`extern crate` epoch lint for Path Clarity RFC (2126)

[Manishearth]

We need to add an epoch lint for RFC 2126

It will lint extern crate foobar;, suggesting removal in root mods, and replacement with use foobar; in non-root mods.

This is blocked on the ability to elide extern crate in the first place.

[est31]

It is also blocked on a feature to force linkage of a crate, see this comment and the discussion afterwards: #44660 (comment)

[Manishearth]

cc @cramertj

[golddranks]

I think the lint should be by default off in cases with macro_use, such as:

#[macro_use] extern crate slog;

This is because of the ergonomics problems with macro_rules! macros and the new macro import system, expressed here: https://internals.rust-lang.org/t/help-stabilize-a-subset-of-macros-2-0/7252/18?u=golddranks

It's bewildering for the migrating users if a lint tells them not to use the extern crate syntax, but then finding out that many of the macros out there have downright bad ergonomics with the use import syntax.

(For clarification: I think this is an undesirable state of affairs, and we should be linting against all extern crates, but if we do that, the ergonomics problem with the macro import should be fixed somehow.)

[alexreg]

I don't think we want extern crate at all, really. If we want to import macros, we should have another means of doing it. Otherwise we have an inconsistency. So yes, I think I agree with your last paragraph above all, @golddranks.

[est31]

@golddranks @alexreg I've discussed this a bit in the tracking issue for the macros import system: #35896 (comment)

[golddranks]

@est31 Thanks, you expressed the exact concern I had in a concise way.

[Manishearth]

Another thing this needs to lint is unlisted crates that are included in Cargo.toml that clash with existing modules.

I'm not quite sure on what the story needs to be here cc @rust-lang/lang

[est31]

Does it? I've thought one of the things the path clarity RFC was about was to have separate namespaces for top level modules and extern crates.

[Manishearth]

The current plan has changed a bit so I'm not sure if that is still true.

(If stuff does change it will probably be RFCd again, there's a bit of experimentation going on)

[est31]

Sigh I hope we won't discuss all of this crap all over again... I don't enjoy that but it is not me who wants to change things.