Sorry for the slow reply, I’ve been at home caring for a baby with daycare plague.
This is a quirk of how Legion works. When we build the query structure in-function, Legion has no way of knowing which world we mean. We’ve explicitly requested a SubWorld (with access to Point and MovingRandomly) - so we know that we need that world (the ecs variable), but the query structure itself doesn’t know that. So we have to tell it, as a parameter to iter_mut.
If I were revising it, I’d probably put the query in the system parameters (with automatic wiring up of the subworld). I’ll make a note for a potential future second edition.
Thanks!