error[E0631]: type mismatch in function arguments
  --> $DIR/fn-variance-1.rs:11:15
   |
LL | fn takes_mut(x: &mut isize) { }
   | --------------------------- found signature of `for<'r> fn(&'r mut isize) -> _`
LL | 
LL | fn apply<T, F>(t: T, f: F) where F: FnOnce(T) {
   |                                     --------- required by this bound in `apply`
...
LL |     apply(&3, takes_mut);
   |               ^^^^^^^^^ expected signature of `fn(&{integer}) -> _`

error[E0631]: type mismatch in function arguments
  --> $DIR/fn-variance-1.rs:15:19
   |
LL | fn takes_imm(x: &isize) { }
   | ----------------------- found signature of `for<'r> fn(&'r isize) -> _`
...
LL | fn apply<T, F>(t: T, f: F) where F: FnOnce(T) {
   |                                     --------- required by this bound in `apply`
...
LL |     apply(&mut 3, takes_imm);
   |                   ^^^^^^^^^ expected signature of `fn(&mut {integer}) -> _`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0631`.