FoundationMacros: use cross-compilation to build for host

Use `ExternalProject` to switch FoundationMacros to cross-compilation.
This allows us to build the macros for the right OS/architecture when
cross-compiling Foundation for other environments.

Author: compnerd

Sources/CMakeLists.txt

@@ -12,12 +12,27 @@
 ##
 ##===----------------------------------------------------------------------===##
 
-add_subdirectory(_FoundationCShims)
-
-# Disable the macro build on Windows until we can correctly build it for the host architecture
-if(NOT CMAKE_SYSTEM_NAME STREQUAL Windows)
-	add_subdirectory(FoundationMacros)
+include(ExternalProject)
+if(CMAKE_HOST_WIN32)
+  set(_FoundationMacrosSwiftFlags -DCMAKE_Swift_FLAGS="-use-ld=lld")
+endif()
+ExternalProject_Add(FoundationMacros
+  SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/FoundationMacros"
+  PREFIX "${CMAKE_BINARY_DIR}/_deps"
+  BINARY_DIR "macros"
+  CMAKE_ARGS
+    -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
+    ${_FoundationMacrosSwiftFlags}
+  INSTALL_COMMAND "")
+ExternalProject_Get_Property(FoundationMacros BINARY_DIR)
+if(CMAKE_HOST_WIN32)
+  set(_SwiftFoundation_PredicateMacro "${BINARY_DIR}/FoundationMacros.exe#PredicateMacro")
+  set(_SwiftFoundation_ExpressionMacro "${BINARY_DIR}/FoundationMacros.exe#ExpressionMacro")
+else()
+  set(_SwiftFoundation_PredicateMacro "${BINARY_DIR}/FoundationMacros#PredicateMacro")
+  set(_SwiftFoundation_ExpressionMacro "${BINARY_DIR}/FoundationMacros#ExpressionMacro")
 endif()
 
+add_subdirectory(_FoundationCShims)
 add_subdirectory(FoundationEssentials)
 add_subdirectory(FoundationInternationalization)

Sources/FoundationMacros/CMakeLists.txt

@@ -12,10 +12,25 @@
 ##
 ##===----------------------------------------------------------------------===##
 
+cmake_minimum_required(VERSION 3.22)
+
+if(POLICY CMP0156)
+    # Deduplicate linked libraries where appropriate
+    cmake_policy(SET CMP0156 NEW)
+endif()
+if(POLICY CMP0157)
+    # New Swift build model: improved incremental build performance and LSP support
+  cmake_policy(SET CMP0157 NEW)
+endif()
+
+project(FoundationMacros
+  LANGUAGES Swift)
+
 # SwiftSyntax Dependency
-include(FetchContent)
 find_package(SwiftSyntax)
 if(NOT SwiftSyntax_FOUND)
+    include(FetchContent)
+
     # If building at desk, check out and link against the SwiftSyntax repo's targets
     FetchContent_Declare(SwiftSyntax
         GIT_REPOSITORY https://github.com/swiftlang/swift-syntax.git