Freezing

When data is immutably borrowed, it also freezes. Frozen data can’t be
modified via the original object until all references to it go out of scope:

  1. fn main() {
  2.     let mut _mutable_integer = 7i32;
  4.     {
  5.         // Borrow `_mutable_integer`
  6.         let _large_integer = &_mutable_integer;
  8.         // Error! `_mutable_integer` is frozen in this scope
  9.         _mutable_integer = 50;
  10.         // FIXME ^ Comment out this line
  12.         // `_large_integer` goes out of scope
  13.     }
  15.     // Ok! `_mutable_integer` is not frozen in this scope
  16.     _mutable_integer = 3;
  17. }