Assorted improvements to nullness annotations.

I don't think any of the Guava changes are user-visible.

Details:
- Many of these fix bugs that will be noticed by the forthcoming version of our hacky internal nullness checker.
- Some of these fix bugs that even the forthcoming version won't notice but that I happened to see.
- Some changes add `@Nullable` to return types of methods that always throw an exception. There's no strict need for this, but we've mostly done it otherwise, so I figured I'd be consistent (and quiet `ReturnMissingNullable`, at least until I quiet it for all such methods with google/error-prone#2910).
- The `NullnessCasts` change is to discourage `ReturnMissingNullable` from adding a `@Nullable` annotation where we don't want it. (But we'll probably never run `ReturnMissingNullable` in the "aggressive" mode over this code, anyway, so there's not likely to be a need for the suppression.)
- The `@ParametricNulless` changes evidently aren't necessary for anyone right now, but they could theoretically be necessary for j2objc users until j2objc further enhances its support for nullness annotations.
- The `AbstractFuture` change removes a suppression that would be necessary under the Checker Framework (which would consider the supermethod's return type to be non-nullable) but isn't necessary for us (because we consider the supermethod's return type to have unspecified nullness).
RELNOTES=n/a
PiperOrigin-RevId: 421129392

android/guava/src/com/google/common/base/Equivalence.java

@@ -153,7 +153,43 @@ public final <F> Equivalence<F> onResultOf(Function<? super F, ? extends @Nullab
    * @since 10.0
    */
   public final <S extends @Nullable T> Wrapper<S> wrap(@ParametricNullness S reference) {
-    return new Wrapper<S>(this, reference);
+    /*
+     * I'm pretty sure that this warning "makes sense" but doesn't indicate a real problem.
+     *
+     * Why it "makes sense": If we pass a `@Nullable Foo`, then we should also pass an
+     * `Equivalence<? super @Nullable Foo>`. And there's no such thing because Equivalence doesn't
+     * permit nullable type arguments.
+     *
+     * Why there's no real problem: Every Equivalence can handle null.
+     *
+     * We could work around this by giving Wrapper 2 type parameters. In the terms of this method,
+     * that would be both the T parameter (from the class) and the S parameter (from this method).
+     * However, such a change would be source-incompatible. (Plus, there's no reason for the S
+     * parameter from the user's perspective, so it would be a wart.)
+     *
+     * We could probably also work around this by making Wrapper non-final and putting the
+     * implementation into a subclass with those 2 type parameters. But we like `final`, if only to
+     * deter users from using mocking frameworks to construct instances. (And could also complicate
+     * serialization, which is discussed more in the next paragraph.)
+     *
+     * We could probably also work around this by having Wrapper accept an instance of a new
+     * WrapperGuts class, which would then be the class that would declare the 2 type parameters.
+     * But that would break deserialization of previously serialized Wrapper instances. And while we
+     * specifically say not to rely on serialization across Guava versions, users sometimes do. So
+     * we'd rather not break them without a good enough reason.
+     *
+     * (We could work around the serialization problem by writing custom serialization code. But
+     * even that helps only the case of serializing with an old version and deserializing with a
+     * new, not vice versa -- unless we introduce WrapperGuts and the logic to handle it today, wait
+     * until "everyone" has picked up a version of Guava with that code, and *then* change to use
+     * WrapperGuts.)
+     *
+     * Anyway, a suppression isn't really a big deal. But I have tried to do some due diligence on
+     * avoiding it :)
+     */
+    @SuppressWarnings("nullness")
+    Wrapper<S> w = new Wrapper<>(this, reference);
+    return w;
   }
 
   /**

android/guava/src/com/google/common/base/FunctionalEquivalence.java

@@ -20,6 +20,7 @@
 import com.google.common.annotations.GwtCompatible;
 import java.io.Serializable;
 import javax.annotation.CheckForNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
  * Equivalence applied on functional result.
@@ -34,11 +35,11 @@ final class FunctionalEquivalence<F, T> extends Equivalence<F> implements Serial
 
   private static final long serialVersionUID = 0;
 
-  private final Function<? super F, ? extends T> function;
+  private final Function<? super F, ? extends @Nullable T> function;
   private final Equivalence<T> resultEquivalence;
 
   FunctionalEquivalence(
-      Function<? super F, ? extends T> function, Equivalence<T> resultEquivalence) {
+      Function<? super F, ? extends @Nullable T> function, Equivalence<T> resultEquivalence) {
     this.function = checkNotNull(function);
     this.resultEquivalence = checkNotNull(resultEquivalence);
   }

android/guava/src/com/google/common/collect/Collections2.java

@@ -695,7 +695,8 @@ private static boolean isPermutation(List<?> first, List<?> second) {
     return true;
   }
 
