rustc: always suggests #[macro_use] for undefined macros

[liigo]

Work in progress. Don't merge.

If people forget adding #[macro_use] (or even don't know about it), give them a suggestion.

// missing `#[macro_use]`
extern crate log;
fn main() {
    info!("hello");
}

play link: https://is.gd/nwB6ty

[rust-highfive]

r? @arielb1

(rust_highfive has picked a reviewer for you, use r? to override)

[steveklabnik]

it looks like @arielb1 never reviewed. Maybe someone else from @rust-lang/compiler ?

[eddyb]

r? @jseyfried

[jseyfried]

@liigo Thanks for the PR!

Let me know here or on IRC if you have any questions about my comments.

[jseyfried]

I think we should keep this here, but it should just say "have you added the #[macro_use] on the module?".

[jseyfried]

I think this should say "have you added the #[macro_use] on the extern crate", but only if there is a macro with that name in an extern crate.

You should be able to use resolver.lookup_import_candidates() (in librustc_resolve/lib.rs) with a filter_fn that checks for Def::Macros to see if there are any relevant macros in extern crates that the user might want to #[macro_use].

[pnkfelix]

Hmm, with this change, if I write:

#[macro_use]
extern crate log;
fn main() {
    infq!("hello");
}

won't the compiler now suggest I put in a #[macro_use] (in addition to suggesting renaming to info!) even though no new #[macro_use] is needed?

---

Update: oops, @jseyfried 's feedback has already covered this; I needed to refresh my page.

[liigo]

I'll try to work on according to review suggestions. Please don't merge util I'm done.

[alexcrichton]

Closing due to inactivity, but feel free to resubmit with comments addressed!