What's new in Kotlin 2.0.20-RC2

Data class copy function to have the same visibility as constructor

Currently, if you create a data class using a private constructor, the automatically generated .copy() function doesn't have the same visibility. This can cause problems later in your code. In future Kotlin releases, we will introduce the behavior that the default visibility of the .copy() function is the same as the constructor. This change will be introduced gradually to help you migrate your code as smoothly as possible.

Our migration plan starts with Kotlin 2.0.20-RC2, where we issue warnings in your code where the visibility will change in the future. For example:

For the latest information about our migration plan, see the corresponding issue in YouTrack.

To give you more control over this behavior, in Kotlin 2.0.20-RC2 we've introduced two annotations:

• @ConsistentCopyVisibility to opt in to the behavior now before we make it the default in a later release.

• @ExposedCopyVisibility to opt out of the behavior and suppress warnings at the declaration site. Note that even with this annotation, the compiler still reports warnings when the .copy() function is called.

If you want to opt in to the new behavior already in 2.0.20-RC2 for a whole module rather than in individual classes, you can use the -Xconsistent-data-class-copy-visibility compiler option. This option has the same effect as adding the @ConsistentCopyVisibility annotation to all data classes in a module.

Phased removal of context receivers feature

In Kotlin 1.6.20, we introduced context receivers as an Experimental feature. After listening to community feedback, we've decided not to continue with it. Instead, we plan to introduce a replacement in future Kotlin releases: context parameters. You can find the proposal for context parameters in the KEEP.

Although context receivers have always been an Experimental feature, we know they are already used by a large number of developers. So we will remove support for context receivers gradually.

Our migration plan starts with Kotlin 2.0.20-RC2, where we issue warnings in your code where context receivers are used when you use the -Xcontext-receivers compiler option. For example:

This warning will become an error in future Kotlin releases.

If you use context receivers in your code, we recommend that you migrate your code to use either:

• Explicit parameters.

  Before	After
  context(ContextReceiverType) fun someFunction() { contextReceiverMember() }	fun someFunction(explicitContext: ContextReceiverType) { explicitContext.contextReceiverMember() }

• Extension member functions, if possible.

  Before	After
  context(ContextReceiverType) fun contextReceiverMember() = TODO() context(ContextReceiverType) fun someFunction() { contextReceiverMember() }	class ContextReceiverType { fun contextReceiverMember() = TODO() } fun ContextReceiverType.someFunction() { contextReceiverMember() }

Static accessors for source sets from the default target hierarchy

Since Kotlin 1.9.20, the default hierarchy template is automatically applied to all Kotlin Multiplatform projects. And for all of the source sets from the default hierarchy template, the Kotlin Gradle plugin provided type-safe accessors. That way, you could finally access source sets for all the specified targets without having to use by getting or by creating constructions.

Kotlin 2.0.20 aims to improve your IDE experience even further. It now provides static accessors in the sourceSets {} block for all the source sets from the default hierarchy template. We believe this change will make accessing source sets by name easier and more predictable.

Each such source set now has a detailed KDoc comment with a sample and a diagnostic message with a warning in case you try to access the source set without declaring the corresponding target first:

Learn more about the hierarchical project structure in Kotlin Multiplatform.

Deprecated compatibility with Kotlin Multiplatform Gradle plugin and Gradle Java plugins

In Kotlin 2.0.20-RC2, we introduce a deprecation warning when you apply the Kotlin Multiplatform Gradle plugin and any of the following Gradle Java plugins to the same project: Java, Java Library, and Application. The warning also appears when another Gradle plugin in your multiplatform project applies a Gradle Java plugin. For example, the Spring Boot Gradle Plugin automatically applies the Application plugin.

We've added this deprecation warning due to fundamental compatibility issues between Kotlin Multiplatform's project model and Gradle's Java ecosystem plugins. Gradle's Java ecosystem plugins currently don't take into account that other plugins may:

• Also publish or compile for the JVM target in a different way than the Java ecosystem plugins.

• Have two different JVM targets in the same project, such as JVM and Android.

