From 1c1e0ecdf6fbaa64c14e858276093f5bddb02332 Mon Sep 17 00:00:00 2001
From: Jonathan Grynspan <jgrynspan@apple.com>
Date: Thu, 9 Jul 2026 15:51:31 -0400
Subject: [PATCH 1/8] Bootstrap a testing harness target.

This PR adds a (stub, unimplemented, incomplete, _etc._) harness executable
target that uses Swift Argument Parser. This target will eventually act as a
host for tests, but for now just throws an error if invoked.
---
 Package.swift                               | 35 ++++++++++++++++-----
 Sources/CMakeLists.txt                      |  1 +
 Sources/Harness/CMakeLists.txt              | 22 +++++++++++++
 Sources/Harness/Harness.swift               | 25 +++++++++++++++
 cmake/modules/shared/CompilerSettings.cmake |  5 +++
 5 files changed, 81 insertions(+), 7 deletions(-)
 create mode 100644 Sources/Harness/CMakeLists.txt
 create mode 100644 Sources/Harness/Harness.swift

diff --git a/Package.swift b/Package.swift
index dff2b7409..7bc0b7b56 100644
--- a/Package.swift
+++ b/Package.swift
@@ -68,29 +68,33 @@ let package = Package(
     var result = [Product]()
 
 #if os(Windows)
-    result.append(
+    result += [
       .library(
         name: "Testing",
         type: .dynamic, // needed so Windows exports ABI entry point symbols
         targets: ["Testing"]
       )
-    )
+    ]
 #else
-    result.append(
+    result += [
       .library(
         name: "Testing",
         targets: ["Testing"]
       )
-    )
+    ]
 #endif
 
-    result.append(
+    result += [
       .library(
         name: "_TestDiscovery",
         type: .static,
         targets: ["_TestDiscovery"]
-      )
-    )
+      ),
+      .executable(
+        name: "swift-testing-harness",
+        targets: ["swift-testing-harness"]
+      ),
+    ]
 
 #if DEBUG
     // Build _TestingInterop for debugging/testing purposes only. It is
@@ -116,6 +120,9 @@ let package = Package(
     // specified semantic version, meaning the most recent "prerelease" tag will
     // always be used.
     .package(url: "https://github.com/swiftlang/swift-syntax.git", from: "604.0.0-latest"),
+
+    // TODO: determine the best version range to depend on here
+    .package(url: "https://github.com/apple/swift-argument-parser.git", .upToNextMajor(from: "1.8.0")),
   ],
 
   targets: [
@@ -289,6 +296,19 @@ let package = Package(
       swiftSettings: .packageSettings() + .enableLibraryEvolution() + .moduleABIName("_Testing_WinSDK")
     ),
 
