Iterate over combinations of entities

Use iter_combinations_mut() which will zip up all combinations of components.

fn interact_bodies(
  mut query: Query<(&Mass, &GlobalTransform, &mut Acceleration)>,
) {
  let mut iter = query.iter_combinations_mut();
  while let Some(
    [(Mass(m1), transform1, mut acc1), (Mass(m2), transform2, mut acc2)],
  ) = iter.fetch_next()
  {
    let delta = transform2.translation() - transform1.translation();
    let distance_sq: f32 = delta.length_squared();

    let f = GRAVITY_CONSTANT / distance_sq;
    let force_unit_mass = delta * f;
    acc1.0 += force_unit_mass * *m2;
    acc2.0 -= force_unit_mass * *m1;
  }
}

This can be particularly useful when building something like a collision system.