Add Sendable conformance

[vlm]

It can be useful to send AsyncParsableCommand as an argument to some function. This is trivial to do in a non-async context. However, in async context the type that is sent as an argument has to be Sendable. This patch adds Sendable conformance so this becomes expressible and warning-free:

struct Args: AsyncParsableCommand, Sendable {
    func run() async throws {
        Task { await otherFunction(arguments: self) }
    }
}

The warnings will become errors in Swift 6, so this is a forward-looking change.

Checklist

• [x] I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

[natecook1000]

Thanks for this, @vlm! Do we not need to declare the Parsed enum as sendable as well? That can indirectly store a user-supplied closure via initializers that include a transform parameter, which could break encapsulation. Should we look at adding @Sendable annotations to those closures at the same time?

[vlm]

@natecook1000 looking...

[vlm]

@natecook1000 these conformances doesn't appear to be required. Added a test to showcase that. No warnings.

https://github.com/apple/swift-argument-parser/pull/582/files#diff-1d0a514c56a2c811f846acdce3410d034f8008d908b6180d6a4fa1d536a279c3R36

[natecook1000]

@swift-ci Please test

[rauhul]

Is it possible to put the additional sendable restrictions behind conditional compilation so this change isn't source breaking?

[natecook1000]

@rauhul None of these changes should be source breaking, since checking is only a warning until you upgrade to Swift 6 mode. Additionally, to turn off the warnings making other source changes, a package that depends on ArgumentParser can add @preconcurrency to their import statement.

[natecook1000]

@swift-ci Please test