-  private static <E> ObjectCountHashMap<E> counts(Collection<E> collection) {
+  private static <E extends @Nullable Object> ObjectCountHashMap<E> counts(
+      Collection<E> collection) {
     ObjectCountHashMap<E> map = new ObjectCountHashMap<>();
     for (E e : collection) {
       map.put(e, map.get(e) + 1);

android/guava/src/com/google/common/collect/ImmutableMap.java

@@ -599,7 +599,9 @@ public ImmutableMap<K, V> buildKeepingLast() {
     }
 
     static <V> void sortEntries(
-        @Nullable Object[] alternatingKeysAndValues, int size, Comparator<V> valueComparator) {
+        @Nullable Object[] alternatingKeysAndValues,
+        int size,
+        Comparator<? super V> valueComparator) {
       @SuppressWarnings({"rawtypes", "unchecked"})
       Entry<Object, V>[] entries = new Entry[size];
       for (int i = 0; i < size; i++) {

android/guava/src/com/google/common/collect/ImmutableMultiset.java

@@ -31,6 +31,7 @@
 import java.util.Iterator;
 import java.util.Set;
 import javax.annotation.CheckForNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
  * A {@link Multiset} whose contents will never change, with many other important properties
@@ -282,7 +283,7 @@ public final boolean setCount(E element, int oldCount, int newCount) {
 
   @GwtIncompatible // not present in emulated superclass
   @Override
-  int copyIntoArray(Object[] dst, int offset) {
+  int copyIntoArray(@Nullable Object[] dst, int offset) {
     for (Multiset.Entry<E> entry : entrySet()) {
       Arrays.fill(dst, offset, offset + entry.getCount(), entry.getElement());
       offset += entry.getCount();

android/guava/src/com/google/common/collect/Maps.java

@@ -528,7 +528,20 @@ SortedMapDifference<K, V> difference(
     onlyOnRight.putAll(right); // will whittle it down
     SortedMap<K, V> onBoth = Maps.newTreeMap(comparator);
     SortedMap<K, MapDifference.ValueDifference<V>> differences = Maps.newTreeMap(comparator);
-    doDifference(left, right, Equivalence.equals(), onlyOnLeft, onlyOnRight, onBoth, differences);
+
+    /*
+     * V is a possibly nullable type, but we decided to declare Equivalence with a type parameter
+     * that is restricted to non-nullable types. Still, this code is safe: We made that decision
+     * about Equivalence not because Equivalence is null-hostile but because *every* Equivalence can
+     * handle null inputs -- and thus it would be meaningless for the type system to distinguish
+     * between "an Equivalence for nullable Foo" and "an Equivalence for non-nullable Foo."
+     *
+     * (And the unchecked cast is safe because Equivalence is contravariant.)
+     */
+    @SuppressWarnings({"nullness", "unchecked"})
+    Equivalence<V> equalsEquivalence = (Equivalence<V>) Equivalence.equals();
+
+    doDifference(left, right, equalsEquivalence, onlyOnLeft, onlyOnRight, onBoth, differences);
     return new SortedMapDifferenceImpl<>(onlyOnLeft, onlyOnRight, onBoth, differences);
   }
 

android/guava/src/com/google/common/collect/NullnessCasts.java

@@ -57,7 +57,7 @@ final class NullnessCasts {
   }
 
   /** Returns {@code null} as any type, even one that does not include {@code null}. */
-  @SuppressWarnings({"nullness", "TypeParameterUnusedInFormals"})
+  @SuppressWarnings({"nullness", "TypeParameterUnusedInFormals", "ReturnMissingNullable"})
   // The warnings are legitimate. Each time we use this method, we document why.
   @ParametricNullness
   static <T extends @Nullable Object> T unsafeNull() {

android/guava/src/com/google/common/collect/RegularImmutableSortedSet.java

@@ -155,7 +155,7 @@ boolean isPartialView() {
   }
 
   @Override
-  int copyIntoArray(Object[] dst, int offset) {
+  int copyIntoArray(@Nullable Object[] dst, int offset) {
     return elements.copyIntoArray(dst, offset);
   }
 

