[breatheoften]

Its not a small change ... binary compatibility has essentially meant “c compatible” for the last several decades ... will any of the new languages currently attempting to implement a binary comparability model succeed in shifting this in a meaningful way?

[zozbot234]

Complete binary compatibility across modules for open records/sums (this is what #[non_exhaustive] boils down to) is quite non-trivial. You end up going through a level of indirection, kind of like objects in Python/Ruby etc. It's not a disaster because it specifically applies to the open case, which ought to be rare; but it's something to be aware of.