Enable MSVC's conformwat /Zc:preprocessor

Enable MSVC's conformant preprocessor (/Zc:preprocessor, available since
VS 2019 16.5) in both CMake and the setuptools extension, and lock the
behaviour in with a build-time _Static_assert self-test so any future
regression (new compiler, new build system, accidental flag removal)
fails the build immediately on every platform — including the MSVC CI
lane.

Related-to: #289
Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>
(cherry picked from commit f795700)

Author: sidcha

CMakeLists.txt

@@ -46,6 +46,12 @@ include(BuildType)
 ## Global settings
 if(NOT MSVC)
 	add_compile_options(-Wall -Wextra)
+else()
+	# MSVC's legacy preprocessor mis-handles the token-paste + rescan used
+	# by IS_ENABLED() in utils/include/utils/utils.h, silently returning 0
+	# for every defined-as-1 flag. /Zc:preprocessor switches to the
+	# C99/C11-conformant preprocessor (VS 2019 16.5+).
+	add_compile_options(/Zc:preprocessor)
 endif()
 
 if (OPT_BUILD_BARE_METAL)

python/setup.py

@@ -6,6 +6,7 @@
 
 import os
 import re
+import sys
 from setuptools import setup, Extension
 import shutil
 import subprocess
@@ -210,6 +211,12 @@ def try_vendor_sources(src_dir, src_files, vendor_dir):
     [ "-D" + define + "=1" for define in definitions ]
 )
 
+# PyPI Windows wheels are built with MSVC via cibuildwheel. Its legacy
+# preprocessor breaks the IS_ENABLED() macro in utils/include/utils/utils.h;
+# /Zc:preprocessor switches to the C99/C11-conformant preprocessor.
+if sys.platform == "win32":
+    compile_args.append("/Zc:preprocessor")
+
 if os.path.exists("README.md"):
     with open("README.md", "r") as f:
         long_description = f.read()