android/guava/src/com/google/common/collect/TopKSelector.java

@@ -184,8 +184,10 @@ private void trim() {
       }
       iterations++;
       if (iterations >= maxIterations) {
+        @SuppressWarnings("nullness") // safe because we pass sort() a range that contains real Ts
+        T[] castBuffer = (T[]) buffer;
         // We've already taken O(k log k), let's make sure we don't take longer than O(k log k).
-        Arrays.sort(buffer, left, right + 1, comparator);
+        Arrays.sort(castBuffer, left, right + 1, comparator);
         break;
       }
     }
@@ -263,7 +265,9 @@ public void offerAll(Iterator<? extends T> elements) {
    * this {@code TopKSelector}. This method returns in O(k log k) time.
    */
   public List<T> topK() {
-    Arrays.sort(buffer, 0, bufferSize, comparator);
+    @SuppressWarnings("nullness") // safe because we pass sort() a range that contains real Ts
+    T[] castBuffer = (T[]) buffer;
+    Arrays.sort(castBuffer, 0, bufferSize, comparator);
     if (bufferSize > k) {
       Arrays.fill(buffer, k, buffer.length, null);
       bufferSize = k;

android/guava/src/com/google/common/util/concurrent/AbstractFuture.java

@@ -1084,7 +1084,6 @@ protected void afterDone() {}
    * method returns @Nullable, too. However, we're not sure if we want to make any changes to that
    * class, since it's in a separate artifact that we planned to release only a single version of.
    */
-  @SuppressWarnings("nullness")
   @CheckForNull
   protected final Throwable tryInternalFastPathGetFailure() {
     if (this instanceof Trusted) {

android/guava/src/com/google/common/util/concurrent/NullnessCasts.java

@@ -65,7 +65,8 @@ final class NullnessCasts {
    * return to a caller, the code needs to a way to return {@code null} from a method that returns
    * "plain {@code T}." This API provides that.
    */
-  @SuppressWarnings("nullness")
+  @SuppressWarnings({"nullness", "TypeParameterUnusedInFormals", "ReturnMissingNullable"})
+  // The warnings are legitimate. Each time we use this method, we document why.
   @ParametricNullness
   static <T extends @Nullable Object> T uncheckedNull() {
     return null;

guava/src/com/google/common/base/Equivalence.java

@@ -165,7 +165,43 @@ public final <F> Equivalence<F> onResultOf(Function<? super F, ? extends @Nullab
    * @since 10.0
    */
   public final <S extends @Nullable T> Wrapper<S> wrap(@ParametricNullness S reference) {
-    return new Wrapper<S>(this, reference);
+    /*
+     * I'm pretty sure that this warning "makes sense" but doesn't indicate a real problem.
+     *
+     * Why it "makes sense": If we pass a `@Nullable Foo`, then we should also pass an
+     * `Equivalence<? super @Nullable Foo>`. And there's no such thing because Equivalence doesn't
+     * permit nullable type arguments.
+     *
+     * Why there's no real problem: Every Equivalence can handle null.
+     *
+     * We could work around this by giving Wrapper 2 type parameters. In the terms of this method,
+     * that would be both the T parameter (from the class) and the S parameter (from this method).
+     * However, such a change would be source-incompatible. (Plus, there's no reason for the S
+     * parameter from the user's perspective, so it would be a wart.)
+     *
+     * We could probably also work around this by making Wrapper non-final and putting the
+     * implementation into a subclass with those 2 type parameters. But we like `final`, if only to
+     * deter users from using mocking frameworks to construct instances. (And could also complicate
+     * serialization, which is discussed more in the next paragraph.)
+     *
+     * We could probably also work around this by having Wrapper accept an instance of a new
+     * WrapperGuts class, which would then be the class that would declare the 2 type parameters.
+     * But that would break deserialization of previously serialized Wrapper instances. And while we
+     * specifically say not to rely on serialization across Guava versions, users sometimes do. So
+     * we'd rather not break them without a good enough reason.
+     *
+     * (We could work around the serialization problem by writing custom serialization code. But
+     * even that helps only the case of serializing with an old version and deserializing with a
+     * new, not vice versa -- unless we introduce WrapperGuts and the logic to handle it today, wait
+     * until "everyone" has picked up a version of Guava with that code, and *then* change to use
+     * WrapperGuts.)
+     *
+     * Anyway, a suppression isn't really a big deal. But I have tried to do some due diligence on
+     * avoiding it :)
+     */
+    @SuppressWarnings("nullness")
+    Wrapper<S> w = new Wrapper<>(this, reference);
+    return w;
   }
 
