warning: unexpected `cfg` condition name: `feature`
  --> $DIR/cargo-feature.rs:10:7
   |
LL | #[cfg(feature = "serde")]
   |       ^^^^^^^^^^^^^^^^^
   |
   = help: consider defining some features in `Cargo.toml`
   = note: `#[warn(unexpected_cfgs)]` on by default

warning: 1 warning emitted