JVM IR: Support parameterless main methods

This commit:

- introduces tests explicating what is and isn't considered a
  proper main method on the JVM backends.
- implements support for parameterless main methods on the JVM IR
  backend
- See KT-34338 for more tests.

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/DeclarationOrigins.kt

@@ -43,4 +43,5 @@ interface JvmLoweredDeclarationOrigin : IrDeclarationOrigin {
     object SYNTHETIC_INLINE_CLASS_MEMBER : IrDeclarationOriginImpl("SYNTHETIC_INLINE_CLASS_MEMBER", isSynthetic = true)
     object INLINE_CLASS_GENERATED_IMPL_METHOD : IrDeclarationOriginImpl("INLINE_CLASS_GENERATED_IMPL_METHOD")
     object GENERATED_ASSERTION_ENABLED_FIELD : IrDeclarationOriginImpl("GENERATED_ASSERTION_ENABLED_FIELD", isSynthetic = true)
+    object GENERATED_MAIN_FOR_PARAMETERLESS_MAIN_METHOD : IrDeclarationOriginImpl("GENERATED_MAIN_FOR_PARAMETERLESS_MAIN_METHOD", isSynthetic = true)
 }

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/JvmLower.kt

@@ -172,8 +172,15 @@ private val syntheticAccessorPhase = makeIrFilePhase(
     prerequisite = setOf(objectClassPhase, staticDefaultFunctionPhase, interfacePhase)
 )
 
+private val mainMethodGenerationPhase = makeIrFilePhase(
+    ::MainMethodGenerationLowering,
+    name = "MainMethodGeneration",
+    description = "Identify parameterless main methods and generate bridge main-methods"
+)
+
 @Suppress("Reformat")
 private val jvmFilePhases =
+        mainMethodGenerationPhase then
         typeAliasAnnotationMethodsPhase then
         stripTypeAliasDeclarationsPhase then
         provisionalFunctionExpressionPhase then

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/MethodSignatureMapper.kt

@@ -10,6 +10,7 @@ import org.jetbrains.kotlin.backend.common.lower.allOverridden
 import org.jetbrains.kotlin.backend.common.lower.parentsWithSelf
 import org.jetbrains.kotlin.backend.jvm.JvmBackendContext
 import org.jetbrains.kotlin.backend.jvm.JvmLoweredDeclarationOrigin
+import org.jetbrains.kotlin.backend.jvm.ir.getJvmNameFromAnnotation
 import org.jetbrains.kotlin.backend.jvm.ir.hasJvmDefault
 import org.jetbrains.kotlin.backend.jvm.ir.propertyIfAccessor
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
@@ -122,11 +123,6 @@ class MethodSignatureMapper(private val context: JvmBackendContext) {
             getJvmModuleNameForDeserializedDescriptor(function.descriptor)
         else null) ?: context.state.moduleName
 
-    private fun IrDeclaration.getJvmNameFromAnnotation(): String? {
-        val const = getAnnotation(DescriptorUtils.JVM_NAME)?.getValueArgument(0) as? IrConst<*>
-        return const?.value as? String
-    }
-
     private fun IrFunction.isPublishedApi(): Boolean =
         propertyIfAccessor.annotations.hasAnnotation(KotlinBuiltIns.FQ_NAMES.publishedApi)
 

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/ir/IrUtils.kt

@@ -18,6 +18,7 @@ import org.jetbrains.kotlin.ir.builders.Scope
 import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.descriptors.IrBuiltIns
 import org.jetbrains.kotlin.ir.expressions.IrStatementOrigin
+import org.jetbrains.kotlin.ir.expressions.IrConst
 import org.jetbrains.kotlin.ir.symbols.IrClassSymbol
 import org.jetbrains.kotlin.ir.symbols.IrSymbol
 import org.jetbrains.kotlin.ir.symbols.IrTypeParameterSymbol
@@ -25,6 +26,7 @@ import org.jetbrains.kotlin.ir.types.*
 import org.jetbrains.kotlin.ir.types.impl.IrSimpleTypeImpl
 import org.jetbrains.kotlin.ir.types.impl.IrStarProjectionImpl
 import org.jetbrains.kotlin.ir.util.*
+import org.jetbrains.kotlin.resolve.DescriptorUtils
 import org.jetbrains.kotlin.resolve.jvm.annotations.JVM_DEFAULT_FQ_NAME
 
 /**
@@ -82,6 +84,12 @@ val IrType.erasedUpperBound: IrClass
         else -> throw IllegalStateException()
     }
 
