[SR-2847] Strings should not be able to be bridged to NSString when Foundation is not imported

[phausler]

Previous ID	SR-2847
Radar	rdar://problem/31418210
Original Reporter	@phausler
Type	New Feature

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	New Feature, AffectsABI, Runtime, SDKOverlay
Assignee	None
Priority	Medium

md5: 6451cacb0a3ffe168105c8113d92efdc

Issue Description:

See https://ci.swift.org/job/swift-PR-osx/3925/consoleFull#106371907ee1a197b-acac-4b17-83cf-a53b95139a76 for the failure in question when attempting to move parts of the bridge into the Foundation overlay.

Any application that is linking Foundation should get the bridgeable behavior and any application that is not importing Foundation should not be getting bridging behavior. This poses a distinct inconsistency with the requested model of the application as well as it can potentially set up the developer for a possible failure when porting to linux.

This is borderline between a bug and a feature request. It should be categorized appropriately.

Fixing this would be a breaking change for applications that are implicitly requiring this behavior so this has a potential ABI and source compatibility impact.

[belkadan]

This is already the behavior we have at the user level. That crash is about an implementation detail of the standard library.

[belkadan]

i.e. the standard library is calling CF directly with an NSString subclass they made. String isn't involved at all in this part of the code (just how it got there).

[phausler]

Hmm looking for occurrences and it seems like String.lowercased() can call this. So perhaps that is merit to actually put the fast-paths fully in the core stdlib instead of the overlay.

[bob-wilson]

@swift-ci create