[Gradle] Associate the textFixtures source set with main and test ones

#KT-34901 Fixed

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/plugin/AbstractKotlinPlugin.kt

@@ -16,6 +16,7 @@ import org.gradle.api.publish.PublishingExtension
 import org.gradle.api.publish.maven.MavenPom
 import org.gradle.api.publish.maven.MavenPublication
 import org.gradle.api.tasks.SourceSet
+import org.gradle.internal.component.external.model.TestFixturesSupport.TEST_FIXTURES_FEATURE_NAME
 import org.gradle.jvm.tasks.Jar
 import org.gradle.tooling.provider.model.ToolingModelBuilderRegistry
 import org.jetbrains.kotlin.gradle.dsl.KotlinSingleJavaTargetExtension
@@ -37,6 +38,7 @@ const val NATIVE_COMPILER_PLUGIN_CLASSPATH_CONFIGURATION_NAME = "kotlinNativeCom
 const val COMPILER_CLASSPATH_CONFIGURATION_NAME = "kotlinCompilerClasspath"
 internal const val BUILD_TOOLS_API_CLASSPATH_CONFIGURATION_NAME = "kotlinBuildToolsApiClasspath"
 internal const val KLIB_COMMONIZER_CLASSPATH_CONFIGURATION_NAME = "kotlinKlibCommonizerClasspath"
+private const val JAVA_TEST_FIXTURES_PLUGIN_ID = "java-test-fixtures"
 
 internal abstract class AbstractKotlinPlugin(
     val tasksProvider: KotlinTasksProvider,
@@ -211,6 +213,8 @@ internal abstract class AbstractKotlinPlugin(
                 getByName(KotlinCompilation.TEST_COMPILATION_NAME).associateWith(getByName(KotlinCompilation.MAIN_COMPILATION_NAME))
             }
 
+            configureJavaTestFixturesSourceSets(kotlinTarget)
+
             // Since the 'java' plugin (as opposed to 'java-library') doesn't known anything about the 'api' configurations,
             // add the API dependencies of the main compilation directly to the 'apiElements' configuration, so that the 'api' dependencies
             // are properly published with the 'compile' scope (KT-28355):
@@ -224,6 +228,29 @@ internal abstract class AbstractKotlinPlugin(
             }
         }
 
+        private fun configureJavaTestFixturesSourceSets(kotlinTarget: KotlinTarget) {
+            val project = kotlinTarget.project
+            project.plugins.withId(JAVA_TEST_FIXTURES_PLUGIN_ID) {
+                kotlinTarget.compilations.run {
+                    val testFixturesSourceSet = findByName(TEST_FIXTURES_FEATURE_NAME)
+                    if (testFixturesSourceSet == null) {
+                        project.logger.warn(
+                            "The `$JAVA_TEST_FIXTURES_PLUGIN_ID` plugin has been detected, " +
+                                    "however the `$TEST_FIXTURES_FEATURE_NAME` source set cannot be found. " +
+                                    "`internal` declarations can be not available in the test fixtures.",
+                        )
+                        return@withId
+                    }
+                    testFixturesSourceSet.associateWith(getByName(KotlinCompilation.MAIN_COMPILATION_NAME))
+                    getByName(KotlinCompilation.TEST_COMPILATION_NAME).associateWith(testFixturesSourceSet)
+                    project.logger.debug(
+                        "The `$JAVA_TEST_FIXTURES_PLUGIN_ID` plugin has been detected, and the `$TEST_FIXTURES_FEATURE_NAME` " +
+                                "source set has been associated with the default source sets to provide `internal` declarations access"
+                    )
+                }
+            }
+        }
+
         private fun configureAttributes(
             kotlinTarget: KotlinWithJavaTarget<*, *>
         ) {