[KPM] Implement test to check if data classes are marked as internal

KT-51262
KT-51220

libraries/tools/kotlin-gradle-plugin-idea/src/main/kotlin/org/jetbrains/kotlin/gradle/kpm/idea/IdeaKotlinFragmentDependency.kt

@@ -8,3 +8,11 @@ package org.jetbrains.kotlin.gradle.kpm.idea
 import java.io.Serializable
 
 interface IdeaKotlinFragmentDependency : Serializable
+
+@Suppress("unused")
+@InternalKotlinGradlePluginApi
+class IdeaKotlinFragmentDependencyImpl: IdeaKotlinFragmentDependency {
+    companion object {
+        const val serialVersionUID = 0L
+    }
+}

libraries/tools/kotlin-gradle-plugin-idea/src/test/kotlin/org/jetbrains/kotlin/gradle/kpm/idea/IdeaKotlinProjectModelObjectGraphTest.kt

@@ -11,6 +11,8 @@ import org.reflections.Reflections
 import java.lang.reflect.Field
 import java.lang.reflect.Modifier
 import kotlin.reflect.KClass
+import kotlin.reflect.KVisibility
+import kotlin.reflect.KVisibility.PUBLIC
 import kotlin.reflect.full.memberProperties
 import kotlin.test.Test
 import kotlin.test.assertEquals
@@ -21,6 +23,8 @@ import kotlin.test.assertTrue
 @RunWith(Parameterized::class)
 class IdeaKotlinProjectModelObjectGraphTest(private val node: KClass<*>, @Suppress("unused_parameter") clazzName: String) {
 
+    private val reflections = Reflections("org.jetbrains.kotlin")
+
     @Test
     fun `test - node implements Serializable`() {
         assertTrue(
@@ -31,12 +35,27 @@ class IdeaKotlinProjectModelObjectGraphTest(private val node: KClass<*>, @Suppre
 
     @Test
     fun `test - node implementations contain serialVersionUID`() {
-        val reflections = Reflections("org.jetbrains.kotlin")
-        reflections.getSubTypesOf(node.java).forEach { subtype ->
-            if (!subtype.isInterface && !Modifier.isAbstract(subtype.modifiers)) {
-                assertNodeImplementationDefinesSerialVersionUID(subtype)
+        val implementations = reflections.getSubTypesOf(node.java).plus(node.java)
+            .filter { subtype -> !subtype.isInterface && !Modifier.isAbstract(subtype.modifiers) }
+
+        assertTrue(
+            implementations.isNotEmpty(),
+            "No implementations found for $node"
+        )
+
+        implementations.forEach { implementation -> assertNodeImplementationDefinesSerialVersionUID(implementation) }
+    }
+
+    @Test
+    fun `test - node implementations are marked with InternalKotlinGradlePluginApi when data class`() {
+        reflections.getSubTypesOf(node.java).plus(node.java)
+            .filter { subtype -> subtype.kotlin.isData && subtype.kotlin.visibility == PUBLIC }
+            .forEach { dataClass ->
+                assertTrue(
+                    dataClass.annotations.any { it.annotationClass.simpleName == "InternalKotlinGradlePluginApi" },
+                    "Expected $dataClass to be annotated with '@InternalKotlinGradlePluginApi'"
+                )
             }
-        }
     }
 
     private fun assertNodeImplementationDefinesSerialVersionUID(implementationClass: Class<*>) {