[cratelyn]

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:

 // 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:

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

[cratelyn]

i preserved the existing all(test, feature = "full") predicate. i think that is probably worthwhile because it seems like a good ergonomic decision to have cargo test --features full continue to deny warnings, as a concise way to run tests and check for warnings when developing a feature.

[cratelyn]

i used two leading underscores in the name of this, following the convention set by previous feature flags like __internal_flaky_tests and __internal_happy_eyeballs_tests.