Use annotation default values with @AutoBuilder and @AutoAnnotation.

You can use `@AutoBuilder` to call an `@AutoAnnotation` method. As usual for `@AutoAnnotation`, each parameter of that method corresponds to an element of the annotation. If that element has a default value, the parameter should default to that same value.

Trivial example:
```java
class Example {
  @interface MyAnnotation {
    String value() default "foo";
  }

  @AutoAnnotation
  static MyAnnotation myAnnotation(String value) {
    return new AutoAnnotation_Example_myAnnotation(value);
  }

  @autobuilder(callMethod = "myAnnotation")
  interface MyAnnotationBuilder {
    MyAnnotationBuilder value(String value);
    MyAnnotation build();
  }

  static MyAnnotationBuilder myAnnotationBuilder() {
    return new AutoBuilder_Example_MyAnnotationBuilder();
  }
}
```

In the example, the call `myAnnotationBuilder().build()` has the same effect as `myAnnotationBuilder().value("foo").build()` because of `value() default "foo"`.

RELNOTES=AutoBuilder now uses annotation defaults when building a call to an `@AutoAnnotation` method.
PiperOrigin-RevId: 395114215

value/src/it/functional/src/test/java/com/google/auto/value/AutoBuilderTest.java

@@ -149,6 +149,12 @@ static MyAnnotation myAnnotation(String value, Truthiness truthiness) {
     return new AutoAnnotation_AutoBuilderTest_myAnnotation(value, truthiness);
   }
 
+  // This method has parameters for all the annotation elements.
+  @AutoAnnotation
+  static MyAnnotation myAnnotationAll(String value, int id, Truthiness truthiness) {
+    return new AutoAnnotation_AutoBuilderTest_myAnnotationAll(value, id, truthiness);
+  }
+
   @AutoBuilder(callMethod = "myAnnotation")
   interface MyAnnotationBuilder {
     MyAnnotationBuilder value(String x);
@@ -159,12 +165,28 @@ interface MyAnnotationBuilder {
   }
 
   static MyAnnotationBuilder myAnnotationBuilder() {
-    return new AutoBuilder_AutoBuilderTest_MyAnnotationBuilder()
-        .truthiness(MyAnnotation.DEFAULT_TRUTHINESS);
+    return new AutoBuilder_AutoBuilderTest_MyAnnotationBuilder();
+  }
+
+  @AutoBuilder(callMethod = "myAnnotationAll")
+  interface MyAnnotationAllBuilder {
+    MyAnnotationAllBuilder value(String x);
+
+    MyAnnotationAllBuilder id(int x);
+
+    MyAnnotationAllBuilder truthiness(Truthiness x);
+
+    MyAnnotation build();
+  }
+
+  static MyAnnotationAllBuilder myAnnotationAllBuilder() {
+    return new AutoBuilder_AutoBuilderTest_MyAnnotationAllBuilder();
   }
 
   @Test
   public void simpleAutoAnnotation() {
+    // We haven't supplied a value for `truthiness`, so AutoBuilder should use the default one in
+    // the annotation.
     MyAnnotation annotation1 = myAnnotationBuilder().value("foo").build();
     assertThat(annotation1.value()).isEqualTo("foo");
     assertThat(annotation1.id()).isEqualTo(MyAnnotation.DEFAULT_ID);
@@ -174,6 +196,15 @@ public void simpleAutoAnnotation() {
     assertThat(annotation2.value()).isEqualTo("bar");
     assertThat(annotation2.id()).isEqualTo(MyAnnotation.DEFAULT_ID);
     assertThat(annotation2.truthiness()).isEqualTo(Truthiness.TRUTHY);
+
+    MyAnnotation annotation3 = myAnnotationAllBuilder().value("foo").build();
+    MyAnnotation annotation4 =
+        myAnnotationAllBuilder()
+            .value("foo")
+            .id(MyAnnotation.DEFAULT_ID)
+            .truthiness(MyAnnotation.DEFAULT_TRUTHINESS)
+            .build();
+    assertThat(annotation3).isEqualTo(annotation4);
   }
 
