fix(ci): deny warnings in 'features' step

fixes #4232.

in #4231, we found that we don't deny warnings when running cargo hack,
because of the predicate that conditionally gates a crate-level
`#![deny(warnings)]` attribute in `lib.rs`:

```rust
 // src/lib.rs
 #![cfg_attr(all(test, feature = "full"), deny(unreachable_pub))]
 #![cfg_attr(all(test, feature = "full"), deny(warnings))]
 #![cfg_attr(all(test, feature = "nightly"), feature(test))]
```

while this prevents warnings from existing when running `cargo test
--features full`, it does not affect other builds. so, this would not be
active in the commands run by the `features` step in CI, which runs the
following two `cargo-hack` commands:

```sh
RUSTFLAGS="-D dead_code -D unused_imports" \
  cargo hack --no-dev-deps check \
  --feature-powerset --depth 2 --skip ffi,tracing

RUSTFLAGS="--cfg hyper_unstable_tracing -D dead_code -D unused_imports" \
  cargo hack --no-dev-deps check \
  --feature-powerset --depth 2 --features tracing --skip ffi
```

this commit introduces a new feature, intended for internal use in CI,
called `__deny_warnings`. this provides another mechanism to deny
warnings when building hyper.

`cargo hack` invocations run in CI are updated to enable this flag.
because `-D dead_code -D unused_imports` are now redundant, the
`RUSTFLAGS` values passed to these steps are also accordingly
simplified.

Signed-off-by: katelyn martin <git@katelyn.world>

Author: cratelyn

.github/workflows/CI.yml

@@ -181,14 +181,12 @@ jobs:
       - uses: Swatinem/rust-cache@v2
 
       - name: check --feature-powerset
-        run: cargo hack --no-dev-deps check --feature-powerset --depth 2 --skip ffi,tracing
-        env:
-          RUSTFLAGS: "-D dead_code -D unused_imports"
+        run: cargo hack --no-dev-deps check --feature-powerset --depth 2 --features __deny_warnings --skip ffi,tracing
 
       - name: check --feature-powerset with tracing feature
-        run: cargo hack --no-dev-deps check --feature-powerset --depth 2 --features tracing --skip ffi
+        run: cargo hack --no-dev-deps check --feature-powerset --depth 2 --features tracing,__deny_warnings --skip ffi
         env:
-          RUSTFLAGS: "--cfg hyper_unstable_tracing -D dead_code -D unused_imports"
+          RUSTFLAGS: "--cfg hyper_unstable_tracing"
 
   ffi:
     name: Test C API (FFI)

Cargo.toml

@@ -96,6 +96,7 @@ capi = []
 tracing = ["dep:tracing"]
 
 # internal features used in CI
+__deny_warnings = []
 nightly = []
 
 [lints.rust.unexpected_cfgs]

src/lib.rs

@@ -1,8 +1,10 @@
 #![deny(missing_docs)]
 #![deny(missing_debug_implementations)]
 #![cfg_attr(test, deny(rust_2018_idioms))]
-#![cfg_attr(all(test, feature = "full"), deny(unreachable_pub))]
-#![cfg_attr(all(test, feature = "full"), deny(warnings))]
+#![cfg_attr(
+    any(all(test, feature = "full"), feature = "__deny_warnings"),
+    deny(warnings, unreachable_pub)
+)]
 #![cfg_attr(all(test, feature = "nightly"), feature(test))]
 #![cfg_attr(docsrs, feature(doc_cfg))]