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

Signed-off-by: Mariusz Zaborski <oshogbo@invisiblethingslab.com>

common/include/nodiscard.h

@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: LGPL-3.0-or-later */
+/* Copyright (C) 2024 Intel Corporation
+ *                    Mariusz Zaborski <oshogbo@invisiblethingslab.com>
+ */
+
+/*
+ * This file contains a definition for NODISCARD, a macro that wraps
+ * 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.
+ */
+
+#pragma once
+
+#if defined(__has_c_attribute)
+#if __has_c_attribute(nodiscard)
+#define NODISCARD [[nodiscard]]
+#endif
+#endif
+
+#ifndef NODISCARD
+#define NODISCARD
+#endif

common/include/pal_error.h

@@ -7,9 +7,10 @@
 
 #pragma once
 
+#include <nodiscard.h>
 #include <stddef.h>
 
-typedef enum _pal_error_t {
+typedef enum NODISCARD _pal_error_t {
     PAL_ERROR_SUCCESS = 0,
     PAL_ERROR_NOTIMPLEMENTED,
     PAL_ERROR_NOTDEFINED,

meson.build

@@ -16,6 +16,15 @@ 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')
+  add_project_arguments('-std=c23', language: 'c')
+elif meson.get_compiler('c').has_argument('-std=c2x')
+  add_project_arguments('-std=c2x', language: 'c')
+endif
+
 # we need this subdir() early, because we need scripts defined there for setting up global vars
 subdir('scripts')