edition lint: migrating extern crate with #[macro_use]

[nrc]

The migration issue is that #[macro_use] extern crate foo; beings macros into scope from foo and extern crate is unidiomatic in the 2018 edition. local_inner_macros is the current solution but as discussed in #50911, we're not 100% that works.

[nikomatsakis]

I can't tell what the status is here. We have a lint (landed in #52275) but as far as I can tell (playground) it does not fire. In fact, I get no warnings at all when using extern crate in Rust 2018, even with idiom lints enabled. Is that expected?

(We do appear to have stabilized use_extern_macros in #50911.)

[nikomatsakis]

I don't think however that this needs to be a Rust 2018 RC2 blocker, since edition lints are not idiom enabled by default.

[nikomatsakis]

Clearing from milestone.

[pnkfelix]

@nikomatsakis is there a specific team we can tag this with?
That, or can you remove the nomination? (Would the team be T-compiler, or is there a different group dedicated to idiom lints?)

[pnkfelix]

I'm going to tag this with T-compiler, under the assumption that they will either address it or punt it to the appropriate other team.

[pnkfelix]

discussed at T-compiler meeting. @nikomatsakis says idiom lints are essentially "deferred". They want a working group focused on them; that, or T-compiler. But in any case, it doesn't need the nomination tag; we'll work our way through the queue of A-edition-2018-lints labelled things eventually

unnominating.

[joshtriplett]

This seems to have fallen through the cracks, and hasn't been looked at in years. We should consider whether, at this point, we can safely upgrade this lint to at least warn.

[nikomatsakis]

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=fd8f7c2bb6a4eb66a0f74bd5dae2a4c7

[nikomatsakis]

The behavior seems to be:

If you don't use any macros, you get a warning that #[macro_use] is not used.

If you do, the macro_use_extern_crate lint fires (but is allow-by-default).

I agree with @joshtriplett that we should at least warn and explain that this is deprecated.

[pnkfelix]

Just to clarify (since this issue hasn't gotten updates and the dialogue from years past led me to think that the lint as-implemented as not working):

• We have an allow-by-default macro_use_extern_crate lint.
• It does not fire unless there is an invocation of a macro imported via #[macro_use] (which explains why niko's playground from 2018 was not firing).
• We are generally in favor of making the lint warn-by-default instead of allow-by-default
• We would also be in favor of a better UX overall (e.g. suggesting the necessary change to the new idiom).

[traviscross]

@rustbot labels -I-lang-nominated

As @pnkfelix said above, we'd like to see a PR making this warn-by-default. We'll do the FCP on that PR, so please nominate it for us.