Yeah, idk how much it's POSSIBLE to have miscompilations in `pre_init`, but it's definitely breaking the environmental contract with the rust abstract machine (e.g., some invariants hold true before Rust "starts", e.g. statics will be initialized either to zero or their initial value).

The stack should be placed already (afaik), and any Rust code will restore the stack to the position it was found in, so the only possible UB cases I can imagine are: 

* interactions with statics, or
* optimizations that lift code or values to statics