Comment by WesolyKubeczek

It’s a little bit more nuanced than this. Your parent commenter is wrong, by the way.

Suppose you want Rust, just not a single Rust driver using PCI. But CONFIG_PCI and CONFIG_RUST both selected will cause the PCI abstractions to get built anyway, even if not a single driver using them is selected. Then if the PCI subsystem introduces a change but the Rust counterpart fails to follow it fast enough, the build will break for the kernel as a whole.

This was illustrated with an example in that email thread.

I have some sad experience with polyglot projects. Unless you enjoy pain and drudgery, it’s an extremely unrewarding thing requiring very careful treading.