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/*
* Copyright (C) 2021 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.common.graph;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.ElementType.PARAMETER;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
import com.google.common.annotations.GwtCompatible;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;
/**
* 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}. 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)
@Target({FIELD, METHOD, PARAMETER})
@javax.annotation.meta.TypeQualifierNickname
@javax.annotation.Nonnull(when = javax.annotation.meta.When.UNKNOWN)
@interface ParametricNullness {}
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