[Concurrency] Don't expose Dispatch or CF executors directly.

Also add `final` to the `CooperativeExecutor` declaration.

Author: al45tair

stdlib/public/Concurrency/CFExecutor.swift

@@ -45,7 +45,7 @@ enum CoreFoundation {
 
 /// A CFRunLoop-based main executor (Apple platforms only)
 @available(StdlibDeploymentTarget 6.2, *)
-public final class CFMainExecutor: DispatchMainExecutor, @unchecked Sendable {
+final class CFMainExecutor: DispatchMainExecutor, @unchecked Sendable {
 
   override public func run() throws {
     CoreFoundation.CFRunLoopRun()
@@ -61,8 +61,8 @@ public final class CFMainExecutor: DispatchMainExecutor, @unchecked Sendable {
 
 /// A `TaskExecutor` to match `CFMainExecutor` (Apple platforms only)
 @available(StdlibDeploymentTarget 6.2, *)
-public final class CFTaskExecutor: DispatchGlobalTaskExecutor,
-                                   @unchecked Sendable {
+final class CFTaskExecutor: DispatchGlobalTaskExecutor,
+                            @unchecked Sendable {
 
 }
 

stdlib/public/Concurrency/CooperativeExecutor.swift

@@ -100,7 +100,7 @@ extension ExecutorJob {
 /// A co-operative executor that can be used as the main executor or as a
 /// task executor.
 @available(StdlibDeploymentTarget 6.2, *)
-class CooperativeExecutor: Executor, @unchecked Sendable {
+final class CooperativeExecutor: Executor, @unchecked Sendable {
   var runQueue: PriorityQueue<UnownedJob>
   #if !$Embedded && !SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY
   var waitQueue: PriorityQueue<UnownedJob>

stdlib/public/Concurrency/DispatchExecutor.swift

@@ -25,8 +25,8 @@ import Swift
 
 /// A Dispatch-based main executor.
 @available(StdlibDeploymentTarget 6.2, *)
-public class DispatchMainExecutor: RunLoopExecutor, SchedulingExecutor,
-                                   @unchecked Sendable {
+class DispatchMainExecutor: RunLoopExecutor, SchedulingExecutor,
+                            @unchecked Sendable {
   var threaded = false
 
   public init() {}
@@ -78,8 +78,8 @@ extension DispatchMainExecutor: MainExecutor {}
 
 /// A Dispatch-based `TaskExecutor`
 @available(StdlibDeploymentTarget 6.2, *)
-public class DispatchGlobalTaskExecutor: TaskExecutor, SchedulingExecutor,
-                                         @unchecked Sendable {
+class DispatchGlobalTaskExecutor: TaskExecutor, SchedulingExecutor,
+                                  @unchecked Sendable {
   public init() {}
 
   public func enqueue(_ job: consuming ExecutorJob) {