error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
  --> $DIR/issue-37139.rs:22:18
   |
LL |             test(&mut x); //~ ERROR cannot borrow immutable
   |                  ^^^^^^
   |                  |
   |                  cannot borrow as mutable
   |                  try removing `&mut` here

error: aborting due to previous error

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