BLI Listbase: Add iterator macro that increments an index

.clang-format

@@ -238,6 +238,7 @@ ForEachMacros:
   - LISTBASE_FOREACH_BACKWARD
   - LISTBASE_FOREACH_MUTABLE
   - LISTBASE_FOREACH_BACKWARD_MUTABLE
+  - LISTBASE_FOREACH_INDEX
   - MAN_ITER_AXES_BEGIN
   - NODE_INSTANCE_HASH_ITER
   - NODE_SOCKET_TYPES_BEGIN

source/blender/blenlib/BLI_listbase.h

@@ -171,6 +171,15 @@ struct LinkData *BLI_genericNodeN(void *data);
 #define LISTBASE_FOREACH(type, var, list) \
   for (type var = (type)((list)->first); var != NULL; var = (type)(((Link *)(var))->next))
 
+/**
+ * A version of #LISTBASE_FOREACH that supports incrementing an index variable at every step.
+ * Including this in the macro helps prevent mistakes where "continue" mistakenly skips the
+ * incrementation.
+ */
+#define LISTBASE_FOREACH_INDEX(type, var, list, index_var) \
+  for (type var = (((void)(index_var = 0)), (type)((list)->first)); var != NULL; \
+       var = (type)(((Link *)(var))->next), index_var++)
+
 #define LISTBASE_FOREACH_BACKWARD(type, var, list) \
   for (type var = (type)((list)->last); var != NULL; var = (type)(((Link *)(var))->prev))