Add a user-friendly report that the JDK tools are missing

Issue #KT-20214 Fixed

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

@@ -198,6 +198,9 @@ abstract class AbstractKotlinCompile<T : CommonCompilerArguments>() : AbstractKo
 
     @TaskAction
     open fun execute(inputs: IncrementalTaskInputs): Unit {
+        // Check that the JDK tools are available in Gradle (fail-fast, instead of a fail during the compiler run):
+        findToolsJar()
+
         val sourceRoots = getSourceRoots()
         val allKotlinSources = sourceRoots.kotlinSourceFiles
 

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

@@ -16,6 +16,7 @@
 
 package org.jetbrains.kotlin.gradle.tasks
 
+import org.gradle.api.GradleException
 import org.gradle.api.Project
 import org.gradle.api.artifacts.Configuration
 import org.gradle.api.artifacts.ResolvedDependency
@@ -86,8 +87,20 @@ internal fun findKotlinScriptJvmClasspath(project: Project): List<File> =
 internal fun findKotlinReflectClasspath(project: Project): List<File> =
         findKotlinModuleJar(project, KOTLIN_REFLECT_EXPECTED_CLASS, KOTLIN_REFLECT)
 
-internal fun findToolsJar(): File? =
+internal fun findToolsJar(): File? {
-        Class.forName("com.sun.tools.javac.util.Context")?.let(::findJarByClass)
+    val javacUtilContextClass =
+        try {
+            Class.forName("com.sun.tools.javac.util.Context")
+        } catch (classNotFound: ClassNotFoundException) {
+            val javaHome = System.getProperty("java.home") // current Java installation path
+            throw GradleException(
+                "Kotlin could not find the required JDK tools in the Java installation ${javaHome?.let { "'$it' " }.orEmpty()}" +
+                        "used by Gradle. Make sure Gradle is running on a JDK, not JRE.",
+                classNotFound
+            )
+        }
+    return javacUtilContextClass?.let(::findJarByClass)
+}
 
 internal fun findCoroutinesClasspath(): List<File> {
     val classLoader = Thread.currentThread().contextClassLoader