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 | let x = call_it(&square, 22); | ------- ^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }` | | | required by a bound introduced by this call | = help: the trait `for<'r> Fn<(&'r isize,)>` is not implemented for fn item `for<'r> unsafe fn(&'r isize) -> isize {square}` = note: unsafe function cannot be called generically without an unsafe block note: required by a bound in `call_it` --> $DIR/unboxed-closures-unsafe-extern-fn.rs:9:15 | LL | fn call_it isize>(_: &F, _: isize) -> isize { | ^^^^^^^^^^^^^^^^^^^ required by this bound in `call_it` 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 | let y = call_it_mut(&mut square, 22); | ----------- ^^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }` | | | required by a bound introduced by this call | = help: the trait `for<'r> FnMut<(&'r isize,)>` is not implemented for fn item `for<'r> unsafe fn(&'r isize) -> isize {square}` = note: unsafe function cannot be called generically without an unsafe block note: required by a bound in `call_it_mut` --> $DIR/unboxed-closures-unsafe-extern-fn.rs:12:19 | LL | fn call_it_mut isize>(_: &mut F, _: isize) -> isize { | ^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `call_it_mut` 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 | let z = call_it_once(square, 22); | ------------ ^^^^^^ call the function in a closure: `|| unsafe { /* code */ }` | | | required by a bound introduced by this call | = help: the trait `for<'r> FnOnce<(&'r isize,)>` is not implemented for fn item `for<'r> unsafe fn(&'r isize) -> isize {square}` = note: unsafe function cannot be called generically without an unsafe block note: required by a bound in `call_it_once` --> $DIR/unboxed-closures-unsafe-extern-fn.rs:15:20 | LL | fn call_it_once isize>(_: F, _: isize) -> isize { | ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `call_it_once` error: aborting due to 3 previous errors For more information about this error, try `rustc --explain E0277`.