warning: unexpected `cfg` condition name: `unknown_key`
  --> $DIR/exhaustive-names.rs:7:7
   |
LL | #[cfg(unknown_key = "value")]
   |       ^^^^^^^^^^^^^^^^^^^^^
   |
   = help: to expect this configuration use `--check-cfg=cfg(unknown_key, values("value"))`
   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
   = note: `#[warn(unexpected_cfgs)]` on by default

warning: 1 warning emitted