Detect broken crate attributes

[nox]

I want to avoid this embarrassing commit next time: servo/hyper_serde@b1f2eac

[oli-obk]

I just made the exact same mistake 2 hours ago...

[oli-obk]

So... that one would have been caught. But it would be nice if we'd detect empty lines between a #[allow(foo)] attribute and the next item. So I'm leaving this open

[mcarton]

Dupe of #770.

[phansch]

This seems straightforward for a first lint, so I'll work on detecting newlines after attributes in the next couple of days 👍

[phansch]

Hmm, as I learned in #1378 that clippy only works on the AST which doesn't include empty lines obviously. This seems a bit more difficult now, and maybe something for rustfmt instead? Or is there some other way to look for empty lines @oli-obk?

[oli-obk]

@phansch you can create a span from the attribute to the following item, then use the snippet_opt function to get the text and check whether there are empty newlines in that text