Use fixated Kotlin compiler for compiling the Gradle plugin.

Relates to KT-60521

Author: ALikhachev

build.gradle.kts

@@ -80,7 +80,7 @@ logger.info("Using Kotlin Gradle Plugin $currentKgpVersion")
 val kotlinVersionOverride = providers.gradleProperty("kotlin_version").getOrNull()
 
 if (kotlinVersionOverride != null) {
-    val versionCatalogKotlinVersion = libs.versions.kotlin.get()
+    val versionCatalogKotlinVersion = libs.versions.kotlin.asProvider().get()
     if (kotlinVersionOverride != versionCatalogKotlinVersion) {
         throw IllegalStateException("Kotlin version in Version Catalog was not overridden. Expected:$kotlinVersionOverride, actual:$versionCatalogKotlinVersion.")
     }

gradle/libs.versions.toml

@@ -2,6 +2,7 @@
 
 # Note: Kotlin version can be overridden by passing `-Pkotlin_version=<version>`
 kotlin = "2.0.20"
+kotlin-for-gradle-plugin = "2.0.20" # Kotlin 2.1 removes support for the used language version / api version: KT-60521
 kotlinx-binaryCompatibilityValidator = "0.16.2"
 kotlinx-teamInfra = "0.4.0-dev-81"
 squareup-kotlinpoet = "1.3.0"

integration/build.gradle.kts

@@ -24,7 +24,7 @@ tasks.test {
     systemProperty("plugin_repo_url", plugin.projectDir.resolve("build/maven").absoluteFile.invariantSeparatorsPath)
     systemProperty("runtime_repo_url", rootProject.buildDir.resolve("maven").absoluteFile.invariantSeparatorsPath)
     systemProperty("kotlin_repo_url", rootProject.properties["kotlin_repo_url"])
-    systemProperty("kotlin_version", libs.versions.kotlin.get())
+    systemProperty("kotlin_version", libs.versions.kotlin.asProvider().get())
     systemProperty("minSupportedGradleVersion", libs.versions.minSupportedGradle.get())
     systemProperty("minSupportedKotlinVersion", libs.versions.minSupportedKotlin.get())
 }

plugin/build.gradle.kts

@@ -1,5 +1,7 @@
 import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 import kotlinx.team.infra.InfraExtension
+import org.jetbrains.kotlin.buildtools.api.ExperimentalBuildToolsApi
+import org.jetbrains.kotlin.gradle.ExperimentalKotlinGradlePluginApi
 
 buildscript {
     repositories {
@@ -41,7 +43,7 @@ extensions.configure<InfraExtension> {
     }
 }
 
-logger.info("Using Kotlin ${libs.versions.kotlin.get()} for project ${project.name}")
+logger.info("Using Kotlin ${libs.versions.kotlin.asProvider().get()} for project ${project.name}")
 
 repositories {
     mavenCentral()
@@ -83,14 +85,26 @@ sourceSets {
     }
 }
 
+kotlin {
+    @OptIn(ExperimentalBuildToolsApi::class, ExperimentalKotlinGradlePluginApi::class)
+    compilerVersion = libs.versions.kotlin.`for`.gradle.plugin.get()
+}
+
 tasks.compileKotlin {
     compilerOptions {
         optIn.addAll(
                 "kotlinx.benchmark.gradle.internal.KotlinxBenchmarkPluginInternalApi",
                 "kotlin.RequiresOptIn",
         )
-        @Suppress("DEPRECATION")
-        apiVersion = KotlinVersion.KOTLIN_1_4 // the version of Kotlin embedded in Gradle
+        /**
+         * Those versions are configured according to https://docs.gradle.org/current/userguide/compatibility.html
+         * and the Kotlin compiler compatibility policy stating that Kotlin 1.4 is compatible with 1.5 binaries
+         */
+        @Suppress("DEPRECATION", "DEPRECATION_ERROR")
+        run {
+            languageVersion = KotlinVersion.KOTLIN_1_5
+            apiVersion = KotlinVersion.KOTLIN_1_4
+        }
     }
 }
 
@@ -126,7 +140,7 @@ val generatePluginConstants by tasks.registering {
     val minSupportedKotlinVersion = libs.versions.minSupportedKotlin
     inputs.property("minSupportedKotlinVersion", minSupportedKotlinVersion)
 
-    val kotlinCompilerVersion = libs.versions.kotlin
+    val kotlinCompilerVersion = libs.versions.kotlin.asProvider()
     inputs.property("kotlinCompilerVersion", kotlinCompilerVersion)
 
     doLast {

plugin/gradle.properties

@@ -2,3 +2,6 @@ group=org.jetbrains.kotlinx
 version=0.5.0-SNAPSHOT
 
 kotlin.code.style=official
+
+# to use fixated compiler for compiling Gradle plugin
+kotlin.compiler.runViaBuildToolsApi=true