"error: source trait is private" (fixed by adding `use` of *struct*)

[pnkfelix]

My attempt to call Any::get_type_id() and use its return value of as an instance of fmt::Debug does not work without import of std::any::TypeId struct: http://doc.rust-lang.org/std/any/struct.TypeId.html

Example code:

use std::thread;
// use std::any::TypeId; // uncomment this line to make the compile work

fn main() {
    let b1 = Box::new(3u64);
    println!("hello world, b1: {:?}", b1);
    let child = thread::Thread::scoped(|| {
        let b2 = Box::new(4u64);
        println!("other thread b2: {:?}", b2);
    });
    match child.join() {
        Ok(()) => {}
        Err(ref panic_arg) => {
            if let Some(s) = panic_arg.downcast_ref::<&str>() {
                println!("whoops child died with str: {}", s);
            } else if let Some(s) = panic_arg.downcast_ref::<String>() {
                println!("whoops child died with String: {}", s);
            } else {
                let id = panic_arg.get_type_id();
                println!("whoops child died with unknown panic arg, id: {:?}", id);
            }
        }
    }
}

Transcript of attempt to compile:

<anon>:19:26: 19:49 error: source trait is private
<anon>:19                 let id = panic_arg.get_type_id();
                                   ^~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
playpen: application terminated with error code 101

[arielb1]

importing std::any::Any also works (as get_type_id comes from it). I think this is an error message problem - it should at least talk about the relevant trait.

[pnkfelix]

@arielb1 but should we be worried that importing a struct fixed things here? I am used to needing to import traits in order to be able to call their methods. I'm not clear on why importing a struct fixed things here.

[steveklabnik]

Triage: updated code, made minimal:

use std::thread;
// use std::any::TypeId; // uncomment this line to make the compile work

fn main() {
    let child = thread::spawn(|| {
    });
    child.join().unwrap_err().get_type_id();
}

[jseyfried]

This happens because resolve only loads the items in an external module if this is needed for name resolution (i.e. if the module appears as a non-final segment in a path). Since we build the set of external exports while loading external modules, unused external modules are not counted as exported, leading the privacy checker to incorrectly consider them to be private.