   static class Overload {

value/src/main/java/com/google/auto/value/AutoAnnotation.java

@@ -71,6 +71,39 @@
  * parameter corresponding to an array-valued annotation member, and the implementation of each such
  * member will also return a clone of the array.
  *
+ * <p>If your annotation has many elements, you may consider using {@code @AutoBuilder} to make it
+ * easier to construct instances. In that case, {@code default} values from the annotation will
+ * become default values for the parameters of the {@code @AutoAnnotation} method. For example:
+ *
+ * <pre>
+ * class Example {
+ *   {@code @interface} MyAnnotation {
+ *     String name() default "foo";
+ *     int number() default 23;
+ *   }
+ *
+ *   {@code @AutoAnnotation}
+ *   static MyAnnotation myAnnotation(String value) {
+ *     return new AutoAnnotation_Example_myAnnotation(value);
+ *   }
+ *
+ *   {@code @AutoBuilder(callMethod = "myAnnotation")}
+ *   interface MyAnnotationBuilder {
+ *     MyAnnotationBuilder name(String name);
+ *     MyAnnotationBuilder number(int number);
+ *     MyAnnotation build();
+ *   }
+ *
+ *   static MyAnnotationBuilder myAnnotationBuilder() {
+ *     return new AutoBuilder_Example_MyAnnotationBuilder();
+ *   }
+ * }
+ * </pre>
+ *
+ * Here, {@code myAnnotationBuilder().build()} is the same as {@code
+ * myAnnotationBuilder().name("foo").number(23).build()} because those are the defaults in the
+ * annotation definition.
+ *
  * @author emcmanus@google.com (Éamonn McManus)
  */
 @Target(ElementType.METHOD)

value/src/main/java/com/google/auto/value/processor/AnnotationOutput.java

@@ -130,13 +130,13 @@ public Void visitType(TypeMirror classConstant, StringBuilder sb) {
   private static class InitializerSourceFormVisitor extends SourceFormVisitor {
     private final ProcessingEnvironment processingEnv;
     private final String memberName;
-    private final Element context;
+    private final Element errorContext;
 
     InitializerSourceFormVisitor(
-        ProcessingEnvironment processingEnv, String memberName, Element context) {
+        ProcessingEnvironment processingEnv, String memberName, Element errorContext) {
       this.processingEnv = processingEnv;
       this.memberName = memberName;
-      this.context = context;
+      this.errorContext = errorContext;
     }
 
     @Override
@@ -148,7 +148,7 @@ public Void visitAnnotation(AnnotationMirror a, StringBuilder sb) {
               "@AutoAnnotation cannot yet supply a default value for annotation-valued member '"
                   + memberName
                   + "'",
-              context);
+              errorContext);
       sb.append("null");
       return null;
     }
@@ -209,9 +209,9 @@ static String sourceFormForInitializer(
       AnnotationValue annotationValue,
       ProcessingEnvironment processingEnv,
       String memberName,
-      Element context) {
+      Element errorContext) {
     SourceFormVisitor visitor =
-        new InitializerSourceFormVisitor(processingEnv, memberName, context);
+        new InitializerSourceFormVisitor(processingEnv, memberName, errorContext);
     StringBuilder sb = new StringBuilder();
     visitor.visit(annotationValue, sb);
     return sb.toString();

value/src/main/java/com/google/auto/value/processor/AutoBuilderProcessor.java

@@ -20,6 +20,7 @@
 import static com.google.auto.common.MoreStreams.toImmutableList;
 import static com.google.auto.common.MoreStreams.toImmutableSet;
 import static com.google.auto.value.processor.AutoValueProcessor.OMIT_IDENTIFIERS_OPTION;
+import static com.google.auto.value.processor.ClassNames.AUTO_ANNOTATION_NAME;
 import static com.google.auto.value.processor.ClassNames.AUTO_BUILDER_NAME;
 import static java.util.stream.Collectors.joining;
 import static java.util.stream.Collectors.toCollection;
@@ -38,6 +39,7 @@
 import com.google.common.base.Ascii;
 import com.google.common.base.VerifyException;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Maps;
 import java.lang.reflect.Field;
@@ -60,6 +62,7 @@
 import javax.lang.model.element.PackageElement;
 import javax.lang.model.element.TypeElement;
 import javax.lang.model.element.VariableElement;
+import javax.lang.model.type.TypeKind;
 import javax.lang.model.type.TypeMirror;
 import net.ltgt.gradle.incap.IncrementalAnnotationProcessor;
 import net.ltgt.gradle.incap.IncrementalAnnotationProcessorType;
@@ -134,7 +137,7 @@ void processType(TypeElement autoBuilderType) {
     Map<String, String> propertyToGetterName =
         Maps.transformValues(classifier.builderGetters(), PropertyGetter::getName);
     AutoBuilderTemplateVars vars = new AutoBuilderTemplateVars();
-    vars.props = propertySet(executable, propertyToGetterName);
+    vars.props = propertySet(autoBuilderType, executable, propertyToGetterName);
     builder.defineVars(vars, classifier);
     vars.identifiers = !processingEnv.getOptions().containsKey(OMIT_IDENTIFIERS_OPTION);
     String generatedClassName = generatedClassName(autoBuilderType, "AutoBuilder_");
@@ -152,7 +155,15 @@ void processType(TypeElement autoBuilderType) {
   }
 
