Should Swift 6 prevent sharing a non-Sendable value via Mutex?

Using Swift
1

The following code produces no warnings or errors under Swift 6, even though NonSendable is being effectively shared across concurrency domains. Is this expected?

import Synchronization

class NonSendable {
    var name = "Fred"
}

let m = Mutex(NonSendable())

func test() {
    Task { @concurrent in
        let ns = m.withLock { $0 }
        ns.name = "joe"
    }
    
    Task { @concurrent in
        let ns = m.withLock { $0 }
        ns.name = "fred"
    }
}

It seems to me that returning $0 from the withLock closure should produce some sort of warning. The closure is -> sending Result, and if I understand sending correctly, we shouldn't be allowed to send $0 while still holding onto a reference to it.

So either there's something missing in the compiler (Swift 6.3), or in Mutex, or there's something missing in my understanding.

4 Likes
2

I believe @stephencelis called this out recently as a regression. A previous version of the compiler did correctly catch this case.

1 Like
3

Your understanding aligns with mine. This looks like a bug to me.

3 Likes
4

I am taking a look at it.

3 Likes
5

Could you also take a look at this issue: [RBI] Capturing and returning non-Sendable values in closures can cause data races · Issue #87918 · swiftlang/swift · GitHub

6

I believe there's already at least one GitHub issue for this.

7
1 Like