error[E0596]: cannot borrow `callback` as mutable, as it is not declared as mutable --> $DIR/issue-80313-mutation-in-closure.rs:6:5 | LL | my_var = true; | ------ calling `callback` requires mutable binding due to mutable borrow of `my_var` LL | }; LL | callback(); | ^^^^^^^^ cannot borrow as mutable | help: consider changing this to be mutable | LL | let mut callback = || { | +++ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0596`.