Is adding a `Sendable` requirement a breaking change?

[haikuty]

Thanks for the reply.

@hborla

This is only a problem if there is no possible way to address a warning. For libraries that have not yet been updated to include concurrency annotations, @preconcurrency is designed to suppress diagnostics in client projects,

Is this from the discussion of SE-0337 still accurate?

@Douglas_Gregor

The intent behind Swift 6 is that it will not silently accept the presence of a potential data race (without unsafe code explicitly involved somewhere), so @preconcurrency must not completely silence the diagnostics. Hence, they'll be warnings in Swift 6.

(sorry I haven't checked, I've been out for an extended period).

[taylorswift]

@hborla

This is only a problem if there is no possible way to address a warning. For libraries that have not yet been updated to include concurrency annotations, @preconcurrency is designed to suppress diagnostics in client projects, which will later return once an explicit annotation has been added or the library has adopted Swift 6. There are other suppression tools such as @unchecked Sendable and nonisolated(unsafe) that can also be judiciously applied when you've taken synchronization into your own hands or just need a temporary opt out from actor isolation checking.

it’s not like i seriously expected this to work in the first place, but no, you cannot @preconcurrency import Swift in order to suppress Sendable warnings within the standard library itself.

@preconcurrency import Swift

var arguments:IndexingIterator<[String]> = 
    Swift.CommandLine.arguments.makeIterator()
// reference to static property 'arguments' is not concurrency-safe 
// because it involves shared mutable state

the great irony of this bug is it actually forces you to use unsafe constructs completely unnecessarily:

let commandLineArgs:[String] = UnsafeBufferPointer<UnsafeMutablePointer<CChar>?>.init(
  start: CommandLine.unsafeArgv,
  count: Int.init(CommandLine.argc)).lazy
  .compactMap { $0 }
  .compactMap { String.init(validatingUTF8: $0) }