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Pseudo-KMP Modules

The monorepo is compiled by JPS, which does not support Kotlin Multiplatform. To emulate KMP expect/actual declarations, the Fleet team created the expects-compiler-plugin. This skill covers how to set up and work with modules that use this mechanism.

Documentation

• Multiplatform Migration Guide — step-by-step migration instructions
• Multiplatform Support README — compiler plugin usage and registration
• ExpectInMultiplatform.kt — linkToActual() and @Actual API docs

Module Configuration Checklist

When creating or migrating a pseudo-KMP module:

1. Register in the structure test. Add your module name to the multiplatformModules list in com.intellij.ideaProjectStructure.fast.MultiplatformModuleStructureTest (tests/ideaProjectStructure/testSrc/com/intellij/ideaProjectStructure/fast/MultiplatformModuleStructureTest.kt).

2. Use only allowed library dependencies. The test enforces a closed set:

   • kotlin-stdlib
   • kotlinx-coroutines-core
   • kotlinx-coroutines-debug
   • jetbrains-annotations
   • kotlinx-serialization-core
   • kotlinx-serialization-json
   • kotlin-reflect
   • kotlinx-collections-immutable

   If you need a new library, add it to allowedMultiplatformLibraryDependencies in the test. Dependencies on other multiplatform modules (those in multiplatformModules) are also allowed.

3. Set up source folders:

   • src/ — common code
   • srcJvm/ or srcJvmMain/ — JVM-specific code (Java code can live here too)
   • srcWasmJs/ or srcWasmJsMain/ — WasmJs-specific code
   • srcJsMain/ — JS-specific code
   • srcNativeMain/ — Native-specific code

   Important: Do NOT add srcWasmJs/srcWasmJsMain as a source root in the .iml file. The IDE would try to compile it in JVM mode.

4. Add the .pseudoCommonKotlinSourceSet marker file to the common source root (e.g., src/.pseudoCommonKotlinSourceSet). This empty file tells the Kotlin plugin that the source set is common code, preventing Optimize Imports from removing imports that are unnecessary on JVM but mandatory in common code (e.g., @JvmStatic, @JvmField).

IML File Setup

Your .iml file needs two things: the multiplatformSupport dependency and the compiler plugin registration.

Use community/platform/util/multiplatform/intellij.platform.util.multiplatform.iml as a reference. To find the current plugin version, check that file rather than hardcoding.

1. Add the support module dependency (PROVIDED scope)

<orderEntry type="module" module-name="intellij.platform.multiplatformSupport" scope="PROVIDED" />

2. Add the Kotlin facet with expects-compiler-plugin

<facet type="kotlin-language" name="Kotlin">
  <configuration version="5" platform="JVM 1.8" allPlatforms="JVM [1.8]" useProjectSettings="false">
    <compilerSettings>
      <option name="additionalArguments" value="-Xjvm-default=all ..." />
    </compilerSettings>
    <compilerArguments>
      <stringArguments>
        <stringArg name="jvmTarget" arg="1.8" />
        <stringArg name="apiVersion" arg="2.3" />
        <stringArg name="languageVersion" arg="2.3" />
      </stringArguments>
      <arrayArguments>
        <arrayArg name="pluginClasspaths">
          <args>$MAVEN_REPOSITORY$/jetbrains/fleet/expects-compiler-plugin/{VERSION}/expects-compiler-plugin-{VERSION}.jar</args>
        </arrayArg>
      </arrayArguments>
    </compilerArguments>
  </configuration>
</facet>

Replace {VERSION} with the version from the reference IML.

Expect/Actual Pattern

Only top-level functions are supported. Both the expect and actual declarations must be in the same module.

Common side (in src/)

Use linkToActual() as the function body:

import fleet.util.multiplatform.linkToActual

internal fun myFunction(param1: String, param2: Int): Boolean = linkToActual()

Platform side (in srcJvm/, srcWasmJs/, etc.)

Annotate with @Actual. The function name must be the expect function name + platform suffix:

import fleet.util.multiplatform.Actual

@Actual
internal fun myFunctionJvm(param1: String, param2: Int): Boolean {
  // JVM implementation
}

Platform suffixes

Platform	Suffix	Source folder
JVM	Jvm	srcJvm/ or srcJvmMain/
WasmJs	WasmJs	srcWasmJs/ or srcWasmJsMain/
JS	Js	srcJsMain/
Native	Native	srcNativeMain/
Multi-platform build	Impl	(rare)

Real example

Common (community/platform/util/base/multiplatform/src/com/intellij/util/text/CharArrayUtilKmp.kt):

internal fun getCharsPlatformSpecific(
  sequence: CharSequence, srcOffset: Int, dst: CharArray, dstOffset: Int, len: Int
): Boolean = linkToActual()

JVM actual (community/platform/util/base/multiplatform/srcJvmMain/com/intellij/util/text/jvm.kt):

@Actual
internal fun getCharsPlatformSpecificJvm(
  string: CharSequence, srcOffset: Int, dst: CharArray, dstOffset: Int, len: Int
): Boolean {
  if (string is CharBuffer) { /* ... */ }
  return false
}

Code Conversion Notes

• The monorepo uses the JVM version of the Kotlin standard library, which differs from the common version. Check which declarations are available in common code.
• @JvmStatic, @JvmField, and similar annotations are not auto-imported in common code. Import them manually. The .pseudoCommonKotlinSourceSet marker prevents the IDE from removing these imports.
• Java code can remain in platform-specific source sets (srcJvm/).

After Editing

1. Run ./build/jpsModelToBazel.cmd after modifying any .iml files.
2. Run MultiplatformModuleStructureTest to validate module structure:

   ./tests.cmd --module tests.ideaProjectStructure --test com.intellij.ideaProjectStructure.fast.MultiplatformModuleStructureTest
   

3. Lint changed files with mcp__ijproxy__lint_files.

Artifact Publication

The monorepo does not publish multiplatform artifacts to Maven Central directly. Separate repositories with Gradle infrastructure extract module sources, compile with true KMP, and publish. See fleet-multiplatform-deps and fleet-parser-collection in JetBrains Space.