Scattered extern-crate/use/mod/… statements

[regexident]

In addition to https://github.com/Manishearth/rust-clippy/issues/879 I'd love to have a lint that enforces my header statements (extern crate …, mod …, use …, …) to be cleanly grouped.

So that this would trigger a warning:

mod …;

use std::…;

mod …;

While this wouldn't:

mod …;
mod …;

use std::…;

Nor would this:

use std::…;

mod …;
mod …;

Bonus points for warning on missing/excessive newlines between groups.

[regexident]

This is would probably be a candidate for pedantic.

[regexident]

Probably another candidate for rustfmt. Oh well. Sorry for spamming (#879, #880, #881). 😇 😞

[llogiq]

No problem. We're just pushing back because we think your time is better spent with other lints. 😄

[regexident]

😘