Add @CanIgnoreReturnValue as appropriate to Gson methods. (#2369)

This annotation indicates that return value of the annotated method does
not need to be used. If it is _not_ present on a non-void method, and if
Error Prone's `CheckReturnValue` is active, then calling the method
without using the result is an error. However, we are not enabling
`CheckReturnValue` by default here.

Also update some code that does ignore return values, so that the
returned value is used, if only by assigning it to an unused variable.

extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java

@@ -16,6 +16,7 @@
 
 package com.google.gson.typeadapters;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.Gson;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
@@ -179,6 +180,7 @@ public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
    * Ensures that this factory will handle not just the given {@code baseType}, but any subtype
    * of that type.
    */
+  @CanIgnoreReturnValue
   public RuntimeTypeAdapterFactory<T> recognizeSubtypes() {
     this.recognizeSubtypes = true;
     return this;
@@ -191,6 +193,7 @@ public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
    * @throws IllegalArgumentException if either {@code type} or {@code label}
    *     have already been registered on this type adapter.
    */
+  @CanIgnoreReturnValue
   public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type, String label) {
     if (type == null || label == null) {
       throw new NullPointerException();
@@ -210,6 +213,7 @@ public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
    * @throws IllegalArgumentException if either {@code type} or its simple name
    *     have already been registered on this type adapter.
    */
+  @CanIgnoreReturnValue
   public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type) {
     return registerSubtype(type, type.getSimpleName());
   }

extras/src/test/java/com/google/gson/graph/GraphAdapterBuilderTest.java

@@ -129,8 +129,8 @@ public final class GraphAdapterBuilderTest {
   @Test
   public void testSerializationWithMultipleTypes() {
     Company google = new Company("Google");
-    new Employee("Jesse", google);
+    Employee unused1 = new Employee("Jesse", google);
-    new Employee("Joel", google);
+    Employee unused2 = new Employee("Joel", google);
 
     GsonBuilder gsonBuilder = new GsonBuilder();
     new GraphAdapterBuilder()

extras/src/test/java/com/google/gson/interceptors/InterceptorTest.java

@@ -110,9 +110,9 @@ public final class InterceptorTest {
 
           @Override public User read(JsonReader in) throws IOException {
             in.beginObject();
-            in.nextName();
+            String unused1 = in.nextName();
             String name = in.nextString();
-            in.nextName();
+            String unused2 = in.nextName();
             String password = in.nextString();
             in.endObject();
             return new User(name, password);

extras/src/test/java/com/google/gson/typeadapters/UtcDateTypeAdapterTest.java

@@ -64,7 +64,7 @@ public final class UtcDateTypeAdapterTest {
     Gson gson = new GsonBuilder()
       .registerTypeAdapter(Date.class, new UtcDateTypeAdapter())
       .create();
-    gson.fromJson("'2014-12-05T04:00:00.000Z'", Date.class);
+    Date unused = gson.fromJson("'2014-12-05T04:00:00.000Z'", Date.class);
   }
 
   @Test

gson/src/main/java/com/google/gson/Gson.java

@@ -1223,7 +1223,7 @@ public final class Gson {
     boolean oldLenient = reader.isLenient();
     reader.setLenient(true);
     try {
-      reader.peek();
+      JsonToken unused = reader.peek();
       isEmpty = false;
       TypeAdapter<T> typeAdapter = getAdapter(typeOfT);
       return typeAdapter.read(reader);

