libnm: expose _nm_connection_get_setting_by_metatype() in internal header

We have several variants to get the NMSetting from an NMConnection. Some
of them are public API (nm_connection_get_setting(), nm_connection_get_setting_by_name()).

The most efficient way is lookup by NMMetaSettingType. Expose that as
internal API, so it can be used. The NMMetaSettingType is internal, but
it exists because it's a very useful enum. Allow others to make use of
it.

Also, add a static assert which prevents various wrong uses at compile
time, for example

  _nm_connection_get_setting_by_metatype(connection, NM_TYPE_SETTING_CONNECTION)

src/libnm-core-impl/nm-connection.c

@@ -285,8 +285,11 @@ _get_setting_by_metatype(NMConnectionPrivate *priv, NMMetaSettingType meta_type)
     return priv->settings[meta_type];
 }
 
-static gpointer
-_nm_connection_get_setting_by_metatype(NMConnection *connection, NMMetaSettingType meta_type)
+/* The "unsafe" part here is that _nm_connection_get_setting_by_metatype() has a compile
+ * time check that meta_type is valid. With the unsafe variant, the caller must ensure that,
+ * and we only get an nm_assert() check -- which is basically nothing. */
+gpointer
+_nm_connection_get_setting_by_metatype_unsafe(NMConnection *connection, NMMetaSettingType meta_type)
 {
     g_return_val_if_fail(NM_IS_CONNECTION(connection), NULL);
 

src/libnm-core-intern/nm-core-internal.h

@@ -455,6 +455,20 @@ _nm_connection_get_setting(NMConnection *connection, GType type)
     return (gpointer) nm_connection_get_setting(connection, type);
 }
 
+gpointer _nm_connection_get_setting_by_metatype_unsafe(NMConnection     *connection,
+                                                       NMMetaSettingType meta_type);
+
+/* This variant is the most efficient one, because it does not require resolving a
+ * name/GType first. The NMMetaSettingType enum allows for a direct lookup. */
+#define _nm_connection_get_setting_by_metatype(connection, meta_type)                       \
+    ({                                                                                      \
+        /* Static assert that meta_type is in the valid range. If you don't want that,
+         * because the argument is no a compile time constant, use  _nm_connection_get_setting_by_metatype_unsafe(). */      \
+        G_STATIC_ASSERT((meta_type) < _NM_META_SETTING_TYPE_NUM && ((int) meta_type) >= 0); \
+                                                                                            \
+        _nm_connection_get_setting_by_metatype_unsafe((connection), (meta_type));           \
+    })
+
 NMSettingIPConfig *nm_connection_get_setting_ip_config(NMConnection *connection, int addr_family);
 
 /*****************************************************************************/