Restore and deprecate Function{n}/ExtensionFunction{n} classes for easier migration

Users' Java code will not break in common cases (when passing functions to
Kotlin), and deprecation warnings will be reported.

Provide an inspection with a quick fix which allows to replace deprecated
function class usages to the new classes. Include this fix to the "code
cleanup" action

generators/src/org/jetbrains/kotlin/generators/builtins/deprecatedJavaFunctions.kt

@@ -0,0 +1,43 @@
+/*
+ * 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.generators.builtins
+
+import org.jetbrains.kotlin.generators.builtins.generateBuiltIns.BuiltInsSourceGenerator
+import java.io.PrintWriter
+
+class GenerateDeprecatedJavaFunction(
+        out: PrintWriter,
+        private val arity: Int,
+        private val extension: Boolean
+) : BuiltInsSourceGenerator(out) {
+    override val language = BuiltInsSourceGenerator.Language.JAVA
+
+    override fun generateBody() {
+        val params = (1..arity).map { "P$it" }
+        val generics = "<" + ((if (extension) listOf("E") else listOf()) + params + listOf("R")).join() + ">"
+
+        val name = if (extension) "ExtensionFunction" else "Function"
+        val superArity = if (extension) arity + 1 else arity
+
+        out.println("/**")
+        out.println(" * @deprecated Use the new function classes from the package kotlin.jvm.functions.")
+        out.println(" */")
+        out.println("@Deprecated")
+        out.println("public interface $name$arity$generics extends kotlin.jvm.functions.Function$superArity$generics {")
+        out.println("}")
+    }
+}

generators/src/org/jetbrains/kotlin/generators/builtins/generateBuiltIns.kt

@@ -16,6 +16,7 @@
 
 package org.jetbrains.kotlin.generators.builtins.generateBuiltIns
 
+import org.jetbrains.kotlin.generators.builtins.GenerateDeprecatedJavaFunction
 import org.jetbrains.kotlin.generators.builtins.arrayIterators.GenerateArrayIterators
 import org.jetbrains.kotlin.generators.builtins.arrays.GenerateArrays
 import org.jetbrains.kotlin.generators.builtins.functionImpl.GenerateFunctionImpl
@@ -34,6 +35,7 @@ fun assertExists(file: File): Unit =
 val BUILT_INS_NATIVE_DIR = File("core/builtins/native/")
 val BUILT_INS_SRC_DIR = File("core/builtins/src/")
 val RUNTIME_JVM_DIR = File("core/runtime.jvm/src/")
+val FUNCTIONS_MIGRATION_DIR = File("core/functions.migration/src/")
 
 abstract class BuiltInsSourceGenerator(val out: PrintWriter) {
     protected abstract fun generateBody(): Unit
@@ -43,7 +45,7 @@ abstract class BuiltInsSourceGenerator(val out: PrintWriter) {
     protected open val language: Language = Language.KOTLIN
 
     enum class Language {
-        KOTLIN
+        KOTLIN,
         JAVA
     }
 
@@ -75,6 +77,13 @@ fun generateBuiltIns(generate: (File, (PrintWriter) -> BuiltInsSourceGenerator)
     generate(File(BUILT_INS_SRC_DIR, "kotlin/ProgressionIterators.kt")) { GenerateProgressionIterators(it) }
     generate(File(BUILT_INS_SRC_DIR, "kotlin/Progressions.kt")) { GenerateProgressions(it) }
     generate(File(BUILT_INS_SRC_DIR, "kotlin/Ranges.kt")) { GenerateRanges(it) }
+
+    for (i in 0..22) {
+        generate(File(FUNCTIONS_MIGRATION_DIR, "kotlin/Function$i.java")) { GenerateDeprecatedJavaFunction(it, i, false) }
+    }
+    for (i in 0..21) {
+        generate(File(FUNCTIONS_MIGRATION_DIR, "kotlin/ExtensionFunction$i.java")) { GenerateDeprecatedJavaFunction(it, i, true) }
+    }
 }
 
 fun main(args: Array<String>) {