`#![crate_id="x"]` attribute emits false (and useless) root module warning

[pnkfelix]

(Spawned off of issue #43106)

Compiling this code (in stable/beta/nightly) results in the following compiler warnings:

#![crate_id = "x"]
fn main() { }

warnings:

warning: unused attribute
 --> <anon>:1:1
  |
1 | #![crate_id = "x"]
  | ^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_attributes)] on by default

warning: crate-level attribute should be in the root module
 --> <anon>:1:1
  |
1 | #![crate_id = "x"]
  | ^^^^^^^^^^^^^^^^^^

The first warning is legitimate, since we killed off #![crate_id] back in RFC 109. The fact that it exists as an attribute in the code base is probably for backwards compatibility.

• (The fact that it still has occurrences in the run-pass test suite is less explicable, since it should have no effect in the current rustc...)

The second warning is quite confusing, since the attribute is in the root module.

Can we easily kill off the second warning in this case?