I have a Vec<Person>, V. Persons will be pushed to V, though never popped from. However, a Person's details may be modified.

V can grow quite large, and I would like to keep indexers to Persons based on their .FirstName (String) and .LastName (String). I will store these indexers for both fields in two BTrees.

I also want to iterate through V in V's order, and in that case I want to be able to see their FirstName and LastName immediately. So it is crucial that FirstName and LastName are stored in Persons themselves.

Now, since it would be quite wasteful to have a String copy for each FirstName and LastName, I want to store the index of a Person in V in these BTrees instead.

This would be doable if BTrees allowed custom comparator Fns. Eg. for FirstName it would look like:

|left : &usize, right : &usize| {
    V[*left].FirstName.cmp(&V[*right].FirstName)
}

But BTrees don't have this functionality.

In similar cases, a wrapper type has been suggested for a custom std::cmp::Ord implementation. But in my case, this can't be solved with wrapper types since these comparisons also require an external argument: &V.

I also can't simply make these BTree element values (index, &V), since V would then be shared-referenced, meaning its mutability is locked.

What options remain?

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