Use a dedicated SWT_NO_ condition that DocC doesn't set

Author: grynspan

Package.swift

@@ -383,6 +383,7 @@ extension Array where Element == PackageDescription.SwiftSetting {
       .define("SWT_NO_DYNAMIC_LINKING", .whenEmbedded(or: .when(platforms: [.wasi]))),
       .define("SWT_NO_PIPES", .whenEmbedded(or: .when(platforms: [.wasi]))),
       .define("SWT_NO_FOUNDATION_FILE_COORDINATION", .whenEmbedded(or: .whenApple(false))),
+      .define("SWT_NO_IMAGE_ATTACHMENTS", .whenEmbedded(or: .when(platforms: [.linux, .custom("freebsd"), .openbsd, .wasi, .android]))),
 
       .define("SWT_NO_LEGACY_TEST_DISCOVERY", .whenEmbedded()),
       .define("SWT_NO_LIBDISPATCH", .whenEmbedded()),

Sources/Testing/Attachments/Images/AttachableImageFormat.swift

@@ -31,7 +31,7 @@
 /// @Metadata {
 ///   @Available(Swift, introduced: 6.3)
 /// }
-#if !(SWT_TARGET_OS_APPLE || os(Windows))
+#if SWT_NO_IMAGE_ATTACHMENTS
 @available(*, unavailable, message: "Image attachments are not available on this platform.")
 #endif
 @available(_uttypesAPI, *)
@@ -80,7 +80,7 @@ public struct AttachableImageFormat: Sendable {
 
 // MARK: -
 
-#if !(SWT_TARGET_OS_APPLE || os(Windows))
+#if SWT_NO_IMAGE_ATTACHMENTS
 @available(*, unavailable, message: "Image attachments are not available on this platform.")
 #endif
 @available(_uttypesAPI, *)

Sources/Testing/Attachments/Images/Attachment+_AttachableAsImage.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-#if !(SWT_TARGET_OS_APPLE || os(Windows))
+#if SWT_NO_IMAGE_ATTACHMENTS
 @available(*, unavailable, message: "Image attachments are not available on this platform.")
 #endif
 @available(_uttypesAPI, *)
@@ -104,7 +104,7 @@ extension Attachment {
 // MARK: -
 
 @_spi(Experimental) // STOP: not part of ST-0014
-#if !(SWT_TARGET_OS_APPLE || os(Windows))
+#if SWT_NO_IMAGE_ATTACHMENTS
 @available(*, unavailable, message: "Image attachments are not available on this platform.")
 #endif
 @available(_uttypesAPI, *)

Sources/Testing/Attachments/Images/ImageAttachmentError.swift

@@ -11,7 +11,7 @@
 private import _TestingInternals
 
 /// A type representing an error that can occur when attaching an image.
-#if !(SWT_TARGET_OS_APPLE || os(Windows))
+#if SWT_NO_IMAGE_ATTACHMENTS
 @available(*, unavailable, message: "Image attachments are not available on this platform.")
 #endif
 package enum ImageAttachmentError: Error {
@@ -42,7 +42,7 @@ package enum ImageAttachmentError: Error {
 #endif
 }
 
-#if !(SWT_TARGET_OS_APPLE || os(Windows))
+#if SWT_NO_IMAGE_ATTACHMENTS
 @available(*, unavailable, message: "Image attachments are not available on this platform.")
 #endif
 extension ImageAttachmentError: CustomStringConvertible {

Sources/Testing/Attachments/Images/_AttachableAsImage.swift

@@ -8,6 +8,14 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
+#if SWT_TARGET_OS_APPLE
+// Image attachments on Apple platforms conform to AttachableAsCGImage.
+#elseif os(Windows)
+// Image attachments on Windows platforms conform to AttachableAsIWICBitmapSource.
+#elseif !SWT_NO_IMAGE_ATTACHMENTS
+#error("Platform-specific misconfiguration: support for image attachments requires a platform-specific implementation")
+#endif
+
 /// A protocol describing images that can be converted to instances of
 /// [`Attachment`](https://developer.apple.com/documentation/testing/attachment).
 ///
@@ -18,7 +26,7 @@
 ///   A future Swift Evolution proposal will promote this protocol to API so
 ///   that we don't need to underscore its name.
 /// }
-#if !(SWT_TARGET_OS_APPLE || os(Windows))
+#if SWT_NO_IMAGE_ATTACHMENTS
 @available(*, unavailable, message: "Image attachments are not available on this platform.")
 #endif
 @available(_uttypesAPI, *)

Sources/Testing/Attachments/Images/_AttachableImageWrapper.swift

@@ -20,7 +20,7 @@
 /// @Comment {
 /// | Windows | [`HBITMAP`](https://learn.microsoft.com/en-us/windows/win32/gdi/bitmaps), [`HICON`](https://learn.microsoft.com/en-us/windows/win32/menurc/icons), [`IWICBitmapSource`](https://learn.microsoft.com/en-us/windows/win32/api/wincodec/nn-wincodec-iwicbitmapsource) (including its subclasses declared by Windows Imaging Component) |
 /// }
-#if !(SWT_TARGET_OS_APPLE || os(Windows))
+#if SWT_NO_IMAGE_ATTACHMENTS
 @available(*, unavailable, message: "Image attachments are not available on this platform.")
 #endif
 @available(_uttypesAPI, *)
@@ -41,7 +41,7 @@ public final class _AttachableImageWrapper<Image>: Sendable where Image: _Attach
   }
 }
 
-#if !(SWT_TARGET_OS_APPLE || os(Windows))
+#if SWT_NO_IMAGE_ATTACHMENTS
 @available(*, unavailable, message: "Image attachments are not available on this platform.")
 #endif
 @available(_uttypesAPI, *)