gson/src/main/java/com/google/gson/GsonBuilder.java

@@ -28,6 +28,7 @@ import static com.google.gson.Gson.DEFAULT_SERIALIZE_NULLS;
 import static com.google.gson.Gson.DEFAULT_SPECIALIZE_FLOAT_VALUES;
 import static com.google.gson.Gson.DEFAULT_USE_JDK_UNSAFE;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.annotations.Since;
 import com.google.gson.annotations.Until;
 import com.google.gson.internal.$Gson$Preconditions;
@@ -159,6 +160,7 @@ public final class GsonBuilder {
    * @see Since
    * @see Until
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setVersion(double version) {
     if (Double.isNaN(version) || version < 0.0) {
       throw new IllegalArgumentException("Invalid version: " + version);
@@ -181,6 +183,7 @@ public final class GsonBuilder {
    * {@link java.lang.reflect.Modifier#STATIC}.
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    */
+  @CanIgnoreReturnValue
   public GsonBuilder excludeFieldsWithModifiers(int... modifiers) {
     Objects.requireNonNull(modifiers);
     excluder = excluder.withModifiers(modifiers);
@@ -196,6 +199,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.3
    */
+  @CanIgnoreReturnValue
   public GsonBuilder generateNonExecutableJson() {
     this.generateNonExecutableJson = true;
     return this;
@@ -210,6 +214,7 @@ public final class GsonBuilder {
    *
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    */
+  @CanIgnoreReturnValue
   public GsonBuilder excludeFieldsWithoutExposeAnnotation() {
     excluder = excluder.excludeFieldsWithoutExposeAnnotation();
     return this;
@@ -222,6 +227,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.2
    */
+  @CanIgnoreReturnValue
   public GsonBuilder serializeNulls() {
     this.serializeNulls = true;
     return this;
@@ -306,6 +312,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.7
    */
+  @CanIgnoreReturnValue
   public GsonBuilder enableComplexMapKeySerialization() {
     complexMapKeySerialization = true;
     return this;
@@ -330,6 +337,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.3
    */
+  @CanIgnoreReturnValue
   public GsonBuilder disableInnerClassSerialization() {
     excluder = excluder.disableInnerClassSerialization();
     return this;
@@ -343,6 +351,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.3
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setLongSerializationPolicy(LongSerializationPolicy serializationPolicy) {
     this.longSerializationPolicy = Objects.requireNonNull(serializationPolicy);
     return this;
@@ -354,6 +363,7 @@ public final class GsonBuilder {
    *
    * <p>This method just delegates to {@link #setFieldNamingStrategy(FieldNamingStrategy)}.
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setFieldNamingPolicy(FieldNamingPolicy namingConvention) {
     return setFieldNamingStrategy(namingConvention);
   }
@@ -370,6 +380,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.3
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setFieldNamingStrategy(FieldNamingStrategy fieldNamingStrategy) {
     this.fieldNamingPolicy = Objects.requireNonNull(fieldNamingStrategy);
     return this;
@@ -383,6 +394,7 @@ public final class GsonBuilder {
    * @see ToNumberPolicy#DOUBLE The default object-to-number strategy
    * @since 2.8.9
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setObjectToNumberStrategy(ToNumberStrategy objectToNumberStrategy) {
     this.objectToNumberStrategy = Objects.requireNonNull(objectToNumberStrategy);
     return this;
@@ -396,6 +408,7 @@ public final class GsonBuilder {
    * @see ToNumberPolicy#LAZILY_PARSED_NUMBER The default number-to-number strategy
    * @since 2.8.9
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setNumberToNumberStrategy(ToNumberStrategy numberToNumberStrategy) {
     this.numberToNumberStrategy = Objects.requireNonNull(numberToNumberStrategy);
     return this;
@@ -427,6 +440,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.4
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setExclusionStrategies(ExclusionStrategy... strategies) {
     Objects.requireNonNull(strategies);
     for (ExclusionStrategy strategy : strategies) {
@@ -450,6 +464,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.7
    */
+  @CanIgnoreReturnValue
   public GsonBuilder addSerializationExclusionStrategy(ExclusionStrategy strategy) {
     Objects.requireNonNull(strategy);
     excluder = excluder.withExclusionStrategy(strategy, true, false);
@@ -471,6 +486,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.7
    */
+  @CanIgnoreReturnValue
   public GsonBuilder addDeserializationExclusionStrategy(ExclusionStrategy strategy) {
     Objects.requireNonNull(strategy);
     excluder = excluder.withExclusionStrategy(strategy, false, true);
@@ -486,6 +502,7 @@ public final class GsonBuilder {
    *
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setPrettyPrinting() {
     return setPrettyPrinting(FormattingStyle.DEFAULT);
   }
@@ -500,6 +517,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since $next-version$
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setPrettyPrinting(FormattingStyle formattingStyle) {
     this.formattingStyle = formattingStyle;
     return this;
@@ -515,6 +533,7 @@ public final class GsonBuilder {
    * @see JsonReader#setLenient(boolean)
    * @see JsonWriter#setLenient(boolean)
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setLenient() {
     lenient = true;
     return this;
@@ -527,6 +546,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.3
    */
+  @CanIgnoreReturnValue
   public GsonBuilder disableHtmlEscaping() {
     this.escapeHtmlChars = false;
     return this;
@@ -548,6 +568,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.2
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setDateFormat(String pattern) {
     // TODO(Joel): Make this fail fast if it is an invalid date format
     this.datePattern = pattern;
@@ -568,6 +589,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.2
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setDateFormat(int style) {
     this.dateStyle = style;
     this.datePattern = null;
@@ -589,6 +611,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.2
    */
+  @CanIgnoreReturnValue
   public GsonBuilder setDateFormat(int dateStyle, int timeStyle) {
     this.dateStyle = dateStyle;
     this.timeStyle = timeStyle;
@@ -618,6 +641,7 @@ public final class GsonBuilder {
    * {@link InstanceCreator}, {@link JsonSerializer}, and a {@link JsonDeserializer} interfaces.
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    */
+  @CanIgnoreReturnValue
   public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
     Objects.requireNonNull(type);
     $Gson$Preconditions.checkArgument(typeAdapter instanceof JsonSerializer<?>
@@ -651,6 +675,7 @@ public final class GsonBuilder {
    *
    * @since 2.1
    */
+  @CanIgnoreReturnValue
   public GsonBuilder registerTypeAdapterFactory(TypeAdapterFactory factory) {
     Objects.requireNonNull(factory);
     factories.add(factory);
@@ -671,6 +696,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.7
    */
+  @CanIgnoreReturnValue
   public GsonBuilder registerTypeHierarchyAdapter(Class<?> baseType, Object typeAdapter) {
     Objects.requireNonNull(baseType);
     $Gson$Preconditions.checkArgument(typeAdapter instanceof JsonSerializer<?>
@@ -707,6 +733,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 1.3
    */
+  @CanIgnoreReturnValue
   public GsonBuilder serializeSpecialFloatingPointValues() {
     this.serializeSpecialFloatingPointValues = true;
     return this;
@@ -728,6 +755,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 2.9.0
    */
+  @CanIgnoreReturnValue
   public GsonBuilder disableJdkUnsafe() {
     this.useJdkUnsafe = false;
     return this;
@@ -753,6 +781,7 @@ public final class GsonBuilder {
    * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
    * @since 2.9.1
    */
+  @CanIgnoreReturnValue
   public GsonBuilder addReflectionAccessFilter(ReflectionAccessFilter filter) {
     Objects.requireNonNull(filter);
     reflectionFilters.addFirst(filter);

gson/src/main/java/com/google/gson/JsonArray.java

@@ -16,6 +16,7 @@
 
 package com.google.gson;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.internal.NonNullElementWrapperList;
 import java.math.BigDecimal;
 import java.math.BigInteger;
@@ -145,6 +146,7 @@ public final class JsonArray extends JsonElement implements Iterable<JsonElement
    * @return the element previously at the specified position
    * @throws IndexOutOfBoundsException if the specified index is outside the array bounds
    */
+  @CanIgnoreReturnValue
   public JsonElement set(int index, JsonElement element) {
     return elements.set(index, element == null ? JsonNull.INSTANCE : element);
   }
@@ -157,6 +159,7 @@ public final class JsonArray extends JsonElement implements Iterable<JsonElement
    * @return true if this array contained the specified element, false otherwise
    * @since 2.3
    */
+  @CanIgnoreReturnValue
   public boolean remove(JsonElement element) {
     return elements.remove(element);
   }
@@ -171,6 +174,7 @@ public final class JsonArray extends JsonElement implements Iterable<JsonElement
    * @throws IndexOutOfBoundsException if the specified index is outside the array bounds
    * @since 2.3
    */
+  @CanIgnoreReturnValue
   public JsonElement remove(int index) {
     return elements.remove(index);
   }

gson/src/main/java/com/google/gson/JsonElement.java

@@ -16,6 +16,7 @@
 
 package com.google.gson;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.internal.Streams;
 import com.google.gson.stream.JsonWriter;
 import java.io.IOException;
@@ -143,6 +144,7 @@ public abstract class JsonElement {
    * @throws IllegalStateException if this element is of another type.
    * @since 1.2
    */
+  @CanIgnoreReturnValue
   public JsonNull getAsJsonNull() {
     if (isJsonNull()) {
       return (JsonNull) this;

gson/src/main/java/com/google/gson/JsonObject.java

@@ -16,6 +16,7 @@
 
 package com.google.gson;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.internal.LinkedTreeMap;
 import java.util.Map;
 import java.util.Set;
@@ -77,6 +78,7 @@ public final class JsonObject extends JsonElement {
    *   member with this name exists.
    * @since 1.3
    */
+  @CanIgnoreReturnValue
   public JsonElement remove(String property) {
     return members.remove(property);
   }

gson/src/main/java/com/google/gson/JsonStreamParser.java

@@ -15,6 +15,7 @@
  */
 package com.google.gson;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.internal.Streams;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonToken;

gson/src/main/java/com/google/gson/internal/LinkedTreeMap.java

@@ -17,6 +17,7 @@
 
 package com.google.gson.internal;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.io.IOException;
 import java.io.InvalidObjectException;
 import java.io.ObjectInputStream;
@@ -107,6 +108,7 @@ public final class LinkedTreeMap<K, V> extends AbstractMap<K, V> implements Seri
     return findByObject(key) != null;
   }
 
