Refactor logic of reporting errors in contracts

Rename 'doCheckContract' into 'parseContractAndReportErrors',
emphasizing that it works with definitely call to 'contract' from
stdlib.
In particular, it means that it should either return non-null
value or report some errors.

Note that this commit doesn't change behavior of this code (modulo cases
when something in 'parseContract' throws exception), but just refactors
the code to be more clear and easy to reason about.

compiler/frontend/src/org/jetbrains/kotlin/contracts/parsing/ContractParsingDiagnosticsCollector.kt

@@ -7,7 +7,6 @@ package org.jetbrains.kotlin.contracts.parsing
 
 import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.config.LanguageVersionSettings
-import org.jetbrains.kotlin.config.LanguageVersionSettingsImpl
 import org.jetbrains.kotlin.diagnostics.Diagnostic
 import org.jetbrains.kotlin.diagnostics.Errors
 import org.jetbrains.kotlin.psi.KtCallExpression
@@ -19,49 +18,52 @@ interface ContractParsingDiagnosticsCollector {
     fun unsupportedFeature(languageVersionSettings: LanguageVersionSettings)
     fun contractNotAllowed(message: String)
     fun badDescription(message: String, reportOn: KtElement)
+    fun addFallbackErrorIfNecessary()
 
-    fun flushDiagnostics(parsingFailed: Boolean)
+    fun flushDiagnostics()
     fun hasErrors(): Boolean
 
 
     object EMPTY : ContractParsingDiagnosticsCollector {
         override fun contractNotAllowed(message: String) {}
         override fun badDescription(message: String, reportOn: KtElement) {}
-        override fun unsupportedFeature(languageVersionSettings: LanguageVersionSettings) { }
+        override fun unsupportedFeature(languageVersionSettings: LanguageVersionSettings) {}
+        override fun addFallbackErrorIfNecessary() { }
 
-        override fun flushDiagnostics(parsingFailed: Boolean) {}
+        override fun flushDiagnostics() {}
 
         override fun hasErrors(): Boolean = false
     }
 }
 
 class TraceBasedCollector(private val bindingTrace: BindingTrace, mainCall: KtExpression) : ContractParsingDiagnosticsCollector {
-    private val diagnostics: MutableList<Diagnostic> = mutableListOf()
+    private val reportedErrors: MutableList<Diagnostic> = mutableListOf()
     private val mainCallReportTarget = (mainCall as? KtCallExpression)?.calleeExpression ?: mainCall
 
     override fun contractNotAllowed(message: String) {
-        diagnostics += Errors.CONTRACT_NOT_ALLOWED.on(mainCallReportTarget, message)
+        reportedErrors += Errors.CONTRACT_NOT_ALLOWED.on(mainCallReportTarget, message)
     }
 
     override fun badDescription(message: String, reportOn: KtElement) {
-        diagnostics += Errors.ERROR_IN_CONTRACT_DESCRIPTION.on(reportOn, message)
+        reportedErrors += Errors.ERROR_IN_CONTRACT_DESCRIPTION.on(reportOn, message)
     }
 
     override fun unsupportedFeature(languageVersionSettings: LanguageVersionSettings) {
-        diagnostics += Errors.UNSUPPORTED_FEATURE.on(
+        reportedErrors += Errors.UNSUPPORTED_FEATURE.on(
             mainCallReportTarget,
             LanguageFeature.AllowContractsForCustomFunctions to languageVersionSettings
         )
     }
 
-    override fun flushDiagnostics(parsingFailed: Boolean) {
-        if (parsingFailed && diagnostics.isEmpty()) {
-            diagnostics += Errors.ERROR_IN_CONTRACT_DESCRIPTION.on(mainCallReportTarget, "Error in contract description")
-        }
+    override fun addFallbackErrorIfNecessary() {
+        if (reportedErrors.isEmpty())
+            reportedErrors += Errors.ERROR_IN_CONTRACT_DESCRIPTION.on(mainCallReportTarget, "Error in contract description")
+    }
 
