[JS IR BE] Implement vararg in JS BE

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/BlockDecomposerLowering.kt

@@ -526,6 +526,47 @@ class BlockDecomposerLowering(val context: JsIrBackendContext) : FunctionLowerin
             return KeptResult
         }
 
+        override fun visitVararg(expression: IrVararg, data: VisitData): VisitResult {
+            var decomposed = 0
+
+            val arguments = expression.elements.map {
+                val expr = (it as? IrSpreadElement)?.expression ?: it as IrExpression
+                Pair(it, expr.accept(this, data).applyIfChanged { decomposed++ })
+            }
+
+            if (decomposed == 0) return KeptResult
+
+            val newStatements = mutableListOf<IrStatement>()
+            val newArguments = arguments.map { (original, result) ->
+                if (decomposed == 0) original
+                else {
+                    val originalExpression = (original as? IrSpreadElement)?.expression ?: original as IrExpression
+                    val newExpression = result.runIfChangedOrDefault(originalExpression) {
+                        newStatements += statements
+                        decomposed--
+                        resultValue
+                    }
+                    val wrapVar = makeTempVar(expression.varargElementType)
+                    newStatements += JsIrBuilder.buildVar(wrapVar).apply { initializer = newExpression }
+                    val newValue = JsIrBuilder.buildGetValue(wrapVar)
+                    if (original is IrSpreadElement) {
+                        IrSpreadElementImpl(original.startOffset, original.endOffset, newValue)
+                    } else {
+                        newValue
+                    }
+                }
+            }
+
+            val newExpression = IrVarargImpl(
+                expression.startOffset,
+                expression.endOffset,
+                expression.type,
+                expression.varargElementType,
+                newArguments)
+
+            return DecomposedResult(newStatements, newExpression)
+        }
+
         override fun visitStringConcatenation(expression: IrStringConcatenation, data: VisitData): VisitResult {
             var decomposed = 0
 

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsExpressionTransformer.kt

@@ -19,6 +19,53 @@ import org.jetbrains.kotlin.util.OperatorNameConventions
 
 class IrElementToJsExpressionTransformer : BaseIrElementToJsNodeTransformer<JsExpression, JsGenerationContext> {
 
+    override fun visitVararg(expression: IrVararg, context: JsGenerationContext): JsExpression {
+        // TODO: perform the dark magic below in the separated lowering
+        if (expression.elements.size == 1) {
+            val element = expression.elements[0]
+            if (element is IrSpreadElement) {
+                // special case, invoke slice()
+                val expr = element.expression.accept(this, context)
+                return JsInvocation(JsNameRef(Namer.SLICE_FUNCTION, expr))
+            }
+        }
+
+        var arrayLiteralElements = mutableListOf<JsExpression>()
+        val concatArguments = mutableListOf<JsExpression>()
+        var qualifier: JsExpression? = null
+
+        expression.elements.forEach {
+            if (it is IrSpreadElement) {
+                val expr = it.expression.accept(this, context)
+                if (qualifier == null) {
+                    if (arrayLiteralElements.isEmpty()) {
+                        qualifier = JsNameRef(Namer.CONCAT_FUNCTION, expr)
+                    } else {
+                        val dispatch = JsArrayLiteral(arrayLiteralElements)
+                        arrayLiteralElements = mutableListOf()
+                        qualifier = JsNameRef(Namer.CONCAT_FUNCTION, dispatch)
+                        concatArguments.add(expr)
+                    }
+                } else {
+                    if (arrayLiteralElements.isNotEmpty()) {
+                        concatArguments.add(JsArrayLiteral(arrayLiteralElements))
+                        arrayLiteralElements = mutableListOf()
+                    }
+                    concatArguments.add(expr)
+                }
+            } else {
+                arrayLiteralElements.add(it.accept(this, context))
+            }
+        }
+
+        return qualifier?.let {
+            if (arrayLiteralElements.isNotEmpty()) {
+                concatArguments.add(JsArrayLiteral(arrayLiteralElements))
+            }
+            return JsInvocation(it, concatArguments)
+        } ?: JsArrayLiteral(arrayLiteralElements)
+    }
+
     override fun visitExpressionBody(body: IrExpressionBody, context: JsGenerationContext): JsExpression =
         body.expression.accept(this, context)
 

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/utils/Namer.kt

@@ -34,6 +34,9 @@ object Namer {
     val CALL_FUNCTION = "call"
     val APPLY_FUNCTION = "apply"
 
+    val SLICE_FUNCTION = "slice"
+    val CONCAT_FUNCTION = "concat"
+
     val UNREACHABLE_NAME = "\$unreachable"
 
     val OUTER_FIELD_NAME = "\$outer"