[Gradle, MPP] Print warning on absent 'android' targets

^KT-41641 Verification Pending

libraries/tools/kotlin-gradle-plugin/src/functionalTest/kotlin/org/jetbrains/kotlin/gradle/KT41641AbsentAndroidTarget.kt

@@ -8,12 +8,13 @@
 package org.jetbrains.kotlin.gradle
 
 import com.android.build.gradle.LibraryExtension
+import org.jetbrains.kotlin.gradle.plugin.runMissingAndroidTargetProjectConfigurationHealthCheck
 import org.jetbrains.kotlin.gradle.targets.android.findAndroidTarget
 import org.junit.Test
-import kotlin.test.assertNull
-import kotlin.test.assertSame
+import kotlin.test.*
 
 class KT41641AbsentAndroidTarget : MultiplatformExtensionTest() {
+
     @Test
     fun `test android plugin without android target`() {
         project.plugins.apply("kotlin-multiplatform")
@@ -29,4 +30,27 @@ class KT41641AbsentAndroidTarget : MultiplatformExtensionTest() {
         assertNull(project.findAndroidTarget())
         assertSame(kotlin.android(), project.findAndroidTarget())
     }
+
+    @Test
+    fun `test runMissingAndroidTargetProjectConfigurationHealthCheck`() {
+        project.plugins.apply("kotlin-multiplatform")
+        project.plugins.apply("android-library")
+
+        /* Arbitrary minimal Android setup */
+        val android = project.extensions.getByName("android") as LibraryExtension
+        android.compileSdkVersion(30)
+
+        kotlin.jvm()
+
+        // Missing android target -> expect warning message
+        var warningMessage: String? = null
+        project.runMissingAndroidTargetProjectConfigurationHealthCheck(warningLogger = { warningMessage = it })
+        assertNotNull(warningMessage, "Expected warning message to be logged")
+
+        // Present android target -> expect no warning message anymore
+        kotlin.android()
+        project.runMissingAndroidTargetProjectConfigurationHealthCheck(warningLogger = {
+            fail("Expected no warning message to be logged. Received: $it")
+        })
+    }
 }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinMultiplatformProjectConfigurationHealthChecks.kt

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2010-2021 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.kotlin.gradle.plugin
+
+import org.gradle.api.Project
+import org.jetbrains.kotlin.gradle.dsl.KotlinMultiplatformExtension
+import org.jetbrains.kotlin.gradle.dsl.kotlinExtension
+import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinMetadataTarget
+import org.jetbrains.kotlin.gradle.targets.android.findAndroidTarget
+import org.jetbrains.kotlin.gradle.utils.androidPluginIds
+import org.jetbrains.kotlin.gradle.utils.runProjectConfigurationHealthCheck
+
+private class KotlinMultiplatformProjectConfigurationException(message: String) : Exception(message)
+
+internal fun Project.runMissingKotlinTargetsProjectConfigurationHealthCheck() = project.runProjectConfigurationHealthCheck {
+    val isNoTargetsInitialized = (project.kotlinExtension as KotlinMultiplatformExtension)
+        .targets
+        .none { it !is KotlinMetadataTarget }
+
+    if (isNoTargetsInitialized) {
+        throw KotlinMultiplatformProjectConfigurationException(
+            """
+                Please initialize at least one Kotlin target in '${project.name} (${project.path})'.
+                Read more https://kotlinlang.org/docs/reference/building-mpp-with-gradle.html#setting-up-targets
+                """.trimIndent()
+        )
+    }
+}
+
+internal fun Project.runMissingAndroidTargetProjectConfigurationHealthCheck(
+    warningLogger: (warningMessage: String) -> Unit = project.logger::warn
+) = project.runProjectConfigurationHealthCheck check@{
+    val androidPluginId = androidPluginIds
+        .firstOrNull { androidPluginId -> plugins.findPlugin(androidPluginId) != null } ?: return@check
+
+    if (findAndroidTarget() != null) return@check
+
+    warningLogger(
+        """
+            Missing 'android()' Kotlin target in ${project.name} (${project.path})'.
+            The Android Gradle plugin was applied without creating a corresponding 'android()' Kotlin Target:
+            
+            ```
+            plugins {
+                id("$androidPluginId")
+                kotlin("multiplatform")
+            }
+            
+            kotlin {
+                android() // <-- please register this Android target
+            }
+            ```
+          
+        """.trimIndent()
+    )
+}

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinPluginWrapper.kt

@@ -50,7 +50,7 @@ import org.jetbrains.kotlin.statistics.metrics.StringMetrics
 import javax.inject.Inject
 import kotlin.reflect.KClass
 
-abstract class KotlinBasePluginWrapper: Plugin<Project> {
+abstract class KotlinBasePluginWrapper : Plugin<Project> {
 
     private val log = Logging.getLogger(this.javaClass)
 
@@ -232,19 +232,9 @@ open class KotlinMultiplatformPluginWrapper : KotlinBasePluginWrapper() {
     override val projectExtensionClass: KClass<out KotlinMultiplatformExtension>
         get() = KotlinMultiplatformExtension::class
 
-    override fun whenBuildEvaluated(project: Project) = project.runProjectConfigurationHealthCheck {
-        val isNoTargetsInitialized = (project.kotlinExtension as KotlinMultiplatformExtension)
-            .targets
-            .none { it !is KotlinMetadataTarget }
-
-        if (isNoTargetsInitialized) {
-            throw GradleException(
-                """
-                Please initialize at least one Kotlin target in '${project.name} (${project.path})'.
-                Read more https://kotlinlang.org/docs/reference/building-mpp-with-gradle.html#setting-up-targets
-                """.trimIndent()
-            )
-        }
+    override fun whenBuildEvaluated(project: Project) {
+        project.runMissingAndroidTargetProjectConfigurationHealthCheck()
+        project.runMissingKotlinTargetsProjectConfigurationHealthCheck()
     }
 }