gmacros: Add G_COUNTED_BY macro

This currently probably does nothing, but in future both gcc and clang
will support the annotation it uses, and use that to do compile-time
bounds checking on flexible arrays.

See https://people.kernel.org/kees/bounded-flexible-arrays-in-c

It’s the same implementation as used in the kernel:
805d849d7c/include/linux/compiler_attributes.h (L97-L108).

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

docs/reference/glib/macros.md

@@ -825,6 +825,25 @@ by application programmers.
 `G_HAVE_GNUC_VISIBILITY`
 :   Defined to 1 if GCC-style visibility handling is supported.
 
+`G_COUNTED_BY(element)`
+
+:   Indicates that the `struct` member being annotated is a flexible length
+    array and that the given `element` contains its length (in elements, not
+    bytes).
+
+    Place the attribute after the declaration of the `struct` member which is a
+    flexible array, just before the semicolon.
+
+        struct
+          {
+            size_t n_doubles;
+            double array_of_doubles[] G_COUNTED_BY(n_doubles);
+          };
+
+    See the [GNU C documentation](https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-element-count-variable-attribute) for more details.
+
+    Since: 2.82
+
 
 ## Deprecation
 

glib/gmacros.h

@@ -405,6 +405,12 @@
 #define G_GNUC_ALLOC_SIZE2(x,y)
 #endif
 
+#if g_macro__has_attribute(__counted_by__)
+#define G_COUNTED_BY(x) __attribute__(__counted_by__(x))
+#else
+#define G_COUNTED_BY(x)
+#endif
+
 /**
  * G_GNUC_PRINTF:
  * @format_idx: the index of the argument corresponding to the