Suggestion in warning for extern function pointers broken

[gnzlbg]

https://play.rust-lang.org/?gist=d8a933822fb81608431b4001e6bae154&version=stable&mode=debug&edition=2015

Given this

#![crate_type = "rlib"]

extern "C" {
    pub static mut foo: fn()->(); // WARNS
    //pub static mut bar: fn "extern"()->(); // ERRORS
    pub static mut baz: extern fn()->(); // OK
}

the warning on foo is:

|
4 |     pub static mut foo: fn()->(); // WARNS
  |                         ^^^^^^^^
  |
  = note: #[warn(improper_ctypes)] on by default
  = help: consider using an `fn "extern"(...) -> ...` function pointer instead

The code suggested does not work though, it should be extern fn(...) -> ... instead.