ci: run coverage via nextest to fix false tracing coverage gaps

.github/workflows/main.yml

@@ -395,13 +395,21 @@ jobs:
           enable-sccache: true
           rust-toolchain: RUST_NIGHTLY
           rust-components: llvm-tools-preview
-          cargo-tools: CARGO_LLVM_COV_VERSION
+          cargo-tools: CARGO_LLVM_COV_VERSION, CARGO_NEXTEST_VERSION
 
       # execute
+      # Coverage is generated with `llvm-cov nextest` (one process per test) instead
+      # of the default `cargo test` harness (all tests in one process). Process
+      # isolation prevents cross-test contamination of tracing's process-global
+      # callsite-interest cache: with the single-process harness, a no-subscriber test
+      # can register a `tracing::event!` callsite as "never interested" before a
+      # subscriber-installing test runs, leaving the macro's enabled branch (its
+      # field-value expressions) recorded as uncovered nondeterministically across
+      # platforms. This matches the `just coverage-measure` recipe (justfiles/coverage.just).
       - name: Generate Coverage (all-features)
-        run: cargo +${{ env.RUST_NIGHTLY }} llvm-cov --all-features --workspace --lcov --output-path lcov-all.info
+        run: cargo +${{ env.RUST_NIGHTLY }} llvm-cov nextest --all-features --workspace --lcov --output-path lcov-all.info
       - name: Generate Coverage (no-default-features)
-        run: cargo +${{ env.RUST_NIGHTLY }} llvm-cov --no-default-features --workspace --lcov --output-path lcov-no-def.info
+        run: cargo +${{ env.RUST_NIGHTLY }} llvm-cov nextest --no-default-features --workspace --lcov --output-path lcov-no-def.info
       - name: Upload Coverage to Codecov
         uses: codecov/codecov-action@v7.0.0
         with:

Cargo.toml

@@ -211,6 +211,15 @@ unreachable_pub = "warn"
 unsafe_op_in_unsafe_fn = "warn"
 unused_lifetimes = "warn"
 
+# Coverage builds (`cargo llvm-cov nextest`) compile the non-test library of
+# leaf crates whose `coverage(off)` attributes are all gated behind cargo
+# features or `cfg(test)`. In feature/target combinations where none of those
+# attributes are compiled, the crate-level `#![feature(coverage_attribute)]`
+# gate (enabled by the `coverage_nightly` cfg) is technically unused. That is a
+# benign artifact of instrumentation, not a real problem, so allow it rather
+# than encoding brittle per-crate feature conditions on every coverage gate.
+unused_features = "allow"
+
 # Allow our special-purpose nonstandard cfg attributes.
 # - coverage / coverage_nightly: set by cargo-llvm-cov to enable nightly coverage attributes.
 # - loom: set via RUSTFLAGS to swap in the loom model-checker for atomics in select crates.

crates/automation/src/lib.rs

@@ -4,7 +4,7 @@
 //! An unpublished crate for shared code used for writing Rust scripts
 
 #![allow(clippy::missing_errors_doc, reason = "this is an internal crate for scripts")]
-#![cfg_attr(all(coverage_nightly, test), feature(coverage_attribute))]
+#![cfg_attr(coverage_nightly, feature(coverage_attribute))]
 #![cfg_attr(coverage_nightly, coverage(off))]
 
 mod cargo;

crates/cachet/src/telemetry/cache.rs

@@ -142,6 +142,10 @@ impl CacheTelemetry {
         not(feature = "logs"),
         expect(clippy::unused_self, reason = "self.logging_enabled is used when logs is enabled")
     )]
+    #[cfg_attr(
+        not(any(feature = "logs", test)),
+        allow(unused_variables, reason = "consumed only by the logging path, which is compiled out here")
+    )]
     fn record_debug_with_duration(&self, cache_name: CacheName, event: &'static str, duration: Duration) {
         #[cfg(any(feature = "logs", test))]
         if self.logging_enabled {
@@ -151,16 +155,16 @@ impl CacheTelemetry {
                 cache.duration_ns = duration.as_nanos()
             );
         }
-        #[cfg(not(any(feature = "logs", test)))]
-        {
-            let _ = (cache_name, event, duration);
-        }
     }
 
     #[cfg_attr(
         not(feature = "logs"),
         expect(clippy::unused_self, reason = "self.logging_enabled is used when logs is enabled")
     )]
+    #[cfg_attr(
+        not(any(feature = "logs", test)),
+        allow(unused_variables, reason = "consumed only by the logging path, which is compiled out here")
+    )]
     fn record_info_with_duration(&self, cache_name: CacheName, event: &'static str, duration: Duration) {
         #[cfg(any(feature = "logs", test))]
         if self.logging_enabled {
@@ -170,16 +174,16 @@ impl CacheTelemetry {
                 cache.duration_ns = duration.as_nanos()
             );
         }
-        #[cfg(not(any(feature = "logs", test)))]
-        {
-            let _ = (cache_name, event, duration);
-        }
     }
 
     #[cfg_attr(
         not(feature = "logs"),
         expect(clippy::unused_self, reason = "self.logging_enabled is used when logs is enabled")
     )]
+    #[cfg_attr(
+        not(any(feature = "logs", test)),
+        allow(unused_variables, reason = "consumed only by the logging path, which is compiled out here")
+    )]
     fn record_error_with_duration(&self, cache_name: CacheName, event: &'static str, duration: Duration) {
         #[cfg(any(feature = "logs", test))]
         if self.logging_enabled {
@@ -189,10 +193,6 @@ impl CacheTelemetry {
                 cache.duration_ns = duration.as_nanos()
             );
         }
-        #[cfg(not(any(feature = "logs", test)))]
-        {
-            let _ = (cache_name, event, duration);
-        }
     }
 
     pub(crate) fn record_hit(&self, tier_name: CacheName, duration: Duration, fallback: bool) {

crates/fetch_hyper/src/connection/hyper_handler.rs

@@ -192,9 +192,7 @@ mod tests {
     use bytes::Bytes;
     use fetch_options::{ConnectionLifetime, PoolIndex, RequestFilter};
     use http::Version;
-    use http_body_util::BodyExt as _;
     use http_extensions::{HttpBodyBuilder, HttpRequestBuilder};
-    use layered::Service as _;
 
     use super::*;
     use crate::HyperTransport;

crates/fetch_hyper/src/lib.rs

@@ -1,7 +1,7 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
-#![cfg_attr(all(coverage_nightly, test), feature(coverage_attribute))]
+#![cfg_attr(coverage_nightly, feature(coverage_attribute))]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![doc(html_logo_url = "https://media.githubusercontent.com/media/microsoft/oxidizer/refs/heads/main/crates/fetch_hyper/logo.png")]
 #![doc(html_favicon_url = "https://media.githubusercontent.com/media/microsoft/oxidizer/refs/heads/main/crates/fetch_hyper/favicon.ico")]