• Have a complex multiplatform project structure with potentially multiple non-JVM targets.

Unfortunately, Gradle doesn't currently provide any API to address these issues.

We previously used some workarounds in Kotlin Multiplatform to help with the integration of Java ecosystem plugins. However, these workarounds never truly solved the compatibility issues, and since the release of Gradle 8.8, these workarounds are no longer possible. For more information, see our YouTrack issue.

While we don't yet know exactly how to resolve this compatibility problem, we are committed to continuing support for some form of Java source compilation in your Kotlin Multiplatform projects. At a minimum, we will support the compilation of Java sources and using Gradle's java-base plugin within your multiplatform projects.

In the meantime, if you see this deprecation warning in your multiplatform project, we recommend that you:

1. Determine whether you actually need the Gradle Java plugin in your project. If not, consider removing it.

2. Check if the Gradle Java plugin is only used for a single task. If so, you might be able to remove the plugin without much effort. For example, if the task uses a Gradle Java plugin to create a Javadoc JAR file, you can define the Javadoc task manually instead.

Otherwise, if you want to use both the Kotlin Multiplatform Gradle plugin and these Gradle plugins for Java in your multiplatform project, we recommend that you:

1. Create a separate subproject in your multiplatform project.

2. In the separate subproject, apply the Gradle plugin for Java.

3. In the separate subproject, add a dependency on your parent multiplatform project.

For example, you have a multiplatform project called my-main-project and you want to use the Application Gradle plugin to run a JVM application.

Once you've created a subproject, let's call it subproject-A, your parent project structure should look like this:

In your subproject's build.gradle.kts file, apply the Application plugin in the plugins {} block:

In your subproject's build.gradle.kts file, add a dependency on your parent multiplatform project:

Your parent project is now set up to work with both plugins.

Concurrent marking in garbage collector

In Kotlin 2.0.20-RC2, the JetBrains team takes another step toward improving Kotlin/Native runtime performance. We've added experimental support for concurrent marking in the garbage collector (GC).

By default, application threads must be paused when GC is marking objects in the heap. This greatly affects the duration of the GC pause time, which is important for the performance of latency-critical applications, such as UI applications built with Compose Multiplatform.

Now, the marking phase of the garbage collection can be run simultaneously with application threads. This should significantly shorten the GC pause time and help improve app responsiveness.

Support for bitcode embedding removed

Starting with Kotlin 2.0.20-RC2, the Kotlin/Native compiler no longer supports bitcode embedding. Bitcode embedding was deprecated in Xcode 14 and removed in Xcode 15 for all Apple targets.

Now, the embedBitcode parameter for the framework configuration, as well as the -Xembed-bitcode and -Xembed-bitcode-marker command line arguments are deprecated.

If you still use earlier versions of Xcode but want to upgrade to Kotlin 2.0.20-RC2, disable bitcode embedding in your Xcode projects.

Changes to monitoring GC performance with signposts

Kotlin 2.0.0 has made it possible to monitor the performance of Kotlin/Native garbage collector (GC) through Xcode Instruments. Instruments include the signposts tool, which can show GC pauses as events. This comes in handy when checking GC-related freezes in your iOS apps.

The feature was enabled by default, but unfortunately, it sometimes led to crashes when the application was run simultaneously with Xcode Instruments. Starting with 2.0.20-RC2, it requires an explicit opt-in with the following compiler option:

Calling Kotlin suspending functions from Swift/Objective-C on non-main threads

Previously, Kotlin/Native had a default restriction, limiting the ability to call Kotlin suspending functions from Swift and Objective-C to only the main thread. Kotlin 2.0.20-RC2 lifts that limitation, allowing you to run Kotlin suspend functions from Swift/Objective-C on any thread.

If you've previously switched the default behavior for non-main threads with the kotlin.native.binary.objcExportSuspendFunctionLaunchThreadRestriction=none binary option, you can now remove it from your gradle.properties file.

Error in default export usage

As part of the migration towards named exports, a warning message was previously printed to the console when using a default import for Kotlin/Wasm exports in JavaScript.

