I believe the closest that exists currently is
let mut list: LinkedList;
let mut tail = list.split_off(idx); // O(n), seek idx and split
list.push_back(el); // O(1), push to back
list.append(&mut tail); // O(1), move tail's elements back to list
Without dependent typing, a safe O(1) insert cannot be provided from the LinkedList itself. Perhaps a Cursor API could be added that allows you to travel up and down the tape, thus separating the seek and insert costs?
(Unquoted context:
O(1) insertion is often the reason why linked list gets chosen as data structure, and is what the quoted [source of unsafety in actix-web] implements.
)
