"Zero-cost abstractions" can be a confusing term and it is often misunderstood, but it has a precise meaning. Zero-cost abstractions doesn't mean that using them has no runtime cost, just that the abstraction itself causes no additional runtime cost.
These can also be quite narrow: Rc is a zero-cost abstraction for refcounting with both strong and weak references allocated with the object on the heap. You cannot implement something the same more efficiently, but you can implement something different but similar that is both faster and lighter than Rc. You can make a CheapRc that only has strong counts, and that will be both lighter and faster by a tiny amount, or a SeparateRc that stores the counts separately on the heap, which offers cheaper conversions to/from Rc.
These can also be quite narrow: Rc is a zero-cost abstraction for refcounting with both strong and weak references allocated with the object on the heap. You cannot implement something the same more efficiently, but you can implement something different but similar that is both faster and lighter than Rc. You can make a CheapRc that only has strong counts, and that will be both lighter and faster by a tiny amount, or a SeparateRc that stores the counts separately on the heap, which offers cheaper conversions to/from Rc.