Thanks for the pointer! My goal in the post was to avoid "just use this crate" approaches, because clearly there are dozens of crates that could help with this

Thanks for the pointer! My goal in the post was to avoid "just use this crate" approaches, because clearly there are dozens of crates that could help with this... even crates that implement BSTs :-) The goal was to look at the underlying implementation of such crates, and as the other comment pointed out it seemed like slotmap and others used approaches that mapped to one of the three I outlined in the post.