Use @NullMarked instead of our custom @ElementTypesAreNonnullByDefault annotations.

This is the next step toward [using JSpecify in Guava](jspecify/jspecify#239 (comment)). At the end of that path, we'll be able to [remove our dependency on JSR-305](#2960) (and on the Checker Framework's annotations), and we'll have one less blocker to [providing a `module-info`](#2970).

`@NullMarked` allows tools like kotlinc to produce errors for code like `ImmutableList<String?>`. (Before releasing this change, I'll conduct some further testing to more fully characterize the effects, both under Kotlin 2.1 and prior.) As we make further changes, it will allow kotlinc to detect even more nullness problems. We will make these changes in a series of incremental releases so that users can pick them up gradually, as we did inside Google. In simple cases, users may wish to pick up all the changes at once instead by upgrading straight from Guava 33.4.0 (or an earlier version) to Guava 33.4.4 (or whatever the version to make the final changes ends up being).

RELNOTES=Replaced our custom `@ElementTypesAreNonnullByDefault` annotations with the JSpecify `@NullMarked` annotation.
PiperOrigin-RevId: 708598410

Author: cpovirk

android/guava-testlib/src/com/google/common/collect/testing/AbstractCollectionTester.java

@@ -19,6 +19,7 @@
 import com.google.common.annotations.GwtCompatible;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import java.util.Collection;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 import org.junit.Ignore;
 
@@ -32,7 +33,7 @@
 @Ignore("test runners must not instantiate and run this directly, only via suites we build")
 // @Ignore affects the Android test runner, which respects JUnit 4 annotations on JUnit 3 tests.
 @SuppressWarnings("JUnit4ClassUsedInJUnit3")
-@ElementTypesAreNonnullByDefault
+@NullMarked
 public abstract class AbstractCollectionTester<E extends @Nullable Object>
     extends AbstractContainerTester<Collection<E>, E> {
 

android/guava-testlib/src/com/google/common/collect/testing/AbstractContainerTester.java

@@ -27,6 +27,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 import org.junit.Ignore;
 
@@ -42,7 +43,7 @@
 @Ignore("test runners must not instantiate and run this directly, only via suites we build")
 // @Ignore affects the Android test runner, which respects JUnit 4 annotations on JUnit 3 tests.
 @SuppressWarnings("JUnit4ClassUsedInJUnit3")
-@ElementTypesAreNonnullByDefault
+@NullMarked
 public abstract class AbstractContainerTester<C, E extends @Nullable Object>
     extends AbstractTester<OneSizeTestContainerGenerator<C, E>> {
   protected SampleElements<E> samples;

android/guava-testlib/src/com/google/common/collect/testing/AbstractIteratorTester.java

@@ -34,6 +34,7 @@
 import java.util.NoSuchElementException;
 import java.util.Set;
 import java.util.Stack;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -45,7 +46,7 @@
  * @author Chris Povirk
  */
 @GwtCompatible
-@ElementTypesAreNonnullByDefault
+@NullMarked
 abstract class AbstractIteratorTester<E extends @Nullable Object, I extends Iterator<E>> {
   private Stimulus<E, ? super I>[] stimuli;
   private final Iterator<E> elementsToInsert;

android/guava-testlib/src/com/google/common/collect/testing/AbstractMapTester.java

@@ -26,6 +26,7 @@
 import java.util.ListIterator;
 import java.util.Map;
 import java.util.Map.Entry;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 import org.junit.Ignore;
 
@@ -43,7 +44,7 @@
 @Ignore("test runners must not instantiate and run this directly, only via suites we build")
 // @Ignore affects the Android test runner, which respects JUnit 4 annotations on JUnit 3 tests.
 @SuppressWarnings("JUnit4ClassUsedInJUnit3")
-@ElementTypesAreNonnullByDefault
+@NullMarked
 public abstract class AbstractMapTester<K extends @Nullable Object, V extends @Nullable Object>
     extends AbstractContainerTester<Map<K, V>, Entry<K, V>> {
   protected Map<K, V> getMap() {

android/guava-testlib/src/com/google/common/collect/testing/AbstractTester.java

@@ -20,6 +20,7 @@
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
 import junit.framework.TestCase;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -34,7 +35,7 @@
  * @author George van den Driessche
  */
 @GwtCompatible(emulated = true)
-@ElementTypesAreNonnullByDefault
+@NullMarked
 public class AbstractTester<G> extends TestCase {
   private G subjectGenerator;
   private String suiteName;

android/guava-testlib/src/com/google/common/collect/testing/DerivedCollectionGenerators.java

@@ -33,6 +33,7 @@
 import java.util.Set;
 import java.util.SortedMap;
 import java.util.SortedSet;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -41,7 +42,7 @@
  * @author George van den Driessche
  */
 @GwtCompatible
-@ElementTypesAreNonnullByDefault
+@NullMarked
 public final class DerivedCollectionGenerators {
   public static class MapEntrySetGenerator<K extends @Nullable Object, V extends @Nullable Object>
       implements TestSetGenerator<Entry<K, V>>, DerivedGenerator {

android/guava-testlib/src/com/google/common/collect/testing/Helpers.java

@@ -44,10 +44,11 @@
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Set;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 @GwtCompatible(emulated = true)
-@ElementTypesAreNonnullByDefault
+@NullMarked
 public class Helpers {
   // Clone of Objects.equal
   static boolean equal(@Nullable Object a, @Nullable Object b) {

android/guava-testlib/src/com/google/common/collect/testing/IgnoreJRERequirement.java

@@ -19,12 +19,13 @@
 import static java.lang.annotation.ElementType.TYPE;
 
 import java.lang.annotation.Target;
+import org.jspecify.annotations.NullMarked;
 
 /**
  * Disables Animal Sniffer's checking of compatibility with older versions of Java/Android.
  *
  * <p>Each package's copy of this annotation needs to be listed in our {@code pom.xml}.
  */
 @Target({METHOD, CONSTRUCTOR, TYPE})
-@ElementTypesAreNonnullByDefault
+@NullMarked
 @interface IgnoreJRERequirement {}

android/guava-testlib/src/com/google/common/collect/testing/IteratorTester.java

@@ -19,6 +19,7 @@
 import com.google.common.annotations.GwtCompatible;
 import java.util.Collections;
 import java.util.Iterator;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -85,7 +86,7 @@
  * @author Chris Povirk
  */
 @GwtCompatible
-@ElementTypesAreNonnullByDefault
+@NullMarked
 public abstract class IteratorTester<E extends @Nullable Object>
     extends AbstractIteratorTester<E, Iterator<E>> {
   /**

android/guava-testlib/src/com/google/common/collect/testing/ListIteratorTester.java

@@ -20,6 +20,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.ListIterator;
+import org.jspecify.annotations.NullMarked;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -36,7 +37,7 @@
  * @author Chris Povirk
  */
 @GwtCompatible
-@ElementTypesAreNonnullByDefault
+@NullMarked
 public abstract class ListIteratorTester<E extends @Nullable Object>
     extends AbstractIteratorTester<E, ListIterator<E>> {
   protected ListIteratorTester(