Improve signatures of methods for converting iterables to arrays.

- Refine the signature of `Iterables.toArray` (and `Iterators.toArray`) by using `@NonNull`.
- Finish fixing the signature of `FluentIterable.toArray`, which we'd started doing in cl/519736884 but didn't finish: In that CL, we'd added `@NonNull` to fix a build error but not removed the `@Nullable` in the return type. Actually, that `@Nullable` probably _never_ made sense? Probably I had blindly added it as part of addressing a build error during cl/388680701 instead of reasoning out that it wasn't necessary. It's especially sad to have gone out of the way to add it when we ended up needing a suppression on the method regardless. I hope I didn't need to uglify any callers in response to the change....

RELNOTES=n/a
PiperOrigin-RevId: 537067780

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

@@ -785,8 +785,8 @@ public final boolean isEmpty() {
    *     copied
    */
   @GwtIncompatible // Array.newArray(Class, int)
-  public final @Nullable E[] toArray(Class<@NonNull E> type) {
-    return Iterables.toArray(getDelegate(), type);
+  public final E[] toArray(Class<@NonNull E> type) {
+    return Iterables.<E>toArray(getDelegate(), type);
   }
 
   /**

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

@@ -36,6 +36,7 @@
 import java.util.RandomAccess;
 import java.util.Set;
 import javax.annotation.CheckForNull;
+import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -323,12 +324,8 @@ public static String toString(Iterable<?> iterable) {
    * @return a newly-allocated array into which all the elements of the iterable have been copied
    */
   @GwtIncompatible // Array.newInstance(Class, int)
-  /*
-   * If we could express Class<@Nonnull T>, we could generalize the type parameter to <T extends
-   * @Nullable Object>, and then we could accept an Iterable<? extends T> and return a plain T[]
-   * instead of a @Nullable T[].
-   */
-  public static <T> @Nullable T[] toArray(Iterable<? extends @Nullable T> iterable, Class<T> type) {
+  public static <T extends @Nullable Object> T[] toArray(
+      Iterable<? extends T> iterable, Class<@NonNull T> type) {
     return toArray(iterable, ObjectArrays.newArray(type, 0));
   }
 

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

@@ -47,6 +47,7 @@
 import java.util.PriorityQueue;
 import java.util.Queue;
 import javax.annotation.CheckForNull;
+import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -344,10 +345,10 @@ public static String toString(Iterator<?> iterator) {
    * @return a newly-allocated array into which all the elements of the iterator have been copied
    */
   @GwtIncompatible // Array.newInstance(Class, int)
-  // For discussion of this signature, see the corresponding overload of *Iterables*.toArray.
-  public static <T> @Nullable T[] toArray(Iterator<? extends @Nullable T> iterator, Class<T> type) {
-    List<@Nullable T> list = Lists.newArrayList(iterator);
-    return Iterables.toArray(list, type);
+  public static <T extends @Nullable Object> T[] toArray(
+      Iterator<? extends T> iterator, Class<@NonNull T> type) {
+    List<T> list = Lists.newArrayList(iterator);
+    return Iterables.<T>toArray(list, type);
   }
 
   /**

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

@@ -777,8 +777,8 @@ public final boolean isEmpty() {
    *     copied
    */
   @GwtIncompatible // Array.newArray(Class, int)
-  public final @Nullable E[] toArray(Class<@NonNull E> type) {
-    return Iterables.toArray(getDelegate(), type);
+  public final E[] toArray(Class<@NonNull E> type) {
+    return Iterables.<E>toArray(getDelegate(), type);
   }
 
   /**

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

@@ -39,6 +39,7 @@
 import java.util.function.Consumer;
 import java.util.stream.Stream;
 import javax.annotation.CheckForNull;
+import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -285,12 +286,8 @@ public static String toString(Iterable<?> iterable) {
    * @return a newly-allocated array into which all the elements of the iterable have been copied
    */
   @GwtIncompatible // Array.newInstance(Class, int)
-  /*
-   * If we could express Class<@Nonnull T>, we could generalize the type parameter to <T extends
-   * @Nullable Object>, and then we could accept an Iterable<? extends T> and return a plain T[]
-   * instead of a @Nullable T[].
-   */
-  public static <T> @Nullable T[] toArray(Iterable<? extends @Nullable T> iterable, Class<T> type) {
+  public static <T extends @Nullable Object> T[] toArray(
+      Iterable<? extends T> iterable, Class<@NonNull T> type) {
     return toArray(iterable, ObjectArrays.newArray(type, 0));
   }
 

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

@@ -47,6 +47,7 @@
 import java.util.PriorityQueue;
 import java.util.Queue;
 import javax.annotation.CheckForNull;
+import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -344,10 +345,10 @@ public static String toString(Iterator<?> iterator) {
    * @return a newly-allocated array into which all the elements of the iterator have been copied
    */
   @GwtIncompatible // Array.newInstance(Class, int)
-  // For discussion of this signature, see the corresponding overload of *Iterables*.toArray.
-  public static <T> @Nullable T[] toArray(Iterator<? extends @Nullable T> iterator, Class<T> type) {
-    List<@Nullable T> list = Lists.newArrayList(iterator);
-    return Iterables.toArray(list, type);
+  public static <T extends @Nullable Object> T[] toArray(
+      Iterator<? extends T> iterator, Class<@NonNull T> type) {
+    List<T> list = Lists.newArrayList(iterator);
+    return Iterables.<T>toArray(list, type);
   }
 
   /**