+    // Testing harness: a process that runs in between a host like SwiftPM and
+    // the actual test process(es) and which manages interactions between them.
+    .executableTarget(
+      name: "swift-testing-harness",
+      dependencies: [
+        "Testing",
+        .product(name: "ArgumentParser", package: "swift-argument-parser"),
+      ],
+      path: "Sources/Harness",
+      exclude: ["CMakeLists.txt"],
+      swiftSettings: .packageSettings()
+    ),
+
     // Utility targets: These are utilities intended for use when developing
     // this package, not for distribution.
     .executableTarget(
@@ -510,6 +530,7 @@ extension Array where Element: _LanguageBuildSetting {
       "SWT_NO_ABI_JSON_SCHEMA": (platforms: .none, embedded: true),
       "SWT_NO_CODABLE": (platforms: .none, embedded: true),
       "SWT_NO_INTEROP": (platforms: .none, embedded: true),
+      "SWT_NO_HARNESS": (platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android], embedded: true),
       "SWT_NO_UNSTRUCTURED_TASKS": (platforms: .none, embedded: true),
       "SWT_NO_GLOBAL_ACTORS": (platforms: .none, embedded: true),
       "SWT_NO_SUSPENDING_CLOCK": (platforms: .none, embedded: true),
diff --git a/Sources/CMakeLists.txt b/Sources/CMakeLists.txt
index 4fc0847b7..87bbcdf11 100644
--- a/Sources/CMakeLists.txt
+++ b/Sources/CMakeLists.txt
@@ -107,4 +107,5 @@ add_subdirectory(_TestDiscovery)
 add_subdirectory(_TestingInterop)
 add_subdirectory(_TestingInternals)
 add_subdirectory(Overlays)
+add_subdirectory(Harness)
 add_subdirectory(Testing)
diff --git a/Sources/Harness/CMakeLists.txt b/Sources/Harness/CMakeLists.txt
new file mode 100644
index 000000000..c15eb41cf
--- /dev/null
+++ b/Sources/Harness/CMakeLists.txt
@@ -0,0 +1,22 @@
+# 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(NOT (CMAKE_SYSTEM_NAME IN_LIST SWT_NO_HARNESS_LIST))
+  include(ModuleABIName)
+
+  find_package(ArgumentParser CONFIG REQUIRED)
+
+  add_executable(swift-testing-harness
+    Harness.swift)
+  target_link_libraries(swift-test PRIVATE
+    ArgumentParser
+    Testing)
+
+  install(TARGETS swift-testing-harness
+    RUNTIME DESTINATION "libexec/swift/pm")
+endif()
diff --git a/Sources/Harness/Harness.swift b/Sources/Harness/Harness.swift
new file mode 100644
index 000000000..c3f019f45
--- /dev/null
+++ b/Sources/Harness/Harness.swift
@@ -0,0 +1,25 @@
+//
+// 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 !SWT_NO_HARNESS
+@_spi(ForToolsIntegrationOnly) private import Testing
+private import ArgumentParser
+
+/// The harness' main command (i.e. its entry point).
+@main struct Harness: Sendable, AsyncParsableCommand {
+  fileprivate static let configuration = CommandConfiguration(
+    commandName: "swift-testing-harness"
+  )
+
+  mutating func run() async throws {
+    throw ValidationError("This tool is currently unimplemented.")
+  }
+}
+#endif
diff --git a/cmake/modules/shared/CompilerSettings.cmake b/cmake/modules/shared/CompilerSettings.cmake
index 480e45f18..2ec58f8d4 100644
--- a/cmake/modules/shared/CompilerSettings.cmake
+++ b/cmake/modules/shared/CompilerSettings.cmake
@@ -64,3 +64,8 @@ if((NOT BUILD_SHARED_LIBS) AND (NOT CMAKE_SYSTEM_NAME STREQUAL "WASI"))
   # When building a static library, Interop is not supported at this time
   add_compile_definitions("SWT_NO_INTEROP")
 endif()
+
+set(SWT_NO_HARNESS_LIST "iOS" "watchOS" "tvOS" "visionOS" "WASI" "Android")
+if(CMAKE_SYSTEM_NAME IN_LIST SWT_NO_HARNESS_LIST)
+  add_compile_definitions("SWT_NO_HARNESS")
+endif()

From 39fd7983fe7305f9daa4036643fa5ade0171a029 Mon Sep 17 00:00:00 2001
From: Jonathan Grynspan <jgrynspan@apple.com>
Date: Thu, 9 Jul 2026 16:05:27 -0400
Subject: [PATCH 2/8] Nominally build the package target for unsupported
 platforms (but not CMake)

---
 Sources/Harness/Harness.swift | 2 --
 1 file changed, 2 deletions(-)

diff --git a/Sources/Harness/Harness.swift b/Sources/Harness/Harness.swift
index c3f019f45..a1d3e3dd4 100644
--- a/Sources/Harness/Harness.swift
+++ b/Sources/Harness/Harness.swift
@@ -8,7 +8,6 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-#if !SWT_NO_HARNESS
 @_spi(ForToolsIntegrationOnly) private import Testing
 private import ArgumentParser
 
@@ -22,4 +21,3 @@ private import ArgumentParser
     throw ValidationError("This tool is currently unimplemented.")
   }
 }
-#endif

From 88568b9d464084e81a0cc809e11ca3d8941f964d Mon Sep 17 00:00:00 2001
From: Jonathan Grynspan <jgrynspan@apple.com>
Date: Thu, 9 Jul 2026 17:01:07 -0400
Subject: [PATCH 3/8] Fix typo

