error: useless lint attribute
 --> $DIR/useless_attribute.rs:5:1
  |
5 | #[allow(dead_code, unused_extern_crates)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![allow(dead_code, unused_extern_crates)]`
  |
  = note: `-D useless-attribute` implied by `-D warnings`