Poor error message with #[warn(ctypes)]

[mrmonday]

The following code:

pub struct Bar;
#[repr(C)]
pub struct Foo {
    bar: Bar
}

extern {
    pub fn extern_func(foo: Foo);
}

fn main() {
}

(Playpen: http://is.gd/ANqwz9)

Gives the error:

<anon>:8:29: 8:32 warning: found type without foreign-function-safe
                                             representation annotation in foreign module, consider adding a #[repr(...)] attribute to the type, #[warn(ctypes)] on by default
<anon>:8     pub fn extern_func(foo: Foo);
                                     ^~~

Despite Foo being marked as #[repr(C)]. This should either:

• Include a note specifying which part of Foo is causing it to not be repr(C), or
• Cause an error, since Foo is not repr(C), despite being marked as such.

[huonw]

Dupe of #16719. (Thanks for filing!)