Work around Windows build failure due to SwiftPM regression.

Works around a link-time bug in SwiftPM that appears to have been introduced in
swiftlang/swift-package-manager#8049.

Author: grynspan

Package.swift

@@ -128,6 +128,8 @@ extension Array where Element == PackageDescription.SwiftSetting {
       .enableExperimentalFeature("AccessLevelOnImport"),
       .enableUpcomingFeature("InternalImportsByDefault"),
 
+      .enableExperimentalFeature("SymbolLinkageMarkers"),
+
       .define("SWT_TARGET_OS_APPLE", .when(platforms: [.macOS, .iOS, .macCatalyst, .watchOS, .tvOS, .visionOS])),
 
       .define("SWT_NO_EXIT_TESTS", .when(platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android])),

Sources/Testing/ABI/EntryPoints/ABIEntryPoint.swift

@@ -60,6 +60,9 @@ extension ABIv0 {
 ///
 /// - Returns: The value of ``ABIv0/entryPoint-swift.type.property`` cast to an
 ///   untyped pointer.
+#if hasFeature(SymbolLinkageMarkers)
+@_used
+#endif
 @_cdecl("swt_abiv0_getEntryPoint")
 @usableFromInline func abiv0_getEntryPoint() -> UnsafeRawPointer {
   unsafeBitCast(ABIv0.entryPoint, to: UnsafeRawPointer.self)
@@ -82,6 +85,9 @@ typealias ABIEntryPoint_v0 = @Sendable (
 /// Xcode 16 Beta 1.
 ///
 /// This function will be removed in a future update.
+#if hasFeature(SymbolLinkageMarkers)
+@_used
+#endif
 @available(*, deprecated, message: "Use ABIv0.entryPoint (swt_abiv0_getEntryPoint()) instead.")
 @_cdecl("swt_copyABIEntryPoint_v0")
 @usableFromInline func copyABIEntryPoint_v0() -> UnsafeMutableRawPointer {

cmake/modules/shared/CompilerSettings.cmake

@@ -15,6 +15,8 @@ add_compile_options(
 add_compile_options(
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend ExistentialAny>"
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend InternalImportsByDefault>")
+add_compile_options(
+  "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-experimental-feature -Xfrontend SymbolLinkageMarkers>")
 add_compile_options(
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-package-name swift_testing>")