+  @CanIgnoreReturnValue
   @Override public V put(K key, V value) {
     if (key == null) {
       throw new NullPointerException("key == null");

gson/src/main/java/com/google/gson/internal/Streams.java

@@ -23,6 +23,7 @@ import com.google.gson.JsonParseException;
 import com.google.gson.JsonSyntaxException;
 import com.google.gson.internal.bind.TypeAdapters;
 import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonToken;
 import com.google.gson.stream.JsonWriter;
 import com.google.gson.stream.MalformedJsonException;
 import java.io.EOFException;
@@ -44,7 +45,7 @@ public final class Streams {
   public static JsonElement parse(JsonReader reader) throws JsonParseException {
     boolean isEmpty = true;
     try {
-      reader.peek();
+      JsonToken unused = reader.peek();
       isEmpty = false;
       return TypeAdapters.JSON_ELEMENT.read(reader);
     } catch (EOFException e) {

gson/src/main/java/com/google/gson/internal/bind/JsonTreeReader.java

@@ -16,6 +16,7 @@
 
 package com.google.gson.internal.bind;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.JsonArray;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonNull;
@@ -153,6 +154,7 @@ public final class JsonTreeReader extends JsonReader {
     return stack[stackSize - 1];
   }
 
+  @CanIgnoreReturnValue
   private Object popStack() {
     Object result = stack[--stackSize];
     stack[stackSize] = null;

gson/src/main/java/com/google/gson/internal/bind/JsonTreeWriter.java

@@ -16,6 +16,7 @@
 
 package com.google.gson.internal.bind;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.JsonArray;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonNull;
@@ -92,6 +93,7 @@ public final class JsonTreeWriter extends JsonWriter {
     }
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter beginArray() throws IOException {
     JsonArray array = new JsonArray();
     put(array);
@@ -99,6 +101,7 @@ public final class JsonTreeWriter extends JsonWriter {
     return this;
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter endArray() throws IOException {
     if (stack.isEmpty() || pendingName != null) {
       throw new IllegalStateException();
@@ -111,6 +114,7 @@ public final class JsonTreeWriter extends JsonWriter {
     throw new IllegalStateException();
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter beginObject() throws IOException {
     JsonObject object = new JsonObject();
     put(object);
@@ -118,6 +122,7 @@ public final class JsonTreeWriter extends JsonWriter {
     return this;
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter endObject() throws IOException {
     if (stack.isEmpty() || pendingName != null) {
       throw new IllegalStateException();
@@ -130,6 +135,7 @@ public final class JsonTreeWriter extends JsonWriter {
     throw new IllegalStateException();
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter name(String name) throws IOException {
     Objects.requireNonNull(name, "name == null");
     if (stack.isEmpty() || pendingName != null) {
@@ -143,6 +149,7 @@ public final class JsonTreeWriter extends JsonWriter {
     throw new IllegalStateException();
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter value(String value) throws IOException {
     if (value == null) {
       return nullValue();
@@ -155,16 +162,19 @@ public final class JsonTreeWriter extends JsonWriter {
     throw new UnsupportedOperationException();
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter nullValue() throws IOException {
     put(JsonNull.INSTANCE);
     return this;
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter value(boolean value) throws IOException {
     put(new JsonPrimitive(value));
     return this;
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter value(Boolean value) throws IOException {
     if (value == null) {
       return nullValue();
@@ -173,6 +183,7 @@ public final class JsonTreeWriter extends JsonWriter {
     return this;
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter value(float value) throws IOException {
     if (!isLenient() && (Float.isNaN(value) || Float.isInfinite(value))) {
       throw new IllegalArgumentException("JSON forbids NaN and infinities: " + value);
@@ -181,6 +192,7 @@ public final class JsonTreeWriter extends JsonWriter {
     return this;
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter value(double value) throws IOException {
     if (!isLenient() && (Double.isNaN(value) || Double.isInfinite(value))) {
       throw new IllegalArgumentException("JSON forbids NaN and infinities: " + value);
@@ -189,11 +201,13 @@ public final class JsonTreeWriter extends JsonWriter {
     return this;
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter value(long value) throws IOException {
     put(new JsonPrimitive(value));
     return this;
   }
 
+  @CanIgnoreReturnValue
   @Override public JsonWriter value(Number value) throws IOException {
     if (value == null) {
       return nullValue();

gson/src/main/java/com/google/gson/stream/JsonReader.java

@@ -1119,7 +1119,7 @@ public class JsonReader implements Closeable {
           return;
         } else if (c == '\\') {
           pos = p;
-          readEscapeCharacter();
+          char unused = readEscapeCharacter();
           p = pos;
           l = limit;
         } else if (c == '\n') {
@@ -1664,7 +1664,7 @@ public class JsonReader implements Closeable {
    */
   private void consumeNonExecutePrefix() throws IOException {
     // fast forward through the leading whitespace
-    nextNonWhitespace(true);
+    int unused = nextNonWhitespace(true);
     pos--;
 
