Support building macro plugin as a library instead of executable via CMake (#517)

This adds support in the CMake rules for building the `TestingMacros`
macro plugin target as a library instead of an executable.

### Motivation:

In preparation for adding Swift Testing to Swift.org toolchains (as
[mentioned](https://github.com/swiftlang/swift-evolution/blob/main/visions/swift-testing.md#distribution)
in the vision document), this modifies the project's CMake rules to
allow building its macro plugin in a way that is consistent with others
in the toolchain — as a shared library rather than an executable. Macros
in the toolchain can be libraries which are directly loaded by the
compiler and avoid the IPC overhead of being a separate executable.

### 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: stmontgomery

Package.swift

@@ -68,6 +68,10 @@ let package = Package(
       ],
       exclude: ["CMakeLists.txt"],
       swiftSettings: .packageSettings + [
+        // When building as a package, the macro plugin always builds as an
+        // executable rather than a library.
+        .define("SWT_NO_LIBRARY_MACRO_PLUGINS"),
+
         // The only target which needs the ability to import this macro
         // implementation target's module is its unit test target. Users of the
         // macros this target implements use them via their declarations in the

Sources/CMakeLists.txt

@@ -12,18 +12,22 @@ if(NOT SwiftTesting_MACRO_MAKE_PROGRAM)
   set(SwiftTesting_MACRO_MAKE_PROGRAM ${CMAKE_MAKE_PROGRAM})
 endif()
 
+find_package(SwiftSyntax CONFIG GLOBAL)
+if(SwiftSyntax_FOUND)
+  set(SwiftTesting_BuildMacrosAsExecutables NO)
+else()
+  set(SwiftTesting_BuildMacrosAsExecutables YES)
+endif()
+
 ExternalProject_Add(TestingMacros
   PREFIX "tm"
   SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/TestingMacros"
   CMAKE_ARGS
     -DCMAKE_MAKE_PROGRAM=${SwiftTesting_MACRO_MAKE_PROGRAM}
+    -DSwiftTesting_BuildMacrosAsExecutables=${SwiftTesting_BuildMacrosAsExecutables}
+    -DSwiftSyntax_DIR=${SwiftSyntax_DIR}
   INSTALL_COMMAND "")
 ExternalProject_Get_Property(TestingMacros BINARY_DIR)
-if(CMAKE_HOST_WIN32)
-  set(TestingMacrosPath "${BINARY_DIR}/TestingMacros.exe#TestingMacros")
-else()
-  set(TestingMacrosPath "${BINARY_DIR}/TestingMacros#TestingMacros")
-endif()
 
 include(AvailabilityDefinitions)
 include(CompilerSettings)

Sources/Testing/CMakeLists.txt

@@ -96,9 +96,17 @@ target_link_libraries(Testing PRIVATE
   _TestingInternals)
 add_dependencies(Testing
   TestingMacros)
-target_compile_definitions(Testing PRIVATE
-  SWT_BUILDING_WITH_CMAKE)
-target_compile_options(Testing PUBLIC
-  -load-plugin-executable "${TestingMacrosPath}")
 target_compile_options(Testing PRIVATE
   -enable-library-evolution)
+
+if(SwiftTesting_BuildMacrosAsExecutables)
+  if(CMAKE_HOST_WIN32)
+    set(_TestingMacros_ExecutableSuffix ".exe")
+  endif()
+
+  target_compile_options(Testing PUBLIC
+    "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-load-plugin-executable ${BINARY_DIR}/TestingMacros${_TestingMacros_ExecutableSuffix}#TestingMacros>")
+else()
+  target_compile_options(Testing PUBLIC
+    "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-plugin-path ${BINARY_DIR}>")
+endif()

Sources/TestingMacros/CMakeLists.txt

@@ -22,9 +22,12 @@ if(WIN32)
   add_compile_definitions(_CRT_SECURE_NO_WARNINGS)
 endif()
 
