Implement and use MBEDTLS_STATIC_ASSERT()

Fixes #3693

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>

library/common.h

@@ -29,6 +29,7 @@
 #include "mbedtls/config.h"
 #endif
 
+#include <assert.h>
 #include <stddef.h>
 #include <stdint.h>
 
@@ -347,4 +348,34 @@ static inline const unsigned char *mbedtls_buffer_offset_const(
     }
 #endif
 
+/* Always provide a static assert macro, so it can be used unconditionally.
+ * Note that it will expand to nothing on some systems.
+ * Can be used outside functions (but don't add a trailing ';' in that case:
+ * the semicolon is included here to avoid triggering -Wextra-semi when
+ * MBEDTLS_STATIC_ASSERT() expands to nothing).
+ * Can't use the C11-style `defined(static_assert)` on FreeBSD, since it
+ * defines static_assert even with -std=c99, but then complains about it.
+ */
+#if defined(static_assert) && !defined(__FreeBSD__)
+#define MBEDTLS_STATIC_ASSERT(expr, msg)    static_assert(expr, msg);
+#elif defined(__COUNTER__)
+/* gcc will say "size of array ‘mbedtls_static_assert_failedN’ is negative"
+ * (and with -pedantic will complain further);
+ * clang will say "'mbedtls_static_assert_failedN' declared as an array with a
+ * negative size";
+ * Visual Studio will just say "error C2118: negative subscript" (without the
+ * mbedtls_static_assert_failedN part)
+ */
+#if defined(__GNUC__)
+#define MBEDTLS_UNUSED __attribute__((unused))
+#else
+#define MBEDTLS_UNUSED
+#endif
+#define MBEDTLS_STATIC_ASSERT2(expr, count) extern int MBEDTLS_UNUSED mbedtls_static_assert_failed ## count [2 * !!(expr) - 1];
+#define MBEDTLS_STATIC_ASSERT1(expr, count) MBEDTLS_STATIC_ASSERT2(expr, count)
+#define MBEDTLS_STATIC_ASSERT(expr, msg)    MBEDTLS_STATIC_ASSERT1(expr, __COUNTER__)
+#else
+#define MBEDTLS_STATIC_ASSERT(expr, msg)
+#endif
+
 #endif /* MBEDTLS_LIBRARY_COMMON_H */