error[E0596]: cannot borrow `tick` as mutable, as it is not declared as mutable
  --> $DIR/unboxed-closures-infer-fnmut-missing-mut.rs:7:5
   |
LL |     let tick = || counter += 1;
   |                   ------- calling `tick` requires mutable binding due to mutable borrow of `counter`
LL |     tick();
   |     ^^^^ cannot borrow as mutable
   |
help: consider changing this to be mutable
   |
LL |     let mut tick = || counter += 1;
   |         +++

error: aborting due to 1 previous error

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