[FIR] Put new contract syntax under feauture flag

^KT-55171 Fixed

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt

@@ -56,6 +56,7 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
             FirTailrecFunctionChecker,
             FirMemberFunctionsChecker,
             FirDataObjectContentChecker,
+            ContractSyntaxV2FunctionChecker,
         )
 
     override val propertyCheckers: Set<FirPropertyChecker>
@@ -73,6 +74,7 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
             FirDelegateUsesExtensionPropertyTypeParameterChecker,
             FirTopLevelPropertiesChecker,
             FirLocalExtensionPropertyChecker,
+            ContractSyntaxV2PropertyChecker,
         )
 
     override val backingFieldCheckers: Set<FirBackingFieldChecker>

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/NiceContractSyntaxChecker.kt

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2010-2023 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.fir.analysis.checkers.declaration
+
+import org.jetbrains.kotlin.KtNodeTypes
+import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
+import org.jetbrains.kotlin.fir.declarations.FirContractDescriptionOwner
+import org.jetbrains.kotlin.fir.declarations.FirProperty
+import org.jetbrains.kotlin.fir.declarations.FirSimpleFunction
+
+object ContractSyntaxV2FunctionChecker : FirSimpleFunctionChecker() {
+    override fun check(declaration: FirSimpleFunction, context: CheckerContext, reporter: DiagnosticReporter) {
+        checkFeatureIsEnabled(declaration, context, reporter)
+    }
+}
+
+object ContractSyntaxV2PropertyChecker : FirPropertyChecker() {
+    override fun check(declaration: FirProperty, context: CheckerContext, reporter: DiagnosticReporter) {
+        declaration.getter?.let { checkFeatureIsEnabled(it, context, reporter) }
+        declaration.setter?.let { checkFeatureIsEnabled(it, context, reporter) }
+    }
+}
+
+private fun checkFeatureIsEnabled(declaration: FirContractDescriptionOwner, context: CheckerContext, reporter: DiagnosticReporter) {
+    val source = declaration.contractDescription.source ?: return
+    if (source.elementType != KtNodeTypes.CONTRACT_EFFECT_LIST) return
+    val languageVersionSettings = context.languageVersionSettings
+    if (!languageVersionSettings.supportsFeature(LanguageFeature.ContractSyntaxV2)) {
+        reporter.reportOn(source, FirErrors.UNSUPPORTED_FEATURE, LanguageFeature.ContractSyntaxV2 to languageVersionSettings, context)
+    }
+}