slide-rs/specs

Hi, I've been working on the movement system and got an issue:

impl<'a> System<'a> for DestinationMovement {
    type SystemData = (
        Entities<'a>,
        WriteStorage<'a, Position>,
        ReadStorage<'a, Speed>,
        ReadStorage<'a, Destination>,
        ReadExpect<'a, Timings>,
        ReadExpect<'a, Map>,
    );

    fn run(
        &mut self,
        (entities, mut positions, speeds, destinations, timings, map): Self::SystemData,
    ) {
                for (_entity, speed, destination, position) in
            (&entities, &speeds, &destinations, &mut positions).join()
        {
                let destination_position = positions.get(destination.0).cloned().unwrap();

                // rest of the code is irrelevant
        }
    }
}

Some entities have a destination, which is wrapper over Entity, and to calculate a path to destination I need to retrieve destination's position. Naturally, I also want to have own position of the current entity itself.

Issue is that iterator mutably borrows positions, so I cannot use it inside loop. For me it looks like a very common issue, so maybe there is known solution for this problem?

I could remove positions dependency from join and get both entity and destination position inside loop, but I don't like that it will relax component requirements for this system. Another potential solution I can think of, but haven't tried, is to collect all new positions into separate Vec and then assign them with another loop. I don't like performance implications of this approach and unsoundness of it.

Is there anything else I can do?