Compiler plugins

Learn the Kotlin compiler plugin concept.

What is a compiler plugin?

A compiler plugin hooks into the Kotlin compiler and can transform code during compilation. Unlike KSP, it has access to the full AST.

The plugin can make classes open, add synthetic constructors, or transform the IR (Intermediate Representation) at compile time. In Kotlin 2.x all official plugins work this way: all-open, no-arg, compose and kotlinx.serialization. A plugin is added in Gradle as a compilerPlugin dependency; the output is modified bytecode. In this lesson you will set up the basic plugin skeleton.

Plugin lifecycle

A compiler plugin hooks into specific phases of the compile lifecycle: Frontend (parsing and resolution), IR (intermediate representation), and Backend (code generation).

Example: the all-open plugin makes classes with certain annotations open in the frontend phase. The compose plugin transforms @Composable functions in the IR phase. Each phase has its own APIs; the most common is IR. Writing a plugin involves implementing ComponentRegistrar and registering an extension that hooks the relevant phase.

Exercise Write the CompilerPlugin skeleton

Define class CompilerPlugin. Method fun load() should call println("plugin loaded"). In main, call CompilerPlugin().load().

Show example solution

class CompilerPlugin {
    fun load() {
        println("plugin loaded")
    }
}
fun main() {
    CompilerPlugin().load()
}

Test yourself

What can a compiler plugin access?
- Only annotations
- Only runtime data
- The full AST
- Only the source file name
Show answer

The full AST — A compiler plugin has access to the full AST; KSP only sees symbols.

Complete the load method

class CompilerPlugin {
    fun ____() {
        println("plugin loaded")
    }
}

Show answer

class CompilerPlugin {
    fun load() {
        println("plugin loaded")
    }
}

Match plugin parts

ComponentRegistrar

Plugin registration
extension

Phase hook
frontend

Parsing and resolution
backend

Code generation

Next lesson: IR

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