Check the MPP model for unused Kotlin source sets, warn when found

Issue #KT-26963 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/NewMultiplatformIT.kt

@@ -5,6 +5,7 @@
 package org.jetbrains.kotlin.gradle
 
 import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinMultiplatformPlugin
+import org.jetbrains.kotlin.gradle.plugin.mpp.UnusedSourceSetsChecker
 import org.jetbrains.kotlin.gradle.plugin.sources.METADATA_CONFIGURATION_NAME_SUFFIX
 import org.jetbrains.kotlin.gradle.plugin.sources.SourceSetConsistencyChecks
 import org.jetbrains.kotlin.gradle.util.checkBytecodeContains
@@ -1062,4 +1063,34 @@ class NewMultiplatformIT : BaseGradleIT() {
             assertFileExists("foo/2.txt")
         }
     }
+
+    @Test
+    fun testUnusedSourceSetsReport() = with(Project("sample-lib", gradleVersion, "new-mpp-lib-and-app")) {
+        setupWorkingDir()
+
+        gradleBuildScript().appendText("\nkotlin { sourceSets { foo { } } }")
+
+        build {
+            assertSuccessful()
+            assertContains(UnusedSourceSetsChecker.WARNING_PREFIX_ONE, UnusedSourceSetsChecker.WARNING_INTRO)
+        }
+
+        gradleBuildScript().appendText("\nkotlin { sourceSets { bar { dependsOn foo } } }")
+
+        build {
+            assertSuccessful()
+            assertContains(UnusedSourceSetsChecker.WARNING_PREFIX_MANY, UnusedSourceSetsChecker.WARNING_INTRO)
+        }
+
+        gradleBuildScript().appendText("\nkotlin { sourceSets { jvm6Main { dependsOn bar } } }")
+
+        build {
+            assertSuccessful()
+            assertNotContains(
+                UnusedSourceSetsChecker.WARNING_PREFIX_ONE,
+                UnusedSourceSetsChecker.WARNING_PREFIX_MANY,
+                UnusedSourceSetsChecker.WARNING_INTRO
+            )
+        }
+    }
 }

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

@@ -94,6 +94,8 @@ class KotlinMultiplatformPlugin(
 
         // propagate compiler plugin options to the source set language settings
         setupCompilerPluginOptions(project)
+
+        UnusedSourceSetsChecker.checkSourceSets(project)
     }
 
     private fun setupCompilerPluginOptions(project: Project) {
@@ -104,14 +106,7 @@ class KotlinMultiplatformPlugin(
                 .compilations.getByName(KotlinCompilation.MAIN_COMPILATION_NAME)
 
         val primaryCompilationsBySourceSet by lazy { // don't evaluate eagerly: Android targets are not created at this point
-            val allCompilationsForSourceSets =
+            val allCompilationsForSourceSets = compilationsBySourceSet(project)
-                project.multiplatformExtension!!.targets
-                    .flatMap { target ->
-                        target.compilations.flatMap { compilation ->
-                            compilation.allKotlinSourceSets.map { it to compilation }
-                        }
-                    }
-                    .groupBy(keySelector = { (sourceSet, _) -> sourceSet }, valueTransform = { (_, compilation) -> compilation })
 
             allCompilationsForSourceSets.mapValues { (_, compilations) -> // choose one primary compilation
                 when (compilations.size) {
@@ -305,4 +300,15 @@ class KotlinMultiplatformPlugin(
     }
 }
 
-internal val KotlinTarget.defaultArtifactId get() = "${project.name}-${name.toLowerCase()}"
+internal val KotlinTarget.defaultArtifactId get() = "${project.name}-${name.toLowerCase()}"
+
+internal fun compilationsBySourceSet(project: Project): Map<KotlinSourceSet, Set<KotlinCompilation>> =
+    HashMap<KotlinSourceSet, MutableSet<KotlinCompilation>>().also { result ->
+        for (target in project.multiplatformExtension!!.targets) {
+            for (compilation in target.compilations) {
+                for (sourceSet in compilation.allKotlinSourceSets) {
+                    result.getOrPut(sourceSet) { mutableSetOf() }.add(compilation)
+                }
+            }
+        }
+    }

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

@@ -0,0 +1,48 @@
+/*
+ * 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.gradle.plugin.mpp
+
+import org.gradle.api.Project
+import org.jetbrains.kotlin.gradle.dsl.kotlinExtension
+import org.jetbrains.kotlin.gradle.plugin.KotlinSourceSet
+
+object UnusedSourceSetsChecker {
+    const val WARNING_PREFIX_ONE =
+        "The Kotlin source set"
+
+    const val WARNING_PREFIX_MANY =
+        "The following Kotlin source sets were"
+
+    const val WARNING_INTRO = "configured but not added to any Kotlin compilation"
+
+    const val WARNING_BOTTOM_LINE =
+        "You can add a source set to a target's compilation by connecting it with the compilation's default source set using 'dependsOn'.\n" +
+                "See https://kotlinlang.org/docs/reference/building-mpp-with-gradle.html#connecting-source-sets"
+
+    private fun reportUnusedSourceSets(project: Project, sourceSets: Set<KotlinSourceSet>) {
+        require(sourceSets.isNotEmpty())
+        val message = when (sourceSets.size) {
+            1 -> "$WARNING_PREFIX_ONE ${sourceSets.single().name} was $WARNING_INTRO. $WARNING_BOTTOM_LINE"
+            else -> {
+                val list = sourceSets.joinToString("\n", "\n", "\n") { " * ${it.name}" }
+                "$WARNING_PREFIX_MANY $WARNING_INTRO:$list$WARNING_BOTTOM_LINE"
+            }
+        }
+        project.logger.warn("\n" + message) // make sure the message stands out
+    }
+
+    fun checkSourceSets(project: Project) {
+        project.afterEvaluate {
+            val compilationsBySourceSet = compilationsBySourceSet(project)
+            val unusedSourceSets = project.kotlinExtension.sourceSets.filter {
+                compilationsBySourceSet[it]?.isEmpty() ?: true
+            }
+            if (unusedSourceSets.isNotEmpty()) {
+                reportUnusedSourceSets(project, unusedSourceSets.toSet())
+            }
+        }
+    }
+}