Add unit tests for the config

.gitignore

@@ -6,3 +6,6 @@ xcuserdata/
 DerivedData/
 .swiftpm/config/registries.json
 .swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
+
+# Fixture packages
+**/Fixtures/**/.swiftpm/

.swiftpm/xcode/xcshareddata/xcschemes/spmgraph-Package.xcscheme

@@ -124,8 +124,45 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
-      shouldUseLaunchSchemeArgsEnv = "YES"
-      shouldAutocreateTestPlan = "YES">
+      shouldUseLaunchSchemeArgsEnv = "YES">
+      <TestPlans>
+         <TestPlanReference
+            reference = "container:spmgraph.xctestplan"
+            default = "YES">
+         </TestPlanReference>
+      </TestPlans>
+      <Testables>
+         <TestableReference
+            skipped = "NO">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "SPMGraphExecutableTests"
+               BuildableName = "SPMGraphExecutableTests"
+               BlueprintName = "SPMGraphExecutableTests"
+               ReferencedContainer = "container:">
+            </BuildableReference>
+         </TestableReference>
+         <TestableReference
+            skipped = "NO">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "SPMGraphLintTests"
+               BuildableName = "SPMGraphLintTests"
+               BlueprintName = "SPMGraphLintTests"
+               ReferencedContainer = "container:">
+            </BuildableReference>
+         </TestableReference>
+         <TestableReference
+            skipped = "NO">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "SPMGraphDescriptionInterfaceTests"
+               BuildableName = "SPMGraphDescriptionInterfaceTests"
+               BlueprintName = "SPMGraphDescriptionInterfaceTests"
+               ReferencedContainer = "container:">
+            </BuildableReference>
+         </TestableReference>
+      </Testables>
    </TestAction>
    <LaunchAction
       buildConfiguration = "Debug"

Package.swift

@@ -145,6 +145,18 @@ let package = Package(
       dependencies: [
         .target(name: "SPMGraphExecutable"),
       ]
+    ),
+    .testTarget(
+      name: "SPMGraphDescriptionInterfaceTests",
+      dependencies: [
+        .target(name: "SPMGraphDescriptionInterface"),
+        .target(name: "Core"),
+        .product(
+          name: "ArgumentParser",
+          package: "swift-argument-parser"
+        ),
+      ],
+      resources: [.copy("Fixtures/PackageFixture")]
     )
   ]
 )

Sources/SPMGraphDescriptionInterface/SPMGraphConfigInterface.swift

@@ -133,15 +133,15 @@ public extension SPMGraphConfig.Lint.Rule {
       validate: { package, excludedSuffixes in
         let liveModules = package.modules
           .filter { !$0.containsOneOf(suffixes: excludedSuffixes) }
-          .filter(\.isLiveModule)
+          .filter(isLiveModule)
           .sorted()
 
         let errors: [SPMGraphConfig.Lint.Error] =
           liveModules
           .map { liveModule in
             liveModule.dependencies
               .compactMap(\.module)
-              .filter(\.isLiveModule == true)
+              .filter(isLiveModule)
               .filter { !excludedDependencies.contains($0.name) }
               .map { dependency in
                 SPMGraphConfig.Lint.Error.liveModuleLiveDependency(
@@ -223,7 +223,7 @@ public extension SPMGraphConfig.Lint.Rule {
 
       - Note: For `@_exported` usages, there will be an error in case only the exported module is used.
       For example, module Networking exports module NetworkingHelpers, if only NetworkingHelpers is used by a target
-      there will be a lint error, while if both Networking and NetworkingHelpers are used there will be no error. 
+      there will be a lint error, while if both Networking and NetworkingHelpers are used there will be no error.
       """,
     validate: { package, excludedSuffixes in
       let errors: [SPMGraphConfig.Lint.Error] = package.modules
@@ -355,7 +355,7 @@ extension Module: @retroactive Comparable {
 }
 
 extension SPMGraphConfig.Lint {
-  enum Error: LocalizedError {
+  enum Error: LocalizedError, Equatable {
     case liveModuleLiveDependency(moduleName: String, liveDependencyName: String)
     case baseOrInterfaceModuleLiveDependency(moduleName: String, liveDependencyName: String)
     case unusedDependencies(moduleName: String, dependencyName: String)

Sources/SPMGraphLint/SPMGraphLint.swift

@@ -24,7 +24,7 @@ import SPMGraphDescriptionInterface
 
 // MARK: - Input
 
-public struct SPMGraphLintInput {
+public struct SPMGraphLintInput: Equatable {
   /// "Directory path of Package.swift file"
   let spmPackageDirectory: AbsolutePath
   /// A custom build directory for the package used to edit and load the SPMGraphConfig.
@@ -40,9 +40,11 @@ public struct SPMGraphLintInput {
   let expectedWarningsCount: UInt
   /// Relative path for an output file with the formatted lint results. By default the errors are added only listed in the sdtout.
   let outputFilePath: String?
+}
 
