error[E0499]: cannot borrow `*r` as mutable more than once at a time
  --> tests/ui/column_borrow.rs:9:29
   |
8  |             let col1 = &mut r[0];
   |                             - first mutable borrow occurs here
9  |             let col2 = &mut r[1];
   |                             ^ second mutable borrow occurs here
10 |             assert_ne!(col1.get_str()?, col2.get_str()?);
   |                        -------------- first borrow later used here