KT-12985 Do not create range instances for 'for' loop in CharSequence.indices

compiler/backend/src/org/jetbrains/kotlin/codegen/ExpressionCodegen.java

@@ -656,6 +656,11 @@ public class ExpressionCodegen extends KtVisitor<StackValue, StackValue> impleme
             assert extensionReceiver != null : "Extension receiver should be non-null for optimizable 'Collection.indices' call";
             return new ForInCollectionIndicesRangeLoopGenerator(forExpression, extensionReceiver);
         }
+        else if (RangeCodegenUtil.isCharSequenceIndices(loopRangeCallee)) {
+            ReceiverValue extensionReceiver = loopRangeCall.getExtensionReceiver();
+            assert extensionReceiver != null : "Extension receiver should be non-null for optimizable 'CharSequence.indices' call";
+            return new ForInCharSequenceIndicesRangeLoopGenerator(forExpression, extensionReceiver);
+        }
 
         return null;
     }
@@ -1198,7 +1203,7 @@ public class ExpressionCodegen extends KtVisitor<StackValue, StackValue> impleme
 
             StackValue receiver = generateReceiverValue(receiverValue, false);
             Type receiverType = asmType(receiverValue.getType());
-            receiver.put(receiverType, v); // NB receiverType is a collection or an array
+            receiver.put(receiverType, v);
             getReceiverSizeAsInt();
             v.iconst(1);
             v.sub(Type.INT_TYPE);
@@ -1233,6 +1238,17 @@ public class ExpressionCodegen extends KtVisitor<StackValue, StackValue> impleme
         }
     }
 
+    private class ForInCharSequenceIndicesRangeLoopGenerator extends ForInOptimizedIndicesLoopGenerator {
+        private ForInCharSequenceIndicesRangeLoopGenerator(@NotNull KtForExpression forExpression, @NotNull ReceiverValue receiverValue) {
+            super(forExpression, receiverValue);
+        }
+
+        @Override
+        protected void getReceiverSizeAsInt() {
+            v.invokeinterface("java/lang/CharSequence", "length", "()I");
+        }
+    }
+
     private class ForInProgressionExpressionLoopGenerator extends AbstractForInProgressionOrRangeLoopGenerator {
         private int incrementVar;
         private Type incrementType;

compiler/backend/src/org/jetbrains/kotlin/codegen/RangeCodegenUtil.java

@@ -165,6 +165,17 @@ public class RangeCodegenUtil {
         return true;
     }
 
+    public static boolean isCharSequenceIndices(@NotNull CallableDescriptor descriptor) {
+        if (!isTopLevelInPackage(descriptor, "indices", "kotlin.text")) return false;
+
+        ReceiverParameterDescriptor extensionReceiver = descriptor.getExtensionReceiverParameter();
+        if (extensionReceiver == null) return false;
+        KotlinType extensionReceiverType = extensionReceiver.getType();
+        if (!KotlinBuiltIns.isCharSequenceOrNullableCharSequence(extensionReceiverType)) return false;
+
+        return true;
+    }
+
     private static boolean isTopLevelInPackage(@NotNull CallableDescriptor descriptor, @NotNull String name, @NotNull String packageName) {
         if (!name.equals(descriptor.getName().asString())) return false;