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

warning: `extern crate` is not idiomatic in the new edition
  --> $DIR/remove-extern-crate.rs:32:5
   |
LL |     extern crate core;
   |     ^^^^^^^^^^^^^^^^^^ help: convert it to a `use`