Adjust script compilation to use jsr305 strict mode

This is what we already do for Kotlin plugins. This change makes
the Gradle DSL more consistent.

gradle/shared-with-buildSrc/mirrors.settings.gradle.kts

@@ -53,7 +53,7 @@ fun withMirrors(handler: RepositoryHandler) {
         if (this is MavenArtifactRepository) {
             originalUrls.forEach { name, originalUrl ->
                 if (normalizeUrl(originalUrl) == normalizeUrl(this.url.toString()) && mirrorUrls.containsKey(name)) {
-                    this.setUrl(mirrorUrls.get(name))
+                    mirrorUrls.get(name)?.let { this.setUrl(it) }
                 }
             }
         }

subprojects/base-services/src/main/java/org/gradle/api/internal/NullableTransformer.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.gradle.api.internal;
+
+import org.gradle.api.Transformer;
+
+import javax.annotation.Nullable;
+
+/**
+ * <p>{@code Transformer} extension that explicitly allows {@code null} return values</p>
+ *
+ * @param <OUT> The type the value is transformed to.
+ * @param <IN> The type of the value to be transformed.
+ */
+public abstract class NullableTransformer<OUT, IN> implements Transformer<OUT, IN> {
+
+    @Nullable
+    @Override
+    public abstract OUT transform(IN in);
+
+    public Transformer<OUT, IN> asTransformer() {
+        return this;
+    }
+}

subprojects/core-api/src/main/java/org/gradle/api/file/ContentFilterable.java

@@ -76,7 +76,12 @@ public interface ContentFilterable {
      * Adds a content filter based on the provided transformer.  The Closure will be called with each line (stripped of line
      * endings) and should return a String to replace the line or {@code null} to remove the line.  If every line is
      * removed, the result will be an empty file, not an absent one.
-     *
+     * <p>
+     * Note, that due to the nullability constraints clients written in Kotlin cannot return null values as it results in a
+     * compile-time error. To fix that, the {@code nullableTransformer} utility method should be used:
+     * <pre>
+     *     filter(nullableTransformer { line -&gt; ... })
+     * </pre>
      * @param transformer to implement line based filtering
      * @return this
      */

subprojects/docs/src/docs/userguide/migration/upgrading_version_7.adoc

@@ -38,6 +38,10 @@ Some plugins will break with this new version of Gradle, for example because the
 
 === Potential breaking changes
 
+=== Kotlin build scripts compilation enforces non-null APIs
+
+If a Kotlin build script uses a Java API that has nullability annotations (JSR-305) then the compilation will enforce those constraints. Previous Gradle versions, however, forgot to add the same enforcement to the Kotlin build script compilation. Starting from Gradle 7.4 the nullability checks are enforced in the script compilation.
+
 ==== Updates to default tool integration versions
 
 - PMD has been updated to https://github.com/pmd/pmd/releases/tag/pmd_releases%2F6.39.0[PMD 6.39.0].

subprojects/docs/src/snippets/files/copy/kotlin/build.gradle.kts

@@ -218,9 +218,9 @@ tasks.register<Copy>("filter") {
         "[$line]"
     }
     // Use a closure to remove lines
-    filter { line: String ->
+    filter(nullableTransformer { line: String ->
         if (line.startsWith('-')) null else line
-    }
+    })
     filteringCharset = "UTF-8"
 }
 // end::filter-files[]

subprojects/docs/src/snippets/java/crossCompilation/kotlin/build.gradle.kts

@@ -21,7 +21,7 @@ java {
 // end::java-cross-compilation[]
 
 tasks.withType<Test>().configureEach {
-    systemProperty("targetJavaVersion", project.findProperty("targetJavaVersion"))
+    project.findProperty("targetJavaVersion")?.let { systemProperty("targetJavaVersion", it) }
 }
 
 tasks.register("checkJavadocOutput") {

subprojects/integ-test/src/integTest/groovy/org/gradle/integtests/GradleKotlinDslIntegrationTest.groovy

@@ -147,7 +147,7 @@ import org.gradle.tooling.provider.model.ToolingModelBuilderRegistry
 
 task("dumpKotlinBuildScriptModelClassPath") {
     doLast {
-        val modelName = KotlinBuildScriptModel::class.qualifiedName
+        val modelName = KotlinBuildScriptModel::class.qualifiedName!!
         val builderRegistry = (project as ProjectInternal).services[ToolingModelBuilderRegistry::class.java]
         val builder = builderRegistry.getBuilder(modelName)
         val model = builder.buildAll(modelName, project) as KotlinBuildScriptModel

subprojects/jacoco/src/main/java/org/gradle/testing/jacoco/plugins/JacocoTaskExtension.java

