CommandLine.arguments and CommandLine.argc should be deprecated and replaced with immutable equivalents.

[Lukasa]

CommandLine.arguments and CommandLine.argc are both declared static var. This is a real footgun: they're unsafe to use in a concurrency context because they may be mutated from any thread at any time. There's no real value in these being mutable so I think it is largely an oversight. We should consider deprecating these and replacing them with thread-safe equivalents.

[grynspan]

I wonder if we could maybe get away with just removing the setters? Hopefully nobody's relying on them…

[CodPro-Sui]

import Foundation

let commandLineArguments = CommandLine.arguments

// Access command-line arguments as an array of strings
for argument in commandLineArguments {
    print("Argument: \(argument)")
}

// Access the number of command-line arguments
let argumentCount = CommandLine.argc
print("Argument count: \(argumentCount)")

[grynspan]

Note argc is immutable. A workaround:

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

[stevapple]

I wonder if we could maybe get away with just removing the setters? Hopefully nobody's relying on them…

I don't think this can get through before Swift 6. Also, we cannot change it directly because of ABI stability.

[grynspan]

We could add an equivalent method (the method-ness can be used to indicate that the value is recomputed on each use?)

extension CommandLine {
  /// Get the command-line arguments passed to this process.
  ///
  /// - Returns: An array of command-line arguments.
  public static func arguments() -> [String] {
    UnsafeBufferPointer(start: unsafeArgv, count: Int(argc)).lazy
      .compactMap { $0 }
      .compactMap { String(validatingUTF8: $0) }
  }
}

We can trivially make such a thing back-deployable or transparent so that it can be used in older Darwin releases too.

[gwynne]

See also #69658