---
 Sources/Harness/CMakeLists.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Sources/Harness/CMakeLists.txt b/Sources/Harness/CMakeLists.txt
index c15eb41cf..2fb4492bb 100644
--- a/Sources/Harness/CMakeLists.txt
+++ b/Sources/Harness/CMakeLists.txt
@@ -13,7 +13,7 @@ if(NOT (CMAKE_SYSTEM_NAME IN_LIST SWT_NO_HARNESS_LIST))
 
   add_executable(swift-testing-harness
     Harness.swift)
-  target_link_libraries(swift-test PRIVATE
+  target_link_libraries(swift-testing-harness PRIVATE
     ArgumentParser
     Testing)
 

From ed880e2685ca5fd22dbdbfabf05490f92c776ff9 Mon Sep 17 00:00:00 2001
From: Jonathan Grynspan <jgrynspan@apple.com>
Date: Fri, 10 Jul 2026 10:24:45 -0400
Subject: [PATCH 4/8] Fetch ArgumentParser if the package is not found by CMake
 (is this completely correct? do we need to bootstrap something?)

---
 Package.swift                  |  7 +++++--
 Sources/Harness/CMakeLists.txt | 11 +++++++++--
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/Package.swift b/Package.swift
index 7bc0b7b56..83a9109e7 100644
--- a/Package.swift
+++ b/Package.swift
@@ -121,8 +121,11 @@ let package = Package(
     // always be used.
     .package(url: "https://github.com/swiftlang/swift-syntax.git", from: "604.0.0-latest"),
 
-    // TODO: determine the best version range to depend on here
-    .package(url: "https://github.com/apple/swift-argument-parser.git", .upToNextMajor(from: "1.8.0")),
+    // Add a dependency on Swift Argument Parser. Note that the rest of the
+    // toolchain (as of this writing) uses 1.5.x, so we match that; if the
+    // toolchain updates its dependency, please update the version number here
+    // and in Sources/Harness/CMakeLists.txt.
+    .package(url: "https://github.com/apple/swift-argument-parser.git", .upToNextMajor(from: "1.5.0")),
   ],
 
   targets: [
diff --git a/Sources/Harness/CMakeLists.txt b/Sources/Harness/CMakeLists.txt
index 2fb4492bb..cfb39eeba 100644
--- a/Sources/Harness/CMakeLists.txt
+++ b/Sources/Harness/CMakeLists.txt
@@ -9,7 +9,14 @@
 if(NOT (CMAKE_SYSTEM_NAME IN_LIST SWT_NO_HARNESS_LIST))
   include(ModuleABIName)
 
-  find_package(ArgumentParser CONFIG REQUIRED)
+  find_package(ArgumentParser CONFIG)
+  if(NOT ArgumentParser_FOUND)
+    message("-- Vending swift-argument-parser")
+    FetchContent_Declare(ArgumentParser
+      GIT_REPOSITORY https://github.com/apple/swift-argument-parser
+      GIT_TAG 1.5.1)
+    FetchContent_MakeAvailable(ArgumentParser)
+  endif()
 
   add_executable(swift-testing-harness
     Harness.swift)
@@ -18,5 +25,5 @@ if(NOT (CMAKE_SYSTEM_NAME IN_LIST SWT_NO_HARNESS_LIST))
     Testing)
 
   install(TARGETS swift-testing-harness
-    RUNTIME DESTINATION "libexec/swift/pm")
+    RUNTIME DESTINATION "libexec/swift/testing")
 endif()

From 22a623098003f4d978c2237ba0e72227eba16cf0 Mon Sep 17 00:00:00 2001
From: Jonathan Grynspan <jgrynspan@apple.com>
Date: Fri, 10 Jul 2026 11:01:33 -0400
Subject: [PATCH 5/8] Explicitly include FetchContent module

