warning: unused extern crate
  --> $DIR/edition-extern-crate-allowed.rs:7:1
   |
LL | extern crate edition_extern_crate_allowed;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
   |
note: lint level defined here
  --> $DIR/edition-extern-crate-allowed.rs:5:9
   |
LL | #![warn(rust_2018_idioms)]
   |         ^^^^^^^^^^^^^^^^
   = note: #[warn(unused_extern_crates)] implied by #[warn(rust_2018_idioms)]