Checking the following code, could you confirm that is does not work concurrently ? The "world" var is read-only, does it mean that all other entities are also readonly (cannot be modified by another thread, for instance) ?

  struct World {
    entities Vec<Entity>;
  }
  func step(world &World, entity in world.entities[] mut) {
    entity.advance();
    let collision = world.get_collision_for_entity(entity);
    entity.resolve(collision);
  }