fix(compile): Ignore unused deps if also transitive (#16935)

arlosi · web-flow · commit 32aa9b5fb068 · 2026-04-23T17:58:08.000Z
### What does this PR try to resolve? This changes `unused_dependencies` lint to reduce the chance of false positives while we work out the story of how we want people to response to false positives (rust-lang/rfcs#3920). ### How to test and review this PR? My assumption is that I will revert the `ignore` lint control added in #16600 after this. Looking at the known cases of false positives, my short term plan is: - transitive version constraint: don't lint - transitive feature activation: don't lint - dynamically used dep (e.g. `build.rs` in `curl`): users must `allow` Longer term, I'd like to replace this change with either - `lib = false` from artifact deps - `cfg(resolver = "versions")`, [#t-cargo > Additional dependency tables @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/Additional.20dependency.20tables/near/590208010) We can evolve this because the exact nature of warning-by-default lints is not stable.
diff --git a/src/cargo/core/compiler/unused_deps.rs b/src/cargo/core/compiler/unused_deps.rs
@@ -98,7 +98,13 @@ impl UnusedDepState {
                 } else {
                     continue;
                 };
-                state.externs.insert(dep.extern_crate_name, manifest_deps);
+                state.externs.insert(
+                    dep.extern_crate_name,
+                    ExternState {
+                        unit: dep.unit.clone(),
+                        manifest_deps,
+                    },
+                );
             }
         }
 
@@ -213,7 +219,7 @@ impl UnusedDepState {
                     continue;
                 }
 
-                for (ext, dependency) in &state.externs {
+                for (ext, extern_state) in &state.externs {
                     if state
                         .unused_externs
                         .values()
@@ -227,9 +233,17 @@ 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) = dependency {
+                    let dependency = if let Some(dependency) = &extern_state.manifest_deps {
                         dependency
                     } else {
                         continue;
@@ -318,7 +332,7 @@ impl UnusedDepState {
 #[derive(Default)]
 struct DependenciesState {
     /// All declared dependencies
-    externs: IndexMap<InternedString, Option<Vec<Dependency>>>,
+    externs: IndexMap<InternedString, ExternState>,
     /// Expected [`Self::unused_externs`] entries to know we've received them all
     ///
     /// To avoid warning in cases where we didn't,
@@ -328,6 +342,12 @@ struct DependenciesState {
     unused_externs: IndexMap<Unit, Vec<InternedString>>,
 }
 
+#[derive(Clone)]
+struct ExternState {
+    unit: Unit,
+    manifest_deps: Option<Vec<Dependency>>,
+}
+
 fn dep_kind_of(unit: &Unit) -> DepKind {
     match unit.target.kind() {
         TargetKind::Lib(_) => match unit.mode {
@@ -352,3 +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
+}
diff --git a/tests/testsuite/lints/unused_dependencies.rs b/tests/testsuite/lints/unused_dependencies.rs
@@ -968,7 +968,9 @@ fn package_selection() {
     Package::new("used_bar", "0.1.0").publish();
     Package::new("used_foo", "0.1.0").publish();
     Package::new("used_external", "0.1.0").publish();
-    Package::new("unused", "0.1.0").publish();
+    Package::new("unused_bar", "0.1.0").publish();
+    Package::new("unused_foo", "0.1.0").publish();
+    Package::new("unused_external", "0.1.0").publish();
     let p = project()
         .file(
             "Cargo.toml",
@@ -988,7 +990,7 @@ fn package_selection() {
             edition = "2018"
 
             [dependencies]
-            unused = "0.1.0"
+            unused_foo = "0.1.0"
             used_foo = "0.1.0"
             bar.path = "../bar"
             external.path = "../external"
@@ -1013,7 +1015,7 @@ fn package_selection() {
             edition = "2018"
 
             [dependencies]
-            unused = "0.1.0"
+            unused_bar = "0.1.0"
             used_bar = "0.1.0"
 
             [lints.cargo]
@@ -1036,7 +1038,7 @@ fn package_selection() {
             edition = "2018"
 
             [dependencies]
-            unused = "0.1.0"
+            unused_external = "0.1.0"
             used_external = "0.1.0"
 
