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 | fn call<F>(f: F) where F : Fn() {
   |               - change this to accept `FnMut` instead of `Fn`
...
LL |     call(|| {
   |     ---- -- in this closure
   |     |
   |     expects `Fn` instead of `FnMut`
LL |         counter += 1;
   |         ^^^^^^^^^^^^ cannot assign

error: aborting due to previous error

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