Feature request: Bridge Kotlin primitive type boxes to Swift native primitives

[suau]

What is the problem?

Primitive Kotlin types are represented in the Objective-C header as regular Objective-C primitives. Swift's Objective-C interop bridges them directly to native Swift primitives. That results in a nice translation of Kotlin's Int to Swift's Int32, Kotlin's Boolean to Swift's Bool etc.

Those Objective-C primitives, however, don't support nullability, nor can they be used as a type argument. Therefore, when a primitive type is nullable, or used as a generic type argument, Kotlin uses a special box type. Each primitive has one defined KotlinInt for Int, KotlinBoolean for Boolean etc. And these don't bridge to native Swift types and have to be explicitly unboxed.

What is the proposed solution?

Due to SKIE's support for extending Objective-C bridging, it should be possible to declare bridges for these primitive box types to map directly to Swift's primitive types.

[TadeasKriz]

Thank you for the report. It's a little inaccurate. SkieSwiftXXXFlow types are expected, but what you probably meant was that the element types supplied are KotlinInt, KotlinBoolean etc. This is caused by how Kotlin handles these types currently with generics and closures.

We should be able to fix this, but it'd be a source-breaking change for the rest of the codebase, because it'd map all KotlinInt to Int32, KotlinBoolean to Bool etc. I'll go ahead and convert this issue into a discussion so we can keep track of it as a proposed feature.

[TadeasKriz]

After some experimentation and internal discussion, we realized this is something we've already tried before and it didn't work. It's possible to get it working in one way, but then it makes calling any Kotlin code that has the box type as parameter impossible from Swift. That's because we can only bridge a single Objective-C type to a single Swift type. Swift's primitive types are already bridged to Objective-C's NSNumber. That's also why we can just tell Swift to bridge KotlinBoolean, KotlinInt etc. and it just works for anything Kotlin sends to Swift. KotlinBoolean, KotlinInt etc. all have the same superclass, NSNumber. But when trying to call a method that accepts KotlinBoolean, KotlinInt etc., it results in a crash of the Swift compiler (not a compilation error, a crash). As far as I can understand, this is because Swift only knows how to bridge Bool, Int etc. to NSNumber, but not to KotlinBoolean, KotlinInt etc. And since bridging is an internal Swift thing, they don't expect new types to be added.

Interestingly, for anything we generate in Swift, we should be able to bridge the type manually. This, however, requires a bit more work on the SKIE side and the only place it'd be useful is the suspend->async conversion.