KMP Kotlin-to-Java direct actualization

Author: nikitabobko

proposals/kmp-kotlin-to-java-direct-actualization.md

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+# KMP Kotlin-to-Java direct actualization
+
+* **Type**: Design proposal
+* **Author**: Nikita Bobko
+* **Contributors**: Dmitriy Novozhilov, Kevin Bierhoff, Mikhail Zarechenskiy, Pavel Kunyavskiy
+* **Discussion**: todo
+* **Status**: Submitted
+* **Related YouTrack issue**: [KT-67202](https://youtrack.jetbrains.com/issue/KT-67202)
+
+**Definition.** ClassId is a class identifier that consists of three independent components: the package where class is declared in, names of all outer classes (if presented), and the name of the class itself.
+In Kotlin, it could be represented as the data class:
+```kotlin
+data class ClassId(val package: String, val outerClasses: List<String>, val className: String)
+```
+
+Two ClassIds are equal if their respective components are equal.
+Example: `ClassId("foo.bar", emptyListOf(), "baz")` and `ClassId("foo", listOf("bar"), "baz")` are different ClassIds
+
+## Introduction
+
+In Kotlin, there are **two ways** to write an actual declaration for the existing expect declaration.
+You can either write an actual declaration with the same ClassId as its appropriate expect and mark the appropriate declarations with `expect` and `actual` keywords (From now on, we will call such actualizations _direct actualizations_),
+or you can use `actual typealias`.
+
+**The first way.**
+_direct actualization_ has a nice property that two declarations share the same ClassId.
+It's good because when users move code between common and platform fragments, their imports stay unchanged.
+But _direct actualization_ has a "downside" that it doesn't allow declaring actuals in external binaries (jars or klibs).
+In other words, expect declaration and its appropriate actual must be located in the same "compilation unit."
+[Below](#direct-actualization-forces-expect-and-actual-to-be-in-the-same-compilation-unit) we say why, in fact, it's not a "downside" but a "by design" restriction that reflects the reality.
+
+**The second way.**
+Contrary, `actual typealias` forces users to change the ClassId of the actual declaration.
+(An attempt to specify the very same ClassId in the `typealias` target leads to `RECURSIVE_TYPEALIAS_EXPANSION` diagnostic)
+But we gain the possibility to declare expect and actual declarations in different "compilation units."
+
+> [!NOTE]
+> Though it's a philosophical question what is "the real actual declaration" in this case.
+> Is it the `actual typealias` itself (which is still declared in the same "compilation unit"), or is it the target of the `actual typealias` (which, in fact, can be declared in external jar or klib)?
+
+|                                                                     | _Direct actualization_ | `actual typealias` |
+|---------------------------------------------------------------------|------------------------|--------------------|
+| Do expect and actual share the same ClassId?                        | Yes                    | No                 |
+| Can expect and actual be declared in different "compilation units"? | No                     | Yes                |
+
+While `actual typealias` already allows actualizing Kotlin expect declarations with Java declarations (Informally: Kotlin-to-Java actualization), _direct actualization_ only allows Kotlin-to-Kotlin actualizations.
+The idea of this proposal is to support _direct actualization_ for Kotlin-to-Java actualizations.
+
+## Motivation
+
+As stated in the [introduction](#introduction), unlike `actual typealias`, _direct actualization_ allows to keep the same ClassIds for common and platform declarations in case of Kotlin-to-Java actualization.
+
+One popular use case for Kotlin-to-Java actualization is KMP-fying existing Java libraries.
