warning: unused variable: `a`
  --> $DIR/attributes-included.rs:17:9
   |
LL |     let a: i32 = "foo";
   |         ^ help: consider prefixing with an underscore: `_a`
   |
note: lint level defined here
  --> $DIR/attributes-included.rs:4:9
   |
LL | #![warn(unused)]
   |         ^^^^^^
   = note: #[warn(unused_variables)] implied by #[warn(unused)]