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`KVec::with_capacity(0)` is doable, so why not here? As long as it doesn't introduce soundness issues I'd say this is the caller's business; a driver with a legitimate empty IdPool use-case would now need to special-case it. Now we do have an actual soundness issue with zero-sized IdPools, which is that `find_unused_id` would panic with `CONFIG_RUST_BITMAP_HARDENED`, but as I said on patch 5 I don't think it should anyway. Another potential issue is that `grow_request` would not grow anything; but that should be fixed there by handling the `capacity =3D=3D 0` case. Actually that would give justification for empty IdPools to exist: just like a vector can start empty and grow, so can an IdPool.