A new policy is born...

In general you shouldn't be making coding changes to accomodate backporting. That is severely frowned upon, but does ocassionally happen regardless.

But, there is a whole set of other grey areas where things get murky. The gcc toolchain has been kept back for alot of different reasons, including backporting.

People tend to think carefully before renaming files or moving code around. I've seen patches pushed back on because of code motion harming backporting. For instance if you send a patch to reorder functions to avoid forward declarations then there is a chance that will be refused as being too trival and too harmful. It depends on the maintainer.

I would say the basic informal agreement we seem to have is that people have to do the best technical work upstream but that with enough patches the backporting will succeed. Rust is a big, big step up of the backport requirement and I think nobody with customers and timelines funding their projects wants to be forced to be the first one that pipe cleans this. Nobody knows what this looks like, or what is possible, and there is a real fear that the project would end up useless for the real world if backporting fails.