error: function `foo` is never used
  --> $DIR/with-core-crate.rs:7:4
   |
LL | fn foo() {
   |    ^^^
   |
note: the lint level is defined here
  --> $DIR/with-core-crate.rs:1:9
   |
LL | #![deny(dead_code)]
   |         ^^^^^^^^^

error: aborting due to previous error