[stdlib] Enforce that atomic representations are BitwiseCopyable

[Azoy]

Now that BitwiseCopyable is a thing, let's enforce that atomic representations must always be bitwise copyable. This avoids a bunch of subtle issues that can occur with @_rawLayout now like not supporting non-bitwise movable values.

Resolves rdar://128296144

[Azoy]

@lorentey @glessard we should amend the bitwise copyable to have 1. Never conform 2. have AtomicRepresentable's representation conform and 3. have AtomicOptionalRepresentable's representation conform.

[Azoy]

@swift-ci please test

[nate-chandler]

FYI Never already conforms: https://github.com/apple/swift-evolution/blame/main/proposals/0426-bitwise-copyable.md#L430 .

[Azoy]

@nate-chandler was the implementation just missed then?

[nate-chandler]

No, conformance is inferred.

[Azoy]

Well without the explicit conformance, the compiler complains it does not conform to AtomicRepresentable.

/Users/alejandro.alonso/Desktop/swift/swift/stdlib/public/Synchronization/AtomicRepresentable.swift:258:1: error: type 'Never' does not conform to protocol 'AtomicRepresentable'
164 |   /// The storage representation type that `Self` encodes to and decodes from
165 |   /// which is a suitable type when used in atomic operations.
166 |   associatedtype AtomicRepresentation: BitwiseCopyable
    |                  `- note: protocol requires nested type 'AtomicRepresentation'; add nested type 'AtomicRepresentation' for conformance
167 | 
168 |   /// Destroys a value of `Self` and prepares an `AtomicRepresentation` storage
    :
256 | 
257 | @available(SwiftStdlib 6.0, *)
258 | extension Never: AtomicRepresentable {
    | `- error: type 'Never' does not conform to protocol 'AtomicRepresentable'
259 |   /// The storage representation type that `Self` encodes to and decodes from
260 |   /// which is a suitable type when used in atomic operations.
261 |   @available(SwiftStdlib 6.0, *)
262 |   public typealias AtomicRepresentation = Never
    |                    `- note: possibly intended match 'Never.AtomicRepresentation' (aka 'Never') does not conform to 'BitwiseCopyable'
263 | 
264 | 

[nate-chandler]

Thanks for bringing that to my attention.

I see that error too now, but only when building for embedded. In the Swift.swiftmodule for non-embedded macosx, various inferred conformances like UnboundedRange_ : Sendable and Never : BitwiseCopyable appear in the swiftinterface (e.g. Swift.swiftmodule/arm64-apple-macos.swiftinterface); they don't appear in the .swiftmodule (e.g. Swift.swiftmodule/arm64-apple-macos.swiftmodule). In the Swift.swiftmodule for embedded, there are no .swiftinterfaces; and the inferred conformances still don't appear in the .swiftmodule files. The result is the actual non-conformance of UnboundedRange_ to Sendable and Never to BitwiseCopyable on embedded.

I filed rdar://128358780 .

In the meantime, explicitly stating the conformance of Never to BitwiseCopyable is harmless.

[nate-chandler]

The issue with inference in the embedded stdlib is fixed by #73756 , so stating Never's conformance explicitly should no longer be needed (though it might perhaps be desirable).