[swift-dev] Renamed types (Swift 3/4 Mix-and-Match)

Jordan Rose jordan_rose at apple.com
Thu Apr 20 18:55:35 CDT 2017

TLDR: Should we just always import C/ObjC types under their Swift 4 names, and use typealiases in Swift 3 mode?


Hi, swift-dev. As my recent PRs have probably indicated, I've been working on the problems that can come up when mixing Swift 3 and Swift 4 code. Most of these problems have to do with C/ObjC APIs that might present themselves differently in Swift 3 and Swift 4, using the "API notes" feature in our downstream branch of Clang, and a good subset of these problems have to do with types getting renamed. (This includes being "renamed" into a member, such as NSNotificationName becoming (NS)Notification.Name in Swift.)

What's the problem? Well, there are a few. First of all, an API defined in terms of the Swift 3 name should still be callable in Swift 4. As an example, let's pretend NSNotification.Name was going to be renamed NSNotification.Identifier in Swift 4.

// Swift 3 library
public func postTestNotification(named name: NSNotification.Name) { … }

// Swift 4 app
let id: Notification.Identifier = …
postTestNotification(named: id) // should work

This means the reference to "NSNotification.Name" in the library's swiftmodule needs to still be resolvable. This isn't too bad if we leave behind a typealias for 'NSNotification.Name'. I have a reasonable (but too broad) implementation at https://github.com/apple/swift/pull/8737 <https://github.com/apple/swift/pull/8737>.

That just leads us to another problem, though: because Swift functions can be overloaded, the symbol name includes the type, and the type has changed. The Swift 3 library exposes a symbol '_T03Lib20postTestNotificationySo14NSNotificationC4NameV5named_tF', but the Swift 4 client expects '_T03Lib20postTestNotificationySo14NSNotificationC10IdentifierV5named_tF'.

My planned approach to combat this was to use the C name of the type in the mangling, producing '_T03Lib20postTestNotificationySo18NSNotificationNamea5named_tF'. This is prototyped in https://github.com/apple/swift/pull/8871 <https://github.com/apple/swift/pull/8871>.

At this point Slava pointed out I was chasing down a lot of issues when there's a much simpler solution for Swift 4: when importing types, always use the Swift 4 name, and use typealiases to handle Swift 3 compatibility. This defines both of the previous issues away, as well as any more that I just haven't thought of yet.

There are some downsides:
- We currently keep people from using Swift 4 names in Swift 3 code, and we wouldn't be able to do that, since the actual declaration of the type always needs to be available.
- We'd probably want to tweak the "aka" printing in diagnostics to not look through these typealiases. That's not hard, though.
- We can't keep doing this once we have ABI stability. Hopefully framework owners aren't going to continue changing Swift names of types, but we'll probably need to implement my "C name in the mangling" plan anyway, just in case.

What do people think?

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