[FIR] Add extension with additional checkers

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/extensions/AbstractFirAdditionalCheckersExtension.kt

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.extensions
+
+import org.jetbrains.kotlin.fir.FirSession
+import kotlin.reflect.KClass
+
+abstract class AbstractFirAdditionalCheckersExtension(session: FirSession) : FirExtension(session) {
+    fun interface Factory : FirExtension.Factory<AbstractFirAdditionalCheckersExtension>
+
+    final override val extensionType: KClass<out FirExtension>
+        get() = AbstractFirAdditionalCheckersExtension::class
+}

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/extensions/FirExtension.kt

@@ -25,7 +25,7 @@ abstract class FirExtension(val session: FirSession) {
 
     internal abstract val extensionType: KClass<out FirExtension>
 
-    fun interface Factory<P : FirExtension> {
+    fun interface Factory<out P : FirExtension> {
         fun create(session: FirSession): P
     }
 

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/extensions/FirExtensionRegistrar.kt

@@ -24,6 +24,11 @@ abstract class FirExtensionRegistrar {
         operator fun ((FirSession) -> FirClassGenerationExtension).unaryPlus() {
             classGenerationExtensions += FirClassGenerationExtension.Factory { this.invoke(it) }
         }
+
+        @JvmName("plusAdditionalCheckersExtension")
+        operator fun ((FirSession) -> AbstractFirAdditionalCheckersExtension).unaryPlus() {
+            additionalCheckersExtensions += AbstractFirAdditionalCheckersExtension.Factory { this.invoke(it) }
+        }
     }
 
     protected abstract fun ExtensionRegistrarContext.configurePlugin()
@@ -32,19 +37,22 @@ abstract class FirExtensionRegistrar {
         ExtensionRegistrarContext().configurePlugin()
         return RegisteredExtensions(
             statusTransformerExtensions,
-            classGenerationExtensions
+            classGenerationExtensions,
+            additionalCheckersExtensions,
         )
     }
 
     private val statusTransformerExtensions: MutableList<FirStatusTransformerExtension.Factory> = mutableListOf()
     private val classGenerationExtensions: MutableList<FirClassGenerationExtension.Factory> = mutableListOf()
+    private val additionalCheckersExtensions: MutableList<AbstractFirAdditionalCheckersExtension.Factory> = mutableListOf()
 
     class RegisteredExtensions(
         val statusTransformerExtensions: List<FirStatusTransformerExtension.Factory>,
-        val classGenerationExtensions: List<FirClassGenerationExtension.Factory>
+        val classGenerationExtensions: List<FirClassGenerationExtension.Factory>,
+        val additionalCheckersExtensions: List<AbstractFirAdditionalCheckersExtension.Factory>,
     ) {
         companion object {
-            val EMPTY = RegisteredExtensions(emptyList(), emptyList())
+            val EMPTY = RegisteredExtensions(emptyList(), emptyList(), emptyList())
         }
     }
 }
@@ -52,4 +60,5 @@ abstract class FirExtensionRegistrar {
 fun FirExtensionsService.registerExtensions(extensions: FirExtensionRegistrar.RegisteredExtensions) {
     registerExtensions(FirStatusTransformerExtension::class, extensions.statusTransformerExtensions)
     registerExtensions(FirClassGenerationExtension::class, extensions.classGenerationExtensions)
+    registerExtensions(AbstractFirAdditionalCheckersExtension::class, extensions.additionalCheckersExtensions)
 }