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 | fn b<F:FnMut(isize, isize) -> isize>(f: F) {
   |                                      - help: consider changing this to be mutable: `mut f`
LL |     f(1, 2);
   |     ^ cannot borrow as mutable

error[E0382]: use of moved value: `f`
  --> $DIR/borrowck-unboxed-closures.rs:12:5
   |
LL | fn c<F:FnOnce(isize, isize) -> 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<F:FnOnce(isize, isize) -> 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`.