tests/ui/borrowck/borrowck-assign-comp.rs

   1 struct Point { x: isize, y: isize }
   2
   3 fn a() {
   4     let mut p = Point {x: 3, y: 4};
   5     let q = &p;
   6
   7     // This assignment is illegal because the field x is not
   8     // inherently mutable; since `p` was made immutable, `p.x` is now
   9     // immutable.  Otherwise the type of &_q.x (&isize) would be wrong.
  10     p.x = 5; //~ ERROR cannot assign to `p.x` because it is borrowed
  11     q.x;
  12 }
  13
  14 fn c() {
  15     // this is sort of the opposite.  We take a loan to the interior of `p`
  16     // and then try to overwrite `p` as a whole.
  17
  18     let mut p = Point {x: 3, y: 4};
  19     let q = &p.y;
  20     p = Point {x: 5, y: 7};//~ ERROR cannot assign to `p` because it is borrowed
  21     p.x; // silence warning
  22     *q; // stretch loan
  23 }
  24
  25 fn d() {
  26     // just for completeness's sake, the easy case, where we take the
  27     // address of a subcomponent and then modify that subcomponent:
  28
  29     let mut p = Point {x: 3, y: 4};
  30     let q = &p.y;
  31     p.y = 5; //~ ERROR cannot assign to `p.y` because it is borrowed
  32     *q;
  33 }
  34
  35 fn main() {
  36 }