std-aux: work around "-Wunused-but-set-variable" warning with clang in nm_auto()

We use the cleanup attribute heavily. It's useful for deferring
deallocation. For example, we have code like:

  gs_unref_object NMBluezManager *self_keep_alive = g_object_ref(self);

where we don't use the variable otherwise, except for owning (and
freeing) the reference. This already lead to a compiler warning about
unused variable, which we would workaround with

  _nm_unused gs_unref_object NMBluezManager *self_keep_alive = g_object_ref(self);

With clang 13.0.0~rc1-1.fc35, this got worse. Now for example also

    static inline void
    nm_strvarray_set_strv(GArray **array, const char *const *strv)
    {
        gs_unref_array GArray *array_old = NULL;

        array_old = g_steal_pointer(array);

        if (!strv || !strv[0])
            return;

        nm_strvarray_ensure(array);
        for (; strv[0]; strv++)
            nm_strvarray_add(*array, strv[0]);
    }

leads to a warning

    ./src/libnm-glib-aux/nm-shared-utils.h:3078:28: error: variable array_old set but not used [-Werror,-Wunused-but-set-variable]
        gs_unref_array GArray *array_old = NULL;
                               ^

This is really annoying. We don't want to plaster our code with _nm_unused,
because that might hide actual issues. But we also want to keep using this
pattern and need to avoid the warning.

A problem is also that GCC usually does not warn about truly unused
variables with cleanup attribute. Clang was very useful here to flag
such variables. But now clang warns about cases which are no bugs, which
is a problem. So this does loose some useful warnings. On the other hand,
a truly unused variable (with cleanup attribute) is ugly, but not an actual
problem.

Now, with clang 13, automatically mark nm_auto() variables as _nm_unused
as workaround.

1 files changed, 10 insertions, 1 deletions

diff --git a/src/libnm-std-aux/nm-std-aux.h b/src/libnm-std-aux/nm-std-aux.h
index a0a7513e1f..ce4d06d25b 100644
--- a/src/libnm-std-aux/nm-std-aux.h
+++ b/src/libnm-std-aux/nm-std-aux.h
@@ -26,7 +26,16 @@
 #define _nm_alignof(type)      __alignof(type)
 #define _nm_alignas(type)      _nm_align(_nm_alignof(type))
 #define _nm_deprecated(msg)    __attribute__((__deprecated__(msg)))
-#define nm_auto(fcn)           __attribute__((__cleanup__(fcn)))
+
+#if defined(__clang__) && __clang_major__ == 13
+/* Clang 13 can emit -Wunused-but-set-variable warning for cleanup variables
+ * that are only assigned (never used otherwise). Hack around */
+#define _nm_auto_extra _nm_unused
+#else
+#define _nm_auto_extra
+#endif
+
+#define nm_auto(fcn) _nm_auto_extra __attribute__((__cleanup__(fcn)))
 
 #define _nm_nil