Do not generate the field for unused parameters in suspend lambdas

.idea/misc.xml

@@ -1,5 +1,9 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
+  <component name="ASMIdeaPluginConfiguration">
+    <asm skipDebug="false" skipFrames="false" skipCode="false" expandFrames="false" />
+    <groovy codeStyle="LEGACY" />
+  </component>
   <component name="ClientPropertiesManager">
     <properties class="javax.swing.JPanel">
       <property name="BorderFactoryClass" class="java.lang.String" />
@@ -82,6 +86,9 @@
       </set>
     </option>
   </component>
+  <component name="SuppressKotlinCodeStyleNotification">
+    <option name="disableForAll" value="true" />
+  </component>
   <component name="WebServicesPlugin" addRequiredLibraries="true" />
   <component name="com.sixrr.metrics.MetricsReloaded">
     <option name="selectedProfile" value="" />

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/codegen/ir/FirBytecodeTextTestGenerated.java

@@ -1525,6 +1525,11 @@ public class FirBytecodeTextTestGenerated extends AbstractFirBytecodeTextTest {
                 runTest("compiler/testData/codegen/bytecodeText/coroutines/cleanup/simple.kt");
             }
 
+            @TestMetadata("unusedParamNotSpill.kt")
+            public void testUnusedParamNotSpill() throws Exception {
+                runTest("compiler/testData/codegen/bytecodeText/coroutines/cleanup/unusedParamNotSpill.kt");
+            }
+
             @TestMetadata("when.kt")
             public void testWhen() throws Exception {
                 runTest("compiler/testData/codegen/bytecodeText/coroutines/cleanup/when.kt");

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/lower/SuspendLambdaLowering.kt

@@ -22,6 +22,7 @@ import org.jetbrains.kotlin.codegen.inline.coroutines.FOR_INLINE_SUFFIX
 import org.jetbrains.kotlin.descriptors.DescriptorVisibilities
 import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.ir.IrElement
+import org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI
 import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
 import org.jetbrains.kotlin.ir.builders.*
 import org.jetbrains.kotlin.ir.builders.declarations.*
@@ -29,11 +30,11 @@ import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.expressions.*
 import org.jetbrains.kotlin.ir.expressions.impl.*
 import org.jetbrains.kotlin.ir.symbols.IrSimpleFunctionSymbol
+import org.jetbrains.kotlin.ir.symbols.impl.IrVariableSymbolImpl
+import org.jetbrains.kotlin.ir.symbols.IrValueParameterSymbol
 import org.jetbrains.kotlin.ir.types.*
 import org.jetbrains.kotlin.ir.util.*
-import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
+import org.jetbrains.kotlin.ir.visitors.*
-import org.jetbrains.kotlin.ir.visitors.IrElementVisitorVoid
-import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
 import org.jetbrains.kotlin.load.java.JavaDescriptorVisibilities
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.name.SpecialNames
@@ -138,6 +139,7 @@ private class SuspendLambdaLowering(context: JvmBackendContext) : SuspendLowerin
             }
         }
 
+    @OptIn(ObsoleteDescriptorBasedAPI::class)
     private fun generateContinuationClassForLambda(reference: IrFunctionReference, parent: IrDeclarationParent): IrClass =
         context.irFactory.buildClass {
             name = SpecialNames.NO_NAME_PROVIDED
@@ -163,10 +165,22 @@ private class SuspendLambdaLowering(context: JvmBackendContext) : SuspendLowerin
                         + context.irBuiltIns.anyNType
             )
             superTypes = listOf(suspendLambda.defaultType, functionNType)
+            val usedParams = mutableListOf<IrSymbolOwner>()
+
+            // marking the parameters referenced in the function
+            function.acceptChildrenVoid(
+                object : IrElementVisitorVoid {
+                    override fun visitElement(element: IrElement) =
+                        if (element is IrDeclarationReference && element.symbol is IrValueParameterSymbol && element.symbol.owner in function.explicitParameters)
+                            usedParams += element.symbol.owner
+                        else
+                            Unit
+                },
+            )
 
             addField(COROUTINE_LABEL_FIELD_NAME, context.irBuiltIns.intType, JavaDescriptorVisibilities.PACKAGE_VISIBILITY)
 
-            val parametersFields = function.explicitParameters.map {
+            val parametersFields = function.explicitParameters.filter { it in usedParams }.map {
                 addField {
                     // Rename `$this` to avoid being caught by inlineCodegenUtils.isCapturedFieldName()
                     name = if (it.index < 0) Name.identifier("p\$") else it.name

compiler/testData/codegen/bytecodeText/coroutines/cleanup/unusedParamNotSpill.kt

@@ -0,0 +1,5 @@
+val f: suspend (Int) -> Unit = { unused ->
+}
+
+// 0 GETFIELD p\$0
+// 0 PUTFIELD p\$0

compiler/tests-gen/org/jetbrains/kotlin/codegen/BytecodeTextTestGenerated.java

@@ -1515,6 +1515,11 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
                 runTest("compiler/testData/codegen/bytecodeText/coroutines/cleanup/simple.kt");
             }
 
+            @TestMetadata("unusedParamNotSpill.kt")
+            public void testUnusedParamNotSpill() throws Exception {
+                runTest("compiler/testData/codegen/bytecodeText/coroutines/cleanup/unusedParamNotSpill.kt");
+            }
+
             @TestMetadata("when.kt")
             public void testWhen() throws Exception {
                 runTest("compiler/testData/codegen/bytecodeText/coroutines/cleanup/when.kt");

compiler/tests-gen/org/jetbrains/kotlin/codegen/ir/IrBytecodeTextTestGenerated.java

@@ -1525,6 +1525,11 @@ public class IrBytecodeTextTestGenerated extends AbstractIrBytecodeTextTest {
                 runTest("compiler/testData/codegen/bytecodeText/coroutines/cleanup/simple.kt");
             }
 
+            @TestMetadata("unusedParamNotSpill.kt")
+            public void testUnusedParamNotSpill() throws Exception {
+                runTest("compiler/testData/codegen/bytecodeText/coroutines/cleanup/unusedParamNotSpill.kt");
+            }
+
             @TestMetadata("when.kt")
             public void testWhen() throws Exception {
                 runTest("compiler/testData/codegen/bytecodeText/coroutines/cleanup/when.kt");