+For library authors, the possibility to keep the same ClassIds between common and platform declarations is highly valuable:
+
+- Since it avoids the creation of two ClassIds that refer to the same object, it avoids the confusion on which ClassId should be used
+- It simplifies the migration of client code from the Java library to a KMP version of the same library (no need to replace imports)
+- It avoid duplication of potentially entire API surface, which can otherwise become cumbersome
+- Later replacing the Java actualization with a Kotlin `actual` class is possible without keeping the previous `actual typealias` in place indefinitely
+
+## The proposal
+
+**(1)** Introduce `kotlin.annotations.jvm.KotlinActual` annotation in `kotlin-annotations-jvm.jar`
+```java
+package kotlin.annotations.jvm;
+
+@Retention(RetentionPolicy.SOURCE)
+@Target({ElementType.TYPE, ElementType.METHOD, ElementType.CONSTRUCTOR})
+public @interface KotlinActual {
+}
+```
+
+- The annotation is intended to be used on Java declarations
+- **Any usage** of the annotation in Kotlin will be prohibited (even as a type, even as a reference)
+- The annotation in Java will function similarly to the `actual` keyword in Kotlin
+- It doesn't make sense to mark the annotation with `@ExperimentalMultiplatform` since `OPT_IN_USAGE_ERROR` is not reported in Java
+- `ElementType.FIELD` annotation target is not specified by design. It's not possible to actualize Kotlin properties with Java fields
+
+**(2)** If Kotlin expect and Java class have the same ClassId, Kotlin compiler should consider Kotlin expect class being actualized with the appropriate Java class.
+In other words, support _direct actualization_ for Kotlin-to-Java actualization.
+
+**(3)** Kotlin compiler should require using `@KotlinActual` on the Java top level class and its respective members.
+The rules must be similar to the one with `actual` keyword requirement in Kotlin-to-Kotlin actualization.
+
+**(4)** If `@KotlinActual` is used on Java class members that don't have respective Kotlin expect members, it should be reported by the Kotlin compiler.
+The rules must be similar to the one with `actual` keyword requirement in Kotlin-to-Kotlin actualization.
+Please note that Kotlin can only detect excessive `@KotlinActual` annotations on methods of the classes that actualize some existing Kotlin expect classes.
+Since Kotlin doesn't traverse all Java files, it's not possible to detect excessive `@KotlinActual` annotation on the top-level Java classes for which a respective Kotlin expect class doesn't exist.
+For the same reason, it's not possible to detect excessive `@KotlinActual` annotation on members of such Java classes.
+For these cases, it's proposed to implement Java IDE inspection.
+
+**Worth noting.**
+1. Java wasn't capable and still is not capable of actualizing Kotlin top-level functions in any way.
+2. Kotlin expect classes are not capable of expressing Java static members [KT-29882](https://youtrack.jetbrains.com/issue/KT-29882)
+
+Example of a valid Kotlin-to-Java direct actualization:
+```kotlin
+// MODULE: common
+expect class Foo() {
+    fun foo()
+}
+
+// MODULE: JVM
+@kotlin.annotations.jvm.KotlinActual public class Foo {
+    @kotlin.annotations.jvm.KotlinActual public Foo() {}
+    @kotlin.annotations.jvm.KotlinActual public void foo() {}
+
+    @Override
+    public String toString() { return "Foo"; } // No @KotlinActual is required
+}
+```
+
+## actual keyword is a virtue
+
+An alternative suggestion is to match only by ClassIds, and to drop `actual` keyword in Kotlin-to-Kotlin actualizations, and to drop `@KotlinActual` annotation in Kotlin-to-Java actualization.
+
+The suggestion was rejected because we consider `actual` keyword being beneficial for readers, much like the `override` keyword.
+
+-   **Misspelling prevention.**
+    The explicit `actual` keyword (or `@KotlinActual` annotation) helps against misspelling
+    (esp. when Kotlin supports expect declarations with bodies [KT-20427](https://youtrack.jetbrains.com/issue/KT-20427))
+-   **Explicit intent.**
+    Declarations may be written solely to fullfil the "expect requirement" but the declarations may not be directly used by the platform code.
+    The `actual` keyword (or `@KotlinActual` annotation) explictly signals the intention to actualize member rather than accidentally defining a new one.
+    (too bad that members of `actual typealias` break the design in this place)
+-   **Safeguards during refactoring.**
+    If the member in expect class changes (e.g. a new parameter added), `actual` keyword (or `@KotlinActual` annotation) helps to quickly identify members in the actual class that needs an update.
+    Without the keyword, there might be already a suitable overload in the actual class that would silently become a new actualization.
+
+To support our arguments, we link Swift community disscussions about the explicit keyword for protocol conformance (as of swift 5.9, no keyword is required):
+[1](https://forums.swift.org/t/pre-pitch-explicit-protocol-fulfilment-with-the-conformance-keyword/60246), [2](https://forums.swift.org/t/keyword-for-protocol-conformance/3837)
+
+## The proposal doesn't cover Kotlin-free pure Java library use case
+
+There are two cases:
+1. The user has a Kotlin-Java mixed project, and they want to KMP-fy it.
