Swift 6 withCheckedContinuation(isolation:)

[swhitty]

Description

With Swift 6 language mode, It is not possible to call withTaskCancellationHandler and withCheckedContinuation(isolation:, body:) within a function on Xcode 16 Beta 6 / Linux. Compilation fails with error:

❌ error: sending 'body' risks causing data races

• ❌ Linux Swift 6 language mode
• ⚠️ Linux Swift 5 language mode (⚠️ warns with .enableExperimentalFeature("StrictConcurrency"))
• ✅ macOS Swift 6 language mode (Xcode 16 beta 5)
• ❌ macOS Swift 6 language mode (Xcode 16 beta 6)
• ⚠️ macOS Swift 5 language mode (Xcode 16 beta 6)

Reproduction

Compile the following using Swift 6 language mode on linux / Xcode 16 beta 6

% swiftc main.swift -swift-version 6

func withContinuation<T>(
  isolation: isolated (any Actor)? = #isolation,
  body: (CheckedContinuation<T, Never>) -> Void
) async -> T {
    await withTaskCancellationHandler {
        await withCheckedContinuation(isolation: isolation) {
            body($0) // ❌ error: sending 'body' risks causing data races
        }
    } onCancel: {

    }
}

Removing withTaskCancellationHandler { } compiles and runs correctly:

func withContinuation<T>(
  isolation: isolated (any Actor)? = #isolation,
  body: (CheckedContinuation<T, Never>) -> Void
) async -> T {
     await withCheckedContinuation(isolation: isolation) {
          body($0)
    }
}

Removing isolation from withCheckedContinuation compiles, but isolation is not preserved and body is executed on another executor.

func withContinuation<T>(
  isolation: isolated (any Actor)? = #isolation,
  body: (CheckedContinuation<T, Never>) -> Void
) async -> T {
    await withTaskCancellationHandler {
        await withCheckedContinuation {
            body($0)
        }
    } onCancel: {

    }
}

actor Foo {
    func bar() async {
        await withContinuation {
            assertIsolated()
            $0.resume()
        }
    }
}

await Foo().bar()  // ❌ Incorrect actor executor assumption 💣 Program crashed:

Expected behavior

Program compiles fine and body closure is executed within the current isolation.

Environment

Swift version 6.0-dev (LLVM a0bfc8c8fd6ffd6, Swift 629aa83)
Target: x86_64-unknown-linux-gnu

Docker

swiftlang/swift   nightly-6.0-jammy   eafb9a2f9d84

Additional information

No response

[swhitty]

FYI the warning can be silenced with nonisolated(unsafe)

func withContinuation<T>(
  isolation: isolated (any Actor)? = #isolation,
  body: (CheckedContinuation<T, Never>) -> Void
) async -> T {
   nonisolated(unsafe) let body = body
   return await withTaskCancellationHandler {
        await withCheckedContinuation(isolation: isolation) {
            body($0) // ✅
        }
    } onCancel: {

    }
}

[guidedways]

This may be related to the crashes we're experiencing when building with Xcode 16: #75952