Kapt: Attach generated Kotlin sources in 'compilation' mode (KT-32535)

plugins/kapt3/kapt3-cli/testData/integration/simple/ap/Processor.kt

@@ -4,14 +4,23 @@ import javax.annotation.processing.*
 import javax.lang.model.SourceVersion
 import javax.lang.model.element.TypeElement
 import javax.tools.Diagnostic.Kind.*
+import javax.tools.StandardLocation
 
 annotation class Anno
 
 class SampleApt : AbstractProcessor() {
     override fun process(annotations: Set<TypeElement>, roundEnv: RoundEnvironment): Boolean {
+        val writeKotlinFiles = processingEnv.options["kapt.test.writeKotlinFiles"] == "true"
+
         for (element in roundEnv.getElementsAnnotatedWith(Anno::class.java)) {
             val generatedSimpleName = element.simpleName.toString().capitalize()
-            processingEnv.filer.createSourceFile("generated.$generatedSimpleName").openWriter().use {
+
+            val file = when (writeKotlinFiles) {
+                true -> processingEnv.filer.createResource(StandardLocation.SOURCE_OUTPUT, "generated", "$generatedSimpleName.kt")
+                false -> processingEnv.filer.createSourceFile("generated.$generatedSimpleName")
+            }
+
+            file.openWriter().use {
                 it.write("package generated;\npublic class $generatedSimpleName {}")
             }
         }
@@ -19,7 +28,7 @@ class SampleApt : AbstractProcessor() {
         return true
     }
 
-    override fun getSupportedOptions() = emptySet<String>()
+    override fun getSupportedOptions() = setOf("kapt.test.writeKotlinFiles")
     override fun getSupportedSourceVersion() = SourceVersion.RELEASE_8
     override fun getSupportedAnnotationTypes() = setOf("apt.Anno")
 }