git-compat-util.h: implement a different ARRAY_SIZE macro for for safely deriving the size of array

To get number of elements in an array git use the ARRAY_SIZE macro
defined as:

       #define ARRAY_SIZE(x) (sizeof(x)/sizeof((x)[0]))

The problem with it is a possibility of mistakenly passing to it a
pointer instead an array. The ARRAY_SIZE macro as conventionally
defined does not provide good type-safety and the open-coded
approach is more fragile, more verbose and provides no improvement in
type-safety.

Use instead a different but compatible ARRAY_SIZE() macro,
which will also break compile if you try to
use it on a pointer. This implemention revert to the original code
if the compiler doesn't know the typeof and __builtin_types_compatible_p
GCC extensions.

This can ensure our code is robust to changes, without
needing a gratuitous macro or constant. A similar
ARRAY_SIZE implementation also exists in the linux kernel.

Credits to Rusty Russell and his ccan library.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -3,6 +3,23 @@
 
 #define _FILE_OFFSET_BITS 64
 
+
+/* Derived from Linux "Features Test Macro" header
+ * Convenience macros to test the versions of gcc (or
+ * a compatible compiler).
+ * Use them like this:
+ *  #if GIT_GNUC_PREREQ (2,8)
+ *   ... code requiring gcc 2.8 or later ...
+ *  #endif
+*/
+#if defined(__GNUC__) && defined(__GNUC_MINOR__)
+# define GIT_GNUC_PREREQ(maj, min) \
+	((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
+#else
+ #define GIT_GNUC_PREREQ(maj, min) 0
+#endif
+
+
 #ifndef FLEX_ARRAY
 /*
  * See if our compiler is known to support flexible array members.
@@ -25,7 +42,42 @@
 #endif
 #endif
 
-#define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
+
+/*
+ * BUILD_ASSERT_OR_ZERO - assert a build-time dependency, as an expression.
+ * @cond: the compile-time condition which must be true.
+ *
+ * Your compile will fail if the condition isn't true, or can't be evaluated
+ * by the compiler.  This can be used in an expression: its value is "0".
+ *
+ * Example:
+ *	#define foo_to_char(foo)					\
+ *		 ((char *)(foo)						\
+ *		  + BUILD_ASSERT_OR_ZERO(offsetof(struct foo, string) == 0))
+ */
+#define BUILD_ASSERT_OR_ZERO(cond) \
+	(sizeof(char [1 - 2*!(cond)]) - 1)
+
+#if defined(__GNUC__) && (__GNUC__ >= 3)
+# if GIT_GNUC_PREREQ(3, 1)
+ /* &arr[0] degrades to a pointer: a different type from an array */
+# define BARF_UNLESS_AN_ARRAY(arr)						\
+	BUILD_ASSERT_OR_ZERO(!__builtin_types_compatible_p(__typeof__(arr), \
+							   __typeof__(&(arr)[0])))
+# else
+#  define BARF_UNLESS_AN_ARRAY(arr) 0
+# endif
+#endif
+/*
+ * ARRAY_SIZE - get the number of elements in a visible array
+ *  <at> x: the array whose size you want.
+ *
+ * This does not work on pointers, or arrays declared as [], or
+ * function parameters.  With correct compiler support, such usage
+ * will cause a build error (see the build_assert_or_zero macro).
+ */
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]) + BARF_UNLESS_AN_ARRAY(x))
+
 #define bitsizeof(x)  (CHAR_BIT * sizeof(x))
 
 #define maximum_signed_value_of_type(a) \