[Wasm] Add interop annotation @JsFun

libraries/stdlib/wasm/builtins/kotlin/String.kt

@@ -51,11 +51,11 @@ public class String constructor(public val string: String) : Comparable<String>,
     public override fun hashCode(): Int = 10
 }
 
-@WasmImport("runtime", "String_plus")
+@JsFun("(it, other) => it + String(other)")
 private fun stringPlusImpl(it: String, other: String): String =
     implementedAsIntrinsic
 
-@WasmImport("runtime", "String_getLength")
+@JsFun("(it) => it.length")
 private fun stringLengthImpl(it: String): Int =
     implementedAsIntrinsic
 

libraries/stdlib/wasm/internal/kotlin/wasm/internal/JsArray.kt

@@ -72,11 +72,11 @@ internal fun JsArray_set_WasmExternRef(array: WasmExternRef, index: Int, value:
 internal fun JsArray_getSize(array: WasmExternRef): Int =
     implementedAsIntrinsic
 
-@WasmImport("runtime", "identity")
+@JsFun("(x) => x")
 internal fun Any?.toWasmExternRef(): WasmExternRef =
     implementedAsIntrinsic
 
-@WasmImport("runtime", "identity")
+@JsFun("(x) => x")
 internal fun WasmExternRefToAny(ref: WasmExternRef): Any? =
     implementedAsIntrinsic
 

libraries/stdlib/wasm/src/kotlin/JsInterop.kt

@@ -0,0 +1,22 @@
+/*
+ * 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 kotlin
+
+/**
+ * Implements annotated function in JavaScript and automatically imports is to Wasm.
+ * [code] string must contain JS expression that evaluates to JS function with signature that matches annotated kotlin function
+ *
+ * For example, a function that adds two Doubles via JS:
+ *
+ *      @JsFun("(x, y) => x + y")
+ *      fun jsAdd(x: Double, y: Double): Double =
+ *          error("...")
+ *
+ * This is a temporary annotation because K/Wasm <-> JS interop is not designed yet.
+ */
+@Target(AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY_GETTER, AnnotationTarget.PROPERTY_SETTER)
+@Retention(AnnotationRetention.BINARY)
+public annotation class JsFun(val code: String)