To fully support named exports, this warning has now been upgraded to an error. If you use a default import, you encounter the following error message:

This change is part of a deprecation cycle to migrate towards named exports. Here's what you can expect during each phase:

• In version 2.0.0: A warning message is printed to the console, explaining that exporting entities via default exports is deprecated.

• In version 2.0.20: An error occurs, requesting the use of the corresponding named import.

• In version 2.1.0: The use of default imports is completely removed.

New location of ExperimentalWasmDsl annotation

Previously, the @ExperimentalWasmDsl annotation for WebAssembly (Wasm) features was placed in this location within the Kotlin Gradle plugin:

In 2.0.20-RC2, the @ExperimentalWasmDsl annotation has been relocated to:

The previous location is now deprecated and might lead to build failures with unresolved references.

To reflect the new location of the @ExperimentalWasmDsl annotation, update the import statement in your Gradle build scripts. Use an explicit import for the new @ExperimentalWasmDsl location:

Alternatively, remove this star import statement from the old package:

Support for static members in JavaScript

Starting with Kotlin 2.0.20-RC2, you can use the @JsStatic annotation. It works similarly to @JvmStatic and instructs the compiler to generate additional static methods for the target declaration. This helps you use static members from your Kotlin code directly in JavaScript.

You can use the @JsStatic annotation for functions defined in named objects, as well as in companion objects declared inside classes and interfaces. The compiler generates both a static method of the object and an instance method in the object itself. For example:

Now, callStatic() is static in JavaScript while callNonStatic() is not:

It's also possible to apply the @JsStatic annotation on a property of an object or a companion object, making its getter and setter methods static members in that object or the class containing the companion object.

Creating Kotlin collections from JavaScript

Kotlin 2.0.0 introduced the ability to export Kotlin collections to JavaScript (and TypeScript). Now, the JetBrains team is taking another step to improve collection interoperability. Starting with Kotlin 2.0.20-RC2, it's possible to create Kotlin collections directly from the JavaScript/TypeScript side.

You can create Kotlin collections from JavaScript and pass them as arguments to the exported constructors or functions. As soon as you mention a collection inside an exported declaration, Kotlin generates a factory for the collection that is available in JavaScript/TypeScript.

Take a look at the following exported function:

Since the MutableMap collection is mentioned, Kotlin generates an object with a factory method available from JavaScript/TypeScript. This factory method then creates a MutableMap from a JavaScript's Map:

This feature is available for Set, Map, and List Kotlin collection types and their mutable counterparts.

Gradle support for versions 8.6–8.8

Kotlin 2.0.20-RC2 is fully compatible with Gradle 6.8.3 through 8.6. Gradle 8.7 and 8.8 are also supported, with only one exception: If you use the Kotlin Multiplatform Gradle plugin, you may see deprecation warnings in your multiplatform projects if you use the withJava() function in the JVM target. We plan to fix this issue as soon as possible.

For more information, see the issue in YouTrack.

Deprecated incremental compilation based on JVM history files

In Kotlin 2.0.20-RC2, the incremental compilation approach based on JVM history files is deprecated in favor of the new incremental compilation approach that has been enabled by default since Kotlin 1.8.20.

The incremental compilation approach based on JVM history files suffered from limitations, such as not working with Gradle's build cache and not supporting compilation avoidance. In contrast, the new incremental compilation approach overcomes these limitations and has performed well since its introduction.

Given that the new incremental compilation approach has been used by default for the last two major Kotlin releases, the kotlin.incremental.useClasspathSnapshot Gradle property is deprecated in Kotlin 2.0.20-RC2. Therefore, if you use it to opt out, you will see a deprecation warning.

Added task dependency for rare cases when the compile task lacks one on an artifact

Prior to 2.0.20-RC2, we found that there were scenarios where a compile task was missing a task dependency for one of its artifact inputs. This meant that the result of the dependent compile task was unstable, as sometimes the artifact had been generated in time, but sometimes, it hadn't.

To fix this issue, the Kotlin Gradle plugin now automatically adds the required task dependency in these scenarios.

