Make Attachment always conform to CustomStringConvertible. (#1367)

This PR ensures that `Attachment` conforms to `CustomStringConvertible`
even if the value it wraps is non-copyable. We accomplish this by
creating a new helper function that is able to box an arbitrary
`~Copyable` value in `Any` if its type is actually `Copyable`. (This
requires a newer runtime on Apple platforms, so we fall back to a
generic (`~Copyable`) implementation on older macOS/iOS/etc.) We then
call this helper function in `Attachment.description` and if it returns
a value, we can stringify it.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Package.swift

@@ -406,6 +406,7 @@ extension Array where Element == PackageDescription.SwiftSetting {
       .enableExperimentalFeature("AvailabilityMacro=_regexAPI:macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0"),
       .enableExperimentalFeature("AvailabilityMacro=_swiftVersionAPI:macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0"),
       .enableExperimentalFeature("AvailabilityMacro=_typedThrowsAPI:macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0"),
+      .enableExperimentalFeature("AvailabilityMacro=_castingWithNonCopyableGenerics:macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0"),
 
       .enableExperimentalFeature("AvailabilityMacro=_distantFuture:macOS 99.0, iOS 99.0, watchOS 99.0, tvOS 99.0, visionOS 99.0"),
     ]

Sources/Testing/Attachments/Attachment.swift

@@ -193,21 +193,17 @@ public struct AnyAttachable: AttachableWrapper, Sendable, Copyable {
 
 // MARK: - Describing an attachment
 
-extension Attachment where AttachableValue: ~Copyable {
-  @_documentation(visibility: private)
-  public var description: String {
-    let typeInfo = TypeInfo(describing: AttachableValue.self)
-    return #""\#(preferredName)": instance of '\#(typeInfo.unqualifiedName)'"#
-  }
-}
-
-extension Attachment: CustomStringConvertible {
+extension Attachment: CustomStringConvertible where AttachableValue: ~Copyable {
   /// @Metadata {
   ///   @Available(Swift, introduced: 6.2)
   ///   @Available(Xcode, introduced: 26.0)
   /// }
   public var description: String {
-    #""\#(preferredName)": \#(String(describingForTest: attachableValue))"#
+    if #available(_castingWithNonCopyableGenerics, *), let attachableValue = boxCopyableValue(attachableValue) {
+      return #""\#(preferredName)": \#(String(describingForTest: attachableValue))"#
+    }
+    let typeInfo = TypeInfo(describing: AttachableValue.self)
+    return #""\#(preferredName)": instance of '\#(typeInfo.unqualifiedName)'"#
   }
 }
 

Sources/Testing/CMakeLists.txt

@@ -76,6 +76,7 @@ add_library(Testing
   Support/Additions/ArrayAdditions.swift
   Support/Additions/CollectionDifferenceAdditions.swift
   Support/Additions/CommandLineAdditions.swift
+  Support/Additions/CopyableAdditions.swift
   Support/Additions/NumericAdditions.swift
   Support/Additions/ResultAdditions.swift
   Support/Additions/TaskAdditions.swift

Sources/Testing/Support/Additions/CopyableAdditions.swift

@@ -0,0 +1,47 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+#if !hasFeature(Embedded)
+/// A helper protocol for ``boxCopyableValue(_:)``.
+private protocol _CopyablePointer {
+  /// Load the value at this address into an existential box.
+  ///
+  /// - Returns: The value at this address.
+  func load() -> Any
+}
+
+extension UnsafePointer: _CopyablePointer where Pointee: Copyable {
+  func load() -> Any {
+    pointee
+  }
+}
+#endif
+
+/// Copy a value to an existential box if its type conforms to `Copyable`.
+///
+/// - Parameters:
+///   - value: The value to copy.
+///
+/// - Returns: A copy of `value` in an existential box, or `nil` if the type of
+///   `value` does not conform to `Copyable`.
+///
+/// When using Embedded Swift, this function always returns `nil`.
+#if !hasFeature(Embedded)
+@available(_castingWithNonCopyableGenerics, *)
+func boxCopyableValue(_ value: borrowing some ~Copyable) -> Any? {
+  withUnsafePointer(to: value) { address in
+    return (address as? any _CopyablePointer)?.load()
+  }
+}
+#else
+func boxCopyableValue(_ value: borrowing some ~Copyable) -> Void? {
+  nil
+}
+#endif

cmake/modules/shared/AvailabilityDefinitions.cmake

@@ -17,4 +17,5 @@ add_compile_options(
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -define-availability -Xfrontend \"_regexAPI:macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0\">"
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -define-availability -Xfrontend \"_swiftVersionAPI:macOS 13.0, iOS 16.0, tvOS 16.0, watchOS 9.0\">"
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -define-availability -Xfrontend \"_typedThrowsAPI:macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0\">"
+  "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -define-availability -Xfrontend \"_castingWithNonCopyableGenerics:macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0\">"
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -define-availability -Xfrontend \"_distantFuture:macOS 99.0, iOS 99.0, watchOS 99.0, tvOS 99.0, visionOS 99.0\">")