* the summary from my PoV is:

- `&self` is more permissive for drivers, more restrictive for impls
- `&mut self` is more restrictive for drivers, more permissivee for impls

it's a "zero sum game"

so the key questions are

A- How much better is `&self` for drivers?
B- How much better is `&mut self` for impls?

If A is "stronger" than B, we should pick `&self`. if B is stronger, we should pick `&mut self`.