Usage of mutable reference having one immutable

1

Hello,
Every time i try to compile implementation of one of advent of code's task I get following error:

error[E0502]: cannot borrow `wires` as mutable because it is also borrowed as immutable
   --> src/main.rs:205:20
    |
195 |   let mut collision = Collision::new(&wires);
    |                                      ------ immutable borrow occurs here
...
205 |   for each_wire in &mut wires {
    |                    ^^^^^^^^^^ mutable borrow occurs here
206 |     each_wire.get_shortest_num_of_moves_for_collision(&collision_pts);
    |

Here is my code:

  let mut collision = Collision::new(&wires);
  let collision_pts: Vec<&Point> = collision.detect_on_wires();
  println!("Found collision points: {:?}", collision_pts);

  // part 1
  let shortest_dist = get_shortest_distance(&collision_pts);
  println!("Shortest distance: {}", shortest_dist);

  // part 2
  let shortest_moves_for_collisions: Vec<u32> = vec![];
  for each_wire in &mut wires {
    each_wire.get_shortest_num_of_moves_for_collision(&collision_pts);
  }

And fragment of Collision structure:

struct Collision <'a> { wires: &'a [Wire] }
impl <'a> Collision <'a> {
  pub fn new(wires: &[Wire]) -> Collision {
    Collision {
      wires: wires
    }
  }

The problem is that Collision::new(.. uses immutable reference to wires, but then I need to perform some operations on wire elements. I know that this is a problem and I can't have mutable and immutable references of same element at the same time, but to be hones I have no idea how to solve that. Could you please explain me how to avoid such situations? Regards!

2

Does Collision only have that one field wires (or did you cut out the rest for simplicity's sake)?

3

Yes, it only has wires field

4

Is Wire your own type or is it contained in an external module?

Also, does Point contain a reference to Wire?

5

Both structures are mine. Here you have some more code:

6

Depending on how Point is defined, it might even be cheaper to copy it than to reference it.

Please post the type definitions in this thread.

7

Get rid of the Collision struct. Convert the methods of Collision into free-standing functions, that take &[Wire] as their first argument. You never mutate Collision and it doesn't own anything, i.e. chances are very high the struct is useless.

You can define a type alias, instead, if you want to:

type Collision<'a> = &'a [Wire];

fn detect_collision_pts(collision: Collision<'_>) { … }
8

Yeah, that is a solution and it will definitely work in this case, but I can think of some cases in which having such struct will be required and my question remains valid. Do you have idea of handling those? I mean, is there any chance in which my implementation may work?
Regards!

9

What happens, if you get rid of the collision variable and instead just use collision like this?

let collision_pts: Vec<&Point> = Collision::new(&wires).detect_on_wires();

You may have to wrap Collision::new() in pair of (). Not sure if Rust likes Type::method().method().

10

Then we have a new error:
219 | let collision_pts: Vec<&Point> = Collision::new(&wires).detect_on_wires();
| ^^^^^^^^^^^^^^^^^^^^^^ - temporary value is freed at the end of this statement
| |
| creates a temporary which is freed while still in use
:smiley:

11

After some analysis it wont solve it either :frowning: Problem is that i still need to borrow 'wires' twice.

  1. First time in
    let collision_pts: Vec<&Point> = detect_on_wires(&wires);
  2. Second time in
    for each_wire in &mut wires {
    each_wire.get_shortest_num_of_moves_for_collision(&collision_pts);
    }

Result is the same -> I have mutable and immutable reference for the same variable :frowning:

12

Return Vec<Point> instead of Vec<&Point>. Derive Copy and Clone for Point.

3 Likes
13

Thank you all for your help :slight_smile:

14

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