Don't use Synchronization.Atomic for deliverExpectationCheckedEvents.

This PR replaces the new use of `Atomic<Int>` with a `Locked<Int>` in the
implementation of `deliverExpectationCheckedEvents`. Why? Because we're running
into some environments where the Synchronization module isn't available (e.g.
older host macOSes) and this is simpler. The performance profile is comparable:
on my system, running the `repeatedlyExpect()` test takes 0.55s instead of 0.49s
to call `#expect()` 1,000,000 times, so it's still a significant win over the
implementation we had earlier.

Author: grynspan

Package.swift

@@ -148,7 +148,6 @@ extension Array where Element == PackageDescription.SwiftSetting {
       .enableExperimentalFeature("AvailabilityMacro=_clockAPI:macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0"),
       .enableExperimentalFeature("AvailabilityMacro=_regexAPI:macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0"),
       .enableExperimentalFeature("AvailabilityMacro=_swiftVersionAPI:macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0"),
-      .enableExperimentalFeature("AvailabilityMacro=_synchronizationAPI:macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0"),
 
       .enableExperimentalFeature("AvailabilityMacro=_distantFuture:macOS 99.0, iOS 99.0, watchOS 99.0, tvOS 99.0, visionOS 99.0"),
     ]

Sources/Testing/Running/Runner.RuntimeState.swift

@@ -8,8 +8,6 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-private import Synchronization
-
 extension Runner {
   /// A type which collects the task-scoped runtime state for a running
   /// ``Runner`` instance, the tests it runs, and other objects it interacts
@@ -113,8 +111,8 @@ extension Configuration {
   /// - Returns: A unique number identifying `self` that can be
   ///   passed to `_removeFromAll(identifiedBy:)`` to unregister it.
   private func _addToAll() -> UInt64 {
-    if deliverExpectationCheckedEvents, #available(_synchronizationAPI, *) {
-      Self._deliverExpectationCheckedEventsCount.add(1, ordering: .sequentiallyConsistent)
+    if deliverExpectationCheckedEvents {
+      Self._deliverExpectationCheckedEventsCount.increment()
     }
     return Self._all.withLock { all in
       let id = all.nextID
@@ -133,8 +131,8 @@ extension Configuration {
     let configuration = Self._all.withLock { all in
       all.instances.removeValue(forKey: id)
     }
-    if let configuration, configuration.deliverExpectationCheckedEvents, #available(_synchronizationAPI, *) {
-      Self._deliverExpectationCheckedEventsCount.subtract(1, ordering: .sequentiallyConsistent)
+    if let configuration, configuration.deliverExpectationCheckedEvents {
+      Self._deliverExpectationCheckedEventsCount.decrement()
     }
   }
 
@@ -143,8 +141,7 @@ extension Configuration {
   ///
   /// On older Apple platforms, this property is not available and ``all`` is
   /// directly consulted instead (which is less efficient.)
-  @available(_synchronizationAPI, *)
-  private static let _deliverExpectationCheckedEventsCount = Atomic(0)
+  private static let _deliverExpectationCheckedEventsCount = Locked(rawValue: 0)
 
   /// Whether or not events of the kind
   /// ``Event/Kind-swift.enum/expectationChecked(_:)`` should be delivered to
@@ -155,11 +152,7 @@ extension Configuration {
   /// for these events, consult the per-instance
   /// ``Configuration/deliverExpectationCheckedEvents`` property.
   static var deliverExpectationCheckedEvents: Bool {
-    if #available(_synchronizationAPI, *) {
-      _deliverExpectationCheckedEventsCount.load(ordering: .sequentiallyConsistent) > 0
-    } else {
-      all.contains(where: \.deliverExpectationCheckedEvents)
-    }
+    _deliverExpectationCheckedEventsCount.rawValue > 0
   }
 }
 

Sources/Testing/Support/Locked.swift

@@ -177,6 +177,15 @@ extension Locked where T: Numeric {
   @discardableResult func increment() -> T {
     add(1)
   }
+
+  /// Decrement the current wrapped value of this instance.
+  ///
+  /// - Returns: The sum of ``rawValue`` and `-1`.
+  ///
+  /// This function is exactly equivalent to `add(-1)`.
+  @discardableResult func decrement() -> T {
+    add(-1)
+  }
 }
 
 extension Locked {

cmake/modules/shared/AvailabilityDefinitions.cmake

@@ -13,5 +13,4 @@ add_compile_options(
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -define-availability -Xfrontend \"_clockAPI:macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0\">"
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -define-availability -Xfrontend \"_regexAPI:macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0\">"
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -define-availability -Xfrontend \"_swiftVersionAPI:macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0\">"
-  "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -define-availability -Xfrontend \"_synchronizationAPI:macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0\">"
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -define-availability -Xfrontend \"_distantFuture:macOS 99.0, iOS 99.0, watchOS 99.0, tvOS 99.0, visionOS 99.0\">")