In very rare cases, we've found that this new behavior can cause a circular dependency error. For example, if you have multiple compilations where one compilation can see all internal declarations of the other, and the generated artifact relies on the output of both compilation tasks, you could see an error like:

To fix this circular dependency error, we've added a Gradle property: archivesTaskOutputAsFriendModule.

By default, this property is set to true to track the task dependency. To disable the use of the artifact in the compilation task, so that no task dependency is required, add the following in your gradle.properties file:

For more information, see the issue in YouTrack.

Option to share JVM artifacts between projects as class files

In Kotlin 2.0.20-RC2, we introduce a new approach that changes the way the outputs of Kotlin/JVM compilations, such as JAR files, are shared between projects. With this approach, Gradle's apiElements configuration now has a secondary variant that provides access to the directory containing compiled .class files. When configured, your project uses this directory instead of requesting the compressed JAR artifact during compilation. This reduces the number of times JAR files are compressed and decompressed, especially for incremental builds.

Our testing shows that this new approach can provide build performance improvements for Linux and macOS hosts. However, on Windows hosts, we have seen a degradation in performance due to how Windows handles I/O operations when working with files.

To try this new approach, add the following property to your gradle.properties file:

By default, this property is set to false and the apiElements variant in Gradle requests the compressed JAR artifact.

We would appreciate your feedback on this new approach. Do you also see performance improvements? Let us know by adding a comment in YouTrack.

Fixed the unnecessary recompositions issue introduced in 2.0.0

Compose compiler 2.0.0 has an issue where it sometimes incorrectly infers the stability of types in multiplatform projects with non-JVM targets. This can lead to unnecessary (or even endless) recompositions. It is strongly recommended to update your Compose apps made for Kotlin 2.0.0 to the 2.0.10 version or newer.

If your app is built with Compose compiler 2.0.10 or newer but uses dependencies built with Compose compiler 2.0.0, these older dependencies may still cause recomposition issues. To prevent this, update your dependencies to versions built with the same Compose compiler as your app.

New way to configure compiler options

We have introduced a new option configuration mechanism to avoid churn of top-level parameters: it's harder for the Compose compiler team to test things out by creating or removing top-level entries for the composeCompiler {} block. So options such as strong skipping mode and non-skipping group optimizations are now enabled through the featureFlags property. This property will be used to test new Compose compiler options that will eventually become default.

This change has also been applied to the Compose compiler Gradle plugin. To configure feature flags going forward, use the following syntax (this code will flip all of the default values):

Or, if you are configuring the Compose compiler directly, use the following syntax:

The enableIntrinsicRemember, enableNonSkippingGroupOptimization, and enableStrongSkippingMode properties have been therefore deprecated.

We would appreciate any feedback you have on this new approach in YouTrack.

Strong skipping mode enabled by default

Strong skipping mode for the Compose compiler is now enabled by default.

Strong skipping mode is a Compose compiler configuration option that changes the rules for what composables can be skipped. With strong skipping mode enabled, composables with unstable parameters can now also be skipped. Strong skipping mode also automatically remembers lambdas used in composable functions, so you should no longer need to wrap your lambdas with remember to avoid recomposition.

For more details, see the strong skipping mode documentation.

Non-skipping group optimizations

This release includes a new compiler option: when enabled, non-skippable and non-restartable composable functions will no longer generate a group around the body of the composable. This leads to less allocations and thus to improved performance. This option is experimental and disabled by default but can be enabled with the feature flag OptimizeNonSkippingGroups as shown above.

This feature flag is now ready for wider testing. Any issues found when enabling the feature can be filed on the Google issue tracker.

abstract Composable functions support default parameters

You can now add default parameters to abstract Composable functions.

Previously, the Compose compiler would report an error when attempting to do this even though it is valid Kotlin. We have now added support for this in the Compose compiler, and the restriction has been removed. This is especially useful for including default Modifier values:

Default parameters for open Composable functions are still restricted in 2.0.20-RC2. This restriction will be addressed in future releases.