util: Allow STATIC_ASSERT() everywhere

Remove -Werror=vla from c_msvc_compat_args so we can use STATIC_ASSERT()
in core code.  We have a CI job for this.

(And arguably we could probably just drop c_msvc_compat_args entirely.)

Signed-off-by: Rob Clark <robdclark@chromium.org>
Acked-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7941>

meson.build

@@ -1090,7 +1090,7 @@ else
   # Check for C and C++ arguments for MSVC compatibility. These are only used
   # in parts of the mesa code base that need to compile with MSVC, mainly
   # common code
-  foreach a : ['-Werror=pointer-arith', '-Werror=vla', '-Werror=gnu-empty-initializer']
+  foreach a : ['-Werror=pointer-arith', '-Werror=gnu-empty-initializer']
     if cc.has_argument(a)
       c_msvc_compat_args += a
     endif

scons/gallium.py

@@ -86,7 +86,6 @@ def install_shared_library(env, sources, version = ()):
 def msvc2013_compat(env):
     if env['gcc']:
         env.Append(CCFLAGS = [
-            '-Werror=vla',
             '-Werror=pointer-arith',
         ])
 

src/util/macros.h

@@ -66,13 +66,26 @@
 
 /**
  * Static (compile-time) assertion.
- * Basically, use COND to dimension an array.  If COND is false/zero the
- * array size will be -1 and we'll get a compilation error.
  */
-#define STATIC_ASSERT(COND) \
+#if defined(_MSC_VER)
-   do { \
+   /* MSVC doesn't like VLA's, but it also dislikes zero length arrays
+    * (which gcc is happy with), so we have to define STATIC_ASSERT()
+    * slightly differently.
+    */
+#  define STATIC_ASSERT(COND) do {         \
+      (void) sizeof(char [(COND) != 0]);   \
+   } while (0)
+#elif defined(__GNUC__)
+   /* This version of STATIC_ASSERT() relies on VLAs.  If COND is
+    * false/zero, the array size will be -1 and we'll get a compile
+    * error
+    */
+#  define STATIC_ASSERT(COND) do {         \
       (void) sizeof(char [1 - 2*!(COND)]); \
    } while (0)
+#else
+#  define STATIC_ASSERT(COND) do { } while (0)
+#endif
 
 
 /**