error[E0277]: expected a `Fn<(&isize,)>` closure, found `for<'r> unsafe fn(&'r isize) -> isize {square}`
  --> $DIR/unboxed-closures-unsafe-extern-fn.rs:20:21
   |
LL | fn call_it<F: Fn(&isize) -> isize>(_: &F, _: isize) -> isize {
   |               ------------------- required by this bound in `call_it`
...
LL |     let x = call_it(&square, 22);
   |                     ^^^^^^^ expected an `Fn<(&isize,)>` closure, found `for<'r> unsafe fn(&'r isize) -> isize {square}`
   |
   = help: the trait `for<'r> Fn<(&'r isize,)>` is not implemented for `for<'r> unsafe fn(&'r isize) -> isize {square}`

error[E0277]: expected a `FnMut<(&isize,)>` closure, found `for<'r> unsafe fn(&'r isize) -> isize {square}`
  --> $DIR/unboxed-closures-unsafe-extern-fn.rs:25:25
   |
LL | fn call_it_mut<F: FnMut(&isize) -> isize>(_: &mut F, _: isize) -> isize {
   |                   ---------------------- required by this bound in `call_it_mut`
...
LL |     let y = call_it_mut(&mut square, 22);
   |                         ^^^^^^^^^^^ expected an `FnMut<(&isize,)>` closure, found `for<'r> unsafe fn(&'r isize) -> isize {square}`
   |
   = help: the trait `for<'r> FnMut<(&'r isize,)>` is not implemented for `for<'r> unsafe fn(&'r isize) -> isize {square}`

error[E0277]: expected a `FnOnce<(&isize,)>` closure, found `for<'r> unsafe fn(&'r isize) -> isize {square}`
  --> $DIR/unboxed-closures-unsafe-extern-fn.rs:30:26
   |
LL | fn call_it_once<F: FnOnce(&isize) -> isize>(_: F, _: isize) -> isize {
   |                    ----------------------- required by this bound in `call_it_once`
...
LL |     let z = call_it_once(square, 22);
   |                          ^^^^^^ expected an `FnOnce<(&isize,)>` closure, found `for<'r> unsafe fn(&'r isize) -> isize {square}`
   |
   = help: the trait `for<'r> FnOnce<(&'r isize,)>` is not implemented for `for<'r> unsafe fn(&'r isize) -> isize {square}`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0277`.