-        diagnostics.forEach { bindingTrace.report(it) }
+    override fun flushDiagnostics() {
+        reportedErrors.forEach { bindingTrace.report(it) }
     }
 
 
-    override fun hasErrors(): Boolean = diagnostics.isNotEmpty()
+    override fun hasErrors(): Boolean = reportedErrors.isNotEmpty()
 }

compiler/frontend/src/org/jetbrains/kotlin/contracts/parsing/ContractParsingServices.kt

@@ -48,16 +48,13 @@ class ContractParsingServices(val languageVersionSettings: LanguageVersionSettin
         // is a *necessary* (but not sufficient, actually) condition for presence of 'LazyContractProvider'
         if (!expression.isContractDescriptionCallPsiCheck()) return
 
-        val callContext = ContractCallContext(expression, isFirstStatement, scope, trace.bindingContext)
+        val callContext = ContractCallContext(expression, isFirstStatement, scope, trace)
         val contractProviderIfAny = (scope.ownerDescriptor as? FunctionDescriptor)?.getUserData(ContractProviderKey)
         var resultingContractDescription: ContractDescription? = null
 
         try {
             if (!callContext.isContractDescriptionCallPreciseCheck()) return
-
-            val collector = TraceBasedCollector(trace, expression)
-            resultingContractDescription = doCheckContract(collector, callContext)
-            collector.flushDiagnostics(parsingFailed = resultingContractDescription == null)
+            resultingContractDescription = parseContractAndReportErrors(callContext)
         } finally {
             contractProviderIfAny?.setContractDescription(resultingContractDescription)
         }
@@ -66,14 +63,32 @@ class ContractParsingServices(val languageVersionSettings: LanguageVersionSettin
     private fun ContractCallContext.isContractDescriptionCallPreciseCheck(): Boolean =
         contractCallExpression.isContractDescriptionCallPreciseCheck(bindingContext)
 
-    private fun doCheckContract(collector: ContractParsingDiagnosticsCollector, callContext: ContractCallContext): ContractDescription? {
-        checkFeatureEnabled(collector)
-        checkContractAllowedHere(collector, callContext)
+    /**
+     * This function deals with some call that is guaranteed to resolve to 'contract' from stdlib, so,
+     * ideally, it should satisfy following condition: null returned <=> at least one error was reported
+     */
+    private fun parseContractAndReportErrors(callContext: ContractCallContext): ContractDescription? {
+        val collector = TraceBasedCollector(callContext.trace, callContext.contractCallExpression)
 
-        return if (!collector.hasErrors())
-            PsiContractParserDispatcher(collector, callContext).parseContract()
-        else
-            null
+        try {
+            checkFeatureEnabled(collector)
+            checkContractAllowedHere(collector, callContext)
+
+            // Small optimization: do not even try to parse contract if we already have errors
+            if (collector.hasErrors()) return null
+
+            val parsedContract = PsiContractParserDispatcher(collector, callContext).parseContract()
+
+            // Make sure that at least generic error will be reported if we couldn't parse contract
+            // (null returned => at least one error was reported)
+            if (parsedContract == null) collector.addFallbackErrorIfNecessary()
+
+            // Make sure that we don't return non-null value if there were some errors
+            // (null returned <= at least one error was reported)
+            return parsedContract?.takeUnless { collector.hasErrors() }
+        } finally {
+            collector.flushDiagnostics()
+        }
     }
 
     private fun checkFeatureEnabled(collector: ContractParsingDiagnosticsCollector) {
@@ -117,8 +132,9 @@ class ContractCallContext(
     val contractCallExpression: KtExpression,
     val isFirstStatement: Boolean,
     val scope: LexicalScope,
-    val bindingContext: BindingContext
+    val trace: BindingTrace
 ) {
     val ownerDescriptor: DeclarationDescriptor = scope.ownerDescriptor
     val functionDescriptor: FunctionDescriptor = ownerDescriptor as FunctionDescriptor
+    val bindingContext: BindingContext = trace.bindingContext
 }