New @WireEnclosingType annotation, so R8 doesn't prune too much

wire-java-generator/src/main/java/com/squareup/wire/java/JavaGenerator.java

@@ -60,6 +60,7 @@
 import com.squareup.wire.ProtoWriter;
 import com.squareup.wire.ReverseProtoWriter;
 import com.squareup.wire.Syntax;
+import com.squareup.wire.WireEnclosingType;
 import com.squareup.wire.WireEnum;
 import com.squareup.wire.WireEnumConstant;
 import com.squareup.wire.WireField;
@@ -113,6 +114,7 @@ public final class JavaGenerator {
   static final ClassName STRING = ClassName.get(String.class);
   static final ClassName LIST = ClassName.get(List.class);
   static final ClassName MESSAGE = ClassName.get(Message.class);
+  static final ClassName WIRE_ENCLOSING_TYPE = ClassName.get(WireEnclosingType.class);
   static final ClassName ANDROID_MESSAGE = MESSAGE.peerClass("AndroidMessage");
   static final ClassName ADAPTER = ClassName.get(ProtoAdapter.class);
   static final ClassName BUILDER = ClassName.get(Message.Builder.class);
@@ -884,7 +886,9 @@ private TypeSpec generateEnclosingType(EnclosingType type) {
     ClassName javaType = (ClassName) typeName(type.getType());
 
     TypeSpec.Builder builder =
-        TypeSpec.classBuilder(javaType.simpleName()).addModifiers(PUBLIC, FINAL);
+        TypeSpec.classBuilder(javaType.simpleName())
+            .addModifiers(PUBLIC, FINAL)
+            .addAnnotation(WIRE_ENCLOSING_TYPE);
     if (javaType.enclosingClassName() != null) {
       builder.addModifiers(STATIC);
     }

wire-java-generator/src/test/java/com/squareup/wire/java/JavaGeneratorTest.java

@@ -604,6 +604,7 @@ public void enclosingTypeIsNotMessage() throws IOException {
     assertThat(javaOutput)
         .contains(
             ""
+                + "@WireEnclosingType\n"
                 + "public final class A {\n"
                 + "  private A() {\n"
                 + "    throw new AssertionError();\n"

wire-kotlin-generator/src/main/java/com/squareup/wire/kotlin/KotlinGenerator.kt

@@ -68,6 +68,7 @@ import com.squareup.wire.ProtoReader32
 import com.squareup.wire.ProtoWriter
 import com.squareup.wire.ReverseProtoWriter
 import com.squareup.wire.Syntax
+import com.squareup.wire.WireEnclosingType
 import com.squareup.wire.WireEnum
 import com.squareup.wire.WireEnumConstant
 import com.squareup.wire.WireField
@@ -2547,6 +2548,7 @@ class KotlinGenerator private constructor(
 
   private fun generateEnclosing(type: EnclosingType): TypeSpec {
     val classBuilder = TypeSpec.classBuilder(type.typeName as ClassName)
+      .addAnnotation(WIRE_ENCLOSING_TYPE)
       .primaryConstructor(FunSpec.constructorBuilder().addModifiers(PRIVATE).build())
 
     type.nestedTypes.forEach { classBuilder.addType(generateType(it)) }
@@ -3037,6 +3039,7 @@ class KotlinGenerator private constructor(
       ProtoType.UINT32 to CodeBlock.of("0"),
     )
     private val MESSAGE = Message::class.asClassName()
+    private val WIRE_ENCLOSING_TYPE = WireEnclosingType::class.asClassName()
     private val ANDROID_MESSAGE = MESSAGE.peerClass("AndroidMessage")
     private val PROTO_READER = ProtoReader::class.asClassName()
     private val PROTO_READER_32 = ProtoReader32::class.asClassName()

wire-kotlin-generator/src/test/java/com/squareup/wire/kotlin/KotlinGeneratorTest.kt

@@ -202,7 +202,12 @@ class KotlinGeneratorTest {
     val kotlinGenerator = KotlinGenerator.invoke(pruned)
     val typeSpec = kotlinGenerator.generateType(pruned.getType("A")!!)
     val code = FileSpec.get("", typeSpec).toString()
-    assertThat(code).contains("class A private constructor() {")
+    assertThat(code).contains(
+      """
+      |@WireEnclosingType
+      |public class A private constructor() {
+      """.trimMargin(),
+    )
     assertWithMessage(code).that(code.contains("class B(.*) : Message<B, Nothing>".toRegex(DOT_MATCHES_ALL))).isTrue()
   }
 

wire-runtime/api/wire-runtime.api

@@ -386,6 +386,9 @@ public final class com/squareup/wire/Wire {
 	public static final fun get (Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
 }
 
+public abstract interface annotation class com/squareup/wire/WireEnclosingType : java/lang/annotation/Annotation {
+}
+
 public abstract interface class com/squareup/wire/WireEnum {
 	public abstract fun getValue ()I
 }

wire-runtime/src/commonMain/kotlin/com/squareup/wire/WireEnclosingType.kt

@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2024 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.wire
+
+/**
+ * When the enclosing message is pruned, Wire still generates a class for that enclosing message to
+ * preserve the class names and structure.
+ *
+ * This marks such messages and is intended for use by tools like R8 to preserve them.
+ *
+ * https://developer.android.com/build/shrink-code
+ */
+annotation class WireEnclosingType

wire-tests/src/jvmJavaPrunedTest/proto-java/com/squareup/wire/protos/roots/E.java

@@ -9,6 +9,7 @@
 import com.squareup.wire.ProtoWriter;
 import com.squareup.wire.ReverseProtoWriter;
 import com.squareup.wire.Syntax;
+import com.squareup.wire.WireEnclosingType;
 import com.squareup.wire.WireField;
 import com.squareup.wire.internal.Internal;
 import java.io.IOException;
@@ -23,6 +24,7 @@
 /**
  * <b>NOTE:</b> This type only exists to maintain class structure for its nested types and is not an actual message.
  */
+@WireEnclosingType
 public final class E {
   private E() {
     throw new AssertionError();