Support gradle incremental processing in code gen

This adds support for incremental compilation in gradle via incap helper and marking the code gen as `ISOLATING`.

Depends on a newer version of KotlinPoet that has square/kotlinpoet#647

Resolves #589

kotlin/codegen/pom.xml

@@ -25,7 +25,15 @@
     <dependency>
       <groupId>com.squareup</groupId>
       <artifactId>kotlinpoet</artifactId>
-      <version>1.1.0</version>
+      <!-- TODO Pending a KotlinPoet release with https://github.com/square/kotlinpoet/pull/647 -->
+      <version>1.2.0-LOCAL</version>
+    </dependency>
+    <dependency>
+      <groupId>net.ltgt.gradle.incap</groupId>
+      <artifactId>incap</artifactId>
+      <version>${incap.version}</version>
+      <scope>provided</scope>
+      <optional>true</optional>
     </dependency>
     <dependency>
       <groupId>com.google.auto</groupId>
@@ -35,7 +43,7 @@
     <dependency>
       <groupId>com.google.auto.service</groupId>
       <artifactId>auto-service</artifactId>
-      <version>1.0-rc4</version>
+      <version>${autoservice.version}</version>
       <scope>provided</scope>
     </dependency>
     <dependency>
@@ -101,7 +109,12 @@
                 <annotationProcessorPath>
                   <groupId>com.google.auto.service</groupId>
                   <artifactId>auto-service</artifactId>
-                  <version>1.0-rc4</version>
+                  <version>${autoservice.version}</version>
+                </annotationProcessorPath>
+                <annotationProcessorPath>
+                  <groupId>net.ltgt.gradle.incap</groupId>
+                  <artifactId>incap-processor</artifactId>
+                  <version>${incap.version}</version>
                 </annotationProcessorPath>
               </annotationProcessorPaths>
             </configuration>

kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/AdapterGenerator.kt

@@ -52,6 +52,7 @@ internal class AdapterGenerator(
 
   private val nameAllocator = NameAllocator()
   private val adapterName = "${className.simpleNames.joinToString(separator = "_")}JsonAdapter"
+  private val originalElement = target.element
   private val originalTypeName = target.element.asType().asTypeName()
 
   private val moshiParam = ParameterSpec.builder(
@@ -97,6 +98,7 @@ internal class AdapterGenerator(
 
   private fun generateType(generatedOption: TypeElement?): TypeSpec {
     val result = TypeSpec.classBuilder(adapterName)
+        .addOriginatingElement(originalElement)
 
     generatedOption?.let {
       result.addAnnotation(AnnotationSpec.builder(it.asClassName())

kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessor.kt

@@ -16,12 +16,12 @@
 package com.squareup.moshi.kotlin.codegen
 
 import com.google.auto.service.AutoService
-import com.squareup.kotlinpoet.TypeSpec
 import com.squareup.moshi.JsonClass
 import me.eugeniomarletti.kotlin.metadata.KotlinMetadataUtils
 import me.eugeniomarletti.kotlin.metadata.declaresDefaultValue
 import me.eugeniomarletti.kotlin.processing.KotlinAbstractProcessor
-import java.io.File
+import net.ltgt.gradle.incap.IncrementalAnnotationProcessor
+import net.ltgt.gradle.incap.IncrementalAnnotationProcessorType.ISOLATING
 import javax.annotation.processing.ProcessingEnvironment
 import javax.annotation.processing.Processor
 import javax.annotation.processing.RoundEnvironment
@@ -42,6 +42,7 @@ import javax.tools.Diagnostic.Kind.ERROR
  * If you don't want this though, you can use the runtime [JsonClass] factory implementation.
  */
 @AutoService(Processor::class)
+@IncrementalAnnotationProcessor(ISOLATING)
 class JsonClassCodegenProcessor : KotlinAbstractProcessor(), KotlinMetadataUtils {
 
   companion object {
@@ -86,7 +87,8 @@ class JsonClassCodegenProcessor : KotlinAbstractProcessor(), KotlinMetadataUtils
       val jsonClass = type.getAnnotation(annotation)
       if (jsonClass.generateAdapter) {
         val generator = adapterGenerator(type) ?: continue
-        generator.generateAndWrite(generatedType)
+        generator.generateFile(generatedType)
+            .writeTo(filer)
       }
     }
 
@@ -123,18 +125,4 @@ class JsonClassCodegenProcessor : KotlinAbstractProcessor(), KotlinMetadataUtils
 
     return AdapterGenerator(type, sortedProperties)
   }
-
-  private fun AdapterGenerator.generateAndWrite(generatedOption: TypeElement?) {
-    val fileSpec = generateFile(generatedOption)
-    val adapterName = fileSpec.members.filterIsInstance<TypeSpec>().first().name!!
-    val outputDir = generatedDir ?: mavenGeneratedDir(adapterName)
-    fileSpec.writeTo(outputDir)
-  }
-
-  private fun mavenGeneratedDir(adapterName: String): File {
-    // Hack since the maven plugin doesn't supply `kapt.kotlin.generated` option
-    // Bug filed at https://youtrack.jetbrains.com/issue/KT-22783
-    val file = filer.createSourceFile(adapterName).toUri().let(::File)
-    return file.parentFile.also { file.delete() }
-  }
 }

kotlin/tests/pom.xml

@@ -108,7 +108,7 @@
               <sourceDirs>
                 <sourceDir>src/test/kotlin</sourceDir>
                 <sourceDir>src/test/java</sourceDir>
-                <sourceDir>target/generated-sources/kaptKotlin/test</sourceDir>
+                <sourceDir>target/generated-sources/kapt/test</sourceDir>
               </sourceDirs>
             </configuration>
           </execution>

pom.xml

@@ -33,6 +33,8 @@
     <kotlin-metadata.version>1.4.0</kotlin-metadata.version>
     <dokka.version>0.9.17</dokka.version>
     <maven-assembly.version>3.1.0</maven-assembly.version>
+    <incap.version>0.2</incap.version>
+    <autoservice.version>1.0-rc4</autoservice.version>
 
     <!-- Dependencies -->
     <okio.version>1.16.0</okio.version>