fix(compile): Ignore unused deps if also transitive

This is to reduce false positives without having to ignore them.
In fact, I plan to revert support for `ignore` after this change.

Author: epage

src/cargo/core/compiler/unused_deps.rs

@@ -98,9 +98,13 @@ impl UnusedDepState {
                 } else {
                     continue;
                 };
-                state
-                    .externs
-                    .insert(dep.extern_crate_name, ExternState { manifest_deps });
+                state.externs.insert(
+                    dep.extern_crate_name,
+                    ExternState {
+                        unit: dep.unit.clone(),
+                        manifest_deps,
+                    },
+                );
             }
         }
 
@@ -229,6 +233,14 @@ impl UnusedDepState {
                         );
                         continue;
                     }
+                    if is_transitive_dep(&extern_state.unit, &state.unused_externs, build_runner) {
+                        debug!(
+                            "pkg {} v{} ({dep_kind:?}): ignoring unused extern `{ext}`, may be activating features",
+                            pkg_id.name(),
+                            pkg_id.version(),
+                        );
+                        continue;
+                    }
 
                     // Implicitly added dependencies (in the same crate) aren't interesting
                     let dependency = if let Some(dependency) = &extern_state.manifest_deps {
@@ -332,6 +344,7 @@ struct DependenciesState {
 
 #[derive(Clone)]
 struct ExternState {
+    unit: Unit,
     manifest_deps: Option<Vec<Dependency>>,
 }
 
@@ -359,3 +372,34 @@ fn unit_desc(unit: &Unit) -> String {
         unit.mode,
     )
 }
+
+#[instrument(skip_all)]
+fn is_transitive_dep(
+    direct_dep_unit: &Unit,
+    unused_externs: &IndexMap<Unit, Vec<InternedString>>,
+    build_runner: &mut BuildRunner<'_, '_>,
+) -> bool {
+    let mut queue = std::collections::VecDeque::new();
+    for root_unit in unused_externs.keys() {
+        for unit_dep in build_runner.unit_deps(root_unit) {
+            if root_unit.pkg.package_id() == unit_dep.unit.pkg.package_id() {
+                continue;
+            }
+            if unit_dep.unit == *direct_dep_unit {
+                continue;
+            }
+            queue.push_back(&unit_dep.unit);
+        }
+    }
+
+    while let Some(dep_unit) = queue.pop_front() {
+        for unit_dep in build_runner.unit_deps(dep_unit) {
+            if unit_dep.unit == *direct_dep_unit {
+                return true;
+            }
+            queue.push_back(&unit_dep.unit);
+        }
+    }
+
+    false
+}

tests/testsuite/lints/unused_dependencies.rs

@@ -1320,17 +1320,6 @@ pub fn fun() -> &'static str {
 [CHECKING] transitive v0.1.1
 [CHECKING] intermediate v0.1.0
 [CHECKING] foo v0.1.0 ([ROOT]/foo)
-[WARNING] unused dependency
-  --> Cargo.toml:10:13
-   |
-10 |             transitive = { version = "0.1.1", features = ["a"] }
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = [NOTE] `cargo::unused_dependencies` is set to `warn` in `[lints]`
-[HELP] remove the dependency
-   |
-10 -             transitive = { version = "0.1.1", features = ["a"] }
-   |
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 
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