helpers: Move static_assert definition to shared

Collect the fallback definitions of static_assert() from desktop-shell
and the test shell, and move them to helpers.h. This allows code
throughout the tree to use static_assert() for build-time assertions,
where it is supported by the compiler.

As GCC goes out of its way to only add static_assert() when C11 has been
explicitly requested - which we don't do - make sure to use the more
widely available _Static_assert() if that is provided.

This will be used in future patches to ensure two array lengths don't go
out of sync.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

desktop-shell/shell.c

@@ -47,10 +47,6 @@
 #define DEFAULT_NUM_WORKSPACES 1
 #define DEFAULT_WORKSPACE_CHANGE_ANIMATION_LENGTH 200
 
-#ifndef static_assert
-#define static_assert(cond, msg)
-#endif
-
 struct focus_state {
 	struct desktop_shell *shell;
 	struct weston_seat *seat;

shared/helpers.h

@@ -100,6 +100,40 @@ extern "C" {
 	(type *)( (char *)__mptr - offsetof(type,member) );})
 #endif
 
+/**
+ * Build-time static assertion support
+ *
+ * A build-time equivalent to assert(), will generate a compilation error
+ * if the supplied condition does not evaluate true.
+ *
+ * The following example demonstrates use of static_assert to ensure that
+ * arrays which are supposed to mirror each other have a consistent
+ * size.
+ *
+ * This is only a fallback definition; support must be provided by the
+ * compiler itself.
+ *
+ * @code
+ * int small[4];
+ * long expanded[4];
+ *
+ * static_assert(ARRAY_LENGTH(small) == ARRAY_LENGTH(expanded),
+ *               "size mismatch between small and expanded arrays");
+ * for (i = 0; i < ARRAY_LENGTH(small); i++)
+ *     expanded[i] = small[4];
+ * @endcode
+ *
+ * @param condition Expression to check for truth
+ * @param msg Message to print on failure
+ */
+#ifndef static_assert
+# ifdef _Static_assert
+#  define static_assert(cond, msg) _Static_assert(cond, msg)
+# else
+#  define static_assert(cond, msg)
+# endif
+#endif
+
 #ifdef  __cplusplus
 }
 #endif

tests/weston-test-desktop-shell.c

@@ -41,10 +41,6 @@
 #include "shared/helpers.h"
 #include "libweston-desktop/libweston-desktop.h"
 
-#ifndef static_assert
-#define static_assert(cond, msg)
-#endif
-
 struct desktest_shell {
 	struct wl_listener compositor_destroy_listener;
 	struct weston_desktop *desktop;