I haven't had need to use it, but an improvement on the "I have an index to an object which has been replaced" has been to add a generation to the collection &

I haven't had need to use it, but an improvement on the "I have an index to an object which has been replaced" has been to add a generation to the collection & index, a la the generational_arena crate. If the generation attached to your index doesn't match the generation in the collection at that index, your index is stale.