swift-mirror/userdocs/diagnostics/nonisolated-nonsending-by-default.md

nonisolated(nonsending) by Default (NonisolatedNonsendingByDefault)

Runs nonisolated async functions on the caller's actor by default.

Overview

Prior to this feature, nonisolated async functions never run on an actor's executor and instead switch to global generic executor. The motivation was to prevent unnecessary serialization and contention for the actor by switching off of the actor to run the nonisolated async function, but this does have a number of unfortunate consequences:

1. nonisolated is difficult to understand
2. Async functions that run on the caller's actor are difficult to express
3. It's easy to write invalid async APIs
4. It's difficult to write higher-order async APIs

Introduced in Swift 6.2, -enable-upcoming-feature NonisolatedNonsendingByDefault changes the execution semantics of nonisolated async functions to always run on the caller's actor by default. A new @concurrent attribute can be added in order to specify that a function must always switch off of an actor to run.

struct S: Sendable {
  func performSync() {}

  // `nonisolated(nonsending)` is the default
  func performAsync() async {}

  @concurrent
  func alwaysSwitch() async {}
}

actor MyActor {
  let s: Sendable

  func call() async {
    s.performSync() // runs on actor's executor

    await s.performAsync() // runs on actor's executor

    s.alwaysSwitch() // switches to global generic executor
  }
}

A nonisolated(nonsending) modifier can also be used prior to enabling this upcoming feature in order to run on the caller's actor. To achieve the same semantics as above, S in this case would instead be:

struct S: Sendable {
  func performSync() {}

  nonisolated(nonsending)
  func performAsync() async {}

  // `@concurrent` is the default
  func alwaysSwitch() async {}
}

Migration

-enable-upcoming-feature NonisolatedNonsendingByDefault:migrate

Enabling migration for NonisolatedNonsendingByDefault adds fix-its that aim to keep the current async semantics by adding @concurrent to all existing nonisolated async functions and closures.

See Also

• SE-0461: Run nonisolated async functions on the caller's actor by default