     if (pos + 5 > limit && !fillBuffer(5)) {

gson/src/main/java/com/google/gson/stream/JsonWriter.java

@@ -24,6 +24,7 @@ import static com.google.gson.stream.JsonScope.NONEMPTY_ARRAY;
 import static com.google.gson.stream.JsonScope.NONEMPTY_DOCUMENT;
 import static com.google.gson.stream.JsonScope.NONEMPTY_OBJECT;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.io.Closeable;
 import java.io.Flushable;
 import java.io.IOException;
@@ -318,6 +319,7 @@ public class JsonWriter implements Closeable, Flushable {
    *
    * @return this writer.
    */
+  @CanIgnoreReturnValue
   public JsonWriter beginArray() throws IOException {
     writeDeferredName();
     return open(EMPTY_ARRAY, '[');
@@ -328,6 +330,7 @@ public class JsonWriter implements Closeable, Flushable {
    *
    * @return this writer.
    */
+  @CanIgnoreReturnValue
   public JsonWriter endArray() throws IOException {
     return close(EMPTY_ARRAY, NONEMPTY_ARRAY, ']');
   }
@@ -338,6 +341,7 @@ public class JsonWriter implements Closeable, Flushable {
    *
    * @return this writer.
    */
+  @CanIgnoreReturnValue
   public JsonWriter beginObject() throws IOException {
     writeDeferredName();
     return open(EMPTY_OBJECT, '{');
@@ -348,6 +352,7 @@ public class JsonWriter implements Closeable, Flushable {
    *
    * @return this writer.
    */
+  @CanIgnoreReturnValue
   public JsonWriter endObject() throws IOException {
     return close(EMPTY_OBJECT, NONEMPTY_OBJECT, '}');
   }
@@ -356,6 +361,7 @@ public class JsonWriter implements Closeable, Flushable {
    * Enters a new scope by appending any necessary whitespace and the given
    * bracket.
    */
+  @CanIgnoreReturnValue
   private JsonWriter open(int empty, char openBracket) throws IOException {
     beforeValue();
     push(empty);
@@ -367,6 +373,7 @@ public class JsonWriter implements Closeable, Flushable {
    * Closes the current scope by appending any necessary whitespace and the
    * given bracket.
    */
+  @CanIgnoreReturnValue
   private JsonWriter close(int empty, int nonempty, char closeBracket)
       throws IOException {
     int context = peek();
@@ -415,6 +422,7 @@ public class JsonWriter implements Closeable, Flushable {
    * @param name the name of the forthcoming value. May not be null.
    * @return this writer.
    */
+  @CanIgnoreReturnValue
   public JsonWriter name(String name) throws IOException {
     Objects.requireNonNull(name, "name == null");
     if (deferredName != null) {
@@ -441,6 +449,7 @@ public class JsonWriter implements Closeable, Flushable {
    * @param value the literal string value, or null to encode a null literal.
    * @return this writer.
    */
+  @CanIgnoreReturnValue
   public JsonWriter value(String value) throws IOException {
     if (value == null) {
       return nullValue();
@@ -462,6 +471,7 @@ public class JsonWriter implements Closeable, Flushable {
    *    writing raw JSON values.
    * @since 2.4
    */
+  @CanIgnoreReturnValue
   public JsonWriter jsonValue(String value) throws IOException {
     if (value == null) {
       return nullValue();
@@ -477,6 +487,7 @@ public class JsonWriter implements Closeable, Flushable {
    *
    * @return this writer.
    */
+  @CanIgnoreReturnValue
   public JsonWriter nullValue() throws IOException {
     if (deferredName != null) {
       if (serializeNulls) {
@@ -496,6 +507,7 @@ public class JsonWriter implements Closeable, Flushable {
    *
    * @return this writer.
    */
+  @CanIgnoreReturnValue
   public JsonWriter value(boolean value) throws IOException {
     writeDeferredName();
     beforeValue();
@@ -509,6 +521,7 @@ public class JsonWriter implements Closeable, Flushable {
    * @return this writer.
    * @since 2.7
    */
+  @CanIgnoreReturnValue
   public JsonWriter value(Boolean value) throws IOException {
     if (value == null) {
       return nullValue();
@@ -530,6 +543,7 @@ public class JsonWriter implements Closeable, Flushable {
    *     #setLenient(boolean) lenient}.
    * @since 2.9.1
    */
+  @CanIgnoreReturnValue
   public JsonWriter value(float value) throws IOException {
     writeDeferredName();
     if (!lenient && (Float.isNaN(value) || Float.isInfinite(value))) {
@@ -549,6 +563,7 @@ public class JsonWriter implements Closeable, Flushable {
    * @throws IllegalArgumentException if the value is NaN or Infinity and this writer is
    *     not {@link #setLenient(boolean) lenient}.
    */
+  @CanIgnoreReturnValue
   public JsonWriter value(double value) throws IOException {
     writeDeferredName();
     if (!lenient && (Double.isNaN(value) || Double.isInfinite(value))) {
@@ -564,6 +579,7 @@ public class JsonWriter implements Closeable, Flushable {
    *
    * @return this writer.
    */
+  @CanIgnoreReturnValue
   public JsonWriter value(long value) throws IOException {
     writeDeferredName();
     beforeValue();
@@ -593,6 +609,7 @@ public class JsonWriter implements Closeable, Flushable {
    *     not {@link #setLenient(boolean) lenient}; or if the {@code toString()} result is not a
    *     valid JSON number.
    */
+  @CanIgnoreReturnValue
   public JsonWriter value(Number value) throws IOException {
     if (value == null) {
       return nullValue();

gson/src/test/java/com/google/gson/JsonStreamParserTest.java

@@ -70,8 +70,8 @@ public class JsonStreamParserTest {
 
   @Test
   public void testCallingNextBeyondAvailableInput() {
-    parser.next();
+    JsonElement unused1 = parser.next();
-    parser.next();
+    JsonElement unused2 = parser.next();
     try {
       parser.next();
       fail("Parser should not go beyond available input");

gson/src/test/java/com/google/gson/MixedStreamTest.java

@@ -88,11 +88,11 @@ public final class MixedStreamTest {
     jsonReader.beginArray();
 
     jsonReader.setLenient(false);
-    gson.fromJson(jsonReader, Car.class);
+    Car unused1 = gson.fromJson(jsonReader, Car.class);
     assertThat(jsonReader.isLenient()).isFalse();
 
     jsonReader.setLenient(true);
-    gson.fromJson(jsonReader, Car.class);
+    Car unused2 = gson.fromJson(jsonReader, Car.class);
     assertThat(jsonReader.isLenient()).isTrue();
   }
 

gson/src/test/java/com/google/gson/functional/ConcurrencyTest.java

@@ -47,7 +47,7 @@ public class ConcurrencyTest {
   public void testSingleThreadSerialization() {
     MyObject myObj = new MyObject();
     for (int i = 0; i < 10; i++) {
-      gson.toJson(myObj); 
+      String unused = gson.toJson(myObj);
     }
   }
 
