warning: unexpected `cfg` condition name: `unknown_key`
  --> $DIR/exhaustive-names.rs:8:7
   |
LL | #[cfg(unknown_key = "value")]
   |       ^^^^^^^^^^^^^^^^^^^^^
   |
   = help: expected names are: `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
   = note: `#[warn(unexpected_cfgs)]` on by default

warning: 1 warning emitted