-include(FetchContent)
 find_package(SwiftSyntax CONFIG GLOBAL)
-if(NOT SwiftSyntax_FOUND)
+
+if(SwiftTesting_BuildMacrosAsExecutables)
+  # When building the macro plugin as an executable, clone and build
+  # swift-syntax.
+  include(FetchContent)
   set(FETCHCONTENT_BASE_DIR ${CMAKE_BINARY_DIR}/_d)
   # TODO: Update GIT_TAG to the 6.0 release tag once it is available.
   FetchContent_Declare(SwiftSyntax
@@ -33,9 +36,31 @@ if(NOT SwiftSyntax_FOUND)
   FetchContent_MakeAvailable(SwiftSyntax)
 endif()
 
+# Include these modules _after_ swift-syntax is declared above, but _before_ the
+# macro plugin target is declared below, so that its settings are not applied to
+# the former but are applied to the latter.
 include(AvailabilityDefinitions)
 include(CompilerSettings)
-add_executable(TestingMacros
+
+if(SwiftTesting_BuildMacrosAsExecutables)
+  # When swift-syntax is built locally, the macro plugin must be built as an
+  # executable.
+  add_executable(TestingMacros)
+
+  set_target_properties(TestingMacros PROPERTIES
+    ENABLE_EXPORTS TRUE)
+
+  # Parse the module as a library, even though it's an executable, because it
+  # uses an `@main` type to define its entry point.
+  target_compile_options(TestingMacros PRIVATE -parse-as-library)
+
+  # Include the .swift file which contains its `@main` entry point type.
+  target_compile_definitions(TestingMacros PRIVATE SWT_NO_LIBRARY_MACRO_PLUGINS)
+else()
+  add_library(TestingMacros SHARED)
+endif()
+
+target_sources(TestingMacros PRIVATE
   ConditionMacro.swift
   SourceLocationMacro.swift
   SuiteDeclarationMacro.swift
@@ -61,14 +86,13 @@ add_executable(TestingMacros
   TagMacro.swift
   TestDeclarationMacro.swift
   TestingMacrosMain.swift)
-set_target_properties(TestingMacros PROPERTIES
-  ENABLE_EXPORTS TRUE)
-target_compile_options(TestingMacros PRIVATE -parse-as-library)
+
 target_link_libraries(TestingMacros PRIVATE
-  SwiftSyntax::SwiftCompilerPlugin
   SwiftSyntax::SwiftSyntax
   SwiftSyntax::SwiftSyntaxMacroExpansion
-  SwiftSyntax::SwiftSyntaxMacros
-  SwiftSyntax509
-  SwiftSyntax510
-  SwiftSyntax600)
+  SwiftSyntax::SwiftSyntaxMacros)
+if(SwiftTesting_BuildMacrosAsExecutables)
+  # Link the 'SwiftCompilerPlugin' target, but only when built as an executable.
+  target_link_libraries(TestingMacros PRIVATE
+    SwiftSyntax::SwiftCompilerPlugin)
+endif()

Sources/TestingMacros/TestingMacrosMain.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-#if canImport(SwiftCompilerPlugin)
+#if SWT_NO_LIBRARY_MACRO_PLUGINS
 import SwiftCompilerPlugin
 import SwiftSyntaxMacros
 

cmake/modules/shared/CompilerSettings.cmake

@@ -6,7 +6,7 @@
 # See http://swift.org/LICENSE.txt for license information
 # See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 
-# Settings intended to be applied to every Swift target in this package.
+# Settings intended to be applied to every Swift target in this project.
 # Analogous to project-level build settings in an Xcode project.
 add_compile_options(
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -require-explicit-sendable>")
@@ -16,7 +16,10 @@ add_compile_options(
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend ExistentialAny>"
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend InternalImportsByDefault>")
 
-# Platform specific defines.
+# Definitions applied unconditionally to files of all languages, in all targets.
+add_compile_definitions("SWT_BUILDING_WITH_CMAKE")
+
+# Platform-specific definitions.
 if(APPLE)
   add_compile_definitions("SWT_TARGET_OS_APPLE")
 endif()