[brundolf]

I don't think the average programmer would think to do the Vec indexing for this situation in the first place

> that's when to turn to other more expensive approaches such as Rc

If you're saying this was done just as an optimization... all I can say is, I hope you benchmarked first. As estebank pointed out, Rc is very fast: https://news.ycombinator.com/item?id=32240478. It can even be faster than mark-and-sweep in some situations. In fact the Swift language only uses reference-counting, not mark-and-sweep, at the language level.

If you profiled and found that the Vec approach solved some performance problem you had with reference-counting, then so be it. But I would be surprised if it meaningfully helped, and shocked if it helped enough to outweigh the extra complexity.