You cannot append to a std::span, or append to a vector-backed span through the span. You have to perform the append on the underlying vector. It is possible

You cannot append to a std::span, or append to a vector-backed span through the span. You have to perform the append on the underlying vector. It is possible to perform an insertion into the middle of a std::vector through std::vector::insert. If you can establish the precondition that the underlying vector will not re-allocate as a result of the append, then it is perfectly safe to perform such an append while holding a reference to one or more elements of the vector. Same thing for insertions: references to elements before the insertion point may still remain valid if you can establish the precondition that the vector will not be resized. In both cases, it is straightforward to establish the precondition through the reserve() member plus some foreknowledge of how much extra capacity the algorithm needs. You can always construct a user-defined reference type which back-doors the borrow checker, such as by storing indexes instead of iterators as you mentioned. If the std.vector is reduced, then they are still just as invalid.