Branch: junie/KT-22815

Overview

This branch adds tests for the FirSerializationTypeAliasChecker in the Kotlin Serialization compiler plugin. The checker reports warnings when the kotlinx.serialization.Serializable annotation is used as a typealias in Kotlin code compiled with K2 (the new Kotlin compiler).

Background

The K2 compiler does not expand annotation typealiases on the COMPILER_REQUIRED_ANNOTATIONS phase. This means that when the Serializable annotation is used through a typealias, it might not be properly processed by the serialization plugin, potentially leading to unexpected behavior.

Changes in this Branch

This branch adds a comprehensive test file (typeAliasedSerializable.fir.kt) that verifies the correct behavior of the FirSerializationTypeAliasChecker. The test file includes:

A typealias for the Serializable annotation:

typealias MySerializable = Serializable

Normal usage of the Serializable annotation (which should not trigger a warning):
```
@Serializable
class NormalClass(val value: String)
```
Usage of the typealiased Serializable annotation (which should trigger a warning):
```
@MySerializable
class TypeAliasedClass(val value: String)
```
Usage of the typealiased Serializable annotation with a custom serializer (which should also trigger a warning):
```
@MySerializable(CustomSerializer::class)
class TypeAliasedClassWithCustomSerializer(val value: String)
```

The test file includes the expected diagnostic markers (<!TYPEALIASED_SERIALIZABLE_ANNOTATION!>) to indicate where the warnings should be reported.

Implementation Details

The FirSerializationTypeAliasChecker is implemented in the Kotlin Serialization compiler plugin and is responsible for detecting and reporting warnings when the Serializable annotation is used through a typealias. The checker:

Examines typealias declarations
Checks if the expanded type is kotlinx.serialization.Serializable
Reports a warning on the typealias declaration if it expands to Serializable

Why This Matters

This warning helps developers avoid potential issues when using the Serialization plugin with K2. By warning about typealiased Serializable annotations, it guides developers toward using the annotation directly, ensuring proper processing by the compiler.

Warning Message

When a developer uses a typealiased Serializable annotation, they will see the following warning:

Typealiased kotlinx.serialization.Serializable annotation is not expanded on the COMPILER_REQUIRED_ANNOTATIONS phase in K2 compiler. Consider using @MetaSerializable instead.

The warning not only explains the issue but also suggests using @MetaSerializable as an alternative approach for creating custom serializable annotations.

Related Issues

KT-22815: Add tests for FirSerializationTypeAliasChecker warnings