[pitch] Strict concurrency for global variables

Joe_Groff · August 23, 2023, 8:04pm

Noncopyable types should eventually allow for a mutex-guarded value to be presented as an "immutable" type which can be stored in a let property, which then wouldn't require a global actor annotation to be safely accessible. There's a prototype example of this here:

github.com

apple/swift/blob/main/test/Prototypes/UnfairLock.swift#L40


      
              @inline(__always)
              borrowing func withLock<R>(_ body: () throws -> R) rethrows -> R {
                  let address = _address
                  os_unfair_lock_lock(address)
                  defer { os_unfair_lock_unlock(address) }
          
                  return try body()
              }
          }
          
          final class Locked<T>: @unchecked Sendable {
              private let lock = UnfairLock()
          
              // Don't need exclusivity checking since accesses always go through the
              // lock
              @exclusivity(unchecked)
              private var value: T
          
              init(initialValue: T) {
                  self.value = consume initialValue
              }