error[E0502]: cannot borrow `f` as immutable because it is also borrowed as mutable --> $DIR/borrowck-unboxed-closures.rs:3:5 | LL | let g = &mut f; | ------ mutable borrow occurs here LL | f(1, 2); | ^ immutable borrow occurs here LL | use_mut(g); | - mutable borrow later used here error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable --> $DIR/borrowck-unboxed-closures.rs:7:5 | LL | f(1, 2); | ^ cannot borrow as mutable | help: consider changing this to be mutable | LL | fn b isize>(mut f: F) { | +++ error[E0382]: use of moved value: `f` --> $DIR/borrowck-unboxed-closures.rs:12:5 | LL | fn c isize>(f: F) { | - move occurs because `f` has type `F`, which does not implement the `Copy` trait LL | f(1, 2); | ------- `f` moved due to this call LL | f(1, 2); | ^ value used here after move | note: this value implements `FnOnce`, which causes it to be moved when called --> $DIR/borrowck-unboxed-closures.rs:11:5 | LL | f(1, 2); | ^ help: consider further restricting this bound | LL | fn c isize + Copy>(f: F) { | ++++++ error: aborting due to 3 previous errors Some errors have detailed explanations: E0382, E0502, E0596. For more information about an error, try `rustc --explain E0382`.