error[E0596]: cannot borrow `b` as mutable, as it is not declared as mutable
  --> $DIR/issue-114896.rs:4:9
   |
LL |         let &b = a;
   |             -- help: consider changing this to be mutable: `&(mut b)`
LL |         b.make_ascii_uppercase();
   |         ^ cannot borrow as mutable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.