Enable warnings as errors where appropriate

Author: qwwdfsad

buildSrc/src/main/kotlin/configure-compilation-conventions.gradle.kts

@@ -4,16 +4,45 @@
 
 import org.jetbrains.kotlin.gradle.tasks.*
 
+val graduallyIntroducedWarningAsErrorProjects = setOf(
+    // UI
+    "kotlinx-coroutines-android",
+    "kotlinx-coroutines-javafx",
+    "kotlinx-coroutines-swing",
+
+    // Reactive
+    "kotlinx-coroutines-jdk9",
+    "kotlinx-coroutines-reactive",
+    "kotlinx-coroutines-reactor",
+    "kotlinx-coroutines-rx2",
+    "kotlinx-coroutines-rx3",
+
+    // Integration
+    "kotlinx-coroutines-guava",
+    "kotlinx-coroutines-jdk8",
+    "kotlinx-coroutines-play-services",
+    "kotlinx-coroutines-slf4j",
+
+    // Top-level
+    "kotlinx-coroutines-debug",
+
+    )
+
 configure(subprojects) {
     if (name in sourceless) return@configure
     apply(plugin = "kotlinx-atomicfu")
+    val projectName = name
     tasks.withType(KotlinCompile::class).all {
+        val isMainTaskName = name == "compileKotlin" || name == "compileKotlinJvm"
         kotlinOptions {
+            if (projectName in graduallyIntroducedWarningAsErrorProjects && isMainTaskName) {
+                allWarningsAsErrors = true
+            }
             val newOptions =
                 listOf(
                     "-progressive", "-Xno-param-assertions", "-Xno-receiver-assertions",
                     "-Xno-call-assertions"
-                ) + optInAnnotations.map { "-Xopt-in=$it" }
+                ) + optInAnnotations.map { "-opt-in=$it" }
             freeCompilerArgs = freeCompilerArgs + newOptions
         }
     }

reactive/kotlinx-coroutines-reactor/src/ReactorFlow.kt

@@ -32,8 +32,7 @@ private class FlowAsFlux<T : Any>(
     private val flow: Flow<T>,
     private val context: CoroutineContext
 ) : Flux<T>() {
-    override fun subscribe(subscriber: CoreSubscriber<in T>?) {
-        if (subscriber == null) throw NullPointerException()
+    override fun subscribe(subscriber: CoreSubscriber<in T>) {
         val hasContext = !subscriber.currentContext().isEmpty
         val source = if (hasContext) flow.flowOn(subscriber.currentContext().asCoroutineContext()) else flow
         subscriber.onSubscribe(FlowSubscription(source, subscriber, context))

reactive/kotlinx-coroutines-rx3/src/RxMaybe.kt

@@ -43,6 +43,7 @@ private class RxMaybeCoroutine<T>(
 ) : AbstractCoroutine<T>(parentContext, false, true) {
     override fun onCompleted(value: T) {
         try {
+            @Suppress("NULLABILITY_MISMATCH_BASED_ON_JAVA_ANNOTATIONS") // KT-54201
             if (value == null) subscriber.onComplete() else subscriber.onSuccess(value)
         } catch (e: Throwable) {
             handleUndeliverableException(e, context)