KT-47347: Kapt processors should not be input files for stub generation

This commit avoids passing kapt processors as sources for stub generation
task, and it also avoid using them as input files for stub generation.
Instead, it adds a new property that simply records if processors are
present (as going from empty to 1+ processors should re-run stubs).

Fixes #KT-47347

Test: Kapt3IT.kt

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/Kapt3IT.kt

@@ -27,6 +27,7 @@ import org.junit.Test
 import java.io.File
 import java.util.regex.Pattern
 import java.util.zip.ZipFile
+import java.util.zip.ZipOutputStream
 import kotlin.test.assertNotNull
 import kotlin.test.assertNull
 import kotlin.test.assertTrue
@@ -817,4 +818,30 @@ open class Kapt3IT : Kapt3BaseIT() {
             }
         }
     }
+
+    /* Regression test for https://youtrack.jetbrains.com/issue/KT-47347. */
+    @Test
+    fun testChangesToKaptConfigurationDoNotTriggerStubGeneration() {
+        val project = Project("localAnnotationProcessor", directoryPrefix = "kapt2")
+
+        project.build("build") {
+            assertSuccessful()
+        }
+
+        ZipOutputStream(project.projectDir.resolve("fake_processor.jar").outputStream()).close()
+        project.projectDir.resolve("example/build.gradle").appendText(
+            """
+            
+            dependencies {
+                kapt files("../fake_processor.jar")
+            }
+        """.trimIndent()
+        )
+
+        project.build("build") {
+            assertSuccessful()
+            assertTasksExecuted(":example:kaptKotlin")
+            assertTasksUpToDate(":example:kaptGenerateStubsKotlin")
+        }
+    }
 }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/kapt/Kapt3KotlinGradleSubplugin.kt

@@ -292,8 +292,6 @@ class Kapt3GradleSubplugin @Inject internal constructor(private val registry: To
         val kaptTaskProvider: TaskProvider<out KaptTask> = context.createKaptKotlinTask(useWorkerApi = project.isUseWorkerApi())
 
         kaptGenerateStubsTaskProvider.configure { kaptGenerateStubsTask ->
-            kaptGenerateStubsTask.source(*kaptConfigurations.toTypedArray())
-
             kaptGenerateStubsTask.dependsOn(*buildDependencies.toTypedArray())
             kaptGenerateStubsTask.dependsOn(
                 project.provider {

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/kapt/KaptGenerateStubsTask.kt

@@ -76,10 +76,13 @@ abstract class KaptGenerateStubsTask : KotlinCompile(KotlinJvmOptionsImpl()) {
     @get:Internal
     lateinit var generatedSourcesDirs: List<File>
 
-    @get:Classpath
-    @get:InputFiles
+    @get:Internal("Not an input, just passed as kapt args. ")
     abstract val kaptClasspath: ConfigurableFileCollection
 
+    /* Used as input as empty kapt classpath should not trigger stub generation, but a non-empty one should. */
+    @Input
+    fun getIfKaptClasspathIsPresent() = !kaptClasspath.isEmpty
+
     @get:Classpath
     @get:InputFiles
     @Suppress("unused")