   /**

guava/src/com/google/common/base/FunctionalEquivalence.java

@@ -20,6 +20,7 @@
 import com.google.common.annotations.GwtCompatible;
 import java.io.Serializable;
 import javax.annotation.CheckForNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
  * Equivalence applied on functional result.
@@ -34,11 +35,11 @@ final class FunctionalEquivalence<F, T> extends Equivalence<F> implements Serial
 
   private static final long serialVersionUID = 0;
 
-  private final Function<? super F, ? extends T> function;
+  private final Function<? super F, ? extends @Nullable T> function;
   private final Equivalence<T> resultEquivalence;
 
   FunctionalEquivalence(
-      Function<? super F, ? extends T> function, Equivalence<T> resultEquivalence) {
+      Function<? super F, ? extends @Nullable T> function, Equivalence<T> resultEquivalence) {
     this.function = checkNotNull(function);
     this.resultEquivalence = checkNotNull(resultEquivalence);
   }

guava/src/com/google/common/cache/LocalCache.java

@@ -2189,7 +2189,7 @@ V waitForLoadingValue(ReferenceEntry<K, V> e, K key, ValueReference<K, V> valueR
       }
     }
 
-    V compute(K key, int hash, BiFunction<? super K, ? super V, ? extends V> function) {
+    V compute(K key, int hash, BiFunction<? super K, ? super @Nullable V, ? extends V> function) {
       ReferenceEntry<K, V> e;
       ValueReference<K, V> valueReference = null;
       ComputingValueReference<K, V> computingValueReference = null;
@@ -3558,7 +3558,7 @@ public V apply(V newValue) {
       }
     }
 
-    public V compute(K key, BiFunction<? super K, ? super V, ? extends V> function) {
+    public V compute(K key, BiFunction<? super K, ? super @Nullable V, ? extends V> function) {
       stopwatch.start();
       V previousValue;
       try {
@@ -4205,7 +4205,7 @@ public V putIfAbsent(K key, V value) {
   }
 
   @Override
-  public V compute(K key, BiFunction<? super K, ? super V, ? extends V> function) {
+  public V compute(K key, BiFunction<? super K, ? super @Nullable V, ? extends V> function) {
     checkNotNull(key);
     checkNotNull(function);
     int hash = hash(key);

guava/src/com/google/common/collect/HashBiMap.java

@@ -489,6 +489,7 @@ private final class KeySet extends Maps.KeySet<K, V> {
     public Iterator<K> iterator() {
       return new Itr<K>() {
         @Override
+        @ParametricNullness
         K output(BiEntry<K, V> entry) {
           return entry.key;
         }
@@ -530,17 +531,20 @@ class MapEntry extends AbstractMapEntry<K, V> {
         }
 
         @Override
+        @ParametricNullness
         public K getKey() {
           return delegate.key;
         }
 
         @Override
+        @ParametricNullness
         public V getValue() {
           return delegate.value;
         }
 
         @Override
-        public V setValue(V value) {
+        @ParametricNullness
+        public V setValue(@ParametricNullness V value) {
           V oldValue = delegate.value;
           int valueHash = smearedHash(value);
           if (valueHash == delegate.valueHash && Objects.equal(value, oldValue)) {
@@ -675,6 +679,7 @@ public boolean remove(@CheckForNull Object o) {
       public Iterator<V> iterator() {
         return new Itr<V>() {
           @Override
+          @ParametricNullness
           V output(BiEntry<K, V> entry) {
             return entry.value;
           }
@@ -703,17 +708,20 @@ class InverseEntry extends AbstractMapEntry<V, K> {
           }
 
           @Override
+          @ParametricNullness
           public V getKey() {
             return delegate.value;
           }
 
           @Override
+          @ParametricNullness
           public K getValue() {
             return delegate.key;
           }
 
           @Override
-          public K setValue(K key) {
+          @ParametricNullness
+          public K setValue(@ParametricNullness K key) {
             K oldKey = delegate.key;
             int keyHash = smearedHash(key);
             if (keyHash == delegate.keyHash && Objects.equal(key, oldKey)) {

guava/src/com/google/common/collect/ImmutableMultiset.java

@@ -325,7 +325,7 @@ public final boolean setCount(E element, int oldCount, int newCount) {
 
   @GwtIncompatible // not present in emulated superclass
   @Override
-  int copyIntoArray(Object[] dst, int offset) {
+  int copyIntoArray(@Nullable Object[] dst, int offset) {
     for (Multiset.Entry<E> entry : entrySet()) {
       Arrays.fill(dst, offset, offset + entry.getCount(), entry.getElement());
       offset += entry.getCount();