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`.