Explicitly test stray include paths from the host.

This in particular affects `__has_include` new in C++17, which is also
how the test internally works. Tested files are chosen so that there is
no chance that other OSes have matching header files by accident.

.github/workflows/CI.yml

@@ -33,6 +33,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
         toolchain: [1.63.0, stable, nightly]
+        # Ideally we should also test against 0.9.0, but it is too unusable to test from CI
         zig: [0.10.1, master]
         exclude:
           # Only test MSRV with zig stable version

tests/bindgen-exhaustive/src/sysdep.h

@@ -1,11 +1,44 @@
+#if defined __has_include
+	#if __has_include(<linux/netfilter.h>)
+		#define HAS_LINUX_HEADER
+	#endif
+	#if __has_include(<winsock2.h>)
+		#define HAS_WINDOWS_HEADER
+	#endif
+	#if __has_include(<mach/mach_time.h>)
+		#define HAS_MACOS_HEADER
+	#endif
+#endif
+
 #ifdef __linux__
-struct zigbuild_is_linux { int x; };
+	struct zigbuild_is_linux { int x; };
+	#if defined __has_include && !defined HAS_LINUX_HEADER
+		#error "linux targets are expected to have <linux/netfilter.h>"
+	#endif
+#else
+	#if defined __has_include && defined HAS_LINUX_HEADER
+		#error "non-linux targets mistakenly have <linux/netfilter.h>, probably from host includes"
+	#endif
 #endif
 
 #ifdef _WIN32
-struct zigbuild_is_win32 { int x; };
+	struct zigbuild_is_win32 { int x; };
+	#if defined __has_include && !defined HAS_WINDOWS_HEADER
+		#error "windows targets are expected to have <winsock2.h>"
+	#endif
+#else
+	#if defined __has_include && defined HAS_WINDOWS_HEADER
+		#error "non-windows targets mistakenly have <winsock2.h>, probably from host includes"
+	#endif
 #endif
 
 #if defined __APPLE__ && defined __MACH__
-struct zigbuild_is_macos { int x; };
+	struct zigbuild_is_macos { int x; };
+	#if defined __has_include && !defined HAS_MACOS_HEADER
+		#error "macos targets are expected to have <mach/mach_time.h>"
+	#endif
+#else
+	#if defined __has_include && defined HAS_MACOS_HEADER
+		#error "non-macos targets mistakenly have <mach/mach_time.h>, probably from host includes"
+	#endif
 #endif