added more test coverage and implemented sam's suggestion

Author: John Bute

Sources/PackageLoading/ManifestLoader+Validation.swift

@@ -309,9 +309,9 @@ extension Basics.Diagnostic {
         return .error("\(messagePrefix) in dependencies of target '\(targetName)'; valid packages are: \(validPackages.map{ "\($0.descriptionForValidation)" }.joined(separator: ", "))")
     }
 
-    static func invalidDependencyOnTestTarget(dependency: String, targetName: String) -> Self {
+    static func invalidDependencyOnTestTarget(dependency: Module.Dependency, targetName: String) -> Self {
         .error(
-            "Invalid dependency: '\(targetName)' cannot depend on test target dependency '\(dependency)'. Only test targets can depend on other test targets"
+            "Invalid dependency: '\(targetName)' cannot depend on test target dependency '\(dependency.name)'. Only test targets can depend on other test targets"
         )
     }
 

Sources/PackageLoading/PackageBuilder.swift

@@ -955,7 +955,7 @@ public final class PackageBuilder {
             for dependency in dependencies {
                 if let depTarget = dependency.module, depTarget.type == .test {
                     self.observabilityScope.emit(.invalidDependencyOnTestTarget(
-                        dependency: dependency.name,
+                        dependency: dependency,
                         targetName: potentialModule.name
                     ))
                 }

Tests/PackageGraphTests/ModulesGraphTests.swift

@@ -446,6 +446,115 @@ final class ModulesGraphTests: XCTestCase {
         }
     }
 
+    func testExecutableInvalidDependencyOnTestTarget() throws {
+        let fs = InMemoryFileSystem(
+            emptyFiles:
+            "/Foo/Sources/Foo/main.swift",
+            "/Foo/Tests/FooTest/FooTest.swift"
+        )
+
+        let observability = ObservabilitySystem.makeForTesting()
+
+        let _ = try loadModulesGraph(
+            fileSystem: fs,
+            manifests: [
+                Manifest.createRootManifest(
+                    displayName: "Foo",
+                    path: "/Foo",
+                    toolsVersion: .v6_0,
+                    targets: [
+                        TargetDescription(name: "Foo", dependencies: ["FooTest"], type: .executable),
+                        TargetDescription(name: "FooTest", type: .test),
+                    ]
+                ),
+            ],
+            observabilityScope: observability.topScope
+        )
+
+        testDiagnostics(observability.diagnostics) { result in
+            result.check(
+                diagnostic: "Invalid dependency: 'Foo' cannot depend on test target dependency 'FooTest'. Only test targets can depend on other test targets",
+                severity: .error
+            )
+        }
+    }
+
+    func testPluginInvalidDependencyOnTestTarget() throws {
+        let fs = InMemoryFileSystem(
+            emptyFiles:
+            "/Foo/Plugins/Foo/main.swift",
+            "/Foo/Tests/FooTest/FooTest.swift"
+        )
+
+        let observability = ObservabilitySystem.makeForTesting()
+
+        let _ = try loadModulesGraph(
+            fileSystem: fs,
+            manifests: [
+                Manifest.createRootManifest(
+                    displayName: "Foo",
+                    path: "/Foo",
+                    toolsVersion: .v6_0,
+                    targets: [
+                        TargetDescription(
+                            name: "Foo",
+                            dependencies: ["FooTest"],
+                            type: .plugin,
+                            pluginCapability: .buildTool
+                        ),
+                        TargetDescription(name: "FooTest", type: .test),
+                    ]
+                ),
+            ],
+            observabilityScope: observability.topScope
+        )
+
+        testDiagnostics(observability.diagnostics) { result in
+            result.check(
+                diagnostic: "Invalid dependency: 'Foo' cannot depend on test target dependency 'FooTest'. Only test targets can depend on other test targets",
+                severity: .error
+            )
+        }
+    }
+    
+    func testMacroInvalidDependencyOnTestTarget() throws {
+        let fs = InMemoryFileSystem(
+            emptyFiles:
+            "/Foo/Sources/Foo/main.swift",
+            "/Foo/Tests/FooTest/FooTest.swift"
+        )
+
+        let observability = ObservabilitySystem.makeForTesting()
+
+        let _ = try loadModulesGraph(
+            fileSystem: fs,
+            manifests: [
+                Manifest.createRootManifest(
+                    displayName: "Foo",
+                    path: "/Foo",
+                    toolsVersion: .v6_0,
+                    targets: [
+                        TargetDescription(
+                            name: "Foo",
+                            dependencies: ["FooTest"],
+                            type: .macro
+                        ),
+                        TargetDescription(name: "FooTest", type: .test),
+                    ]
+                ),
+            ],
+            observabilityScope: observability.topScope
+        )
+
+        testDiagnostics(observability.diagnostics) { result in
+            result.check(
+                diagnostic: "Invalid dependency: 'Foo' cannot depend on test target dependency 'FooTest'. Only test targets can depend on other test targets",
+                severity: .error
+            )
+        }
+    }
+
+
     func testValidDependencyOnTestTarget() throws {
         let fs = InMemoryFileSystem(
             emptyFiles: