Description
Recently we got the unused_extern_crate lint (#42588). It still has an issue though with false positives around crates that contain natively linkable stuff, like -sys crates for example. The issue has been discussed in PR #42588 already, however it has been dismissed, because its a similar situation to the bad_style lint. There is a difference though, which is that you disable the lint at the declaration site, not the use site.
The difference became a very visible in my werk crate. I was having a bunch of integration tests each only linking to my werk crate to get the native API it offers, and I had to add #![allow(unused_extern_crate)] to every single integration test, while I had to allow the bad_style lint only in once place. That's not nice!
It would be cool if the false positive doesn't get fixed, there at least was a feature where I could do something with the werk crate so that the "unused" extern crate werk doesn't become a warning. This is not just interesting for my particular use case which is probably a bit niche compared to mainstream. Its also interesting if you look into the future (#44660), where #[allow(unused_extern_crate)] extern crate foo; becomes legacy, and you'd probably have to think about either having to add a way to disable the lint in Cargo.toml, or the only way to disable the lint will be globally for all dependencies by doing #![allow(unused_extern_crate)] in {lib,main}.rs. I think what I suggest is better than those two alternatives.
cc @aturon
cc @ishitatsuyuki