fixup! [meson,common] Enable C23 and nodiscard feature when available

Signed-off-by: Mariusz Zaborski <[email protected]>

common/include/nodiscard.h

@@ -8,16 +8,13 @@
  * the nodiscard attribute introduced in C23. However, because Gramine supports
  * older systems that might not have support for C23, we have to wrap it on our
  * own and change it to a no-op on systems that don't support it.
+ * TODO: Remove this after dropping *EL8 and Ubuntu 20.04 support.
  */
 
 #pragma once
 
-#if defined(__has_c_attribute)
-#if __has_c_attribute(nodiscard)
+#if GRAMINE_HAS_NODISCARD
 #define NODISCARD [[nodiscard]]
-#endif
-#endif
-
-#ifndef NODISCARD
+#else
 #define NODISCARD
 #endif

meson.build

@@ -17,12 +17,26 @@ project(
 )
 
 # If C23 (or experimetal C23 - C2x) is available, use it.
-# XXX: Gramine supports older versions of Ubuntu, so newer versions of compilers
-#      may not be available.
-if meson.get_compiler('c').has_argument('-std=c23')
+# TODO: Gramine supports older versions of gcc, so newer versions of compilers
+#       may not be available.
+#       We can change to c2x when we drop *EL8 and Ubuntu 20.04.
+#       We can't change to c23 for any supported versions yet (requires at
+#       leaast gcc 14).
+nodiscard_code = '[[nodiscard]] int func() { return 0; }'
+if meson.get_compiler('c').compiles(
+    nodiscard_code,
+    args : '-std=c23',
+    name : 'check for [[nodiscard]] and c23',
+  )
   add_project_arguments('-std=c23', language: 'c')
-elif meson.get_compiler('c').has_argument('-std=c2x')
+  add_global_arguments('-DGRAMINE_HAS_NODISCARD', language : 'c')
+elif meson.get_compiler('c').compiles(
+    nodiscard_code,
+    args : '-std=c2x',
+    name : 'check for [[nodiscard]] and c2x',
+  )
   add_project_arguments('-std=c2x', language: 'c')
+  add_global_arguments('-DGRAMINE_HAS_NODISCARD', language : 'c')
 endif
 
 # we need this subdir() early, because we need scripts defined there for setting up global vars