[5.10][SE-0411] Promote IsolatedDefaultValues from an experimental feature to an upcoming feature.

[hborla]

• Explanation:

SE-0411: Isolated Default Values closed an important hole in Swift's static data-race safety model which prevents actor-isolated default stored property values from being synchronously evaluated from outside the actor. For example, the following code is currently valid, but it will crash at runtime at the call to MainActor.assertIsolated():

@MainActor func requiresMainActor() -> Int {
  MainActor.assertIsolated()
  return 0
}

@MainActor struct S {
  var x = requiresMainActor()
  var y: Int
}

nonisolated func call() async {
  let s = await S(y: 10)
}

await call()

This happens because requiresMainActor() is used as a default argument to the member-wise initializer of S, but default arguments are always evaluated in the caller. In this case, the caller is on the generic executor, so the default argument evaluation crashes. Under -enable-upcoming-feature IsolatedDefaultValues, this code is still value, but callees that have isolated default arguments will hop to the isolation domain before evaluating the default arguments.

• Scope: Only impacts code that builds with -strict-concurrency=complete. Code can also opt into this feature using -enable-upcoming-feature IsolatedDefaultValues. More specifically, this change impacts structs and classes whose stored properties are global actor isolated and their initial values must run on that global actor.
• Risk:

Medium. This is technically a source breaking change for code that uses isolated stored property initial values in a nonisolated initializer:

@MainActor func requiresMainActor() -> Int {
  MainActor.assertIsolated()
  return 0
}

class C {
  @MainActor var x = requiresMainActor()

  init() {}
}

In Swift 5.9, the above code is valid. With this change, it's invalid under -strict-concurrency=complete:

error: return from initializer without initializing all stored properties
  init() {}
          ^
note: 'self.x' not initialized
  @MainActor var x = requiresMainActor()
                 ^

However, I believe such code is rare in practice, and the common code pattern that currently exhibits data races is like the above example, which remains valid (but safe) under this proposal.

• Testing: Updated existing tests that use -strict-concurrency=complete and added new unit tests for IsolatedDefaultValues. The main PR for enabling IsolatedDefaultValues under -strict-concurrency=complete also passed the source compatibility suite.
• Issue: Resolves [SR-15916] @MainActor-isolated default arguments for @MainActor-isolated functions #58177.
• Main branch PRs:
  • [Concurrency] Allow isolated default arguments to be used from across isolation boundaries. #69391
  • [SE-0411] Fixes for isolated default values. #69974
  • [SE-0411] Promote IsolatedDefaultValues from an experimental feature to an upcoming feature. #70257
  • [Concurrency] Compute actor isolation for var decls before checking decl attributes. #70222

[hborla]

@swift-ci please test

[hborla]

@swift-ci please test