@@ -58,7 +58,7 @@ public class ConcurrencyTest {
   @Test
   public void testSingleThreadDeserialization() {
     for (int i = 0; i < 10; i++) {
-      gson.fromJson("{'a':'hello','b':'world','i':1}", MyObject.class); 
+      MyObject unused = gson.fromJson("{'a':'hello','b':'world','i':1}", MyObject.class);
     }
   }
 
@@ -79,7 +79,7 @@ public class ConcurrencyTest {
           try {
             startLatch.await();
             for (int i = 0; i < 10; i++) {
-              gson.toJson(myObj);
+              String unused = gson.toJson(myObj);
             }
           } catch (Throwable t) {
             failed.set(true);
@@ -110,7 +110,7 @@ public class ConcurrencyTest {
           try {
             startLatch.await();
             for (int i = 0; i < 10; i++) {
-              gson.fromJson("{'a':'hello','b':'world','i':1}", MyObject.class); 
+              MyObject unused = gson.fromJson("{'a':'hello','b':'world','i':1}", MyObject.class);
             }
           } catch (Throwable t) {
             failed.set(true);

gson/src/test/java/com/google/gson/functional/DefaultTypeAdaptersTest.java

@@ -127,7 +127,7 @@ public class DefaultTypeAdaptersTest {
     URL target = gson.fromJson(json, URL.class);
     assertThat(target.toExternalForm()).isEqualTo(urlValue);
 
-    gson.fromJson('"' + urlValue + '"', URL.class);
+    URL unused = gson.fromJson('"' + urlValue + '"', URL.class);
     assertThat(target.toExternalForm()).isEqualTo(urlValue);
   }
 

gson/src/test/java/com/google/gson/functional/InheritanceTest.java

@@ -17,6 +17,7 @@ package com.google.gson.functional;
 
 import static com.google.common.truth.Truth.assertThat;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.Gson;
 import com.google.gson.JsonArray;
 import com.google.gson.JsonElement;
@@ -267,6 +268,7 @@ public class InheritanceTest {
       return sb.toString();
     }
 
+    @CanIgnoreReturnValue
     private StringBuilder append(StringBuilder sb, Collection<?> c) {
       sb.append("[");
       boolean first = true;

gson/src/test/java/com/google/gson/functional/JsonAdapterAnnotationOnFieldsTest.java

@@ -118,7 +118,7 @@ public final class JsonAdapterAnnotationOnFieldsTest {
       out.value("PartJsonFieldAnnotationAdapter");
     }
     @Override public Part read(JsonReader in) throws IOException {
-      in.nextString();
+      String unused = in.nextString();
       return new Part("PartJsonFieldAnnotationAdapter");
     }
   }
@@ -131,7 +131,7 @@ public final class JsonAdapterAnnotationOnFieldsTest {
         }
         @SuppressWarnings("unchecked")
         @Override public T read(JsonReader in) throws IOException {
-          in.nextString();
+          String unused = in.nextString();
           return (T) new Part("GizmoPartTypeAdapterFactory");
         }
       };
@@ -158,7 +158,7 @@ public final class JsonAdapterAnnotationOnFieldsTest {
       out.value("UserClassAnnotationAdapter");
     }
     @Override public User read(JsonReader in) throws IOException {
-      in.nextString();
+      String unused = in.nextString();
       return new User("UserClassAnnotationAdapter");
     }
   }
@@ -177,7 +177,7 @@ public final class JsonAdapterAnnotationOnFieldsTest {
       out.value("UserFieldAnnotationAdapter");
     }
     @Override public User read(JsonReader in) throws IOException {
-      in.nextString();
+      String unused = in.nextString();
       return new User("UserFieldAnnotationAdapter");
     }
   }
@@ -187,7 +187,7 @@ public final class JsonAdapterAnnotationOnFieldsTest {
       out.value("RegisteredUserAdapter");
     }
     @Override public User read(JsonReader in) throws IOException {
-      in.nextString();
+      String unused = in.nextString();
       return new User("RegisteredUserAdapter");
     }
   }
@@ -307,7 +307,7 @@ public final class JsonAdapterAnnotationOnFieldsTest {
         }
         @SuppressWarnings("unchecked")
         @Override public T read(JsonReader in) throws IOException {
-          in.nextString();
+          String unused = in.nextString();
           return (T) Arrays.asList(new Part("GizmoPartTypeAdapterFactory"));
         }
       };

gson/src/test/java/com/google/gson/functional/ObjectTest.java

@@ -539,17 +539,17 @@ public class ObjectTest {
     Gson gson = new Gson();
     Product product = new Product();
     assertThat(gson.toJson(product)).isEqualTo("{\"attributes\":[],\"departments\":[]}");
-    gson.fromJson(gson.toJson(product), Product.class);
+    Product unused1 = gson.fromJson(gson.toJson(product), Product.class);
 
     product.departments.add(new Department());
     assertThat(gson.toJson(product))
         .isEqualTo("{\"attributes\":[],\"departments\":[{\"name\":\"abc\",\"code\":\"123\"}]}");
-    gson.fromJson(gson.toJson(product), Product.class);
+    Product unused2 = gson.fromJson(gson.toJson(product), Product.class);
 
     product.attributes.add("456");
     assertThat(gson.toJson(product))
         .isEqualTo("{\"attributes\":[\"456\"],\"departments\":[{\"name\":\"abc\",\"code\":\"123\"}]}");
-    gson.fromJson(gson.toJson(product), Product.class);
+    Product unused3 = gson.fromJson(gson.toJson(product), Product.class);
   }
 
   static final class Department {
@@ -611,7 +611,7 @@ public class ObjectTest {
   @Test
   public void testStaticFieldDeserialization() {
     // By default Gson should ignore static fields
-    gson.fromJson("{\"s\":\"custom\"}", ClassWithStaticField.class);
+    ClassWithStaticField unused = gson.fromJson("{\"s\":\"custom\"}", ClassWithStaticField.class);
     assertThat(ClassWithStaticField.s).isEqualTo("initial");
 
     Gson gson = new GsonBuilder()

gson/src/test/java/com/google/gson/functional/ReadersWritersTest.java

@@ -18,6 +18,7 @@ package com.google.gson.functional;
 import static com.google.common.truth.Truth.assertThat;
 import static org.junit.Assert.fail;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
 import com.google.gson.JsonStreamParser;
@@ -155,12 +156,14 @@ public class ReadersWritersTest {
       final StringBuilder stringBuilder = new StringBuilder();
       int toStringCallCount = 0;
 
+      @CanIgnoreReturnValue
       @Override
       public Appendable append(char c) throws IOException {
         stringBuilder.append(c);
         return this;
       }
 