---
 Sources/Harness/CMakeLists.txt | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Sources/Harness/CMakeLists.txt b/Sources/Harness/CMakeLists.txt
index cfb39eeba..2d9699a6d 100644
--- a/Sources/Harness/CMakeLists.txt
+++ b/Sources/Harness/CMakeLists.txt
@@ -7,6 +7,7 @@
 # See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 
 if(NOT (CMAKE_SYSTEM_NAME IN_LIST SWT_NO_HARNESS_LIST))
+  include(FetchContent)
   include(ModuleABIName)
 
   find_package(ArgumentParser CONFIG)

From 932b2aeb8b6d70097bf0f146740d5af8862dc4d4 Mon Sep 17 00:00:00 2001
From: Jonathan Grynspan <jgrynspan@apple.com>
Date: Fri, 10 Jul 2026 12:57:09 -0400
Subject: [PATCH 6/8] FetchContent fails in CI, okay then

---
 Sources/Harness/CMakeLists.txt | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/Sources/Harness/CMakeLists.txt b/Sources/Harness/CMakeLists.txt
index 2d9699a6d..9ab0606f8 100644
--- a/Sources/Harness/CMakeLists.txt
+++ b/Sources/Harness/CMakeLists.txt
@@ -7,17 +7,9 @@
 # See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 
 if(NOT (CMAKE_SYSTEM_NAME IN_LIST SWT_NO_HARNESS_LIST))
-  include(FetchContent)
   include(ModuleABIName)
 
-  find_package(ArgumentParser CONFIG)
-  if(NOT ArgumentParser_FOUND)
-    message("-- Vending swift-argument-parser")
-    FetchContent_Declare(ArgumentParser
-      GIT_REPOSITORY https://github.com/apple/swift-argument-parser
-      GIT_TAG 1.5.1)
-    FetchContent_MakeAvailable(ArgumentParser)
-  endif()
+  find_package(ArgumentParser CONFIG REQUIRED)
 
   add_executable(swift-testing-harness
     Harness.swift)

From 2fd68a9d0f531b3bc3f0f266431f21e510d76899 Mon Sep 17 00:00:00 2001
From: Jonathan Grynspan <jgrynspan@apple.com>
Date: Fri, 10 Jul 2026 14:50:02 -0400
Subject: [PATCH 7/8] Don't build with CMake, we'll build as a package instead
 after SwiftPM

---
 Package.swift                  | 10 ++++++++--
 Sources/CMakeLists.txt         |  1 -
 Sources/Harness/CMakeLists.txt | 22 ----------------------
 3 files changed, 8 insertions(+), 25 deletions(-)
 delete mode 100644 Sources/Harness/CMakeLists.txt

diff --git a/Package.swift b/Package.swift
index 83a9109e7..ae55af20e 100644
--- a/Package.swift
+++ b/Package.swift
@@ -20,6 +20,10 @@ let git = Context.gitInformation
 /// distribution as a package dependency.
 let buildingForDevelopment = (git?.currentTag == nil)
 
+/// Whether or not to use package dependencies checked out next to the testing
+/// library's package directory instead of downloading them on demand.
+let useLocalDependencies = Context.environment["SWIFTCI_USE_LOCAL_DEPS"] != nil
+
 /// Whether or not this package is being built for Embedded Swift.
 ///
 /// This value is `true` if `SWT_EMBEDDED` is set in the environment to `true`
@@ -110,7 +114,10 @@ let package = Package(
     return result
   }(),
 
-  dependencies: [
+  dependencies: useLocalDependencies ? [
+    .package(path: "../swift-syntax"),
+    .package(path: "../swift-argument-parser"),
+  ] : [
     // swift-syntax periodically publishes a new tag with a suffix of the format
     // "-prerelease-YYYY-MM-DD". We always want to use the most recent tag
     // associated with a particular Swift version, without needing to hardcode
@@ -308,7 +315,6 @@ let package = Package(
         .product(name: "ArgumentParser", package: "swift-argument-parser"),
       ],
       path: "Sources/Harness",
-      exclude: ["CMakeLists.txt"],
       swiftSettings: .packageSettings()
     ),
 
