Update code to prepare for nullness annotations in Guava (Kotlin#3026)

Leverage the fact that `FutureCallback<T>.onSuccess` can only accept
`null` when `T` is `null`, to remove the unchecked casts.

Author: cpovirk

integration/kotlinx-coroutines-guava/README.md

@@ -62,6 +62,6 @@ Integration with Guava [ListenableFuture](https://github.com/google/guava/wiki/L
 
 <!--- INDEX com.google.common.util.concurrent -->
 
-[com.google.common.util.concurrent.ListenableFuture]: https://kotlin.github.io/kotlinx.coroutines/https://google.github.io/guava/releases/28.0-jre/api/docs/com/google/common/util/concurrent/ListenableFuture.html
+[com.google.common.util.concurrent.ListenableFuture]: https://kotlin.github.io/kotlinx.coroutines/https://google.github.io/guava/releases/31.0.1-jre/api/docs/com/google/common/util/concurrent/ListenableFuture.html
 
 <!--- END -->

integration/kotlinx-coroutines-guava/build.gradle.kts

@@ -2,12 +2,17 @@
  * Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
-val guavaVersion = "28.0-jre"
+val guavaVersion = "31.0.1-jre"
 
 dependencies {
     compile("com.google.guava:guava:$guavaVersion")
 }
 
+java {
+    targetCompatibility = JavaVersion.VERSION_1_8
+    sourceCompatibility = JavaVersion.VERSION_1_8
+}
+
 externalDocumentationLink(
     url = "https://google.github.io/guava/releases/$guavaVersion/api/docs/"
 )

integration/kotlinx-coroutines-guava/src/ListenableFuture.kt

@@ -135,10 +135,8 @@ public fun <T> ListenableFuture<T>.asDeferred(): Deferred<T> {
     // Finally, if this isn't done yet, attach a Listener that will complete the Deferred.
     val deferred = CompletableDeferred<T>()
     Futures.addCallback(this, object : FutureCallback<T> {
-        override fun onSuccess(result: T?) {
-            // Here we work with flexible types, so we unchecked cast to trick the type system
-            @Suppress("UNCHECKED_CAST")
-            runCatching { deferred.complete(result as T) }
+        override fun onSuccess(result: T) {
+            runCatching { deferred.complete(result) }
                 .onFailure { handleCoroutineException(EmptyCoroutineContext, it) }
         }
 
@@ -351,7 +349,7 @@ private class JobListenableFuture<T>(private val jobToCancel: Job): ListenableFu
      *
      * To preserve Coroutine's [CancellationException], this future points to either `T` or [Cancelled].
      */
-    private val auxFuture = SettableFuture.create<Any>()
+    private val auxFuture = SettableFuture.create<Any?>()
 
     /**
      * `true` if [auxFuture.get][ListenableFuture.get] throws [ExecutionException].
@@ -436,7 +434,7 @@ private class JobListenableFuture<T>(private val jobToCancel: Job): ListenableFu
     }
 
     /** See [get()]. */
-    private fun getInternal(result: Any): T = if (result is Cancelled) {
+    private fun getInternal(result: Any?): T = if (result is Cancelled) {
         throw CancellationException().initCause(result.exception)
     } else {
         // We know that `auxFuture` can contain either `T` or `Cancelled`.