What's new in Kotlin 2.2.0-RC

Kotlin/Native's memory allocator can now reserve memory on a per-object basis. In some cases, it may help you avoid strict memory limitations or high memory consumption on the application's startup.

The new feature is designed to replace the -Xallocator=std compiler option that enabled the system memory allocator instead of the default one. Now you can disable buffering (paging of allocations) without switching memory allocators.

The feature is currently Experimental. To enable it, set the following option in your gradle.properties file:

kotlin.native.binary.pagedAllocator=false

Please report any problems to our issue tracker YouTrack.

LLVM update from 16 to 19

In Kotlin 2.2.0-RC, we updated LLVM from version 16 to 19. The new version includes performance improvements, bug fixes, and security updates.

This update shouldn't affect your code, but if you encounter any issues, please report them to our issue tracker.

Kotlin/Wasm: wasmJs target separated from js target

Before, the wasmJs target shared the same infrastructure as the js target. As a result, both targets were hosted in the same directory (build/js) and used the same NPM tasks and configurations.

Now, the wasmJs target has its own infrastructure separate from the js target. This allows the Wasm tasks and types to be distinct from the JavaScript ones, enabling independent configuration.

Additionally, the Wasm-related project files and NPM dependencies are now in a separate build/wasm directory.

New NPM-related tasks have been introduced for Wasm, while existing JavaScript tasks are now dedicated only to JavaScript:

Similarly, new Wasm-specific declarations are introduced:

You can now work with the Wasm target independently of the JavaScript target, which simplifies the configuration.

This change is enabled by default and requires no additional configuration.

Kotlin/JS

Fix for copy() in @JsPlainObject interfaces

Kotlin/JS has an experimental plugin called js-plain-objects, which introduced a copy() function for interfaces annotated with @JsPlainObject. You can use the copy() function to manipulate objects.

However, the initial implementation of copy() was not compatible with inheritance, and this caused issues when a @JsPlainObject interface extended other interfaces.

To avoid limitations on plain objects, the copy() function has been moved from the object itself to its companion object:

@JsPlainObject
external interface User {
    val name: String
    val age: Int
}

fun main() {
    val user = User(name = "SomeUser", age = 21)
    // This syntax is not valid anymore
    val copy = user.copy(age = 35)      
    // This is the correct syntax
    val copy = User.copy(user, age = 35)
}

This change resolves conflicts in the inheritance hierarchy and eliminates ambiguity. It is enabled by default starting from Kotlin 2.2.0-RC.

Support for typealiases in files with @JsModule annotation

Previously, files annotated with @JsModule to import declarations from JavaScript modules were restricted to external declarations only. Meaning, you couldn't declare a typealias in such files.

Starting with Kotlin 2.2.0-RC, you can declare typealiases inside files marked with @JsModule:

@file:JsModule("somepackage")
package somepackage
typealias SomeClass = Any

This change reduces an aspect of Kotlin/JS interoperability limitations, and more improvements are planned for future releases.

Support for typealiases in files with @JsModule is enabled by default.

Gradle

Kotlin 2.2.0-RC is fully compatible with Gradle 7.6.3 through 8.14. You can also use Gradle versions up to the latest Gradle release. However, be aware that doing so may result in deprecation warnings, and some new Gradle features might not work.

Support for rich output in console for Kotlin Gradle plugin

In Kotlin 2.2.0-RC, we support color and other rich output in the console during the Gradle build process, making it easier to read and understand the reported diagnostics. Rich output is available in supported terminal emulators for Linux and macOS. We're working on adding support for Windows.

This feature is enabled by default, but if you want to override it, add the following Gradle property to your gradle.properties file:

org.gradle.console=plain

For more information about this property and its options, see Gradle's documentation on Customizing log format.

Integration of Problems API within KGP diagnostics

Previously, the Kotlin Gradle Plugin (KGP) reported diagnostics—such as warnings and errors—only as plain text output to the console or logs.

Starting with 2.2.0-RC, KGP introduces an additional reporting mechanism: it now uses Gradle's Problems API, a standardized way to report rich, structured problem information during the build process.

KGP diagnostics are now easier to read and more consistently displayed across different interfaces like the Gradle CLI and IntelliJ IDEA.

This integration is enabled by default, starting with Gradle 8.6 or later. As the API is still evolving, use the most recent Gradle version to benefit from the latest improvements.

KGP compatibility with '--warning-mode'

The Kotlin Gradle Plugin (KGP) diagnostics reported issues using fixed severity levels, meaning Gradle's --warning-mode command-line option had no effect on how KGP displayed errors.

Now, KGP diagnostics are compatible with the --warning-mode option, providing more flexibility. For example, you can convert all warnings into errors or disable warnings entirely.

With this change, KGP diagnostics adjust the output based on the selected warning mode:

• When you set --warning-mode=fail, diagnostics with Severity.Warning are now elevated to Severity.Error.
• When you set --warning-mode=none, diagnostics with Severity.Warning are not logged.

This behavior is enabled by default starting with 2.2.0-RC.

To ignore the --warning-mode option, set kotlin.internal.diagnostics.ignoreWarningMode=true in your Gradle properties.

Kotlin standard library: Stable Base64 and HexFormat APIs

In Kotlin 2.2.0-RC, the Base64 API and HexFormat API are now Stable.

Base64 encoding and decoding

Kotlin 1.8.20 introduced Experimental support for Base64 encoding and decoding. In Kotlin 2.2.0-RC, the Base64 API is now Stable and includes four encoding schemes, with the new Base64.Pem added in this release:

• Base64.Default uses the standard Base64 encoding scheme.

  The Base64.Default is the companion object of the Base64 class. As a result, you can call its functions with Base64.encode() and Base64.decode() instead of Base64.Default.encode() and Base64.Default.decode().

• Base64.UrlSafe uses the "URL and Filename safe" encoding scheme.

• Base64.Mime uses the MIME encoding scheme, inserting a line separator every 76 characters during encoding and skipping illegal characters during decoding.
• Base64.Pem encodes data like Base64.Mime but limits the line length to 64 characters.

You can use the Base64 API to encode binary data into a Base64 string and decode it back into bytes.

Here's an example:

val foBytes = "fo".map { it.code.toByte() }.toByteArray()
Base64.Default.encode(foBytes) // "Zm8="
// Alternatively:
// Base64.encode(foBytes)

val foobarBytes = "foobar".map { it.code.toByte() }.toByteArray()
Base64.UrlSafe.encode(foobarBytes) // "Zm9vYmFy"

Base64.Default.decode("Zm8=") // foBytes
// Alternatively:
// Base64.decode("Zm8=")

Base64.UrlSafe.decode("Zm9vYmFy") // foobarBytes

On the JVM, use the .encodingWith() and .decodingWith() extension functions to encode and decode Base64 with input and output streams:

import kotlin.io.encoding.*
import java.io.ByteArrayOutputStream

fun main() {
    val output = ByteArrayOutputStream()
    val base64Output = output.encodingWith(Base64.Default)

    base64Output.use { stream ->
        stream.write("Hello World!!".encodeToByteArray())
    }

    println(output.toString())
    // SGVsbG8gV29ybGQhIQ==
}

Hexadecimal parsing and formatting with the HexFormat API

The HexFormat API introduced in Kotlin 1.9.0 is now Stable. You can use it to convert between numerical values and hexadecimal strings.

For example:

fun main() {
    //sampleStart
    println(93.toHexString())
    //sampleEnd
}

For more information, see New HexFormat class to format and parse hexadecimals.