Traditional memory allocators set aside some of the memory for metadata (for instance to keep track of allocated and free memory regions), I guess that Vale stores the generation count associated with an "allocation item" in a similar way, e.g. somewhere else than the actual items.

Also, those are 'generational indices', not pointers. This seems to indicate that items of the same type (or at least same size) are grouped into arrays (and since it's an index anyway, the metadata could be stored in one or multiple separate arrays at the same index).

PS: I already linked it elsewhere, but here's how the same can be achieved without language/compiler support (at least it's the same general idea): https://floooh.github.io/2018/06/17/handles-vs-pointers.html

The big step forward is that the compiler can elide most of the 'dangling checks', the method outlined in the blog post requires a few rules-of-thumb the coder must follow when using a pointer that's been looked up from a generational-index.