[cmake] Use CMAKE_TRY_COMPILE_PLATFORM_VARIABLES

[smeenai]

Summary

Now that our minimum required CMake version is past 3.6, we can use
CMAKE_TRY_COMPILE_PLATFORM_VARIABLES instead of relying on environment
variable trickery. The two aren't entirely equivalent because
CMAKE_TRY_COMPILE_PLATFORM_VARIABLES is only used for try_compiles of
source files and not whole projects, but I ran LLVM configures before
and after this change and the generated CMakeCache.txt files were
identical, so this should be NFC for us.

[thakis]

lgtm

llvm/cmake/platforms/WinMsvc.cmake

 # Windows libraries and headers are not case-correct, this toolchain file sets
 # up a VFS overlay for the SDK headers and case-correcting symlinks for the
 # libraries when running on a case-sensitive filesystem.
 
 include_guard(GLOBAL)
 
 # When configuring CMake with a toolchain file against a top-level CMakeLists.txt,
 # it will actually run CMake many times, once for each small test program used to
-# determine what features a compiler supports.  Unfortunately, none of these
+# determine what features a compiler supports. By default, none of these
 # invocations share a CMakeCache.txt with the top-level invocation, meaning they
-# won't see the value of any arguments the user passed via -D.  Since these are
+# won't see the value of any arguments the user passed via -D. Since these are
 # necessary to properly configure MSVC in both the top-level configuration as well as
-# all feature-test invocations, we set environment variables with the values so that
-# these environments get inherited by child invocations. We can switch to
-# CMAKE_TRY_COMPILE_PLATFORM_VARIABLES once our minimum supported CMake version
-# is 3.6 or greater.
-function(init_user_prop prop)
-  if(${prop})
-    set(ENV{_${prop}} "${${prop}}")
-  else()
-    set(${prop} "$ENV{_${prop}}" PARENT_SCOPE)
-  endif()
-endfunction()
+# all feature-test invocations, we include them in CMAKE_TRY_COMPILE_PLATFORM_VARIABLES,
+# so that they get inherited by child invocations.
+list(APPEND CMAKE_TRY_COMPILE_PLATFORM_VARIABLES
+  HOST_ARCH
+  LLVM_NATIVE_TOOLCHAIN
+  LLVM_WINSYSROOT
+  MSVC_VER
+  WINSDK_VER
+  winsdk_lib_symlinks_dir
+  winsdk_vfs_overlay_path
+  )
 
 function(generate_winsdk_vfs_overlay winsdk_include_dir output_path)
   set(include_dirs)
   file(GLOB_RECURSE entries LIST_DIRECTORIES true "${winsdk_include_dir}/*")
   foreach(entry ${entries})
     if(IS_DIRECTORY "${entry}")
       list(APPEND include_dirs "${entry}")
     endif()
foreach(entry ${entries})
     endif()
   endforeach()
 endfunction()
 
 set(CMAKE_SYSTEM_NAME Windows)
 set(CMAKE_SYSTEM_VERSION 10.0)
 set(CMAKE_SYSTEM_PROCESSOR AMD64)
 
-init_user_prop(HOST_ARCH)
-init_user_prop(LLVM_NATIVE_TOOLCHAIN)
-init_user_prop(LLVM_WINSYSROOT)
-init_user_prop(MSVC_VER)
-init_user_prop(WINSDK_VER)
-
 if(NOT HOST_ARCH)
   set(HOST_ARCH x86_64)
 endif()
 if(HOST_ARCH STREQUAL "aarch64" OR HOST_ARCH STREQUAL "arm64")
   set(TRIPLE_ARCH "aarch64")
   set(WINSDK_ARCH "arm64")
 elseif(HOST_ARCH STREQUAL "armv7" OR HOST_ARCH STREQUAL "arm")
   set(TRIPLE_ARCH "armv7")
set(COMPILE_FLAGS
     --target=${TRIPLE_ARCH}-windows-msvc
     -fms-compatibility-version=19.20
     -vctoolsversion ${MSVC_VER}
     -winsdkversion ${WINSDK_VER}
     -winsysroot ${LLVM_WINSYSROOT})
 
 if(case_sensitive_filesystem)
   # Ensure all sub-configures use the top-level VFS overlay instead of generating their own.
-  init_user_prop(winsdk_vfs_overlay_path)
   if(NOT winsdk_vfs_overlay_path)
     set(winsdk_vfs_overlay_path "${CMAKE_BINARY_DIR}/winsdk_vfs_overlay.yaml")
     generate_winsdk_vfs_overlay("${WINSDK_INCLUDE}" "${winsdk_vfs_overlay_path}")
-    init_user_prop(winsdk_vfs_overlay_path)
   endif()
   list(APPEND COMPILE_FLAGS
        -Xclang -ivfsoverlay -Xclang "${winsdk_vfs_overlay_path}")
 endif()
 
 string(REPLACE ";" " " COMPILE_FLAGS "${COMPILE_FLAGS}")
 string(APPEND CMAKE_C_FLAGS_INIT " ${COMPILE_FLAGS}")
 string(APPEND CMAKE_CXX_FLAGS_INIT " ${COMPILE_FLAGS}")
 
 set(LINK_FLAGS
     # Prevent CMake from attempting to invoke mt.exe. It only recognizes the slashed form and not the dashed form.
     /manifest:no
 
     -libpath:"${ATLMFC_LIB}/${WINSDK_ARCH}"
     -libpath:"${MSVC_LIB}/${WINSDK_ARCH}"
     -libpath:"${WINSDK_LIB}/ucrt/${WINSDK_ARCH}"
     -libpath:"${WINSDK_LIB}/um/${WINSDK_ARCH}")
 
 if(case_sensitive_filesystem)
   # Ensure all sub-configures use the top-level symlinks dir instead of generating their own.
-  init_user_prop(winsdk_lib_symlinks_dir)
   if(NOT winsdk_lib_symlinks_dir)
     set(winsdk_lib_symlinks_dir "${CMAKE_BINARY_DIR}/winsdk_lib_symlinks")
     generate_winsdk_lib_symlinks("${WINSDK_LIB}/um/${WINSDK_ARCH}" "${winsdk_lib_symlinks_dir}")
-    init_user_prop(winsdk_lib_symlinks_dir)
   endif()
   list(APPEND LINK_FLAGS
        -libpath:"${winsdk_lib_symlinks_dir}")
 endif()
 
 string(REPLACE ";" " " LINK_FLAGS "${LINK_FLAGS}")
 string(APPEND CMAKE_EXE_LINKER_FLAGS_INIT " ${LINK_FLAGS}")
 string(APPEND CMAKE_MODULE_LINKER_FLAGS_INIT " ${LINK_FLAGS}")