Ensure plug-in C modules don't mix with target ones.

Sources/Build/BuildDescription/ModuleBuildDescription.swift

@@ -192,4 +192,17 @@ extension ModuleBuildDescription {
         }
         return dependencies
     }
+
+    package func recursiveLinkDependencies(using plan: BuildPlan) -> [Dependency] {
+        var dependencies: [Dependency] = []
+        plan.traverseDependencies(of: self) {
+            // Filter out plugin dependencies
+            $0.module?.type != .plugin
+        } onProduct: { product, _, description in
+            dependencies.append(.product(product, description))
+        } onModule: { module, _, description in
+            dependencies.append(.module(module, description))
+        }
+        return dependencies
+    }
 }

Sources/Build/BuildDescription/SwiftModuleBuildDescription.swift

@@ -1018,4 +1018,10 @@ extension SwiftModuleBuildDescription {
     ) -> [ModuleBuildDescription.Dependency] {
         ModuleBuildDescription.swift(self).recursiveDependencies(using: plan)
     }
+
+    package func recursiveLinkDependencies(
+        using plan: BuildPlan
+    ) -> [ModuleBuildDescription.Dependency] {
+        ModuleBuildDescription.swift(self).recursiveLinkDependencies(using: plan)
+    }
 }

Sources/Build/BuildPlan/BuildPlan+Swift.swift

@@ -20,12 +20,13 @@ extension BuildPlan {
     func plan(swiftTarget: SwiftModuleBuildDescription) throws {
         // We need to iterate recursive dependencies because Swift compiler needs to see all the targets a target
         // depends on.
-        for case .module(let dependency, let description) in swiftTarget.recursiveDependencies(using: self) {
+        for case .module(let dependency, let description) in swiftTarget.recursiveLinkDependencies(using: self) {
             switch dependency.underlying {
             case let underlyingTarget as ClangModule where underlyingTarget.type == .library:
                 guard case let .clang(target)? = description else {
                     throw InternalError("unexpected clang target \(underlyingTarget)")
                 }
+
                 // Add the path to modulemap of the dependency. Currently we require that all Clang targets have a
                 // modulemap but we may want to remove that requirement since it is valid for a target to exist without
                 // one. However, in that case it will not be importable in Swift targets. We may want to emit a warning

Sources/Build/BuildPlan/BuildPlan.swift

@@ -1142,6 +1142,7 @@ extension BuildPlan {
 
     package func traverseDependencies(
         of description: ModuleBuildDescription,
+        filter: (ResolvedModule.Dependency) -> Bool = { _ in true },
         onProduct: (ResolvedProduct, BuildParameters.Destination, ProductBuildDescription?) -> Void,
         onModule: (ResolvedModule, BuildParameters.Destination, ModuleBuildDescription?) -> Void
     ) {
@@ -1163,6 +1164,7 @@ extension BuildPlan {
         ) -> [TraversalNode] {
             module
                 .dependencies(satisfying: description.buildParameters.buildEnvironment)
+                .filter({ filter($0) })
                 .reduce(into: [TraversalNode]()) { partial, dependency in
                     switch dependency {
                     case .product(let product, _):

Tests/BuildTests/BuildPlanTests.swift

@@ -6736,4 +6736,74 @@ final class BuildPlanTests: XCTestCase {
             )
         }
     }
+
+    func testHostTargetCrossingStreams() async throws {
+        let fs = InMemoryFileSystem(emptyFiles: [
+            "/Pkg/Plugins/CCrossingStreams/plugin.swift",
+            "/Pkg/Sources/CLibrary/lib.c",
+            "/Pkg/Sources/CLibrary/include/lib.h",
+            "/Pkg/Sources/Exe/main.swift",
+            "/Pkg/Sources/Tool/main.swift",
+        ])
+
+        let observability = ObservabilitySystem.makeForTesting()
+
+        let graph = try loadModulesGraph(
+            fileSystem: fs,
+            manifests: [
+                .createRootManifest(
+                    displayName: "Pkg",
+                    path: "/Pkg",
+                    targets: [
+                        .init(name: "CLibrary"),
+                        .init(
+                            name: "Tool",
+                            dependencies: [
+                                "CLibrary"
+                            ],
+                            type: .executable
+                        ),
+                        .init(
+                            name: "CCrossingStreams",
+                            dependencies: [
+                                .target(name: "Tool"),
+                            ],
+                            type: .plugin,
+                            pluginCapability: .buildTool
+                        ),
+                        .init(
+                            name: "Exe",
+                            dependencies: [
+                                .target(name: "CLibrary"),
+                            ],
+                            type: .executable,
+                            pluginUsages: [
+                                .plugin(name: "CCrossingStreams", package: nil),
+                            ]
+                        )
+                    ]
+                )
+            ],
+            observabilityScope: observability.topScope
+        )
+
+        let result = try await BuildPlanResult(plan: mockBuildPlan(
+          graph: graph,
+          fileSystem: fs,
+          observabilityScope: observability.topScope
+        ))
+
+        let exe = try result.moduleBuildDescription(for: "Exe").swift()
+        // Ensure the exe doesn't have the tool version of the CLibrary modulemap in it's flags
+        XCTAssertFalse(exe.additionalFlags.contains(where: { $0.hasSuffix("CLibrary-tool.build/module.modulemap")}))
+        XCTAssertTrue(exe.additionalFlags.contains(where: { $0.hasSuffix("CLibrary.build/module.modulemap")}))
+        // Also ensure the include path isn't there twice
+        XCTAssertEqual(exe.additionalFlags.filter({ $0 == "/Pkg/Sources/CLibrary/include" }).count, 1)
+
+        // And make sure the plugin does get the tool version
+        // Note, there are two Tools modules, one for host, one for target.
+        let pluginTool = try XCTUnwrap(result.targetMap.first(where: { $0.module.name == "Tool" && $0.destination == .host })).swift()
+        XCTAssertTrue(pluginTool.additionalFlags.contains(where: { $0.hasSuffix("CLibrary-tool.build/module.modulemap") }))
+        XCTAssertEqual(pluginTool.additionalFlags.filter({ $0 == "/Pkg/Sources/CLibrary/include" }).count, 1)
+    }
 }