+public extension SPMGraphLintInput {
   /// Makes an instance of ``SPMGraphLintInput``
-  public init(
+  init(
     spmPackageDirectory: String,
     configBuildDirectory: String?,
     excludedSuffixes: [String],

Tests/SPMGraphDescriptionInterfaceTests/Fixtures/Package+Fixture.swift

@@ -0,0 +1,38 @@
+//
+//
+//
+//  Copyright (c) 2025 GetYourGuide GmbH
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+//
+
+import Basics
+import Foundation
+import PackageModel
+import Workspace
+
+func loadFixturePackage() async throws -> Package {
+  let path = Bundle.module
+    .path(forResource: "Package", ofType: "swift", inDirectory: "Fixtures/PackageFixture")?
+    .replacingOccurrences(of: "/Package.swift", with: "")
+  let fixturePackagePath = try AbsolutePath(validating: path!)
+
+  let observability = ObservabilitySystem { print("\($0): \($1)") }
+  let workspace = try Workspace(forRootPackage: fixturePackagePath)
+
+  return try await workspace.loadRootPackage(
+    at: fixturePackagePath,
+    observabilityScope: observability.topScope
+  )
+}

Tests/SPMGraphDescriptionInterfaceTests/Fixtures/PackageFixture/Package.swift

@@ -0,0 +1,67 @@
+// swift-tools-version: 6.0
+
+import PackageDescription
+
+let package = Package(
+  name: "LintFixturePackage",
+  products: [
+    .library(
+      name: "LintLibrary",
+      targets: [
+        "BaseModule",
+        "InterfaceModule",
+        "FeatureModule",
+        "NetworkingLive",
+      ]
+    ),
+  ],
+  targets: [
+    // Base module - should not depend on Live modules
+    .target(
+      name: "BaseModule",
+      dependencies: []
+    ),
+
+    // Interface module - should not depend on Live modules
+    .target(
+      name: "InterfaceModule",
+      dependencies: ["BaseModule"]
+    ),
+
+    // Feature module - can depend on Live modules
+    .target(
+      name: "FeatureModule",
+      dependencies: [
+        "InterfaceModule",
+        "NetworkingLive",
+      ]
+    ),
+
+    // Live module - should not depend on other Live modules
+    .target(
+      name: "NetworkingLive",
+      dependencies: ["InterfaceModule"]
+    ),
+
+    // Another Live module for testing Live -> Live dependency rule
+    .target(
+      name: "StorageLive",
+      dependencies: ["NetworkingLive"] // This violates the rule
+    ),
+
+    // Module with unused dependency
+    .target(
+      name: "ModuleWithUnusedDep",
+      dependencies: [
+        "BaseModule", // This is unused in the actual code
+        "InterfaceModule",
+      ]
+    ),
+
+    // Test targets (should be excluded)
+    .testTarget(
+      name: "BaseModuleTests",
+      dependencies: ["BaseModule"]
+    ),
+  ]
+)

Tests/SPMGraphDescriptionInterfaceTests/Fixtures/PackageFixture/Sources/BaseModule/BaseModule.swift

@@ -0,0 +1,7 @@
+public struct BaseModule {
+  public init() {}
+
+  public func baseFunction() -> String {
+    "Base Module"
+  }
+}

Tests/SPMGraphDescriptionInterfaceTests/Fixtures/PackageFixture/Sources/FeatureModule/FeatureModule.swift

@@ -0,0 +1,13 @@
+import InterfaceModule
+import LiveModule
+
+public struct FeatureModule {
+  private let interface = InterfaceModule()
+  private let live = LiveModule()
+
+  public init() {}
+
+  public func featureFunction() -> String {
+    "Feature: \(interface.interfaceFunction()) + \(live.liveFunction())"
+  }
+}

Tests/SPMGraphDescriptionInterfaceTests/Fixtures/PackageFixture/Sources/InterfaceModule/InterfaceModule.swift

@@ -0,0 +1,15 @@
+import BaseModule
+
+public protocol InterfaceModuleProtocol {
+  func interfaceFunction() -> String
+}
+
+public struct InterfaceModule: InterfaceModuleProtocol {
+  private let base = BaseModule()
+
+  public init() {}
+
+  public func interfaceFunction() -> String {
+    "Interface Module with \(base.baseFunction())"
+  }
+}