How to connect 2 entities with a line

[myname1111]

So i want to make a physics, and i have created basic velocity drag and collisions. Now i want a constraint between 2 entities so i can make say a double pendulum. How ever i need to connect 2 entities with a line and modify their position to fit the constraints, how do i do that?

[myname1111]

Okay i have found the answer. I am able to do it using an ID system.

let mut pos : Vec<(Vec<f32>, &Line)> = vec![];

for (mut vox, mut vel, line) in voxel_set.iter_mut() {
    let mut nx = vel.0 * delta.0 + vox.translation.x;
    let mut ny = vel.1 * delta.0 + vox.translation.y;
    ... do physics ...
    pos.push((vec![vox.translation.x, vox.translation.y], line));

let mut i = 0;
for (mut xy, line) in pos.clone() {
      println!("{}, {} -> {}, {}", xy[0], xy[1], pos[line.0.0 as usize].0[0], pos[line.0.0 as usize].0[1])
      i += 1;
}

basically i make a vector of all the things i need (which is the pos and line).
then i loop through all of them and modify them