chore: fix unexpected cfg warning

.github/workflows/CI.yml

@@ -220,6 +220,22 @@ jobs:
       - name: Ensure that hyper.h is up to date
         run: ./capi/gen_header.sh --verify
 
+  unexpected-cfgs:
+    runs-on: ubuntu-latest
+    needs: [style]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@nightly
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo check --all-features
+        env:
+          RUSTFLAGS: |
+            -D unexpected_cfgs
+            --cfg check_unexpected_cfgs
+            --cfg hyper_unstable_tracing
+            --cfg hyper_unstable_ffi
+            --check-cfg=cfg(check_unexpected_cfgs,hyper_unstable_tracing,hyper_unstable_ffi)
+
   doc:
     name: Build docs
     needs: [style, test]

src/lib.rs

@@ -1,6 +1,6 @@
 #![deny(missing_docs)]
 #![deny(missing_debug_implementations)]
-#![allow(unexpected_cfgs)]
+#![cfg_attr(not(check_unexpected_cfgs), allow(unexpected_cfgs))]
 #![cfg_attr(test, deny(rust_2018_idioms))]
 #![cfg_attr(all(test, feature = "full"), deny(unreachable_pub))]
 #![cfg_attr(all(test, feature = "full"), deny(warnings))]