   private ImmutableSet<Property> propertySet(
-      ExecutableElement executable, Map<String, String> propertyToGetterName) {
+      TypeElement autoBuilderType,
+      ExecutableElement executable,
+      Map<String, String> propertyToGetterName) {
+    boolean autoAnnotation =
+        MoreElements.getAnnotationMirror(executable, AUTO_ANNOTATION_NAME).isPresent();
+    ImmutableMap<String, String> builderInitializers =
+        autoAnnotation
+            ? autoAnnotationInitializers(autoBuilderType, executable)
+            : ImmutableMap.of();
     // Fix any parameter names that are reserved words in Java. Java source code can't have
     // such parameter names, but Kotlin code might, for example.
     Map<VariableElement, String> identifiers =
@@ -161,18 +172,58 @@ private ImmutableSet<Property> propertySet(
     fixReservedIdentifiers(identifiers);
     return executable.getParameters().stream()
         .map(
-            v ->
-                newProperty(
-                    v, identifiers.get(v), propertyToGetterName.get(v.getSimpleName().toString())))
+            v -> {
+              String name = v.getSimpleName().toString();
+              return newProperty(
+                  v,
+                  identifiers.get(v),
+                  propertyToGetterName.get(name),
+                  Optional.ofNullable(builderInitializers.get(name)));
+            })
         .collect(toImmutableSet());
   }
 
-  private Property newProperty(VariableElement var, String identifier, String getterName) {
+  private Property newProperty(
+      VariableElement var,
+      String identifier,
+      String getterName,
+      Optional<String> builderInitializer) {
     String name = var.getSimpleName().toString();
     TypeMirror type = var.asType();
     Optional<String> nullableAnnotation = nullableAnnotationFor(var, var.asType());
     return new Property(
-        name, identifier, TypeEncoder.encode(type), type, nullableAnnotation, getterName);
+        name,
+        identifier,
+        TypeEncoder.encode(type),
+        type,
+        nullableAnnotation,
+        getterName,
+        builderInitializer);
+  }
+
+  private ImmutableMap<String, String> autoAnnotationInitializers(
+      TypeElement autoBuilderType, ExecutableElement autoAnnotationMethod) {
+    // We expect the return type of an @AutoAnnotation method to be an annotation type. If it isn't,
+    // AutoAnnotation will presumably complain, so we don't need to complain further.
+    TypeMirror returnType = autoAnnotationMethod.getReturnType();
+    if (!returnType.getKind().equals(TypeKind.DECLARED)) {
+      return ImmutableMap.of();
+    }
+    // This might not actually be an annotation (if the code is wrong), but if that's the case we
+    // just won't see any contained ExecutableElement where getDefaultValue() returns something.
+    TypeElement annotation = MoreTypes.asTypeElement(returnType);
+    ImmutableMap.Builder<String, String> builder = ImmutableMap.builder();
+    for (ExecutableElement method : methodsIn(annotation.getEnclosedElements())) {
+      AnnotationValue defaultValue = method.getDefaultValue();
+      if (defaultValue != null) {
+        String memberName = method.getSimpleName().toString();
+        builder.put(
+            memberName,
+            AnnotationOutput.sourceFormForInitializer(
+                defaultValue, processingEnv, memberName, autoBuilderType));
+      }
+    }
+    return builder.build();
   }
 
   private ExecutableElement findExecutable(

value/src/main/java/com/google/auto/value/processor/AutoValueOrBuilderTemplateVars.java

