src/librustc_error_codes/error_codes/E0524.md

   1 A variable which requires unique access is being used in more than one closure
   2 at the same time.
   3
   4 Erroneous code example:
   5
   6 ```compile_fail,E0524
   7 fn set(x: &mut isize) {
   8     *x += 4;
   9 }
  10
  11 fn dragoooon(x: &mut isize) {
  12     let mut c1 = || set(x);
  13     let mut c2 = || set(x); // error!
  14
  15     c2();
  16     c1();
  17 }
  18 ```
  19
  20 To solve this issue, multiple solutions are available. First, is it required
  21 for this variable to be used in more than one closure at a time? If it is the
  22 case, use reference counted types such as `Rc` (or `Arc` if it runs
  23 concurrently):
  24
  25 ```
  26 use std::rc::Rc;
  27 use std::cell::RefCell;
  28
  29 fn set(x: &mut isize) {
  30     *x += 4;
  31 }
  32
  33 fn dragoooon(x: &mut isize) {
  34     let x = Rc::new(RefCell::new(x));
  35     let y = Rc::clone(&x);
  36     let mut c1 = || { let mut x2 = x.borrow_mut(); set(&mut x2); };
  37     let mut c2 = || { let mut x2 = y.borrow_mut(); set(&mut x2); }; // ok!
  38
  39     c2();
  40     c1();
  41 }
  42 ```
  43
  44 If not, just run closures one at a time:
  45
  46 ```
  47 fn set(x: &mut isize) {
  48     *x += 4;
  49 }
  50
  51 fn dragoooon(x: &mut isize) {
  52     { // This block isn't necessary since non-lexical lifetimes, it's just to
  53       // make it more clear.
  54         let mut c1 = || set(&mut *x);
  55         c1();
  56     } // `c1` has been dropped here so we're free to use `x` again!
  57     let mut c2 = || set(&mut *x);
  58     c2();
  59 }
  60 ```