Extensible mechanism for plugin metadata during descriptor serialization

A mechanism that allows kotlinx.serialization plugin to preserve the
correct (program) order of properties after serializing/deserializing
descriptors to kotlin metadata, which is needed for correct and stable
json serialization of class hierarchies in incremental/multi-module scenario.
It uses protobuf extensions.

compiler/serialization/src/org/jetbrains/kotlin/serialization/DescriptorSerializer.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * 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.
  */
 
@@ -45,7 +45,8 @@ class DescriptorSerializer private constructor(
     private val extension: SerializerExtension,
     val typeTable: MutableTypeTable,
     private val versionRequirementTable: MutableVersionRequirementTable?,
-    private val serializeTypeTableToFunction: Boolean
+    private val serializeTypeTableToFunction: Boolean,
+    val plugins: List<DescriptorSerializerPlugin> = emptyList()
 ) {
     private val contractSerializer = ContractSerializer()
 
@@ -152,6 +153,8 @@ class DescriptorSerializer private constructor(
 
         extension.serializeClass(classDescriptor, builder, versionRequirementTable, this)
 
+        plugins.forEach { it.afterClass(classDescriptor, builder, versionRequirementTable, this, extension) }
+
         writeVersionRequirementForInlineClasses(classDescriptor, builder, versionRequirementTable)
 
         val versionRequirementTableProto = versionRequirementTable.serialize()
@@ -741,6 +744,13 @@ class DescriptorSerializer private constructor(
     )
 
     companion object {
+        private val plugins: MutableSet<DescriptorSerializerPlugin> = mutableSetOf()
+
+        @JvmStatic
+        fun registerSerializerPlugin(plugin: DescriptorSerializerPlugin) {
+            plugins.add(plugin)
+        }
+
         @JvmStatic
         fun createTopLevel(extension: SerializerExtension): DescriptorSerializer =
             DescriptorSerializer(
@@ -775,7 +785,8 @@ class DescriptorSerializer private constructor(
                 MutableTypeTable(),
                 if (container is ClassDescriptor && !isVersionRequirementTableWrittenCorrectly(extension.metadataVersion))
                     parent.versionRequirementTable else MutableVersionRequirementTable(),
-                serializeTypeTableToFunction = false
+                serializeTypeTableToFunction = false,
+                plugins.toList()
             )
             for (typeParameter in descriptor.declaredTypeParameters) {
                 serializer.typeParameters.intern(typeParameter)

compiler/serialization/src/org/jetbrains/kotlin/serialization/DescriptorSerializerPlugin.kt

@@ -0,0 +1,21 @@
+/*
+ * 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.serialization
+
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
+import org.jetbrains.kotlin.metadata.ProtoBuf
+import org.jetbrains.kotlin.metadata.serialization.MutableVersionRequirementTable
+
+interface DescriptorSerializerPlugin {
+    fun afterClass(
+        descriptor: ClassDescriptor,
+        proto: ProtoBuf.Class.Builder,
+        versionRequirementTable: MutableVersionRequirementTable,
+        childSerializer: DescriptorSerializer,
+        extension: SerializerExtension
+    ) {
+    }
+}