src/librustc_error_codes/error_codes/E0502.md

   1 A variable already borrowed as immutable was borrowed as mutable.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0502
   6 fn bar(x: &mut i32) {}
   7 fn foo(a: &mut i32) {
   8     let ref y = a; // a is borrowed as immutable.
   9     bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
  10             //        as immutable
  11     println!("{}", y);
  12 }
  13 ```
  14
  15 To fix this error, ensure that you don't have any other references to the
  16 variable before trying to access it mutably:
  17
  18 ```
  19 fn bar(x: &mut i32) {}
  20 fn foo(a: &mut i32) {
  21     bar(a);
  22     let ref y = a; // ok!
  23     println!("{}", y);
  24 }
  25 ```
  26
  27 For more information on Rust's ownership system, take a look at the
  28 [References & Borrowing][references-and-borrowing] section of the Book.
  29
  30 [references-and-borrowing]: https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html