swiftinterface generator confuses `@exclusivity(unchecked)` and `@_optimize(none)` on Windows

[grynspan]

After merging swiftlang/swift-testing#655, CI started failing with a very curious error:

T:/105/swift/Testing.swiftinterface:156:2: error: '@_optimize(none)' attribute cannot be applied to stored properties
154 | }
155 | @usableFromInline
156 | @_optimize(none) nonisolated(unsafe) internal var failureBreakpointValue: Swift.Int
    |  `- error: '@_optimize(none)' attribute cannot be applied to stored properties
157 | @usableFromInline
158 | @inline(never) internal func failureBreakpoint()

T:/105/swift/Testing.swiftinterface:4:1: error: failed to verify module interface of 'Testing' due to the errors above; the textual interface may be broken by project issues or a compiler bug
  2 | // swift-compiler-version: Swift version 6.1-dev (LLVM bb83b24e1f07337, Swift c3368800501a32d)
  3 | // swift-module-flags: -target x86_64-unknown-windows-msvc -disable-objc-interop -enable-library-evolution -module-link-name Testing -swift-version 6 -O -enable-experimental-feature AccessLevelOnImport -enable-upcoming-feature ExistentialAny -enable-upcoming-feature InternalImportsByDefault -module-name Testing
  4 | public import Swift
    | `- error: failed to verify module interface of 'Testing' due to the errors above; the textual interface may be broken by project issues or a compiler bug

  5 | public import _Concurrency

  6 | public import _StringProcessing

<unknown>:0: error: verify-module-interface command failed with exit code 1 (use -v to see invocation)

This makes no sense because failureBreakpointValue does not have @_optimize(none) on it, it has @exclusivity(unchecked). This issue appears to only affect Windows builds so far.

CI compiler info:

  2 | // swift-compiler-version: Swift version 6.1-dev (LLVM fb5d83939c06f77, Swift a3589c0f0595cad)
  3 | // swift-module-flags: -target x86_64-unknown-windows-msvc -disable-objc-interop -enable-library-evolution -module-link-name Testing -swift-version 6 -O -enable-experimental-feature AccessLevelOnImport -enable-upcoming-feature ExistentialAny -enable-upcoming-feature InternalImportsByDefault -module-name Testing

[stmontgomery]

We've gotten reports now that this is affecting Android and FreeBSD, and in some cases Linux (rdar://151357567).

[stmontgomery]

There's a recent build log here: https://ci.swift.org/job/swift-bootstrap-ubuntu-24_04-x86_64/142/console

In that log, I notice

warning: using (deprecated) legacy driver, Swift installation does not contain swift-driver at: '/home/build-user/stages/stage2/usr/bin/swift-driver-new'

I have wondered if perhaps the legacy Swift driver is related to this issue, because we recently saw a similar issue with parsing macro declarations from a .swiftinterface (#81346) and that only reproduced when using the legacy driver.

[stmontgomery]

Minimal reproducer:

> cat test.swift
@exclusivity(unchecked)
@usableFromInline nonisolated(unsafe) var failureBreakpointValue = 0

> cat test2.swift
// Intentionally empty.

> swiftc --version
Apple Swift version 6.1 (swift-6.1-RELEASE)

> swiftc -disallow-use-new-driver -v -emit-module -emit-module-interface-path MyMod.swiftinterface -enable-library-evolution -module-name MyMod test.swift test2.swift

...

MyMod.swiftinterface:10:2: error: '@_optimize(none)' attribute cannot be applied to stored properties
 8 | import _SwiftConcurrencyShims
 9 | @usableFromInline
10 | @_optimize(none) nonisolated(unsafe) internal var failureBreakpointValue: Swift.Int
   |  `- error: '@_optimize(none)' attribute cannot be applied to stored properties
11 | 

MyMod.swiftinterface:1:1: error: failed to verify module interface of 'MyMod' due to the errors above; the textual interface may be broken by project issues or a compiler bug
 1 | // swift-interface-format-version: 1.0
   | `- error: failed to verify module interface of 'MyMod' due to the errors above; the textual interface may be broken by project issues or a compiler bug
 2 | // swift-compiler-version: Apple Swift version 6.1 (swift-6.1-RELEASE)
 3 | // swift-module-flags: -target arm64-apple-darwin24.2.0 -enable-objc-interop -enable-library-evolution -module-name MyMod

[tshortli]

This is not a textual interfaces bug but rather a bug with -merge-modules (which is a deprecated compilation mode that is only supported by the legacy Swift driver) causing the AST to be incorrect. I think the recommendation here is to adopt -wmo if use of the legacy Swift driver can't be avoided. I don't think fixing bugs in -merge-modules is something we want to do unless absolutely necessary, but I defer to @xymus on that.