get a moment?

[MaxDesiatov]

@swift-ci please test

[millenomi]

@Molanda we support running SCF for debugging/patch writing purposes only on Darwin (via the Xcode workspace/project in the repository), so please make sure you don't break that. Note that it's a bit of a weird environment, but you will not have Swift code importing the 'real' Darwin Foundation in your process, so it will likely be safe to have the extension around — and you may actually need it since you are using SCF and the bundled CF.

[millenomi]

@swift-ci please test

[Molanda]

Thank you.

The builds failed with...

/Users/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-macOS/branch-master/swift-corelibs-foundation/Sources/Foundation/NSString.swift:1651:59: error: class 'CFString' cannot be used in an '@inlinable' function because 'CoreFoundation' was imported implementation-only
18:25:01         return _encodeBitsAsWords(unsafeBitCast(self, to: CFString.self))

It seems @_implementationOnly is a recent change. Any advice on how this should be handled now?

For %@ to work, _cVarArgEncoding needs to return a CFString.

[Molanda]

I can build with the @_implementationOnly change if I remove @inlinable from the extension functions.

On the Swift stdlib side, lorentey had made this suggestion...

...and I'm 99% sure we'd also want to remove @inlinable from all builder methods. (Exposing the vararg builder's class members in the ABI was a regrettable step.)

@millenomi do you agree with removing @inlinable or do you have a different suggestion?

(An unrelated note... while looking at the recent merges, I noticed RunLoop.swift has os(Linux) but not os(Android) in the #if conditional.)

[millenomi]

@Molanda sorry for the on-and-off; yes, please remove @inlinable here, or use unsafeBitCast(…, to: AnyObject.self) instead.

[millenomi]

(CFString and AnyObject have the same representation — an unmodified pointer to the object.)

[MaxDesiatov]

@swift-ci please test

[millenomi]

The failing test is LLDB, not us.

[millenomi]

@swift-ci please test linux

[Molanda]

@millenomi Looks good. Thank you for the help!