Support main entry-point without arguments in JVM

#KT-26574 In Progress

compiler/backend/src/org/jetbrains/kotlin/codegen/codegenUtil.kt

@@ -484,16 +484,25 @@ fun generateBridgeForMainFunctionIfNecessary(
 ) {
     val originElement = origin.element ?: return
     if (functionDescriptor.name.asString() != "main" || !DescriptorUtils.isTopLevelDeclaration(functionDescriptor)) return
-    if (!state.mainFunctionDetector.isMain(functionDescriptor.unwrapInitialDescriptorForSuspendFunction(), false, true)) return
 
-    if (!functionDescriptor.isSuspend) return
+    val unwrappedFunctionDescriptor = functionDescriptor.unwrapInitialDescriptorForSuspendFunction()
+    val isParameterless =
+        unwrappedFunctionDescriptor.extensionReceiverParameter == null && unwrappedFunctionDescriptor.valueParameters.isEmpty()
 
-    val lambdaInternalName = generateLambdaForRunSuspend(
+    if (!functionDescriptor.isSuspend && !isParameterless) return
+    if (!state.mainFunctionDetector.isMain(unwrappedFunctionDescriptor, false, true)) return
+
+    val lambdaInternalName =
+        if (functionDescriptor.isSuspend)
+            generateLambdaForRunSuspend(
                 state,
                 originElement,
                 packagePartClassBuilder.thisName,
-        signatureOfRealDeclaration
+                signatureOfRealDeclaration,
+                isParameterless
             )
+        else
+            null
 
     packagePartClassBuilder.newMethod(
         Synthetic(originElement, functionDescriptor),
@@ -502,6 +511,8 @@ fun generateBridgeForMainFunctionIfNecessary(
         METHOD_DESCRIPTOR_FOR_MAIN, null, null
     ).apply {
         visitCode()
+
+        if (lambdaInternalName != null) {
             visitTypeInsn(NEW, lambdaInternalName)
             visitInsn(DUP)
             visitVarInsn(ALOAD, 0)
@@ -522,6 +533,17 @@ fun generateBridgeForMainFunctionIfNecessary(
                 ),
                 false
             )
+        } else {
+            visitMethodInsn(
+                INVOKESTATIC,
+                packagePartClassBuilder.thisName, "main",
+                Type.getMethodDescriptor(
+                    Type.VOID_TYPE
+                ),
+                false
+            )
+        }
+
         visitInsn(RETURN)
         visitEnd()
     }
@@ -531,7 +553,8 @@ private fun generateLambdaForRunSuspend(
     state: GenerationState,
     originElement: PsiElement,
     packagePartClassInternalName: String,
-    signatureOfRealDeclaration: JvmMethodGenericSignature
+    signatureOfRealDeclaration: JvmMethodGenericSignature,
+    parameterless: Boolean
 ): String {
     val internalName = "$packagePartClassInternalName$$\$main"
     val lambdaBuilder = state.factory.newVisitor(
@@ -592,6 +615,9 @@ private fun generateLambdaForRunSuspend(
     ).apply {
         visitCode()
 
+        if (!parameterless) {
+            // Actually, the field for arguments may also be removed in case of parameterless main,
+            // but probably it'd much easier when IR is ready
             visitVarInsn(ALOAD, 0)
             visitFieldInsn(
                 GETFIELD,
@@ -599,6 +625,7 @@ private fun generateLambdaForRunSuspend(
                 "args",
                 ARRAY_OF_STRINGS_TYPE.descriptor
             )
+        }
 
         visitVarInsn(ALOAD, 1)
         val continuationInternalName = state.languageVersionSettings.continuationAsmType().internalName

compiler/testData/integration/smoke/helloAppOldAndParameterlessMain/hello.compile.expected

