error[E0277]: expected a `std::ops::Fn<()>` closure, found `extern "C" fn() {f}`
  --> $DIR/extern-wrong-value-type.rs:9:5
   |
LL | fn is_fn<F>(_: F) where F: Fn() {}
   | ------------------------------- required by `is_fn`
...
LL |     is_fn(f);
   |     ^^^^^ expected an `Fn<()>` closure, found `extern "C" fn() {f}`
   |
   = help: the trait `std::ops::Fn<()>` is not implemented for `extern "C" fn() {f}`
   = note: wrap the `extern "C" fn() {f}` in a closure with no arguments: `|| { /* code */ }

error: aborting due to previous error

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