Restore old code style settings action (KT-23400)

Can't use `KotlinCodeStyleSettings` instead `KtCodeStyleSettings`,
because current `KotlinCodeStyleSettings` class is already used in third
party plugin.

idea/formatter/src/org/jetbrains/kotlin/idea/core/formatter/KotlinCodeStyleSettings.java

@@ -170,6 +170,10 @@ public class KotlinCodeStyleSettings extends CustomCodeStyleSettings {
         return tempSettings;
     }
 
+    public boolean canRestore() {
+        return settingsAgainstPreviousDefaults != null;
+    }
+
     public void restore() {
         if (settingsAgainstPreviousDefaults != null) {
             copyFrom(settingsAgainstPreviousDefaults);

idea/formatter/src/org/jetbrains/kotlin/idea/formatter/KotlinCommonCodeStyleSettings.java

@@ -231,6 +231,10 @@ public class KotlinCommonCodeStyleSettings extends CommonCodeStyleSettings {
         return provider == null ? null : provider.getSupportedFields();
     }
 
+    public boolean canRestore() {
+        return settingsAgainstPreviousDefaults != null;
+    }
+
     public void restore() {
         if (settingsAgainstPreviousDefaults != null) {
             CompatibilityKt.copyFromEx(this, settingsAgainstPreviousDefaults);

idea/formatter/src/org/jetbrains/kotlin/idea/formatter/KtCodeStyleSettings.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. 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.kotlin.idea.formatter
+
+import com.intellij.openapi.project.Project
+import com.intellij.psi.codeStyle.CodeStyleSettingsManager
+import org.jetbrains.kotlin.idea.KotlinLanguage
+import org.jetbrains.kotlin.idea.core.formatter.KotlinCodeStyleSettings
+
+data class KtCodeStyleSettings(
+    val custom: KotlinCodeStyleSettings,
+    val common: KotlinCommonCodeStyleSettings
+)
+
+fun KtCodeStyleSettings.canRestore(): Boolean {
+    return custom.canRestore() || common.canRestore()
+}
+
+fun KtCodeStyleSettings.restore() {
+    custom.restore()
+    common.restore()
+}
+
+fun ktCodeStyleSettings(project: Project): KtCodeStyleSettings? {
+    @Suppress("DEPRECATION") // Suggested update is not supported in 173. BUNCH: 181
+    val settings = CodeStyleSettingsManager.getSettings(project)
+
+    val ktCommonSettings = settings.getCommonSettings(KotlinLanguage.INSTANCE) as KotlinCommonCodeStyleSettings
+    val ktCustomSettings = settings.getCustomSettings(KotlinCodeStyleSettings::class.java)
+
+    return KtCodeStyleSettings(ktCustomSettings, ktCommonSettings)
+}

idea/idea-jvm/src/org/jetbrains/kotlin/idea/configuration/ui/notifications/NewCodeStyleNotification.kt

@@ -6,12 +6,15 @@
 package org.jetbrains.kotlin.idea.configuration.ui.notifications
 
 import com.intellij.ide.util.PropertiesComponent
-import com.intellij.notification.Notification
+import com.intellij.notification.*
-import com.intellij.notification.NotificationDisplayType
+import com.intellij.openapi.actionSystem.AnActionEvent
-import com.intellij.notification.NotificationType
-import com.intellij.notification.NotificationsConfiguration
 import com.intellij.openapi.application.ApplicationManager
+import com.intellij.openapi.application.runWriteAction
+import com.intellij.openapi.diagnostic.Logger
 import com.intellij.openapi.project.Project
+import org.jetbrains.kotlin.idea.formatter.canRestore
+import org.jetbrains.kotlin.idea.formatter.ktCodeStyleSettings
+import org.jetbrains.kotlin.idea.formatter.restore
 import org.jetbrains.kotlin.idea.util.isDefaultOfficialCodeStyle
 
 private const val KOTLIN_UPDATE_CODE_STYLE_GROUP_ID = "Update Kotlin code style"
@@ -49,4 +52,27 @@ private class KotlinCodeStyleChangedNotification(val project: Project) : Notific
         """.trimIndent(),
     NotificationType.WARNING,
     null
-)
+) {
+    init {
+        val ktFormattingSettings = ktCodeStyleSettings(project)
+        if (ktFormattingSettings != null && ktFormattingSettings.canRestore()) {
+            addAction(object : NotificationAction("Restore old settings") {
+                override fun actionPerformed(e: AnActionEvent, notification: Notification) {
+                    restore(project)
+                }
+            })
+        }
+    }
+
+    companion object {
+        val LOG = Logger.getInstance("KotlinCodeStyleChangedNotification")
+
+        private fun restore(project: Project) {
+            val ktSettings = ktCodeStyleSettings(project) ?: return
+
+            runWriteAction {
+                ktSettings.restore()
+            }
+        }
+    }
+}