[SR-4354] New String design

[bob-wilson]

Previous ID	SR-4354
Radar	rdar://problem/20486780
Original Reporter	@bob-wilson
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Improvement, AffectsABI
Assignee	@airspeedswift
Priority	Medium

md5: 22ab97c83270399b4f65d192960b83b9

relates to:

• SR-2081 String views should have ASCII optimizations

Issue Description:

We are working to revise the design of String for Swift 4. This includes a variety of changes, including:

https://bugs.swift.org/browse/SR-4003 Optimize the representation of String
https://bugs.swift.org/browse/SR-4004 Create a shim to ICU
https://bugs.swift.org/browse/SR-4006 Enable String to store Latin1 characters with 8-bit encoding
https://bugs.swift.org/browse/SR-4007 Implement Small String optimization

I am filing this issue to track the overall effort for the purpose of ABI stability.

[bob-wilson]

At this point all of the String changes relevant to the ABI are done in Swift 4.2.

[bob-wilson]

I should clarify that: there may still be further changes that affect String but no more so than for other standard library types. The big changes for the 2-word implementation and the small string optimization are done.

[milseman]

Done in Swift 5