fix: proc-macro example from dep no longer affects feature resolution

Previously when checking if a dependency is a proc-macro,
the v2 feature resolve resolver v2 looks for `proc-macro = true`
for every target of the dependency.
However, it's impossible to depend on a non-lib target as a proc-macro.

This fix switches to only check if `proc-macro = true` for `[lib]`
target for a dependency.

src/cargo/core/resolver/features.rs

@@ -854,7 +854,7 @@ impl<'a, 'gctx> FeatureResolver<'a, 'gctx> {
                         // for various targets which are either specified in the manifest
                         // or on the cargo command-line.
                         let lib_fk = if fk == FeaturesFor::default() {
-                            (self.track_for_host && (dep.is_build() || self.has_any_proc_macro(dep_id)))
+                            (self.track_for_host && (dep.is_build() || self.has_proc_macro_lib(dep_id)))
                                 .then(|| FeaturesFor::HostDep)
                                 .unwrap_or_default()
                         } else {
@@ -966,4 +966,17 @@ impl<'a, 'gctx> FeatureResolver<'a, 'gctx> {
             .expect("packages downloaded")
             .proc_macro()
     }
+
+    /// Whether the given package is a proc macro lib target.
+    ///
+    /// This is useful for checking if a dependency is a proc macro,
+    /// as it is not possible to depend on a non-lib target as a proc-macro.
+    fn has_proc_macro_lib(&self, package_id: PackageId) -> bool {
+        self.package_set
+            .get_one(package_id)
+            .expect("packages downloaded")
+            .library()
+            .map(|lib| lib.proc_macro())
+            .unwrap_or_default()
+    }
 }

tests/testsuite/features2.rs

@@ -2686,9 +2686,13 @@ fn dont_unify_proc_macro_example_from_dependency() {
         .build();
 
     p.cargo("check")
-        .with_status(101)
-        .with_stderr_contains(
-            "[..]activated_features for invalid package: features did not find PackageId [..]pm_helper[..]NormalOrDev[..]"
+        .with_stderr(
+            "\
+[LOCKING] 2 packages to latest compatible versions
+[CHECKING] pm_helper v0.0.0 ([CWD]/pm_helper)
+[CHECKING] foo v0.0.0 ([CWD])
+[FINISHED] `dev` [..]
+",
         )
         .run();
 }