+2. The user has a pure Java project, and they want to KMP-fy it.
+
+The first case is handled by [the proposal](#the-proposal).
+
+In the second case, the common part of the project is Kotlin sources that depend on `kotlin-stdlib.jar`.
+The common part of the project may also define additional regular non-expect Kotlin declarations.
+JVM part of the project depends on common.
+
+If users want to keep their JVM part free of Kotlin, they have to be accurate and avoid accidental usages of `kotlin-stdlib.jar`, and avoid declaring additional non-expect declarations in the common.
+[The proposal](#the-proposal) doesn't cover that case well since the design would become more complicated.
+The current proposal is a small incremental addition to the existing model, and it doesn't block us from covering the second case later if needed.
+
+`KotlinActual` annotation has `SOURCE` retention by design.
+This way, the annotation is least invasive for the Java sources, and it should be enough to have compile-only dependency on `kotlin-annotations-jvm.jar`.
+Which doesn't contradict the "pure Java project" case.
+
+## Kotlin-to-Java expect-actual incompatibilities diagnostics reporting
+
+**Invariant 1.** Kotlin compiler cannot report compilation errors in non-kt files.
+
+In Kotlin-to-Kotlin actualization, expect-actual incompatibilities are reported on the actual side.
+
+In Kotlin-to-Java actualization, it's proposed to report incompatibilities on the expect side.
+It's inconsistent with Kotlin-to-Kotlin actualizations, but we don't believe that Kotlin-to-Java actualization is significant enough to break the _invariant 1_.
+The reporting may be improved in future versions of Kotlin.
+
+## Direct actualization forces expect and actual to be in the same compilation unit
+
+In the [introduction](#introduction), we mentioned that _direct actualization_ forces expect and actual to be in the same "compilation unit."
+It's an implementation limitation that we believe is beneficial, because it reflects the reality.
+
+It's a common pattern for libraries to use a unique package prefix.
+We want people to stick to that pattern.
+_Direct actualization_ for external declarations encourages wrong behavior.
+
+-   Imagine that it is possible to write an expect declaration for the existing JVM library via _direct actualization_ mechanism.
+    Users may as easily decide to declare non-expect declarations in the same package, which leads to the "split package" problem in JPMS.
+-   Test case: there is an existing JVM library A.
+    Library B is a KMP wrapper around library A.
+    Library B provides expect declarations for the library A.
+    Later, Library A decides to provide its own KMP API.
+    If Library B could use _direct actualization_, it would lead to declarations clash between A and B.
+
+## The frontend technical limitation of Kotlin-to-Java direct actualization
+
+Frontend transformers are run only on Kotlin sources.
+Java sources are visited lazily only if they are referenced from Kotlin sources.
+
+Given frontend technical restriction, it's proposed to implement Kotlin-to-Java _direct actualization_ matching and checking only on IR backend.
+
+## Alternatives considered
+
+-   Implicitly match Kotlin-to-Java with the same ClassId if some predefined annotation is presented on the expect declaration.
+-   `actual typealias` in Kotlin without target in RHS.
+-   `actual` declaration that doesn't generate class files.
+
+    It could be some special annotation that says that bytecode shouldn't be generated for the class.
+    The idea is useful by itself, for example, in stdlib, to declare `kotlin.collections.List`.
+
+    The disadvantages are clear: unlike the current proposal, there will be no compilation time checks;
+    compared to the current proposal, it will result in excessive code duplication in the expect-actual case.
+
+See [actual keyword is a virtue](#actual-keyword-is-a-virtue) to understand why alternatives were discarded.
+Besides, the proposed solution resembles the already familiar Kotlin-to-Kotlin _direct actualization_, but makes it available for Java.
+
+## Unused declaration inspection in Java
+
+IntelliJ IDEA implements an unused declaration inspection.
+The `javac` itself doesn't emit warnings for unused declarations.
+
+The inspection in IDEA should be changed to account for declarations annotated with `@kotlin.annotations.jvm.KotlinActual`.
+
+## Feature interaction with hierarchical multiplatform
+
+There is no feature interaction. It's not possible to have Java files in intermediate fragments.