[5.10][Concurrency] Suppress diagnostics about non-Sendable async iterator arguments in implicit calls to next().

[hborla]

• Explanation:

#67730 closed a hole in strict concurrency checking that failed to diagnose non-Sendable self arguments crossing isolation boundaries. When that hole was closed, the compiler started diagnosing code that iterates over an AsyncSequence type from an actor isolated context (including global actors):

@MainActor func iterate(over stream: AsyncStream<Int>) async {
  for await element in stream { ... } // warning: Passing argument of non-sendable type 'inout AsyncStream<Int>.Iterator' outside of main actor-isolated context may introduce data races
}

This is correct - nearly all AsyncIteratorProtocol types are not Sendable, and AsyncIteratorProtocol.next() is nonisolated async, so any call to next() from an actor-isolated context will pass a non-Sendable iterator value over an isolation boundary! There's a whole big pitch discussion in Swift evolution right now about fixing this properly by making next() polymorphic over actor isolation.

However, in Swift 5.10, there's nothing that programmers can do to to resolve this warning. So instead, this change applies the nonisolated(unsafe) opt out to the generated iterator variable during constraint generation to suppress the warning. Note that I cherry picked the remainder of the implementation of nonisolated(unsafe) (most of the implementation had already been cherry picked) and enabled it in order to use this opt out. The feature has been accepted in Swift Evolution and enabled on main, and programmers will surely need nonisolated(unsafe) in Swift 5.10 given the vast number of Sendable checking holes that have been closed in Swift 5.10, leading to more warnings just like this one.

• Scope: Only impacts Sendable checking, and async iteration in particular
• Risk: Very low. The remaining implementation of nonisolated(unsafe) does not impact other language features, and the only effect of applying it to async iterator variables is a suppressed warning under -strict-concurrency=complete.
• Testing: Added new test cases to show that no warnings are produced for async iteration in an actor isolated context under -stict-concurrency=complete.
• Reviewer: @sophiapoirier
• Main branch PR:
  • [Sema] nonisolated(unsafe) for local variables #70135
  • [Concurrency] unguard nonisolated(unsafe) attribute from GlobalConcurrency experimental feature #70453
  • [Concurrency] Suppress diagnostics about non-Sendable async iterator arguments in implicit calls to next(). #70697

[hborla]

@swift-ci please test

[hborla]

/home/build-user/swift/test/Concurrency/experimental_feature_strictconcurrency.swift:1:1: error: unexpected error produced: new Swift parser generated errors for code that C++ parser accepted

Looks like I'm missing a swift-syntax cherry pick

[hborla]

swiftlang/swift-syntax#2417

@swift-ci please test

[hborla]

swiftlang/swift-syntax#2417

@swift-ci please test