+
+fun IrDeclaration.getJvmNameFromAnnotation(): String? {
+    val const = getAnnotation(DescriptorUtils.JVM_NAME)?.getValueArgument(0) as? IrConst<*>
+    return const?.value as? String
+}
+
 val IrFunction.propertyIfAccessor: IrDeclaration
     get() = (this as? IrSimpleFunction)?.correspondingPropertySymbol?.owner ?: this
 

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

@@ -0,0 +1,84 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.backend.jvm.lower
+
+import org.jetbrains.kotlin.backend.common.ClassLoweringPass
+import org.jetbrains.kotlin.backend.common.ir.allParameters
+import org.jetbrains.kotlin.backend.common.lower.createIrBuilder
+import org.jetbrains.kotlin.backend.jvm.JvmBackendContext
+import org.jetbrains.kotlin.backend.jvm.JvmLoweredDeclarationOrigin
+import org.jetbrains.kotlin.backend.jvm.ir.getJvmNameFromAnnotation
+import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.config.languageVersionSettings
+import org.jetbrains.kotlin.descriptors.Modality
+import org.jetbrains.kotlin.descriptors.Visibilities
+import org.jetbrains.kotlin.ir.builders.declarations.addFunction
+import org.jetbrains.kotlin.ir.builders.declarations.addValueParameter
+import org.jetbrains.kotlin.ir.builders.irBlockBody
+import org.jetbrains.kotlin.ir.builders.irCall
+import org.jetbrains.kotlin.ir.builders.irReturn
+import org.jetbrains.kotlin.ir.declarations.IrClass
+import org.jetbrains.kotlin.ir.declarations.IrDeclarationOrigin
+import org.jetbrains.kotlin.ir.declarations.IrSimpleFunction
+import org.jetbrains.kotlin.ir.expressions.impl.IrConstImpl
+import org.jetbrains.kotlin.ir.types.*
+import org.jetbrains.kotlin.ir.util.functions
+import org.jetbrains.kotlin.ir.util.getAnnotation
+import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.resolve.DescriptorUtils
+import org.jetbrains.kotlin.types.Variance
+
+internal class MainMethodGenerationLowering(val context: JvmBackendContext) : ClassLoweringPass {
+
+    override fun lower(irClass: IrClass) {
+        if (!context.configuration.languageVersionSettings.supportsFeature(LanguageFeature.ExtendedMainConvention)) return
+        if (irClass.origin != IrDeclarationOrigin.FILE_CLASS) return
+
+        val parameterlessMain = irClass.functions.find { it.isParameterlessMainMethod() } ?: return
+
+        if (irClass.functions.any { it.isMainMethod() }) return
+
+        generateMainMethod(irClass, parameterlessMain)
+    }
+
+    private fun generateMainMethod(irClass: IrClass, parameterlessMain: IrSimpleFunction) {
+        irClass.addFunction {
+            name = Name.identifier("main")
+            visibility = Visibilities.PUBLIC
+            returnType = context.irBuiltIns.unitType
+            modality = Modality.FINAL
+            origin = JvmLoweredDeclarationOrigin.GENERATED_MAIN_FOR_PARAMETERLESS_MAIN_METHOD
+        }.apply {
+            addValueParameter {
+                name = Name.identifier("args")
+                type = context.irBuiltIns.arrayClass.typeWith(context.irBuiltIns.stringType)
+            }
+            body = context.createIrBuilder(this.symbol).irBlockBody {
+                +irReturn(this.irCall(parameterlessMain))
+            }
+        }
+    }
+
+    private fun IrSimpleFunction.isParameterlessMainMethod(): Boolean =
+        typeParameters.isEmpty() && valueParameters.isEmpty() && returnType.isUnit() && name.asString() == "main"
+
+
+    private fun IrSimpleFunction.isMainMethod(): Boolean {
+        if (getJvmNameFromAnnotation() ?: name.asString() != "main") return false
+        if (!returnType.isUnit()) return false
+
+        val parameter = allParameters.singleOrNull() ?: return false
+        if (!parameter.type.isArray() && !parameter.type.isNullableArray()) return false
+
+        val argType = (parameter.type as IrSimpleType).arguments.first()
+        return when (argType) {
+            is IrTypeProjection -> {
+                (argType.variance != Variance.IN_VARIANCE) && argType.type.isStringClassType()
+            }
+            else -> false
+        }
+    }
+}