[guppy] fix an edge case with feature activation

As explained in the comment -- same-package features should not activate build
dependencies.

Discovered via upgrade to Cargo 0.81.0.

guppy/src/graph/cargo/build.rs

@@ -403,7 +403,26 @@ impl<'a> CargoSetBuildState<'a> {
             let proc_macro_redirect = follow_target && to.package().is_proc_macro();
 
             // Build dependencies are evaluated against the host platform.
-            let build_dep_redirect = is_enabled(&link, DependencyKind::Build, host_platform);
+            let build_dep_redirect = {
+                // If this is a dependency like:
+                //
+                // ```
+                // [build-dependencies]
+                // cc = { version = "1.0", optional = true }
+                //
+                // [features]
+                // bundled = ["cc"]
+                // ```
+                //
+                // Then, there is an implicit named feature here called "cc" on the target platform,
+                // which enables the optional dependency "cc". But this does not mean that this
+                // package itself is built on the host platform!
+                //
+                // Detect this situation by ensuring that the package ID of the `from` and `to`
+                // nodes are different.
+                from.package_id() != to.package_id()
+                    && is_enabled(&link, DependencyKind::Build, host_platform)
+            };
 
             // Finally, process what needs to be done.
             if build_dep_redirect || proc_macro_redirect {

guppy/src/graph/feature/graph_impl.rs

@@ -593,7 +593,7 @@ impl<'g> fmt::Display for FeatureLabel<'g> {
 }
 
 /// Metadata for a feature within a package.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy)]
 pub struct FeatureMetadata<'g> {
     graph: DebugIgnore<FeatureGraph<'g>>,
     node: FeatureNode,
@@ -641,6 +641,14 @@ impl<'g> FeatureMetadata<'g> {
     }
 }
 
+impl fmt::Debug for FeatureMetadata<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("FeatureMetadata")
+            .field("id", &self.feature_id())
+            .finish()
+    }
+}
+
 /// A graph representing every possible feature of every package, and the connections between them.
 #[derive(Clone, Debug)]
 pub(in crate::graph) struct FeatureGraphImpl {
@@ -722,7 +730,7 @@ impl FeatureGraphImpl {
 /// If a dependency, for example `unix-dep` above, is optional, an implicit feature is created in
 /// the package `main` with the name `unix-dep`. In this case, the dependency from `main/feat` to
 /// `main/unix-dep` is also a `ConditionalLink` representing the same `cfg(unix)` condition.
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone)]
 pub struct ConditionalLink<'g> {
     graph: DebugIgnore<FeatureGraph<'g>>,
     from: &'g FeatureMetadataImpl,
@@ -832,6 +840,18 @@ impl<'g> ConditionalLink<'g> {
     }
 }
 
+impl fmt::Debug for ConditionalLink<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("ConditionalLink")
+            .field("from", &self.from())
+            .field("to", &self.to())
+            .field("normal", &self.normal())
+            .field("build", &self.build())
+            .field("dev", &self.dev())
+            .finish()
+    }
+}
+
 // ---
 
 /// A combination of a package ID and a feature name, forming a node in a `FeatureGraph`.