Static property wrappers and strict concurrency in 5.10

Using Swift
30

Adding my two-cents here!

I have this simple property wrapper:

/// A property wrapper that allows access to a Sample File.
/// Before the file is accessed, it's first copied into it's on mutable copy so we don't affect any other tests.
@propertyWrapper
public struct SampleFile: Sendable {

    private let fileName: String

    init(fileName: String) {
        self.fileName = fileName
    }

    public var wrappedValue: Data {
        let file = fileName.split(separator: ".").first!
        let fileExtension = fileName.split(separator: ".").last!
        let url = Bundle.module.url(forResource: String(file), withExtension: String(fileExtension))!
        return try! Data(contentsOf: url)
    }
}
  • It's nonmutating
  • It's Sendable

Yet, it's not working with Strict Concurrency enabled.

  • nonisolated(unsafe) does not work with Property Wrappers
  • The compiler is still not smart enough to do:
  • get only or nonmutating set property wrappers would have their backing storage defined with a let.

That would solve this and sounds like a reasonable way forward.

But how about Macros?
Well, yes, that would solve it. Honestly, I can very easily rewrite the above example as well. However, Macros are more complicated to create compared to property wrappers and if this is the way forward, I feel like we should consider property wrappers to be deprecated or such since I don't see a way now to make them work with Strict Concurrency.

I'd love to get some more thoughts on this and the current state.

@Douglas_Gregor what is your vision on Property Wrappers and Strict Concurrency?

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