Add IDE diagnostic on @Serializable classes for non-applied serialization plugin

plugins/kotlin-serialization/kotlin-serialization-ide/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationPluginIDEDeclarationChecker.kt

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlinx.serialization.compiler.diagnostic
+
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
+import org.jetbrains.kotlinx.serialization.idea.getIfEnabledOn
+
+class SerializationPluginIDEDeclarationChecker : SerializationPluginDeclarationChecker() {
+    override fun serializationPluginEnabledOn(descriptor: ClassDescriptor): Boolean {
+        // In the IDE, plugin is always in the classpath, but enabled only if corresponding compiler settings
+        // were imported into project model from Gradle.
+        return getIfEnabledOn(descriptor) { true } == true
+    }
+}

plugins/kotlin-serialization/kotlin-serialization-ide/src/org/jetbrains/kotlinx/serialization/compiler/extensions/SerializationIDEContainerContributor.kt

@@ -10,7 +10,7 @@ import org.jetbrains.kotlin.container.useInstance
 import org.jetbrains.kotlin.descriptors.ModuleDescriptor
 import org.jetbrains.kotlin.extensions.StorageComponentContainerContributor
 import org.jetbrains.kotlin.platform.TargetPlatform
-import org.jetbrains.kotlinx.serialization.compiler.diagnostic.SerializationPluginDeclarationChecker
+import org.jetbrains.kotlinx.serialization.compiler.diagnostic.SerializationPluginIDEDeclarationChecker
 
 class SerializationIDEContainerContributor : StorageComponentContainerContributor {
     override fun registerModuleComponents(
@@ -18,6 +18,6 @@ class SerializationIDEContainerContributor : StorageComponentContainerContributo
         platform: TargetPlatform,
         moduleDescriptor: ModuleDescriptor
     ) {
-        container.useInstance(SerializationPluginDeclarationChecker())
+        container.useInstance(SerializationPluginIDEDeclarationChecker())
     }
 }