Don't link plugin dependencies to products

Fixtures/Miscellaneous/Plugins/IncorrectDependencies/Package.swift

@@ -0,0 +1,21 @@
+// swift-tools-version:5.9
+import PackageDescription
+
+let package = Package(
+    name: "111920845-sample",
+    platforms: [
+        .macOS(.v10_15), // example uses swift concurrency which is only available in 10.15 or newer
+    ],
+    products: [
+        .executable(name: "MyPluginExecutable", targets: ["MyPluginExecutable"]),
+        .plugin(name: "MyPlugin", targets: ["MyPlugin"]),
+    ],
+    targets: [
+        .executableTarget(name: "MyPluginExecutable"),
+        .plugin(name: "MyPlugin", capability: .buildTool, dependencies: ["MyPluginExecutable"]),
+
+        .target(name: "MyLibrary", plugins: ["MyPlugin"]),
+        .executableTarget(name: "MyExecutable", dependencies: ["MyLibrary"]),
+        .testTarget(name: "MyExecutableTests", dependencies: ["MyExecutable"]),
+    ]
+)

Fixtures/Miscellaneous/Plugins/IncorrectDependencies/Plugins/MyPlugin/MyPlugin.swift

@@ -0,0 +1,15 @@
+import PackagePlugin
+
+@main
+struct MyPlugin: BuildToolPlugin {
+    func createBuildCommands(context: PluginContext, target: Target) async throws -> [Command] {
+        return [.buildCommand(
+            displayName: "Running MyPluginExecutable",
+            executable: try context.tool(named: "MyPluginExecutable").path,
+            arguments: [],
+            environment: [:],
+            inputFiles: [],
+            outputFiles: []
+        )]
+    }
+}

Fixtures/Miscellaneous/Plugins/IncorrectDependencies/Sources/MyExecutable/MyExecutable.swift

@@ -0,0 +1,8 @@
+import MyLibrary
+
+@main
+struct MyExecutable {
+    static func main() async throws {
+        print("Hello from executable.")
+    }
+}

Fixtures/Miscellaneous/Plugins/IncorrectDependencies/Sources/MyLibrary/Empty.swift

@@ -0,0 +1 @@
+

Fixtures/Miscellaneous/Plugins/IncorrectDependencies/Sources/MyPluginExecutable/MyPluginExecutable.swift

@@ -0,0 +1,6 @@
+@main
+struct MyExecutable {
+    static func main() async throws {
+        print("Hello from plugin executable.")
+    }
+}

Fixtures/Miscellaneous/Plugins/IncorrectDependencies/Tests/MyExecutableTests/MyExecutableTests.swift

@@ -0,0 +1,8 @@
+import XCTest
+@testable import MyExecutable
+
+final class MyExecutableTests: XCTestCase {
+    func test() throws {
+        XCTAssertTrue(true)
+    }
+}

Sources/Build/BuildPlan.swift

@@ -695,25 +695,42 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
         libraryBinaryPaths: Set<AbsolutePath>,
         availableTools: [String: AbsolutePath]
     ) {
+        /* Prior to tools-version 5.9, we used to errorneously recursively traverse plugin dependencies and statically include their
+         targets. For compatibility reasons, we preserve that behavior for older tools-versions. */
+        let shouldExcludePlugins: Bool
+        if let toolsVersion = self.graph.package(for: product)?.manifest.toolsVersion {
+            shouldExcludePlugins = toolsVersion >= .v5_9
+        } else {
+            shouldExcludePlugins = false
+        }
 
         // Sort the product targets in topological order.
         let nodes: [ResolvedTarget.Dependency] = product.targets.map { .target($0, conditions: []) }
         let allTargets = try topologicalSort(nodes, successors: { dependency in
             switch dependency {
             // Include all the dependencies of a target.
             case .target(let target, _):
+                if target.type == .macro {
+                    return []
+                }
+                if shouldExcludePlugins, target.type == .plugin {
+                    return []
+                }
                 return target.dependencies.filter { $0.satisfies(self.buildEnvironment) }
 
             // For a product dependency, we only include its content only if we
-            // need to statically link it or if it's a plugin.
+            // need to statically link it.
             case .product(let product, _):
                 guard dependency.satisfies(self.buildEnvironment) else {
                     return []
                 }
 
+                let productDependencies: [ResolvedTarget.Dependency] = product.targets.map { .target($0, conditions: []) }
                 switch product.type {
-                case .library(.automatic), .library(.static), .plugin:
-                    return product.targets.map { .target($0, conditions: []) }
+                case .library(.automatic), .library(.static):
+                    return productDependencies
+                case .plugin:
+                    return shouldExcludePlugins ? [] : productDependencies
                 case .library(.dynamic), .test, .executable, .snippet, .macro:
                     return []
                 }

Tests/FunctionalTests/PluginTests.swift

@@ -969,6 +969,15 @@ class PluginTests: XCTestCase {
         }
     }
 
+    func testIncorrectDependencies() throws {
+        try XCTSkipIf(!UserToolchain.default.supportsSwiftConcurrency(), "skipping because test environment doesn't support concurrency")
+
+        try fixture(name: "Miscellaneous/Plugins") { path in
+            let (stdout, stderr) = try executeSwiftBuild(path.appending("IncorrectDependencies"), extraArgs: ["--build-tests"])
+            XCTAssert(stdout.contains("Build complete!"), "output:\n\(stderr)\n\(stdout)")
+        }
+    }
+
     func testSandboxViolatingBuildToolPluginCommands() throws {
         #if !os(macOS)
         try XCTSkipIf(true, "sandboxing tests are only supported on macOS")