Extract ClassMapperLite, add enlightening comment

core/descriptor.loader.java/src/org/jetbrains/kotlin/serialization/jvm/ClassMapperLite.kt

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2010-2015 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.kotlin.serialization.jvm
+
+import org.jetbrains.kotlin.builtins.KotlinBuiltIns
+import org.jetbrains.kotlin.name.ClassId
+import org.jetbrains.kotlin.platform.JavaToKotlinClassMap
+import org.jetbrains.kotlin.resolve.jvm.JvmPrimitiveType
+
+// The purpose of this class is to map Kotlin classes to JVM bytecode desc strings, as JetTypeMapper does in the backend.
+// It's used as an optimization during serialization/deserialization: if there's no JVM signature for a method/property/constructor,
+// it means that the JVM signature should be trivially computable from the Kotlin signature with this class.
+// It's not required to support everything in JetTypeMapper, but the more it does, the more we save on JVM signatures in proto metadata.
+// Note that improving the behavior of this class may break binary compatibility of code compiled by Kotlin, because it may make
+// the new compiler skip writing the signatures it now thinks are trivial, and the old compiler would recreate them incorrectly.
+object ClassMapperLite {
+    @JvmStatic
+    fun mapClass(classId: ClassId): String {
+        val internalName = classId.asString().replace('.', '$')
+        val simpleName = internalName.removePrefix("kotlin/")
+        if (simpleName != internalName) {
+            for (jvmPrimitive in JvmPrimitiveType.values) {
+                val primitiveType = jvmPrimitive.primitiveType
+                if (simpleName == primitiveType.typeName.asString()) return jvmPrimitive.desc
+                if (simpleName == primitiveType.arrayTypeName.asString()) return "[" + jvmPrimitive.desc
+            }
+
+            if (simpleName == KotlinBuiltIns.FQ_NAMES.unit.shortName().asString()) return "V"
+        }
+
+        val javaClassId = JavaToKotlinClassMap.INSTANCE.mapKotlinToJava(classId.asSingleFqName().toUnsafe())
+        if (javaClassId != null) {
+            return "L" + javaClassId.asString().replace('.', '$') + ";"
+        }
+
+        return "L$internalName;"
+    }
+}

core/descriptor.loader.java/src/org/jetbrains/kotlin/serialization/jvm/JvmProtoBufUtil.kt

@@ -17,11 +17,7 @@
 package org.jetbrains.kotlin.serialization.jvm
 
 import com.google.protobuf.ExtensionRegistryLite
-import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.load.kotlin.JvmNameResolver
-import org.jetbrains.kotlin.name.ClassId
-import org.jetbrains.kotlin.platform.JavaToKotlinClassMap
-import org.jetbrains.kotlin.resolve.jvm.JvmPrimitiveType
 import org.jetbrains.kotlin.serialization.ClassData
 import org.jetbrains.kotlin.serialization.PackageData
 import org.jetbrains.kotlin.serialization.ProtoBuf
@@ -122,28 +118,6 @@ public object JvmProtoBufUtil {
     data class PropertySignature(val name: String, val desc: String)
 
     private fun mapTypeDefault(type: ProtoBuf.Type, nameResolver: NameResolver): String? {
-        return if (type.hasClassName()) mapClassIdDefault(nameResolver.getClassId(type.className)) else null
-    }
-
-    @JvmStatic
-    fun mapClassIdDefault(classId: ClassId): String {
-        val internalName = classId.asString().replace('.', '$')
-        val simpleName = internalName.removePrefix("kotlin/")
-        if (simpleName != internalName) {
-            for (jvmPrimitive in JvmPrimitiveType.values()) {
-                val primitiveType = jvmPrimitive.primitiveType
-                if (simpleName == primitiveType.typeName.asString()) return jvmPrimitive.desc
-                if (simpleName == primitiveType.arrayTypeName.asString()) return "[" + jvmPrimitive.desc
-            }
-
-            if (simpleName == KotlinBuiltIns.FQ_NAMES.unit.shortName().asString()) return "V"
-        }
-
-        val javaClassId = JavaToKotlinClassMap.INSTANCE.mapKotlinToJava(classId.asSingleFqName().toUnsafe())
-        if (javaClassId != null) {
-            return "L" + javaClassId.asString().replace('.', '$') + ";"
-        }
-
-        return "L$internalName;"
+        return if (type.hasClassName()) ClassMapperLite.mapClass(nameResolver.getClassId(type.className)) else null
     }
 }