must_be.h: Add must_be_array() macro

This macro statically asserts that the argument is an array.

Link: <https://stackoverflow.com/a/57537491>
Cc: Christian Göttsche <cgzones@googlemail.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/must_be.h

@@ -58,4 +58,41 @@
 )
 
 
+/*
+ * SYNOPSIS
+ *	int must_be_array(a);
+ *
+ * ARGUMENTS
+ *	a	Array.
+ *
+ * DESCRIPTION
+ *	This macro fails compilation if 'a' is not an array.  It is
+ *	useful in macros that accept an array as a parameter, where this
+ *	macro can validate the macro argument.  It prevent passing a
+ *	pointer to such macros, which would otherwise produce silent
+ *	bugs.
+ *
+ * RETURN VALUE
+ *	0
+ *
+ * ERRORS
+ *	If 'a' is not an array, the compilation will fail.
+ *
+ * EXAMPLES
+ *	int a[10];
+ *	int *p;
+ *
+ *	must_be_array(a);  // Ok
+ *	must_be_array(p);  // Compile-time error
+ *
+ * SEE ALSO
+ *	must_be()
+ */
+
+
+#define is_same_type(a, b)  __builtin_types_compatible_p(typeof(a), typeof(b))
+#define is_array(a)         (!is_same_type((a), &(a)[0]))
+#define must_be_array(a)    must_be(is_array(a))
+
+
 #endif  // include guard