Comment by rcxdude

It's not at all easy to implement as an optimisation, because it changes a lot of semantics, especially around references and pointers. It is something that you can e.g. implement using rust procedural macros, but it's far from transparent to switch between the two representations.

(It's also not always a win: it can work really well if you primarily operate on the 'columns', and on each column more or less once per update loop, but otherwise you can run into memory bandwidth limitations. For example, games with a lot of heavily interacting systems and an entity list that doesn't fit in cache will probably be better off with trying to load and update each entity exactly once per loop. Factorio is a good example of a game which is limited by this, though it is a bit of an outlier in terms of simulation size.)