+      @CanIgnoreReturnValue
       @Override
       public Appendable append(CharSequence csq) throws IOException {
         if (csq == null) {
@@ -170,6 +173,7 @@ public class ReadersWritersTest {
         return this;
       }
 
+      @CanIgnoreReturnValue
       @Override
       public Appendable append(CharSequence csq, int start, int end) throws IOException {
         if (csq == null) {

gson/src/test/java/com/google/gson/functional/RuntimeTypeAdapterFactoryFunctionalTest.java

@@ -17,6 +17,7 @@ package com.google.gson.functional;
 
 import static com.google.common.truth.Truth.assertThat;
 
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.Gson;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
@@ -124,6 +125,7 @@ public final class RuntimeTypeAdapterFactoryFunctionalTest {
      * @throws IllegalArgumentException if either {@code type} or {@code label}
      *     have already been registered on this type adapter.
      */
+    @CanIgnoreReturnValue
     public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type, String label) {
       if (type == null || label == null) {
         throw new NullPointerException();
@@ -143,6 +145,7 @@ public final class RuntimeTypeAdapterFactoryFunctionalTest {
      * @throws IllegalArgumentException if either {@code type} or its simple name
      *     have already been registered on this type adapter.
      */
+    @CanIgnoreReturnValue
     public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type) {
       return registerSubtype(type, type.getSimpleName());
     }

gson/src/test/java/com/google/gson/functional/TypeHierarchyAdapterTest.java

@@ -136,7 +136,7 @@ public final class TypeHierarchyAdapterTest {
   /** This behaviour changed in Gson 2.1; it used to throw. */
   @Test
   public void testRegisterSubTypeFirstAllowed() {
-    new GsonBuilder()
+    Gson unused = new GsonBuilder()
         .registerTypeHierarchyAdapter(Manager.class, new ManagerAdapter())
         .registerTypeHierarchyAdapter(Employee.class, new EmployeeAdapter())
         .create();

gson/src/test/java/com/google/gson/internal/bind/JsonElementReaderTest.java

@@ -332,9 +332,9 @@ public final class JsonElementReaderTest {
       fail();
     } catch (IllegalStateException expected) {
     }
-    reader.nextName();
+    String unused1 = reader.nextName();
     assertThat(new JsonPrimitive(1)).isEqualTo(reader.nextJsonElement());
-    reader.nextName();
+    String unused2 = reader.nextName();
     reader.beginObject();
     try {
       reader.nextJsonElement();
@@ -342,7 +342,7 @@ public final class JsonElementReaderTest {
     } catch (IllegalStateException expected) {
     }
     reader.endObject();
-    reader.nextName();
+    String unused3 = reader.nextName();
     reader.beginArray();
     try {
       reader.nextJsonElement();

gson/src/test/java/com/google/gson/metrics/PerformanceTest.java

@@ -123,7 +123,7 @@ public class PerformanceTest {
     for (int i = 0; i < count; ++i) {
       list.add(new CollectionEntry("name"+i,"value"+i));
     }
-    gson.toJson(list);
+    String unused = gson.toJson(list);
   }
 
   /**
@@ -163,7 +163,7 @@ public class PerformanceTest {
       for (int i = 0; i < size; ++i) {
         ba[i] = 0x05;
       }
-      gson.toJson(ba);
+      String unused = gson.toJson(ba);
       System.out.printf("Gson could serialize a byte array of size: %d\n", size);
     }
   }
@@ -248,7 +248,7 @@ public class PerformanceTest {
     System.out.printf("Large object serialized in: %d ms\n", (t2 - t1));
 
     t1 = System.currentTimeMillis();
-    gson.fromJson(json, new TypeToken<Map<String, Long>>() {}.getType());
+    Map<String, Long> unused = gson.fromJson(json, new TypeToken<Map<String, Long>>() {}.getType());
     t2 = System.currentTimeMillis();
     System.out.printf("Large object deserialized in: %d ms\n", (t2 - t1));
 
@@ -297,7 +297,7 @@ public class PerformanceTest {
     Gson gson = new Gson();
     String json = gson.toJson(original);
     Type longToLong = new TypeToken<Map<Long, Long>>(){}.getType();
-    gson.fromJson(json, longToLong);
+    Map<Long, Long> unused = gson.fromJson(json, longToLong);
   }
 
   private String buildJsonForClassWithList() {

gson/src/test/java/com/google/gson/regression/JsonAdapterNullSafeTest.java

@@ -30,7 +30,7 @@ public class JsonAdapterNullSafeTest {
   @Test
   public void testNullSafeBugSerialize() {
     Device device = new Device("ec57803e");
-    gson.toJson(device);
+    String unused = gson.toJson(device);
   }
 
   @Test

gson/src/test/java/com/google/gson/stream/JsonReaderPathTest.java

@@ -51,34 +51,34 @@ public class JsonReaderPathTest {
     reader.beginObject();
     assertThat(reader.getPreviousPath()).isEqualTo("$.");
     assertThat(reader.getPath()).isEqualTo("$.");
-    reader.nextName();
+    String unused1 = reader.nextName();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a");
     assertThat(reader.getPath()).isEqualTo("$.a");
     reader.beginArray();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[0]");
     assertThat(reader.getPath()).isEqualTo("$.a[0]");
-    reader.nextInt();
+    int unused2 = reader.nextInt();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[0]");
     assertThat(reader.getPath()).isEqualTo("$.a[1]");
-    reader.nextBoolean();
+    boolean unused3 = reader.nextBoolean();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[1]");
     assertThat(reader.getPath()).isEqualTo("$.a[2]");
-    reader.nextBoolean();
+    boolean unused4 = reader.nextBoolean();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[2]");
     assertThat(reader.getPath()).isEqualTo("$.a[3]");
     reader.nextNull();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[3]");
     assertThat(reader.getPath()).isEqualTo("$.a[4]");
-    reader.nextString();
+    String unused5 = reader.nextString();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[4]");
     assertThat(reader.getPath()).isEqualTo("$.a[5]");
     reader.beginObject();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[5].");
     assertThat(reader.getPath()).isEqualTo("$.a[5].");
-    reader.nextName();
+    String unused6 = reader.nextName();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[5].c");
     assertThat(reader.getPath()).isEqualTo("$.a[5].c");
-    reader.nextString();
+    String unused7 = reader.nextString();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[5].c");
     assertThat(reader.getPath()).isEqualTo("$.a[5].c");
     reader.endObject();
@@ -87,7 +87,7 @@ public class JsonReaderPathTest {
     reader.beginArray();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[6][0]");
     assertThat(reader.getPath()).isEqualTo("$.a[6][0]");
-    reader.nextInt();
+    int unused8 = reader.nextInt();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a[6][0]");
     assertThat(reader.getPath()).isEqualTo("$.a[6][1]");
     reader.endArray();
@@ -106,49 +106,49 @@ public class JsonReaderPathTest {
     assertThat(reader.getPreviousPath()).isEqualTo("$");
     assertThat(reader.getPath()).isEqualTo("$");
 