@@ -123,7 +123,7 @@ public void setDestinationFile(Provider<File> destinationFile) {
         this.destinationFile.set(destinationFile);
     }
 
-    public void setDestinationFile(File destinationFile) {
+    public void setDestinationFile(@Nullable File destinationFile) { // nullability must match on getter and setter argument to end up with a writable Kotlin property
         this.destinationFile.set(destinationFile);
     }
 

subprojects/kotlin-dsl/src/integTest/kotlin/org/gradle/kotlin/dsl/support/KotlinScriptCompilerIntegrationTest.kt

@@ -0,0 +1,34 @@
+package org.gradle.kotlin.dsl.support
+
+import org.gradle.kotlin.dsl.fixtures.AbstractKotlinIntegrationTest
+import org.junit.Test
+import spock.lang.Issue
+
+
+class KotlinScriptCompilerIntegrationTest : AbstractKotlinIntegrationTest() {
+
+    @Test
+    @Issue("https://github.com/gradle/gradle/issues/18714")
+    fun `Build scripts are compiled with jsr305 strict mode`() {
+
+        // given:
+        withFile(
+            "buildSrc/src/main/java/StringTransformer.java",
+            """
+                @org.gradle.api.NonNullApi
+                public interface StringTransformer {
+                    String transform(String input);
+                }
+            """
+        )
+
+        withBuildScript("StringTransformer { input -> null }")
+
+        // when:
+        val result = buildAndFail("help")
+
+        // then:
+        result.assertHasDescription("Script compilation error")
+        result.hasErrorOutput("Null can not be a value of a non-null type String")
+    }
+}

subprojects/kotlin-dsl/src/main/kotlin/org/gradle/kotlin/dsl/ContentFilterableExtensions.kt

@@ -15,7 +15,10 @@
  */
 package org.gradle.kotlin.dsl
 
+import org.gradle.api.Incubating
+import org.gradle.api.Transformer
 import org.gradle.api.file.ContentFilterable
+import org.gradle.api.internal.NullableTransformer
 
 import java.io.FilterReader
 import kotlin.reflect.KClass
@@ -111,3 +114,19 @@ fun <T : FilterReader> ContentFilterable.filter(filterType: KClass<T>, vararg pr
 fun <T : FilterReader> ContentFilterable.filter(filterType: KClass<T>, properties: Map<String, Any?>) =
     if (properties.isEmpty()) filter(filterType.java)
     else filter(properties, filterType.java)
+
+
+/**
+ * Creates a new transformer that can return null and can be used in the context of [ContentFilterable.filter].
+ *
+ * @param transformer the spec of the transformer to be returned.
+ * @since 7.4
+ */
+@Incubating
+fun ContentFilterable.nullableTransformer(transformer: (String) -> String?): Transformer<String, String> {
+    return object : NullableTransformer<String, String>() {
+        override fun transform(input: String): String? {
+            return transformer.invoke(input)
+        }
+    }.asTransformer()
+}

subprojects/kotlin-dsl/src/main/kotlin/org/gradle/kotlin/dsl/support/KotlinCompiler.kt

@@ -57,6 +57,9 @@ import org.jetbrains.kotlin.config.LanguageVersion
 import org.jetbrains.kotlin.config.LanguageVersionSettingsImpl
 
 import org.jetbrains.kotlin.extensions.StorageComponentContainerContributor.Companion.registerExtension
+import org.jetbrains.kotlin.load.java.JavaTypeEnhancementState
+import org.jetbrains.kotlin.load.java.Jsr305Settings
+import org.jetbrains.kotlin.load.java.ReportLevel
 
 import org.jetbrains.kotlin.name.NameUtils
 
@@ -352,6 +355,7 @@ val gradleKotlinDslLanguageVersionSettings = LanguageVersionSettingsImpl(
     apiVersion = ApiVersion.KOTLIN_1_4,
     analysisFlags = mapOf(
         AnalysisFlags.skipMetadataVersionCheck to true,
+        JvmAnalysisFlags.javaTypeEnhancementState to JavaTypeEnhancementState(Jsr305Settings(ReportLevel.STRICT, ReportLevel.STRICT)) { ReportLevel.STRICT },
         JvmAnalysisFlags.jvmDefaultMode to JvmDefaultMode.ENABLE,
     ),
     specificFeatures = mapOf(