IR: Add an extension point after IR construction

The existing IrGenerationExtension is in the middle of IR construction
and basically requires plugins to work with the (deprecated) descriptors.

Since there are plans to change the IR extension point it makes sense to
introduce a temporary work-around to allow pure IR plugins to be
developed now, which could be ported to a more permanent API later.

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/compiler/KotlinCoreEnvironment.kt

@@ -57,6 +57,7 @@ import org.jetbrains.kotlin.asJava.LightClassGenerationSupport
 import org.jetbrains.kotlin.asJava.classes.FacadeCache
 import org.jetbrains.kotlin.asJava.finder.JavaElementFinder
 import org.jetbrains.kotlin.backend.common.extensions.IrGenerationExtension
+import org.jetbrains.kotlin.backend.common.extensions.PureIrGenerationExtension
 import org.jetbrains.kotlin.cli.common.CLIConfigurationKeys
 import org.jetbrains.kotlin.cli.common.CliModuleVisibilityManagerImpl
 import org.jetbrains.kotlin.cli.common.KOTLIN_COMPILER_ENVIRONMENT_KEEPALIVE_PROPERTY
@@ -556,6 +557,7 @@ class KotlinCoreEnvironment private constructor(
             CollectAdditionalSourcesExtension.registerExtensionPoint(project)
             ExtraImportsProviderExtension.registerExtensionPoint(project)
             IrGenerationExtension.registerExtensionPoint(project)
+            PureIrGenerationExtension.registerExtensionPoint(project)
             ScriptEvaluationExtension.registerExtensionPoint(project)
             ShellExtension.registerExtensionPoint(project)
             TypeResolutionInterceptor.registerExtensionPoint(project)

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/extensions/IrGenerationExtension.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.backend.common.extensions
 
+import org.jetbrains.kotlin.backend.common.CommonBackendContext
 import org.jetbrains.kotlin.backend.common.ir.BuiltinSymbolsBase
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.descriptors.ModuleDescriptor
@@ -37,4 +38,15 @@ interface IrGenerationExtension : IrExtensionGenerator {
         moduleFragment: IrModuleFragment,
         pluginContext: IrPluginContext
     )
-}
+}
+
+// Extension point for plugins which run before any lowerings, but after the Ir has been constructed.
+interface PureIrGenerationExtension {
+    companion object :
+        ProjectExtensionDescriptor<PureIrGenerationExtension>("org.jetbrains.kotlin.pureIrGenerationExtension", PureIrGenerationExtension::class.java)
+
+    fun generate(
+        moduleFragment: IrModuleFragment,
+        context: CommonBackendContext
+    )
+}

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/JvmBackendFacade.kt

@@ -8,6 +8,7 @@ package org.jetbrains.kotlin.backend.jvm
 import org.jetbrains.kotlin.backend.common.CodegenUtil
 import org.jetbrains.kotlin.backend.common.extensions.IrGenerationExtension
 import org.jetbrains.kotlin.backend.common.extensions.IrPluginContext
+import org.jetbrains.kotlin.backend.common.extensions.PureIrGenerationExtension
 import org.jetbrains.kotlin.backend.common.phaser.PhaseConfig
 import org.jetbrains.kotlin.backend.jvm.codegen.ClassCodegen
 import org.jetbrains.kotlin.backend.jvm.lower.MultifileFacadeFileEntry
@@ -105,6 +106,10 @@ object JvmBackendFacade {
             context.typeMapper.mapType(context.referenceClass(descriptor).defaultType)
         }
 
+        for (extension in PureIrGenerationExtension.getInstances(state.project)) {
+            extension.generate(irModuleFragment, context)
+        }
+
         JvmLower(context).lower(irModuleFragment)
 
         for (generateMultifileFacade in listOf(true, false)) {