Create helper to post test events to fallback event handler

As part of the strategy for enabling interoperability, unhandled issues
can be turned into events and sent to the fallback event handler.

Also link against the _TestingInterop framework/library (depending on
the platform), which provides a function to lookup the currently
installed fallback event handler.

Author: jerryjrchen

Package.swift

@@ -142,9 +142,20 @@ let package = Package(
       exclude: ["CMakeLists.txt", "Testing.swiftcrossimport"],
       cxxSettings: .packageSettings,
       swiftSettings: .packageSettings + .enableLibraryEvolution(),
-      linkerSettings: [
-        .linkedLibrary("execinfo", .when(platforms: [.custom("freebsd"), .openbsd]))
-      ]
+      linkerSettings: {
+        var result = [LinkerSetting]()
+        result += [
+          .linkedLibrary("execinfo", .when(platforms: [.custom("freebsd"), .openbsd]))
+        ]
+#if compiler(>=6.3)
+        result += [
+          .linkedFramework("_TestingInterop", .whenApple()),
+          .linkedLibrary("_TestingInterop", .whenApple(false)),
+        ]
+#endif
+
+        return result
+      }()
     ),
     .testTarget(
       name: "TestingTests",

Sources/Testing/CMakeLists.txt

@@ -31,6 +31,7 @@ add_library(Testing
   Attachments/Attachment.swift
   Events/Clock.swift
   Events/Event.swift
+  Events/Event+FallbackHandler.swift
   Events/Recorder/Event.AdvancedConsoleOutputRecorder.swift
   Events/Recorder/Event.ConsoleOutputRecorder.swift
   Events/Recorder/Event.HumanReadableOutputRecorder.swift

Sources/Testing/Events/Event+FallbackHandler.swift

@@ -0,0 +1,48 @@
+//
+// 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
+//
+
+private import _TestingInternals
+
+extension Event {
+  private static let _fallbackEventHandler: FallbackEventHandler? = {
+      _swift_testing_getFallbackEventHandler()
+  }()
+
+  /// Post this event to the currently-installed fallback event handler.
+  ///
+  /// - Parameters:
+  ///   - context: The context associated with this event.
+  ///
+  /// - Returns: Whether or not the fallback event handler was invoked. If the
+  ///   currently-installed handler belongs to the testing library, returns
+  ///   `false`.
+  borrowing func postToFallbackHandler(in context: borrowing Context) -> Bool {
+#if canImport(_TestingInterop)
+      guard let fallbackEventHandler = Self._fallbackEventHandler else {
+        return false
+      }
+
+      // Encode the event as JSON and pass it to the handler.
+      let encodeAndInvoke = ABI.CurrentVersion.eventHandler(encodeAsJSONLines: false) {
+        recordJSON in
+        fallbackEventHandler(
+          String(describing: ABI.CurrentVersion.versionNumber),
+          recordJSON.baseAddress!,
+          recordJSON.count,
+          nil
+        )
+      }
+      encodeAndInvoke(self, context)
+      return true
+#else
+      return false
+#endif
+  }
+}

Sources/_TestingInternals/include/Stubs.h

@@ -180,6 +180,32 @@ static int swt_setfdflags(int fd, int flags) {
 }
 #endif
 
+// __SWIFT_COMPILER_VERSION is a packed value. The comparison below is >= 6.3
+#if __SWIFT_COMPILER_VERSION >= 6003000000000 && !SWT_NO_INTEROP
+
+/// A type describing a fallback event handler that testing API can invoke as an
+/// alternate method of reporting test events to the current test runner.
+/// Shadows the type with the same name in _TestingInterop.
+///
+/// - Parameters:
+///   - recordJSONSchemaVersionNumber: The JSON schema version used to encode
+///     the event record.
+///   - recordJSONBaseAddress: A pointer to the first byte of the encoded event.
+///   - recordJSONByteCount: The size of the encoded event in bytes.
+///   - reserved: Reserved for future use.
+typedef void (* FallbackEventHandler)(const char *recordJSONSchemaVersionNumber,
+                                      const void *recordJSONBaseAddress,
+                                      long recordJSONByteCount,
+                                      const void *_Nullable reserved);
+
+/// Get the current fallback event handler.
+/// Shadows the function with the same name in _TestingInterop.
+///
+/// - Returns: The currently-set handler function, if any.
+SWT_EXTERN FallbackEventHandler _Nullable _swift_testing_getFallbackEventHandler(void);
+
+#endif
+
 SWT_ASSUME_NONNULL_END
 
 #endif

Sources/_TestingInterop/FallbackEventHandler.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-#if !SWT_NO_INTEROP
+#if compiler(>=6.3) && !SWT_NO_INTEROP
 #if SWT_TARGET_OS_APPLE && !SWT_NO_OS_UNFAIR_LOCK && !hasFeature(Embedded)
 private import _TestingInternals
 #else