Generate for-in-array loops with proper semantics in 1.3+

In Kotlin 1.3+, assignment to the for-in-array loop range variable in
the loop body doesn't affect loop execution (as if it was a loop on an
array iterator, or some other container).

 #KT-21354 In Progress
 #KT-21321 In Progress

compiler/backend/src/org/jetbrains/kotlin/codegen/range/ArrayRangeValue.kt

@@ -23,9 +23,12 @@ import org.jetbrains.kotlin.codegen.range.inExpression.InExpressionGenerator
 import org.jetbrains.kotlin.psi.KtForExpression
 import org.jetbrains.kotlin.psi.KtSimpleNameExpression
 
-class ArrayRangeValue(private val canCacheArrayLength: Boolean) : RangeValue {
+class ArrayRangeValue(
+        private val canCacheArrayLength: Boolean,
+        private val shouldAlwaysStoreArrayInNewVar: Boolean
+) : RangeValue {
     override fun createForLoopGenerator(codegen: ExpressionCodegen, forExpression: KtForExpression) =
-            ForInArrayLoopGenerator(codegen, forExpression, canCacheArrayLength)
+            ForInArrayLoopGenerator(codegen, forExpression, canCacheArrayLength, shouldAlwaysStoreArrayInNewVar)
 
     override fun createInExpressionGenerator(codegen: ExpressionCodegen, operatorReference: KtSimpleNameExpression): InExpressionGenerator =
             CallBasedInExpressionGenerator(codegen, operatorReference)

compiler/backend/src/org/jetbrains/kotlin/codegen/range/RangeValues.kt

@@ -18,8 +18,10 @@ package org.jetbrains.kotlin.codegen.range
 
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.codegen.*
+import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.descriptors.CallableDescriptor
 import org.jetbrains.kotlin.descriptors.impl.LocalVariableDescriptor
+import org.jetbrains.kotlin.descriptors.impl.SyntheticFieldDescriptor
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.resolve.BindingContext
@@ -46,8 +48,15 @@ fun ExpressionCodegen.createRangeValueForExpression(rangeExpression: KtExpressio
 
     val builtIns = state.module.builtIns
     return when {
-        asmRangeType.sort == Type.ARRAY ->
-            ArrayRangeValue(!isLocalVarReference(rangeExpression, bindingContext))
+        asmRangeType.sort == Type.ARRAY -> {
+            val properForInArraySemantics =
+                    state.languageVersionSettings.supportsFeature(LanguageFeature.ProperForInArrayLoopRangeVariableAssignmentSemantic)
+            ArrayRangeValue(
+                    properForInArraySemantics || !isLocalVarReference(rangeExpression, bindingContext),
+                    properForInArraySemantics
+            )
+        }
+
         isPrimitiveRange(rangeType) ->
             PrimitiveRangeRangeValue()
         isPrimitiveProgression(rangeType) ->
@@ -64,7 +73,10 @@ fun ExpressionCodegen.createRangeValueForExpression(rangeExpression: KtExpressio
 fun isLocalVarReference(rangeExpression: KtExpression, bindingContext: BindingContext): Boolean {
     if (rangeExpression !is KtSimpleNameExpression) return false
     val resultingDescriptor = rangeExpression.getResolvedCall(bindingContext)?.resultingDescriptor ?: return false
-    return resultingDescriptor is LocalVariableDescriptor && resultingDescriptor.isVar
+    return resultingDescriptor is LocalVariableDescriptor &&
+           resultingDescriptor !is SyntheticFieldDescriptor &&
+           !resultingDescriptor.isDelegated &&
+           resultingDescriptor.isVar
 }
 
 private fun isSubtypeOfString(type: KotlinType, builtIns: KotlinBuiltIns) =

compiler/backend/src/org/jetbrains/kotlin/codegen/range/forLoop/ForInArrayLoopGenerator.kt

@@ -29,9 +29,8 @@ import org.jetbrains.org.objectweb.asm.Type
 class ForInArrayLoopGenerator(
         codegen: ExpressionCodegen,
         forExpression: KtForExpression,
-        // We can cache array length if the corresponding range expression is not a local var.
-        // See https://youtrack.jetbrains.com/issue/KT-21354.
-        private val canCacheArrayLength: Boolean
+        private val canCacheArrayLength: Boolean,
+        private val shouldAlwaysStoreArrayInNewVar: Boolean
 ) : AbstractForLoopGenerator(codegen, forExpression) {
     private var indexVar: Int = 0
     private var arrayVar: Int = 0
@@ -46,7 +45,7 @@ class ForInArrayLoopGenerator(
         val loopRange = forExpression.loopRange
         val value = codegen.gen(loopRange)
         val asmLoopRangeType = codegen.asmType(loopRangeType)
-        if (value is StackValue.Local && value.type == asmLoopRangeType) {
+        if (!shouldAlwaysStoreArrayInNewVar && value is StackValue.Local && value.type == asmLoopRangeType) {
             arrayVar = value.index // no need to copy local variable into another variable
         }
         else {