edition idioms: Incorrect span in extern crate removal

[alexcrichton]

First reported upstream this code:

#![warn(rust_2018_idioms)]

#[cfg_attr(test, macro_use)]
extern crate itertools;

use itertools::Itertools;

fn main() {
    println!("{:?}", (0..1).collect_vec());
}

when compiled yields:

warning: unused extern crate
 --> src/main.rs:4:1
  |
4 | extern crate itertools;
  | ^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
  |
note: lint level defined here
 --> src/main.rs:1:9
  |
1 | #![warn(rust_2018_idioms)]
  |         ^^^^^^^^^^^^^^^^
  = note: #[warn(unused_extern_crates)] implied by #[warn(rust_2018_idioms)]

but the suggestion is incorrect! Both the extern crate and the attribute on the item should be removed.

[ExpHP]

Both the extern crate and the attribute on the item should be removed.

That works for the example above, but not if a #[test] uses macros. My original report would have required something like

#[cfg(test)] #[macro_use] extern crate itertools;

to fix it.

[estebank]

We could suggest both removal (with the correct span) or alternatively leaving it behind a cfg flag, using test as an example.

I think full removal suggestions that span multiple lines don't work as well, we should show probably one or two lines before and one or two lines after the removed code to make it clear of where the removal took place.

[ehuss]

See also #57420 for some more examples where cfg attributes cause problems with unused_extern_crates. In those cases, the lint probably shouldn't fire at all.