[Task cancellation] Adopt nonisolated(nonsending) and sending

Author: DougGregor

stdlib/public/Concurrency/TaskCancellation.swift

@@ -76,15 +76,14 @@ import Swift
 @available(SwiftStdlib 5.1, *)
 @export(implementation)
 nonisolated(nonsending)
-public func withTaskCancellationHandler<T, E>(
-  operation: () async throws(E) -> T,
-  onCancel handler: @Sendable () -> Void
-) async throws(E) -> T {
+public func withTaskCancellationHandler<Return, Failure>(
+  operation: nonisolated(nonsending) () async throws(Failure) -> Return,
+  onCancel handler: sending () -> Void
+) async throws(Failure) -> Return {
   // unconditionally add the cancellation record to the task.
   // if the task was already cancelled, it will be executed right away.
   let record = unsafe Builtin.taskAddCancellationHandler(handler: handler)
   defer { unsafe Builtin.taskRemoveCancellationHandler(record: record) }
-
   return try await operation()
 }
 

test/Concurrency/actor_withCancellationHandler.swift

@@ -38,7 +38,7 @@ actor Container {
       num += 1 // no error, this runs synchronously on caller context
     } onCancel: {
       // this should error because cancellation is invoked concurrently
-      num += 10 // expected-error{{actor-isolated property 'num' can not be mutated from a Sendable closure}}
+      num += 10 // expected-error{{actor-isolated property 'num' can not be mutated from a nonisolated context}}
     }
   }
 }