@@ -0,0 +1 @@
+Return code: 0

compiler/testData/integration/smoke/helloAppOldAndParameterlessMain/hello.kt

@@ -0,0 +1,9 @@
+package Hello
+
+fun main(args: Array<String>) {
+    System.out.println("Hello!")
+}
+
+fun main() {
+    System.out.println("Fail")
+}

compiler/testData/integration/smoke/helloAppOldAndParameterlessMain/hello.run.expected

@@ -0,0 +1,4 @@
+OUT:
+Hello!
+
+Return code: 0

compiler/testData/integration/smoke/helloAppParameterlessMain/hello.compile.expected

@@ -0,0 +1 @@
+Return code: 0

compiler/testData/integration/smoke/helloAppParameterlessMain/hello.kt

@@ -0,0 +1,5 @@
+package Hello
+
+fun main() {
+    System.out.println("Hello!")
+}

compiler/testData/integration/smoke/helloAppParameterlessMain/hello.run.expected

@@ -0,0 +1,4 @@
+OUT:
+Hello!
+
+Return code: 0

compiler/testData/integration/smoke/helloAppSuspendParameterlessMain/hello.compile.expected

@@ -0,0 +1 @@
+Return code: 0

compiler/testData/integration/smoke/helloAppSuspendParameterlessMain/hello.kt

@@ -0,0 +1,36 @@
+package Hello
+
+import kotlin.concurrent.thread
+import kotlin.coroutines.suspendCoroutine
+import kotlin.coroutines.resume
+import kotlin.reflect.jvm.javaMethod
+
+@kotlin.jvm.Volatile
+private var result = ""
+@kotlin.jvm.Volatile
+private var callback: Function0<Unit>? = null
+
+suspend fun appendAndSuspend(s: String) {
+    result += s
+
+    suspendCoroutine<Unit> { continuation ->
+        callback = {
+            continuation.resume(Unit)
+        }
+    }
+}
+
+suspend fun main() {
+    thread(isDaemon = true) {
+        while (true) {
+            val c = callback
+            c?.invoke()
+            Thread.sleep(500)
+        }
+    }
+
+    appendAndSuspend("O")
+    appendAndSuspend("K")
+    println(result)
+    callback = null
+}

compiler/testData/integration/smoke/helloAppSuspendParameterlessMain/hello.run.expected

@@ -0,0 +1,4 @@
+OUT:
+OK
+
+Return code: 0

compiler/tests/org/jetbrains/kotlin/integration/CompilerSmokeTest.java

@@ -48,6 +48,27 @@ public class CompilerSmokeTest extends CompilerSmokeTestBase {
         run("hello.run", "-cp", jar, "Hello.HelloKt", "O", "K");
     }
 
+    public void testHelloAppParameterlessMain() throws Exception {
+        String jar = tmpdir.getAbsolutePath() + File.separator + "hello.jar";
+
+        assertEquals("compilation failed", 0, runCompiler("hello.compile", "-include-runtime", "hello.kt", "-d", jar));
+        run("hello.run", "-cp", jar, "Hello.HelloKt");
+    }
+
+    public void testHelloAppOldAndParameterlessMain() throws Exception {
+        String jar = tmpdir.getAbsolutePath() + File.separator + "hello.jar";
+
+        assertEquals("compilation failed", 0, runCompiler("hello.compile", "-include-runtime", "hello.kt", "-d", jar));
+        run("hello.run", "-cp", jar, "Hello.HelloKt");
+    }
+
+    public void testHelloAppSuspendParameterlessMain() throws Exception {
+        String jar = tmpdir.getAbsolutePath() + File.separator + "hello.jar";
+
+        assertEquals("compilation failed", 0, runCompiler("hello.compile", "-include-runtime", "hello.kt", "-d", jar));
+        run("hello.run", "-cp", jar, "Hello.HelloKt", "O", "K");
+    }
+
     public void testCompilationFailed() throws Exception {
         String jar = tmpdir.getAbsolutePath() + File.separator + "smoke.jar";