-    reader.peek();
+    JsonToken unused1 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$");
     assertThat(reader.getPath()).isEqualTo("$");
     reader.beginObject();
     assertThat(reader.getPreviousPath()).isEqualTo("$.");
     assertThat(reader.getPath()).isEqualTo("$.");
 
-    reader.peek();
+    JsonToken unused2 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$.");
     assertThat(reader.getPath()).isEqualTo("$.");
-    reader.nextName();
+    String unused3 = reader.nextName();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a");
     assertThat(reader.getPath()).isEqualTo("$.a");
 
-    reader.peek();
+    JsonToken unused4 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a");
     assertThat(reader.getPath()).isEqualTo("$.a");
-    reader.nextInt();
+    int unused5 = reader.nextInt();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a");
     assertThat(reader.getPath()).isEqualTo("$.a");
 
-    reader.peek();
+    JsonToken unused6 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a");
     assertThat(reader.getPath()).isEqualTo("$.a");
-    reader.nextName();
+    String unused7 = reader.nextName();
     assertThat(reader.getPreviousPath()).isEqualTo("$.b");
     assertThat(reader.getPath()).isEqualTo("$.b");
 
-    reader.peek();
+    JsonToken unused8 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$.b");
     assertThat(reader.getPath()).isEqualTo("$.b");
-    reader.nextInt();
+    int unused9 = reader.nextInt();
     assertThat(reader.getPreviousPath()).isEqualTo("$.b");
     assertThat(reader.getPath()).isEqualTo("$.b");
 
-    reader.peek();
+    JsonToken unused10 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$.b");
     assertThat(reader.getPath()).isEqualTo("$.b");
     reader.endObject();
     assertThat(reader.getPreviousPath()).isEqualTo("$");
     assertThat(reader.getPath()).isEqualTo("$");
 
-    reader.peek();
+    JsonToken unused11 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$");
     assertThat(reader.getPath()).isEqualTo("$");
     reader.close();
@@ -161,35 +161,35 @@ public class JsonReaderPathTest {
     assertThat(reader.getPreviousPath()).isEqualTo("$");
     assertThat(reader.getPath()).isEqualTo("$");
 
-    reader.peek();
+    JsonToken unused1 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$");
     assertThat(reader.getPath()).isEqualTo("$");
     reader.beginArray();
     assertThat(reader.getPreviousPath()).isEqualTo("$[0]");
     assertThat(reader.getPath()).isEqualTo("$[0]");
 
-    reader.peek();
+    JsonToken unused2 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$[0]");
     assertThat(reader.getPath()).isEqualTo("$[0]");
-    reader.nextInt();
+    int unused3 = reader.nextInt();
     assertThat(reader.getPreviousPath()).isEqualTo("$[0]");
     assertThat(reader.getPath()).isEqualTo("$[1]");
 
-    reader.peek();
+    JsonToken unused4 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$[0]");
     assertThat(reader.getPath()).isEqualTo("$[1]");
-    reader.nextInt();
+    int unused5 = reader.nextInt();
     assertThat(reader.getPreviousPath()).isEqualTo("$[1]");
     assertThat(reader.getPath()).isEqualTo("$[2]");
 
-    reader.peek();
+    JsonToken unused6 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$[1]");
     assertThat(reader.getPath()).isEqualTo("$[2]");
     reader.endArray();
     assertThat(reader.getPreviousPath()).isEqualTo("$");
     assertThat(reader.getPath()).isEqualTo("$");
 
-    reader.peek();
+    JsonToken unused7 = reader.peek();
     assertThat(reader.getPreviousPath()).isEqualTo("$");
     assertThat(reader.getPath()).isEqualTo("$");
     reader.close();
@@ -249,11 +249,11 @@ public class JsonReaderPathTest {
     reader.beginObject();
     assertThat(reader.getPreviousPath()).isEqualTo("$.");
     assertThat(reader.getPath()).isEqualTo("$.");
-    reader.nextName();
+    String unused1 = reader.nextName();
     reader.skipValue();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a");
     assertThat(reader.getPath()).isEqualTo("$.a");
-    reader.nextName();
+    String unused2 = reader.nextName();
     assertThat(reader.getPreviousPath()).isEqualTo("$.b");
     assertThat(reader.getPath()).isEqualTo("$.b");
   }
@@ -261,7 +261,7 @@ public class JsonReaderPathTest {
   @Test public void skipObjectEnd() throws IOException {
     JsonReader reader = factory.create("{\"a\":{},\"b\":2}");
     reader.beginObject();
-    reader.nextName();
+    String unused = reader.nextName();
     reader.beginObject();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a.");
     assertThat(reader.getPath()).isEqualTo("$.a.");
@@ -353,37 +353,37 @@ public class JsonReaderPathTest {
     reader.beginObject();
     assertThat(reader.getPreviousPath()).isEqualTo("$.");
     assertThat(reader.getPath()).isEqualTo("$.");
-    reader.nextName();
+    String unused1 = reader.nextName();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a");
     assertThat(reader.getPath()).isEqualTo("$.a");
     reader.beginObject();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a.");
     assertThat(reader.getPath()).isEqualTo("$.a.");
-    reader.nextName();
+    String unused2 = reader.nextName();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a.a1");
     assertThat(reader.getPath()).isEqualTo("$.a.a1");
-    reader.nextInt();
+    int unused3 = reader.nextInt();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a.a1");
     assertThat(reader.getPath()).isEqualTo("$.a.a1");
-    reader.nextName();
+    String unused4 = reader.nextName();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a.a2");
     assertThat(reader.getPath()).isEqualTo("$.a.a2");
-    reader.nextInt();
+    int unused5 = reader.nextInt();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a.a2");
     assertThat(reader.getPath()).isEqualTo("$.a.a2");
     reader.endObject();
     assertThat(reader.getPreviousPath()).isEqualTo("$.a");
     assertThat(reader.getPath()).isEqualTo("$.a");
-    reader.nextName();
+    String unused6 = reader.nextName();
     assertThat(reader.getPreviousPath()).isEqualTo("$.b");
     assertThat(reader.getPath()).isEqualTo("$.b");
     reader.beginObject();
     assertThat(reader.getPreviousPath()).isEqualTo("$.b.");
     assertThat(reader.getPath()).isEqualTo("$.b.");
-    reader.nextName();
+    String unused7 = reader.nextName();
     assertThat(reader.getPreviousPath()).isEqualTo("$.b.b1");
     assertThat(reader.getPath()).isEqualTo("$.b.b1");
-    reader.nextInt();
+    int unused8 = reader.nextInt();
     assertThat(reader.getPreviousPath()).isEqualTo("$.b.b1");
     assertThat(reader.getPath()).isEqualTo("$.b.b1");
     reader.endObject();

