Confusing error message calling indirect function pointer

[emoon]

Hi,

When compiling this code

extern crate libc;

#[repr(C)]
pub struct ApiFromC {
    some_data: *mut libc::c_void,
    some_function: extern fn(data: *mut libc::c_void),
}

struct RustAPI {
    api_from_c: *mut ApiFromC,
}

impl RustAPI {
    fn call_c_api(&self) {
        unsafe {
            (*self.api_from_c).some_function((*self.api_from_c).some_data)
        }
    }
}

Playpen link: http://is.gd/BwFkak

The error message produced

<anon>:17:23: 17:66 error: no method named `some_function` found for type `ApiFromC` in the current scope
<anon>:17           (*self.api_from_c).some_function((*self.api_from_c).some_data)
                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:17:23: 17:66 note: did you mean to write `(*self.api_from_c).some_function`?
<anon>:17           (*self.api_from_c).some_function((*self.api_from_c).some_data)
                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
playpen: application terminated with error code 101

So the correct way to fix this code is by changing

    (*self.api_from_c).some_function((*self.api_from_c).some_data)

to

    ((*self.api_from_c).some_function)((*self.api_from_c).some_data)

But it's from the error message not very clear that's the issue.

[huonw]

Strangely enough, there is a custom error message to handle this case but it seems that it isn't emitted for fields that are extern fns, e.g. if the extern is removed from api_from_c's type, the output looks like:

<anon>:17:23: 17:66 error: no method named `some_function` found for type `ApiFromC` in the current scope
<anon>:17           (*self.api_from_c).some_function((*self.api_from_c).some_data)
                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:17:23: 17:66 note: use `((*self.api_from_c).some_function)(...)` if you meant to call the function stored in the `some_function` field
<anon>:17           (*self.api_from_c).some_function((*self.api_from_c).some_data)

So presumably some match or if somewhere in the compiler needs to understand that extern fns are fns.

[emoon]

@huonw Ah, that is interesting. Thanks for the info and shouldn't be that hard to fix then.