Add support for the counted_by attribute

This is a new attribute being developed.

docs/markdown/Reference-tables.md

@@ -283,6 +283,7 @@ which are supported by GCC, Clang, and other compilers.
 | const                       |
 | constructor                 |
 | constructor_priority        |
+| counted_by                  |
 | deprecated                  |
 | destructor                  |
 | error                       |
@@ -338,6 +339,8 @@ which are supported by GCC, Clang, and other compilers.
 
 ⁷ *New in 1.5.0*
 
+⁸ *New in 1.6.0*
+
 ### MSVC __declspec
 
 These values are supported using the MSVC style `__declspec` annotation,

docs/markdown/snippets/add_counted_by_attribute.md

@@ -0,0 +1,4 @@
+## Support for the `counted_by` attribute
+
+`compiler.has_function_attribute()` has grown support for the new `counted_by`
+attribute.

mesonbuild/compilers/c_function_attributes.py

@@ -30,6 +30,13 @@
         'int foo(void) __attribute__((constructor));',
     'constructor_priority':
         'int foo( void ) __attribute__((__constructor__(65535/2)));',
+    'counted_by':
+        '''
+        struct foo {
+            unsigned int count;
+            char bar[] __attribute__((counted_by(count)));
+        };
+        ''',
     'deprecated':
         'int foo(void) __attribute__((deprecated("")));',
     'destructor':

test cases/common/197 function attributes/meson.build

@@ -117,7 +117,16 @@ if ['gcc', 'intel'].contains(c.get_id())
   endif
 endif
 
+if c.get_id() == 'clang'
+  if c.version().version_compare('>= 18.0.0')
+    attributes += 'counted_by'
+  endif
+endif
+
 if c.get_id() == 'gcc'
+  if c.version().version_compare('>= 15.1')
+    attributes += 'counted_by'
+  endif
   if c.version().version_compare('>= 14.1')
     attributes += 'null_terminated_string_arg'
   endif