             [lints.cargo]
@@ -1056,30 +1058,33 @@ fn package_selection() {
         .with_stderr_data(
             str![[r#"
 [UPDATING] `dummy-registry` index
+[LOCKING] 7 packages to latest compatible versions
 [DOWNLOADING] crates ...
-[CHECKING] bar v0.1.0 ([ROOT]/foo/bar)
-[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
-[LOCKING] 5 packages to latest compatible versions
-[DOWNLOADED] used_foo v0.1.0 (registry `dummy-registry`)
-[DOWNLOADED] used_external v0.1.0 (registry `dummy-registry`)
+[DOWNLOADED] unused_bar v0.1.0 (registry `dummy-registry`)
+[DOWNLOADED] unused_external v0.1.0 (registry `dummy-registry`)
+[DOWNLOADED] unused_foo v0.1.0 (registry `dummy-registry`)
 [DOWNLOADED] used_bar v0.1.0 (registry `dummy-registry`)
-[DOWNLOADED] unused v0.1.0 (registry `dummy-registry`)
-[CHECKING] unused v0.1.0
+[DOWNLOADED] used_external v0.1.0 (registry `dummy-registry`)
+[DOWNLOADED] used_foo v0.1.0 (registry `dummy-registry`)
 [CHECKING] used_bar v0.1.0
+[CHECKING] unused_external v0.1.0
 [CHECKING] used_external v0.1.0
+[CHECKING] unused_bar v0.1.0
 [CHECKING] used_foo v0.1.0
+[CHECKING] unused_foo v0.1.0
+[CHECKING] bar v0.1.0 ([ROOT]/foo/bar)
 [CHECKING] external v0.1.0 ([ROOT]/foo/external)
 [CHECKING] foo v0.1.0 ([ROOT]/foo/foo)
 [WARNING] unused dependency
  --> bar/Cargo.toml:9:13
   |
-9 |             unused = "0.1.0"
-  |             ^^^^^^^^^^^^^^^^
+9 |             unused_bar = "0.1.0"
+  |             ^^^^^^^^^^^^^^^^^^^^
   |
   = [NOTE] `cargo::unused_dependencies` is set to `warn` in `[lints]`
 [HELP] remove the dependency
   |
-9 -             unused = "0.1.0"
+9 -             unused_bar = "0.1.0"
   |
 [WARNING] unused dependency
   --> foo/Cargo.toml:11:13
@@ -1107,13 +1112,14 @@ fn package_selection() {
 [WARNING] unused dependency
  --> foo/Cargo.toml:9:13
   |
-9 |             unused = "0.1.0"
-  |             ^^^^^^^^^^^^^^^^
+9 |             unused_foo = "0.1.0"
+  |             ^^^^^^^^^^^^^^^^^^^^
   |
 [HELP] remove the dependency
   |
-9 -             unused = "0.1.0"
+9 -             unused_foo = "0.1.0"
   |
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 
 "#]]
             .unordered(),
@@ -1124,7 +1130,6 @@ fn package_selection() {
         .masquerade_as_nightly_cargo(&["cargo-lints"])
         .with_stderr_data(
             str![[r#"
-[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 [WARNING] unused dependency
   --> foo/Cargo.toml:11:13
    |
@@ -1151,13 +1156,14 @@ fn package_selection() {
 [WARNING] unused dependency
  --> foo/Cargo.toml:9:13
   |
-9 |             unused = "0.1.0"
-  |             ^^^^^^^^^^^^^^^^
+9 |             unused_foo = "0.1.0"
+  |             ^^^^^^^^^^^^^^^^^^^^
   |
 [HELP] remove the dependency
   |
-9 -             unused = "0.1.0"
+9 -             unused_foo = "0.1.0"
   |
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 
 "#]]
             .unordered(),
@@ -1168,18 +1174,18 @@ fn package_selection() {
         .masquerade_as_nightly_cargo(&["cargo-lints"])
         .with_stderr_data(
             str![[r#"
-[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 [WARNING] unused dependency
  --> bar/Cargo.toml:9:13
   |
-9 |             unused = "0.1.0"
-  |             ^^^^^^^^^^^^^^^^
+9 |             unused_bar = "0.1.0"
+  |             ^^^^^^^^^^^^^^^^^^^^
   |
   = [NOTE] `cargo::unused_dependencies` is set to `warn` in `[lints]`
 [HELP] remove the dependency
   |
-9 -             unused = "0.1.0"
+9 -             unused_bar = "0.1.0"
   |
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 
 "#]]
             .unordered(),
@@ -1314,17 +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
 
 "#]]
