I think it's more a degree of how much effort it is to adjust to the new interface. If it's just 'added a new parameter to a function and there's an obvious default for existing code', then it'll (potentially mechanically) be applied to all the users. If it's 'completely changed around the abstraction and you need to think carefully about how to port your driver to the new interface', then that's something where there needs to be at least some longer-term migration plan, if only because there's not likely one person who can actually understand all the driver code and make the change.

(I do have experience with this causing regressions: someone updates a set of drivers to a new API, and because of the differences and lack of a good way to test, breaks some detail of the driver)

This isn't true; internal API's change all the time (e.g. adding extra arguments) Try running out of tree drivers on bleeding edge kernels to see for yourself.

Of course, for trivial mechanical changes like adding an argument the Rust binding changes are also trivial. If you've just spent half an hour walking through driver code for hardware you've never heard of changing stuff like

quaff(something, 5, Q_DOOP) ... into ... quaff(something, 5, 0, Q_DEFAULT | (Q_DOOP << 4))

Then it's not beyond the wits of a C programmer to realise that the Rust binding

quaff(var1, n, maybe_doop) ... can be ... quaff(var1, n, 0, Q_DEFAULT | (maybe_doop << 4))

Probably the Rust maintainer will be horrified and emit a patch to do something more idiomatic for binding your new API but there's an excellent chance that meanwhile your minimal patch builds and works since now it has the right number and type of arguments.

> If you've just spent half an hour walking through driver code for hardware you've never heard of changing stuff [...].

Isn’t the point of Coccinelle that you don’t have to spend time walking through (C) driver code you’ve never heard of?

I have never used Coccinelle but yes, sort of. However, you're on the hook for the patch you submit, Coccinelle isn't a person so if you blindly send out a patch Coccinelle generated, without even eyeballing it, you should expect some risk of thrown tomatoes if (unknown to you) this utterly broke some clever code using your previous API in a way you hadn't anticipated in a driver you don't run.