gson/src/test/java/com/google/gson/stream/JsonReaderTest.java

@@ -33,6 +33,7 @@ import java.io.IOException;
 import java.io.Reader;
 import java.io.StringReader;
 import java.util.Arrays;
+import com.google.gson.stream.JsonToken;
 import org.junit.Ignore;
 import org.junit.Test;
 
@@ -855,8 +856,8 @@ public final class JsonReaderTest {
     try {
       JsonReader reader = new JsonReader(reader("{\"a\":true}"));
       reader.beginObject();
-      reader.nextName();
+      String unused1 = reader.nextName();
-      reader.peek();
+      JsonToken unused2 = reader.peek();
       reader.close();
       reader.nextBoolean();
       fail();
@@ -869,13 +870,13 @@ public final class JsonReaderTest {
     JsonReader reader = new JsonReader(reader("{\"a\":true}"));
     reader.beginObject();
     try {
-      reader.nextString();
+      String unused = reader.nextString();
       fail();
     } catch (IllegalStateException expected) {
     }
     assertThat(reader.nextName()).isEqualTo("a");
     try {
-      reader.nextName();
+      String unused = reader.nextName();
       fail();
     } catch (IllegalStateException expected) {
     }
@@ -1234,8 +1235,8 @@ public final class JsonReaderTest {
     JsonReader reader = new JsonReader(reader("[true;true]"));
     reader.beginArray();
     try {
-      reader.nextBoolean();
+      boolean unused1 = reader.nextBoolean();
-      reader.nextBoolean();
+      boolean unused2 = reader.nextBoolean();
       fail();
     } catch (IOException expected) {
     }
@@ -1268,8 +1269,8 @@ public final class JsonReaderTest {
     reader.beginObject();
     assertThat(reader.nextName()).isEqualTo("a");
     try {
-      reader.nextBoolean();
+      boolean unused1 = reader.nextBoolean();
-      reader.nextName();
+      String unused2 = reader.nextName();
       fail();
     } catch (IOException expected) {
     }
@@ -1601,9 +1602,9 @@ public final class JsonReaderTest {
     JsonReader reader1 = new JsonReader(reader(json));
     reader1.setLenient(true);
     reader1.beginArray();
-    reader1.nextString();
+    String unused1 = reader1.nextString();
     try {
-      reader1.peek();
+      JsonToken unused2 = reader1.peek();
       fail();
     } catch (IOException expected) {
       assertThat(expected.getMessage()).isEqualTo(message);
@@ -1615,7 +1616,7 @@ public final class JsonReaderTest {
     reader2.beginArray();
     reader2.skipValue();
     try {
-      reader2.peek();
+      JsonToken unused3 = reader2.peek();
       fail();
     } catch (IOException expected) {
       assertThat(expected.getMessage()).isEqualTo(message);
@@ -1626,14 +1627,14 @@ public final class JsonReaderTest {
   public void testFailWithPositionDeepPath() throws IOException {
     JsonReader reader = new JsonReader(reader("[1,{\"a\":[2,3,}"));
     reader.beginArray();
-    reader.nextInt();
+    int unused1 = reader.nextInt();
     reader.beginObject();
-    reader.nextName();
+    String unused2 = reader.nextName();
     reader.beginArray();
-    reader.nextInt();
+    int unused3 = reader.nextInt();
-    reader.nextInt();
+    int unused4 = reader.nextInt();
     try {
-      reader.peek();
+      JsonToken unused5 = reader.peek();
       fail();
     } catch (IOException expected) {
       assertThat(expected.getMessage()).isEqualTo("Expected value at line 1 column 14 path $[1].a[2]");

proto/src/main/java/com/google/gson/protobuf/ProtoTypeAdapter.java

@@ -20,6 +20,7 @@ import static java.util.Objects.requireNonNull;
 
 import com.google.common.base.CaseFormat;
 import com.google.common.collect.MapMaker;
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.JsonArray;
 import com.google.gson.JsonDeserializationContext;
 import com.google.gson.JsonDeserializer;
@@ -102,6 +103,7 @@ public class ProtoTypeAdapter
       setFieldNameSerializationFormat(fromFieldNameFormat, toFieldNameFormat);
     }
 
+    @CanIgnoreReturnValue
     public Builder setEnumSerialization(EnumSerialization enumSerialization) {
       this.enumSerialization = requireNonNull(enumSerialization);
       return this;
@@ -122,6 +124,7 @@ public class ProtoTypeAdapter
      * n__id_ct       nIdCt
      * }</pre>
      */
+    @CanIgnoreReturnValue
     public Builder setFieldNameSerializationFormat(CaseFormat fromFieldNameFormat,
         CaseFormat toFieldNameFormat) {
       this.protoFormat = fromFieldNameFormat;
@@ -141,6 +144,7 @@ public class ProtoTypeAdapter
      * ...the adapter will serialize the field using '{@code appId}' instead of the default '
      * {@code clientAppId}'. This lets you customize the name serialization of any proto field.
      */
+    @CanIgnoreReturnValue
     public Builder addSerializedNameExtension(
         Extension<FieldOptions, String> serializedNameExtension) {
       serializedNameExtensions.add(requireNonNull(serializedNameExtension));
@@ -166,6 +170,7 @@ public class ProtoTypeAdapter
      * Note that you need to set the enum serialization of this adapter to
      * {@link EnumSerialization#NAME}, otherwise these annotations will be ignored.
      */
+    @CanIgnoreReturnValue
     public Builder addSerializedEnumValueExtension(
         Extension<EnumValueOptions, String> serializedEnumValueExtension) {
       serializedEnumValueExtensions.add(requireNonNull(serializedEnumValueExtension));