error[E0594]: cannot assign to `counter`, as it is a captured variable in a `Fn` closure
  --> $DIR/unboxed-closures-mutated-upvar-from-fn-closure.rs:11:9
   |
LL |         counter += 1;
   |         ^^^^^^^^^^^^ cannot assign
   |
help: consider changing this to accept closures that implement `FnMut`
  --> $DIR/unboxed-closures-mutated-upvar-from-fn-closure.rs:10:10
   |
LL |       call(|| {
   |  __________^
LL | |         counter += 1;
LL | |
LL | |     });
   | |_____^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0594`.