I really like this advice, but aren’t these two examples the same, but yet different advice?

// Good? for walrus in walruses { walrus.frobnicate() }

Is essentially equivalent to

// BAD for walrus in walruses { frobnicate(walrus) }

And this is good,

// GOOD frobnicate_batch(walruses)

So should the first one really be something more like

// impl FrobicateAll for &[Walrus] walruses.frobicate_all()