@@ -109,7 +109,8 @@ abstract class AutoValueOrBuilderTemplateVars extends AutoValueishTemplateVars {
    *
    * <ul>
    *   <li>it is {@code @Nullable} (in which case it defaults to null);
-   *   <li>it is {@code Optional} (in which case it defaults to empty);
+   *   <li>it has a builder initializer (for example it is {@code Optional}, which will have an
+   *       initializer of {@code Optional.empty()});
    *   <li>it has a property-builder method (in which case it defaults to empty).
    * </ul>
    */

value/src/main/java/com/google/auto/value/processor/AutoValueishProcessor.java

@@ -160,23 +160,48 @@ public static class Property {
     private final Optional<String> nullableAnnotation;
     private final Optionalish optional;
     private final String getter;
+    private final String builderInitializer; // empty, or with initial ` = `.
 
     Property(
         String name,
         String identifier,
         String type,
         TypeMirror typeMirror,
         Optional<String> nullableAnnotation,
-        String getter) {
+        String getter,
+        Optional<String> maybeBuilderInitializer) {
       this.name = name;
       this.identifier = identifier;
       this.type = type;
       this.typeMirror = typeMirror;
       this.nullableAnnotation = nullableAnnotation;
       this.optional = Optionalish.createIfOptional(typeMirror);
+      this.builderInitializer =
+          maybeBuilderInitializer.isPresent()
+              ? " = " + maybeBuilderInitializer.get()
+              : builderInitializer();
       this.getter = getter;
     }
 
+    /**
+     * Returns the appropriate initializer for a builder property. Builder properties are never
+     * primitive; if the built property is an {@code int} the builder property will be an {@code
+     * Integer}. So the default value for a builder property will be null unless there is an
+     * initializer. The caller of the constructor may have supplied an initializer, but otherwise we
+     * supply one only if this property is an {@code Optional} and is not {@code @Nullable}. In that
+     * case the initializer sets it to {@code Optional.empty()}.
+     */
+    private String builderInitializer() {
+      if (nullableAnnotation.isPresent()) {
+        return "";
+      }
+      Optionalish optional = Optionalish.createIfOptional(typeMirror);
+      if (optional == null) {
+        return "";
+      }
+      return " = " + optional.getEmpty();
+    }
+
     /**
      * Returns the name of the property as it should be used when declaring identifiers (fields and
      * parameters). If the original getter method was {@code foo()} then this will be {@code foo}.
@@ -218,6 +243,14 @@ public Optionalish getOptional() {
       return optional;
     }
 
+    /**
+     * Returns a string to be used as an initializer for a builder field for this property,
+     * including the leading {@code =}, or an empty string if there is no explicit initializer.
+     */
+    public String getBuilderInitializer() {
+      return builderInitializer;
+    }
+
     /**
      * Returns the string to use as a method annotation to indicate the nullability of this
      * property. It is either the empty string, if the property is not nullable, or an annotation
@@ -266,7 +299,8 @@ public static class GetterProperty extends Property {
           type,
           method.getReturnType(),
           nullableAnnotation,
-          method.getSimpleName().toString());
+          method.getSimpleName().toString(),
+          Optional.empty());
       this.method = method;
       this.fieldAnnotations = fieldAnnotations;
       this.methodAnnotations = methodAnnotations;

value/src/main/java/com/google/auto/value/processor/BuilderSpec.java

@@ -333,7 +333,7 @@ void defineVars(AutoValueOrBuilderTemplateVars vars, BuilderMethodClassifier<?>
       vars.builderRequiredProperties =
           vars.props.stream()
               .filter(p -> !p.isNullable())
-              .filter(p -> p.getOptional() == null)
+              .filter(p -> p.getBuilderInitializer().isEmpty())
               .filter(p -> !vars.builderPropertyBuilders.containsKey(p.getName()))
               .collect(toImmutableSet());
     }

value/src/main/java/com/google/auto/value/processor/builder.vm

@@ -40,7 +40,7 @@ class ${builderName}${builderFormalTypes} ##
 
   #if ($p.kind.primitive)
 
-  private $types.boxedClass($p.typeMirror).simpleName $p;
+  private $types.boxedClass($p.typeMirror).simpleName $p $p.builderInitializer;
 
   #else
 
@@ -54,7 +54,7 @@ class ${builderName}${builderFormalTypes} ##
 
     #end
 
-  private $p.type $p #if ($p.optional && !$p.nullable) = $p.optional.empty #end ;
+  private $p.type $p $p.builderInitializer;
 
   #end
 #end