Is there a way to have a Rust closure that moves only some variables into it?

Question

I have a general struct with settings and an extra variable setting that I want to tune and play around with.

For all possible values in an integer ra

Answer 1

Yes, it is possible to move only one or some variables into a closure (rather than all or none).

Yes, this can be used to "circumvent" reference counting.

I found an answer in the documentation of rayon::scope that turns out to be exactly about this problem: 'Accessing the stack data [from within a scoped threads scope]'. That page also has an example that is clearer than the pseudocode in this question.

It turns out that you can either:

• Use a move closure but refer to variables in the outer scope by shadowing them with a reference, therefore capturing them by reference rather than by value, using let settings = &settings:

  crossbeam::scope(|scope| {
      let settings = &settings; // refer to outer variable by reference
      for score in 0..MAX_FEASIBLE_SCORE {
          scope.spawn(move |_| {
              let work_result = do_cool_computation(settings, score);
              println!("{:?}", work_result);
          });
      }
  })
  .unwrap();
  

• Use a normal closure, and only move the required variables by shadowing them inside the closure using let score = score:

  crossbeam::scope(|scope| {
      for score in 0..MAX_FEASIBLE_SCORE {
          scope.spawn(|_| {
              let score = score; // capture only score
              let work_result = do_cool_computation(&settings, score);
              println!("{:?}", work_result);
          });
      }
  })
  .unwrap();