You forgot about the third option: specify "stable" and properly documented API boundaries in relevant subsystems. Developers of Rust drivers would have to rely

You forgot about the third option: specify "stable" and properly documented API boundaries in relevant subsystems. Developers of Rust drivers would have to rely only on those APIs, in turn, if subsystem maintainers want to change those APIs, they would have to synchronize it with Rust developers. The problem is that some subsystem maintainers do not want to even properly document existing APIs (sic!) and want to reserve the right to willy-nilly hack some changes into "their" subsystems without discussing it with other stakeholders.