diff --git a/Sources/CMakeLists.txt b/Sources/CMakeLists.txt
index 87bbcdf11..4fc0847b7 100644
--- a/Sources/CMakeLists.txt
+++ b/Sources/CMakeLists.txt
@@ -107,5 +107,4 @@ add_subdirectory(_TestDiscovery)
 add_subdirectory(_TestingInterop)
 add_subdirectory(_TestingInternals)
 add_subdirectory(Overlays)
-add_subdirectory(Harness)
 add_subdirectory(Testing)
diff --git a/Sources/Harness/CMakeLists.txt b/Sources/Harness/CMakeLists.txt
deleted file mode 100644
index 9ab0606f8..000000000
--- a/Sources/Harness/CMakeLists.txt
+++ /dev/null
@@ -1,22 +0,0 @@
-# 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(NOT (CMAKE_SYSTEM_NAME IN_LIST SWT_NO_HARNESS_LIST))
-  include(ModuleABIName)
-
-  find_package(ArgumentParser CONFIG REQUIRED)
-
-  add_executable(swift-testing-harness
-    Harness.swift)
-  target_link_libraries(swift-testing-harness PRIVATE
-    ArgumentParser
-    Testing)
-
-  install(TARGETS swift-testing-harness
-    RUNTIME DESTINATION "libexec/swift/testing")
-endif()

From 601a828af2538052aa6cca0b8eff2be95a1d0fea Mon Sep 17 00:00:00 2001
From: Jonathan Grynspan <jgrynspan@apple.com>
Date: Fri, 10 Jul 2026 14:54:46 -0400
Subject: [PATCH 8/8] Put common case first in ternary operation in
 Package.swift

---
 Package.swift | 51 ++++++++++++++++++++++-----------------------------
 1 file changed, 22 insertions(+), 29 deletions(-)

diff --git a/Package.swift b/Package.swift
index ae55af20e..c8dcfec7d 100644
--- a/Package.swift
+++ b/Package.swift
@@ -43,31 +43,24 @@ let buildingForEmbedded: Bool = {
 let package = Package(
   name: "swift-testing",
 
-  platforms: {
-    if !buildingForEmbedded {
-      [
-        .macOS(.v14),
-        .iOS(.v17),
-        .watchOS(.v10),
-        .tvOS(.v17),
-        .macCatalyst(.v17),
-        .visionOS(.v1),
-      ]
-    } else {
-      // Open-source main-branch toolchains (currently required to build this
-      // package for Embedded Swift) have higher Apple platform deployment
-      // targets than we would otherwise require.
-      [
-        .macOS(.v14),
-        .iOS(.v18),
-        .watchOS(.v10),
-        .tvOS(.v18),
-        .macCatalyst(.v18),
-        .visionOS(.v1),
-      ]
-    }
-  }(),
-
+  platforms: !buildingForEmbedded ? [
+    .macOS(.v14),
+    .iOS(.v17),
+    .watchOS(.v10),
+    .tvOS(.v17),
+    .macCatalyst(.v17),
+    .visionOS(.v1),
+  ] : [
+    // Open-source main-branch toolchains (currently required to build this
+    // package for Embedded Swift) have higher Apple platform deployment
+    // targets than we would otherwise require.
+    .macOS(.v14),
+    .iOS(.v18),
+    .watchOS(.v10),
+    .tvOS(.v18),
+    .macCatalyst(.v18),
+    .visionOS(.v1),
+  ],
   products: {
     var result = [Product]()
 
@@ -114,10 +107,7 @@ let package = Package(
     return result
   }(),
 
-  dependencies: useLocalDependencies ? [
-    .package(path: "../swift-syntax"),
-    .package(path: "../swift-argument-parser"),
-  ] : [
+  dependencies: !useLocalDependencies ? [
     // swift-syntax periodically publishes a new tag with a suffix of the format
     // "-prerelease-YYYY-MM-DD". We always want to use the most recent tag
     // associated with a particular Swift version, without needing to hardcode
@@ -133,6 +123,9 @@ let package = Package(
     // toolchain updates its dependency, please update the version number here
     // and in Sources/Harness/CMakeLists.txt.
     .package(url: "https://github.com/apple/swift-argument-parser.git", .upToNextMajor(from: "1.5.0")),
+  ] : [
+    .package(path: "../swift-syntax"),
+    .package(path: "../swift-argument-parser"),
   ],
 
   targets: [