Fix UnusedSymbolInspection for parameterless and suspend main functions

#KT-26574 Fixed

idea/src/org/jetbrains/kotlin/idea/inspections/UnusedSymbolInspection.kt

@@ -46,6 +46,7 @@ import org.jetbrains.kotlin.idea.core.toDescriptor
 import org.jetbrains.kotlin.idea.findUsages.KotlinFindUsagesHandlerFactory
 import org.jetbrains.kotlin.idea.findUsages.handlers.KotlinFindClassUsagesHandler
 import org.jetbrains.kotlin.idea.imports.importableFqName
+import org.jetbrains.kotlin.idea.isMainFunction
 import org.jetbrains.kotlin.idea.project.languageVersionSettings
 import org.jetbrains.kotlin.idea.quickfix.RemoveUnusedFunctionParameterFix
 import org.jetbrains.kotlin.idea.references.mainReference
@@ -86,6 +87,12 @@ class UnusedSymbolInspection : AbstractKotlinInspection() {
         fun isEntryPoint(declaration: KtNamedDeclaration): Boolean {
             if (declaration.hasKotlinAdditionalAnnotation()) return true
             if (declaration is KtClass && declaration.declarations.any { it.hasKotlinAdditionalAnnotation() }) return true
+
+            // Some of the main-function-cases are covered by 'javaInspection.isEntryPoint(lightElement)' call
+            // but not all of them: light method for parameterless main still points to parameterless name
+            // that is not an actual entry point from Java language point of view
+            if (declaration.isMainFunction()) return true
+
             val lightElement: PsiElement? = when (declaration) {
                 is KtClassOrObject -> declaration.toLightClass()
                 is KtNamedFunction, is KtSecondaryConstructor -> LightClassUtil.getLightClassMethod(declaration as KtFunction)

idea/testData/inspections/unusedSymbol/functionMain/inspectionData/expected.xml

@@ -0,0 +1,11 @@
+<problems>
+  <problem>
+    <file>topLevelBothMains.kt</file>
+    <line>3</line>
+    <module>light_idea_test_case</module>
+    <package>&lt;default&gt;</package>
+    <entry_point TYPE="file" FQNAME="temp:///src/topLevelBothMains.kt" />
+    <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Unused symbol</problem_class>
+    <description>Function &quot;main&quot; is never used</description>
+  </problem>
+</problems>

idea/testData/inspections/unusedSymbol/functionMain/inspectionData/inspections.test

@@ -0,0 +1,2 @@
+// LANGUAGE_VERSION: 1.3
+// INSPECTION_CLASS: org.jetbrains.kotlin.idea.inspections.UnusedSymbolInspection

idea/testData/inspections/unusedSymbol/functionMain/topLevelBothMains.kt

@@ -0,0 +1,4 @@
+package a
+
+fun main() {}
+fun main(args: Array<String>) {}

idea/testData/inspections/unusedSymbol/functionMain/topLevelParameterlessMain.kt

@@ -0,0 +1,3 @@
+fun main() {
+
+}

idea/testData/inspections/unusedSymbol/functionMain/topLevelSuspendMain.kt

@@ -0,0 +1,3 @@
+suspend fun main(args: Array<String>) {
+
+}

idea/tests/org/jetbrains/kotlin/idea/codeInsight/AbstractInspectionTest.kt

@@ -25,7 +25,9 @@ import com.intellij.testFramework.TestLoggerFactory
 import org.jdom.Document
 import org.jdom.input.SAXBuilder
 import org.jetbrains.kotlin.idea.inspections.runInspection
-import org.jetbrains.kotlin.idea.test.*
+import org.jetbrains.kotlin.idea.test.KotlinLightCodeInsightFixtureTestCase
+import org.jetbrains.kotlin.idea.test.TestFixtureExtension
+import org.jetbrains.kotlin.idea.test.configureCompilerOptions
 import org.jetbrains.kotlin.idea.util.application.runWriteAction
 import org.jetbrains.kotlin.idea.versions.bundledRuntimeVersion
 import org.jetbrains.kotlin.test.InTextDirectivesUtils
@@ -68,6 +70,8 @@ abstract class AbstractInspectionTest : KotlinLightCodeInsightFixtureTestCase()
 
         val fixtureClasses = InTextDirectivesUtils.findListWithPrefixes(options, "// FIXTURE_CLASS: ")
 
+        configureCompilerOptions(options, project, module)
+
         val inspectionsTestDir = optionsFile.parentFile!!
         val srcDir = inspectionsTestDir.parentFile!!
 

idea/tests/org/jetbrains/kotlin/idea/codeInsight/InspectionTestGenerated.java

@@ -409,6 +409,11 @@ public class InspectionTestGenerated extends AbstractInspectionTest {
             runTest("idea/testData/inspections/unusedSymbol/function/inspectionData/inspections.test");
         }
 
+        @TestMetadata("unusedSymbol/functionMain/inspectionData/inspections.test")
+        public void testUnusedSymbol_functionMain_inspectionData_Inspections_test() throws Exception {
+            runTest("idea/testData/inspections/unusedSymbol/functionMain/inspectionData/inspections.test");
+        }
+
         @TestMetadata("unusedSymbol/js/inspectionData/inspections.test")
         public void testUnusedSymbol_js_inspectionData_Inspections_test() throws Exception {
             runTest("idea/testData/inspections/unusedSymbol/js/inspectionData/inspections.test");