lint request: unnecessary extern crate

[Zaechus]

With the release of Rust 2018, many uses of extern crate are useless. Could there be a lint to recommend removing these occurrences?

[sondr3]

Isn't this something that cargo fix should take care of?

[Zaechus]

@sondr3 , I agree. This would be an easy cargo fix lint.

[mati865]

Rustfix already does it but it's buggy.

[oli-obk]

The compiler already has the rust_2018_idioms lint group. You can try if that already solves most of what you want. If not, please open an issue for concrete problems against rustc.