Explicit opt-in switch for Kapt caching.

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

@@ -34,6 +34,7 @@ import java.io.File
 import org.jetbrains.kotlin.gradle.internal.Kapt3GradleSubplugin.Companion.getKaptGeneratedSourcesDir
 import org.jetbrains.kotlin.gradle.internal.Kapt3GradleSubplugin.Companion.getKaptGeneratedClassesDir
 import org.jetbrains.kotlin.gradle.internal.Kapt3GradleSubplugin.Companion.getKaptGeneratedKotlinSourcesDir
+import org.jetbrains.kotlin.gradle.tasks.shouldEnableGradleCache
 import org.jetbrains.kotlin.gradle.tasks.useBuildCacheIfSupported
 import java.io.ByteArrayOutputStream
 import java.io.ObjectOutputStream
@@ -313,6 +314,14 @@ class Kapt3KotlinGradleSubplugin : KotlinGradleSubplugin<KotlinCompile> {
 
         kaptTask.kaptClasspath = kaptClasspath
 
+        kaptTask.useBuildCache = kaptExtension.useBuildCache
+
+        if (shouldEnableGradleCache()) {
+            val reason = "Caching is disabled by default for kapt because of arbitrary behavior of external " +
+                         "annotation processors. You can enable it by adding 'kapt.useBuildCache = true' to the build script."
+            kaptTask.outputs.doNotCacheIf(reason) { !kaptTask.useBuildCache }
+        }
+
         buildAndAddOptionsTo(kaptTask.pluginOptions, aptMode = "apt")
 
         return kaptTask

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

@@ -56,6 +56,9 @@ open class KaptTask : ConventionTask(), CompilerArgumentAware<K2JVMCompilerArgum
     val classpath: FileCollection
         get() = kotlinCompileTask.classpath
 
+    @get:Internal
+    var useBuildCache: Boolean = false
+
     @get:Classpath @get:InputFiles @Suppress("unused")
     internal val kotlinTaskPluginClasspaths get() = kotlinCompileTask.pluginClasspath
 

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KaptExtension.kt

@@ -32,12 +32,20 @@ open class KaptExtension {
 
     open var processors: String = ""
 
+    /** Explicit opt-in switch for Kapt caching. Should be used when annotation processors used by this project are
+     * certain NOT to use anything aside from the task inputs in their logic and are guaranteed to produce the same
+     * output on subsequent runs without input changes. */
+    var useBuildCache: Boolean = false
+
     private val apOptionsActions =
             mutableListOf<(KaptAnnotationProcessorOptions) -> Unit>()
 
     private val javacOptionsActions =
             mutableListOf<(KaptJavacOptionsDelegate) -> Unit>()
 
+    private var apOptionsClosure: Closure<*>? = null
+    private var javacOptionsClosure: Closure<*>? = null
+
     open fun arguments(closure: Closure<*>) {
         apOptionsActions += { apOptions ->
             apOptions.execute(closure)