NoCopy Compiler Plugin(K2)

This example project demonstrates how to prohibit the usage of the copy method on a data class with a private constructor by extending the K2 Kotlin Compiler.

This repository might be useful as a reference for implementing FirAdditionalCheckersExtension.

Overview

The NoCopy Compiler Plugin (K2) is designed to help developers enforce rules in their Kotlin codebase. Specifically, it prevents the invocation of the copy method on data classes with private constructors, ensuring that such classes maintain their intended immutability.

data class A private constructor(val a: Int) {
    companion object {
        fun create(a: Int) = A(a)
    }
}

val a = A(10)
val b = a.copy(20) // This line will not be detected as an error before Kotlin 2.0.20-Beta1.

Motivation

Prior to Kotlin 2.0.20-Beta1, there was an issue where instances of data classes with private constructors could still be created using the copy method. This plugin addresses that issue by providing a way to enforce this restriction at compile time, ensuring that private constructors maintain their intended level of control over instance creation.

data class A private constructor(val a: Int) {
    companion object {
        fun create(a: Int) = A(a)
    }
}

val a = A(10)
val b = a.copy(20) // The NoCopy Compiler Plugin will report this line as an error! ❌

How to Examine

You can enable or disable the NoCopy compiler plugin by using a Kotlin Compiler option.

In test/build.gradle.kts:

kotlin {
    jvm()

    @OptIn(ExperimentalKotlinGradlePluginApi::class)
    compilerOptions.freeCompilerArgs.addAll(
        "-P",
        // try replacing 'true' with 'false'. Then it will compile without any errors.
        "plugin:com.github.kitakkun.nocopy.compiler:enabled=true",
    )
}

Additional Resources

• Speaker Deck

License

This project is licenced under the MIT license. See the LICENSE file for details.