[Concurrency] Fall back to allocating from the heap, fix a test.

`CooperativeExecutor` should fall back to heap allocations if the task
allocator isn't available when it's enqueuing delayed jobs.

Also, remove the reference to `DispatchGlobalExecutor` from the
custom executor test.

rdar://141348916

Author: al45tair

stdlib/public/Concurrency/CooperativeExecutor.swift

@@ -71,7 +71,13 @@ extension ExecutorJob {
   fileprivate mutating func setupCooperativeExecutorTimestamp() {
     // If a Timestamp won't fit, allocate
     if cooperativeExecutorTimestampIsIndirect {
-      let ptr = unsafe allocator!.allocate(as: CooperativeExecutor.Timestamp.self)
+      let ptr: UnsafeMutablePointer<CooperativeExecutor.Timestamp>
+      // Try to use the task allocator if it has one
+      if let allocator {
+        ptr = unsafe allocator.allocate(as: CooperativeExecutor.Timestamp.self)
+      } else {
+        ptr = .allocate(capacity: 1)
+      }
       unsafe self.cooperativeExecutorTimestampPointer = ptr
     }
   }
@@ -80,7 +86,11 @@ extension ExecutorJob {
     // If a Timestamp won't fit, deallocate
     if cooperativeExecutorTimestampIsIndirect {
       let ptr = unsafe self.cooperativeExecutorTimestampPointer
-      unsafe allocator!.deallocate(ptr)
+      if let allocator {
+        unsafe allocator.deallocate(ptr)
+      } else {
+        unsafe ptr.deallocate()
+      }
     }
   }
 }

test/Concurrency/Runtime/custom_main_executor.swift

@@ -20,7 +20,7 @@ struct SimpleExecutorFactory: ExecutorFactory {
   }
   public static var defaultExecutor: any TaskExecutor {
     print("Creating task executor")
-    return DispatchGlobalTaskExecutor()
+    return SimpleTaskExecutor()
   }
 }
 
@@ -62,9 +62,9 @@ final class SimpleMainExecutor: MainExecutor, @unchecked Sendable {
 }
 
 @available(SwiftStdlib 6.2, *)
-final class FatalExecutor: TaskExecutor, @unchecked Sendable {
+final class SimpleTaskExecutor: TaskExecutor, @unchecked Sendable {
   func enqueue(_ job: consuming ExecutorJob) {
-    fatalError("We should never get here")
+    MainActor.executor.enqueue(job)
   }
 }
 
@@ -84,9 +84,9 @@ func myAsyncFunction() async {
 // CHECK: Creating main executor
 // CHECK-NEXT: Creating task executor
 // CHECK-NEXT: Hello
-// CHECK-NEXT: Running
-// CHECK-NEXT: Hello World
 // CHECK-NEXT: Enqueued job
+// CHECK-NEXT: Running
 // CHECK-NEXT: Running job
+// CHECK-NEXT: Hello World
 // CHECK-NEXT: Goodbye