1 files changed, 38 insertions, 5 deletions
diff --git a/android/guava/src/com/google/common/cache/ParametricNullness.java b/android/guava/src/com/google/common/cache/ParametricNullness.java
index 64682de55..eaf194d4a 100644
--- a/android/guava/src/com/google/common/cache/ParametricNullness.java
+++ b/android/guava/src/com/google/common/cache/ParametricNullness.java
@@ -26,23 +26,56 @@ import java.lang.annotation.Retention;
 import java.lang.annotation.Target;
 
 /**
- * Marks a "top-level" type-variable usage as the closest we can get to "non-nullable when
- * non-nullable; nullable when nullable" (like the Android <a
- * href="https://android.googlesource.com/platform/libcore/+/master/luni/src/main/java/libcore/util/NullFromTypeParam.java">{@code
- * NullFromTypeParam}</a>).
+ * Annotates a "top-level" type-variable usage that takes its nullness from the type argument
+ * supplied by the user of the class. For example, {@code Multiset.Entry.getElement()} returns
+ * {@code @ParametricNullness E}, which means:
+ *
+ * <ul>
+ *   <li>{@code getElement} on a {@code Multiset.Entry<@NonNull String>} returns {@code @NonNull
+ *       String}.
+ *   <li>{@code getElement} on a {@code Multiset.Entry<@Nullable String>} returns {@code @Nullable
+ *       String}.
+ * </ul>
+ *
+ * This is the same behavior as type-variable usages have to Kotlin and to the Checker Framework.
+ * Contrast the method above to:
+ *
+ * <ul>
+ *   <li>methods whose return type is a type variable but which can never return {@code null},
+ *       typically because the type forbids nullable type arguments: For example, {@code
+ *       ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly,
+ *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)
+ *   <li>methods whose return type is a type variable but which can return {@code null} regardless
+ *       of the type argument supplied by the user of the class: For example, {@code
+ *       ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null}
+ *       even on an {@code ImmutableMap<K, @NonNull String>}.
+ * </ul>
  *
  * <p>Consumers of this annotation include:
  *
  * <ul>
  *   <li>Kotlin, for which it makes the type-variable usage (a) a Kotlin platform type when the type
  *       argument is non-nullable and (b) nullable when the type argument is nullable. We use this
- *       to "undo" {@link ElementTypesAreNonnullByDefault}.
+ *       to "undo" {@link ElementTypesAreNonnullByDefault}. It is the best we can do for Kotlin
+ *       under our current constraints.
+ *   <li>NullAway, which will <a
+ *       href="https://github.com/google/guava/issues/6126#issuecomment-1204399671">treat it
+ *       identically to {@code Nullable} as of version 0.9.9</a>. To treat it that way before then,
+ *       you can set {@code
+ *       -XepOpt:NullAway:CustomNullableAnnotations=com.google.common.base.ParametricNullness,...,com.google.common.util.concurrent.ParametricNullness},
+ *       where the {@code ...} contains the names of all the other {@code ParametricNullness}
+ *       annotations in Guava. Or you might prefer to omit Guava from your {@code AnnotatedPackages}
+ *       list.
  *   <li><a href="https://developers.google.com/j2objc">J2ObjC</a>
  *   <li>{@code NullPointerTester}, at least in the Android backport (where the type-use annotations
  *       {@code NullPointerTester} would need are not available) and in case of <a
  *       href="https://bugs.openjdk.java.net/browse/JDK-8202469">JDK-8202469</a>
  * </ul>
  *
+ * <p>This annotation is a temporary hack. We will remove it after we're able to adopt the <a
+ * href="https://jspecify.dev/">JSpecify</a> nullness annotations and <a
+ * href="https://github.com/google/guava/issues/6126#issuecomment-1203145963">tools no longer need
+ * it</a>.
  */
 @GwtCompatible
 @Retention(RUNTIME)