Debug not implemeted for function pointers

[gkoz]

extern fn foo() { }

fn main() {
    let ext: extern fn() = foo;
    let f: fn() = main;
    print!("{:?} {:?} {:?} {:?}", ext, foo, f, main);
}

<anon>:6:35: 6:38 error: the trait `core::fmt::Debug` is not implemented for the type `extern "C" fn()` [E0277]
<anon>:6     print!("{:?} {:?} {:?} {:?}", ext, foo, f, main);
                                           ^~~
<anon>:6:40: 6:43 error: the trait `core::fmt::Debug` is not implemented for the type `extern "C" fn() {foo}` [E0277]
<anon>:6     print!("{:?} {:?} {:?} {:?}", ext, foo, f, main);
                                                ^~~
<anon>:6:45: 6:46 error: the trait `core::fmt::Debug` is not implemented for the type `fn()` [E0277]
<anon>:6     print!("{:?} {:?} {:?} {:?}", ext, foo, f, main);
                                                     ^
<anon>:6:48: 6:52 error: the trait `core::fmt::Debug` is not implemented for the type `fn() {main}` [E0277]
<anon>:6     print!("{:?} {:?} {:?} {:?}", ext, foo, f, main);
                                                        ^~~~

While it might be possible to explain away the last three errors, the first one makes it impossible to derive Debug for structs that contain C callbacks.

[petrochenkov]

rust-lang/rust#26082

[gkoz]

@petrochenkov
Thanks.
So it seems the general situation is gloomy but the extern "C" fns that don't use references could still get their impls.

[petrochenkov]

Resolved by rust-lang/rust#28268

[alexcrichton]

Thanks @petrochenkov