Diagnostics: add diagnostic for reporting contract description blocks in old frontend

compiler/frontend/src/org/jetbrains/kotlin/resolve/PlatformConfiguratorBase.kt

@@ -37,7 +37,8 @@ private val DEFAULT_DECLARATION_CHECKERS = listOf(
     TrailingCommaDeclarationChecker,
     MissingDependencySupertypeChecker.ForDeclarations,
     FunInterfaceDeclarationChecker(),
-    DeprecatedSinceKotlinAnnotationChecker
+    DeprecatedSinceKotlinAnnotationChecker,
+    ContractDescriptionBlockChecker
 )
 
 private val DEFAULT_CALL_CHECKERS = listOf(

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/ContractDescriptionBlockChecker.kt

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2010-2020 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.resolve.checkers
+
+import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
+import org.jetbrains.kotlin.diagnostics.Errors
+import org.jetbrains.kotlin.psi.KtDeclaration
+import org.jetbrains.kotlin.psi.KtNamedFunction
+import org.jetbrains.kotlin.psi.KtPropertyAccessor
+
+object ContractDescriptionBlockChecker: DeclarationChecker {
+    override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
+        val contractDescription = when (declaration) {
+            is KtNamedFunction -> declaration.contractDescription
+            is KtPropertyAccessor -> declaration.contractDescription
+            else -> null
+        }
+        if (contractDescription != null) {
+            context.trace.report(Errors.UNSUPPORTED.on(contractDescription, "Contract description blocks are not supported"))
+        }
+    }
+}