Annotate formattingDiffsUsing methods as supporting nullable element/value types.

core/src/main/java/com/google/common/truth/Correspondence.java

@@ -325,13 +325,13 @@ public String toString() {
    * diff-formatting functionality to it. See e.g. {@link IterableSubject#formattingDiffsUsing}.
    */
   @SuppressWarnings("unchecked") // safe covariant cast
-  static <T> Correspondence<T, T> equality() {
+  static <T extends @Nullable Object> Correspondence<T, T> equality() {
     return (Equality<T>) Equality.INSTANCE;
   }
 
-  private static final class Equality<T> extends Correspondence<T, T> {
+  private static final class Equality<T extends @Nullable Object> extends Correspondence<T, T> {
 
-    private static final Equality<Object> INSTANCE = new Equality<>();
+    private static final Equality<@Nullable Object> INSTANCE = new Equality<>();
 
     @Override
     public boolean compare(T actual, T expected) {

core/src/main/java/com/google/common/truth/IterableSubject.java

@@ -950,7 +950,7 @@ UsingCorrespondence<A, E> comparingElementsUsing(
    *
    * @since 1.1
    */
-  public <T> UsingCorrespondence<T, T> formattingDiffsUsing(
+  public <T extends @Nullable Object> UsingCorrespondence<T, T> formattingDiffsUsing(
       DiffFormatter<? super T, ? super T> formatter) {
     return comparingElementsUsing(Correspondence.<T>equality().formattingDiffsUsing(formatter));
   }

core/src/main/java/com/google/common/truth/MapSubject.java

@@ -528,7 +528,7 @@ UsingCorrespondence<A, E> comparingValuesUsing(
    *
    * @since 1.1
    */
-  public final <V> UsingCorrespondence<V, V> formattingDiffsUsing(
+  public final <V extends @Nullable Object> UsingCorrespondence<V, V> formattingDiffsUsing(
       DiffFormatter<? super V, ? super V> formatter) {
     return comparingValuesUsing(Correspondence.<V>equality().formattingDiffsUsing(formatter));
   }