Reimplement _TestingInterop in C++.

Sources/Testing/Events/Event+FallbackEventHandler.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-private import _TestingInternals
+private import _TestingInternals.InteropOnly
 
 extension Event {
   /// Attempt to handle an event encoded as JSON as if it had been generated in

Sources/_TestingInternals/include/FallbackEventHandler.h

@@ -0,0 +1,52 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 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 !defined(SWT_NO_INTEROP)
+#include "Defines.h"
+#include "Includes.h"
+
+SWT_ASSUME_NONNULL_BEGIN
+
+/// 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 (* SWTFallbackEventHandler)(const char *recordJSONSchemaVersionNumber,
+                                      const void *recordJSONBaseAddress,
+                                      size_t recordJSONByteCount,
+                                      const void *_Nullable reserved);
+
+/// Set the current fallback event handler if one has not already been set.
+///
+/// - Parameters:
+///   - handler: The handler function to set.
+///
+/// - Returns: Whether or not `handler` was installed.
+///
+/// The fallback event handler can only be installed once per process, typically
+/// by the first testing library to run. If this function has already been
+/// called and the handler set, it does not replace the previous handler.
+SWT_EXTERN bool _swift_testing_installFallbackEventHandler(SWTFallbackEventHandler handler);
+
+/// 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 SWTFallbackEventHandler _Nullable _swift_testing_getFallbackEventHandler(void);
+
+SWT_ASSUME_NONNULL_END
+
+#endif

Sources/_TestingInternals/include/Stubs.h

@@ -275,43 +275,6 @@ static const char *_Nullable swt_getExitCodeName(int exitCode) {
 #undef SWT_SYSEXIT_CODE
 };
 
-#if !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 (* SWTFallbackEventHandler)(const char *recordJSONSchemaVersionNumber,
-                                      const void *recordJSONBaseAddress,
-                                      size_t recordJSONByteCount,
-                                      const void *_Nullable reserved);
-
-/// Set the current fallback event handler if one has not already been set.
-///
-/// - Parameters:
-///   - handler: The handler function to set.
-///
-/// - Returns: Whether or not `handler` was installed.
-///
-/// The fallback event handler can only be installed once per process, typically
-/// by the first testing library to run. If this function has already been
-/// called and the handler set, it does not replace the previous handler.
-SWT_EXTERN bool _swift_testing_installFallbackEventHandler(SWTFallbackEventHandler handler);
-
-/// 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 SWTFallbackEventHandler _Nullable _swift_testing_getFallbackEventHandler(void);
-
-#endif
-
 SWT_ASSUME_NONNULL_END
 
 #endif

Sources/_TestingInternals/include/module.modulemap

@@ -16,4 +16,9 @@ module _TestingInternals {
     header "Stubs.h"
     export *
   }
+
+  explicit module InteropOnly {
+    header "FallbackEventHandler.h"
+    export *
+  }
 }

Sources/_TestingInterop/CMakeLists.txt

@@ -8,10 +8,19 @@
 
 include(ModuleABIName)
 add_library(_TestingInterop
-  FallbackEventHandler.swift)
+  FallbackEventHandler.cpp)
+if("${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "MSVC" OR
+   "${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
+  target_compile_options(_TestingInternals PRIVATE
+    /EHa-c)
+elseif(CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")
+  target_compile_options(_TestingInternals PRIVATE
+    -fno-exceptions -fPIC)
+else()
+  target_compile_options(_TestingInternals PRIVATE
+    -fno-exceptions)
+endif()
 
-target_link_libraries(_TestingInterop PRIVATE
-  _TestingInternals)
 if(NOT BUILD_SHARED_LIBS)
   # When building a static library, tell clients to autolink the internal
   # libraries.

Sources/_TestingInterop/FallbackEventHandler.cpp

@@ -0,0 +1,27 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025–2026 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 !defined(SWT_NO_INTEROP)
+#include "../_TestingInternals/include/FallbackEventHandler.h"
+
+#include <atomic>
+
+/// Storage for the fallback event handler.
+static std::atomic<SWTFallbackEventHandler> fallbackEventHandler { nullptr };
+
+bool _swift_testing_installFallbackEventHandler(SWTFallbackEventHandler handler) {
+  SWTFallbackEventHandler nullptrValue = nullptr;
+  return fallbackEventHandler.compare_exchange_strong(nullptrValue, handler, std::memory_order_seq_cst, std::memory_order_relaxed);
+}
+
+SWTFallbackEventHandler _swift_testing_getFallbackEventHandler(void) {
+  return fallbackEventHandler.load(std::memory_order_seq_cst);
+}
+#endif

Sources/_TestingInterop/include/Empty.h

@@ -0,0 +1 @@
+

Tests/TestingTests/EventHandlingInteropTests.swift

@@ -9,7 +9,7 @@
 //
 
 @testable @_spi(ForToolsIntegrationOnly) import Testing
-private import _TestingInternals
+private import _TestingInternals.InteropOnly
 
 #if canImport(Foundation)
 import Foundation