Feature request: Sendable conformance

[russellporter]

We've been looking at enabling stronger concurrency checking in our Swift codebase, to help spot issues and get ready for Swift 6. We have many immutable data classes in our KMP library which we assume are safe to be marked as @unchecked Sendable.

Currently we annotate the KMP types from our Swift codebase. However it is far away from the type declaration in Kotlin, which makes it fragile. For example:

extension Exercise: @unchecked Sendable {}

It seems like SKIE potentially could help a lot here:

• adding @UncheckedSendable annotation for kotlin types. Being able to declare it directly on the type in Kotlin code would make the API contract more explicit.
• adding ability to annotate Kotlin interfaces as Sendable. I couldn't find a way to do this yet.
• ideal world: add @CheckedSendable annotation for kotlin types. This would provide a compile time guarantee that a kotlin type is fully Sendable, similar to how Swift compiler does it. I assume this is very hard if not impossible.

[frankois944]

@russellporter you can use @preconcurrency import Shared https://www.avanderlee.com/concurrency/preconcurrency-checking-swift/

You won't need to extend your DataClass with Sendable but some cases still need some migration from SKIE (async/await) or Kotlin cinterop itself.

[el-hoshino]

@frankois944 Unfortunately this is not helping anything; it’s essentially no different from using @unchecked and ultimately does nothing to contribute to data race safety.

[frankois944]

Yes, it doesn't make the library fully compatible with Swift6, the @preconcurrency is a temporary solution in some cases.

[Carrione]

Also, the ability to mark closures as @Sendable would be helpful. Or does such option already exist?

[PhilippeWeidmann]

This should be doable on the objc side with the macro NS_SWIFT_SENDABLE -> https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/incrementaladoption/#CObjective-C

I also added a feature request here:
https://youtrack.jetbrains.com/issue/KT-74440