build_bug.h: add wrapper for _Static_assert

BUILD_BUG_ON() is a little annoying, since it cannot be used outside
function scope.  So one cannot put assertions about the sizeof() a
struct next to the struct definition, but has to hide that in some more
or less arbitrary function.

Since gcc 4.6 (which is now also the required minimum), there is support
for the C11 _Static_assert in all C modes, including gnu89.  So add a
simple wrapper for that.

_Static_assert() requires a message argument, which is usually quite
redundant (and I believe that bug got fixed at least in newer C++
standards), but we can easily work around that with a little macro
magic, making it optional.

For example, adding

  static_assert(sizeof(struct printf_spec) == 8);

in vsprintf.c and modifying that struct to violate it, one gets

./include/linux/build_bug.h:78:41: error: static assertion failed: "sizeof(struct printf_spec) == 8"
 #define __static_assert(expr, msg, ...) _Static_assert(expr, "" msg "")

godbolt.org suggests that _Static_assert() has been support by clang
since at least 3.0.0.

Link: http://lkml.kernel.org/r/20190208203015.29702-1-linux@rasmusvillemoes.dkSigned-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/build_bug.h

@@ -58,4 +58,23 @@
  */
 #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
 
+/**
+ * static_assert - check integer constant expression at build time
+ *
+ * static_assert() is a wrapper for the C11 _Static_assert, with a
+ * little macro magic to make the message optional (defaulting to the
+ * stringification of the tested expression).
+ *
+ * Contrary to BUILD_BUG_ON(), static_assert() can be used at global
+ * scope, but requires the expression to be an integer constant
+ * expression (i.e., it is not enough that __builtin_constant_p() is
+ * true for expr).
+ *
+ * Also note that BUILD_BUG_ON() fails the build if the condition is
+ * true, while static_assert() fails the build if the expression is
+ * false.
+ */
+#define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
+#define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
+
 #endif	/* _LINUX_BUILD_BUG_H */