Suggest possible fix for missing library link

[hellertime]

I'm attempting to learn to write programs using Rust. One problem I encountered early on was errors about failed imports due to unresolved names. The problem was I overlooked the need to declare the link to std before importing names into my module.

I thought rustc could use a more helpful diagnostic message when it encounters this issue, so I went ahead and implemented a reference fix. Hopefully the message will be useful to future Rust programmers.

[hellertime]

So I've built a small test for this case, but I'm not entirely sure how to set it up for the test runner. The docs say that using error-pattern: ... is frowned upon and instead to make use of //~ ERROR comments. However, I am reporting my message as a note, should I still use an //~ ERROR comment?

The documentation on this matter is rather thin. At least any documentation I have found. Pointers to more relevant info would be welcome.

[hellertime]

I cleaned up the history of this pull request, but it seems that comments are attached to the history as well -- which I wasn't aware was the case. So apologies for screwing up the comment history of this request.

For future reference, is it acceptable to rewrite pull request history, or would it be preferred to close out a pull request and open a new one with the rewritten history?

[jdm]

Rewritten is fine.

[kud1ing]

Good idea

[hellertime]

Thanks. I wasn't sure if note_span is the best approach to this. Ideally there could be multiline messages output much like GHC's warning format. I've see some wrapped comments in the source but the output isn't formatted nicely. However enhancing the diagnostic formatter is a project unto itself.

[bstrie]

Will this suggest that people do extern mod foo; in the cases where what they really want is just mod foo; ? Is the same code path used for each?

[hellertime]

I'm still new enough to rust that I need to ask if you could provide an example of such a case, I can certainly test it.

[brson]

Per @bstrie , this seems like it could invoke additional confusion.

e.g. if you write

mod foo {
    mod bar {
        use baz::*;
    }
    mod baz {
    }
}

It will suggest you extern mod baz, but you really probably wanted `use super::baz::*.

Also:

mod foo {
   use std::*;
}

rustc will suggest you add extern mod std, so you write

mod foo {
    extern mod std;
    use std::*;
}

That of course doesn't work because use searches from the top level.

If we're going to suggest extern mod foo it would be pretty easy to first search for a crate matching foo using metadata::loader::find_library_crate.

[brson]

Ok, I think this branch needs a little more work, but this seems like a useful idea so I opened #6379 on this subject. Closing for now.