FIR: add OptIn checkers draft + some annotation / SinceKotlin utilities

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/bad/returnsImplies/notNull.kt

@@ -1,3 +1,4 @@
+// !USE_EXPERIMENTAL: kotlin.RequiresOptIn
 import kotlin.contracts.*
 
 @OptIn(ExperimentalContracts::class)

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/bad/returnsImplies/propertyGetter.kt

@@ -1,3 +1,4 @@
+// !USE_EXPERIMENTAL: kotlin.RequiresOptIn
 import kotlin.contracts.*
 
 @OptIn(ExperimentalContracts::class)

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/good/returnsImplies/conditionLogic.kt

@@ -1,3 +1,4 @@
+// !USE_EXPERIMENTAL: kotlin.RequiresOptIn
 import kotlin.contracts.*
 
 @OptIn(ExperimentalContracts::class)

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/good/returnsImplies/inapplicable.kt

@@ -1,3 +1,4 @@
+// !USE_EXPERIMENTAL: kotlin.RequiresOptIn
 import kotlin.contracts.*
 
 @OptIn(ExperimentalContracts::class)

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/good/returnsImplies/notNull.kt

@@ -1,3 +1,4 @@
+// !USE_EXPERIMENTAL: kotlin.RequiresOptIn
 import kotlin.contracts.*
 
 @OptIn(ExperimentalContracts::class)

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/good/returnsImplies/safeCall.kt

@@ -1,3 +1,4 @@
+// !USE_EXPERIMENTAL: kotlin.RequiresOptIn
 import kotlin.contracts.*
 
 @OptIn(ExperimentalContracts::class)

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/good/returnsImplies/trickyCases.kt

@@ -1,3 +1,4 @@
+// !USE_EXPERIMENTAL: kotlin.RequiresOptIn
 import kotlin.contracts.*
 
 @OptIn(ExperimentalContracts::class)

compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt

@@ -23,6 +23,7 @@ import org.jetbrains.kotlin.fir.symbols.impl.*
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.lexer.KtKeywordToken
 import org.jetbrains.kotlin.lexer.KtModifierKeywordToken
+import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.resolve.ForbiddenNamedArgumentsTarget
@@ -212,6 +213,36 @@ object DIAGNOSTICS_LIST : DiagnosticList("FirErrors") {
         val INAPPLICABLE_FILE_TARGET by error<KtAnnotationEntry>(PositioningStrategy.ANNOTATION_USE_SITE)
     }
 
+    val EXPERIMENTAL by object : DiagnosticGroup("OptIn-related") {
+        val EXPERIMENTAL_API_USAGE by warning<PsiElement>(PositioningStrategy.REFERENCE_BY_QUALIFIED) {
+            parameter<FqName>("optInMarkerFqName")
+            parameter<String>("message")
+        }
+        val EXPERIMENTAL_API_USAGE_ERROR by error<PsiElement>(PositioningStrategy.REFERENCE_BY_QUALIFIED) {
+            parameter<FqName>("optInMarkerFqName")
+            parameter<String>("message")
+        }
+        val EXPERIMENTAL_OVERRIDE by warning<PsiElement> {
+            parameter<FqName>("optInMarkerFqName")
+            parameter<String>("message")
+        }
+        val EXPERIMENTAL_OVERRIDE_ERROR by error<PsiElement> {
+            parameter<FqName>("optInMarkerFqName")
+            parameter<String>("message")
+        }
+        val EXPERIMENTAL_IS_NOT_ENABLED by warning<KtAnnotationEntry>(PositioningStrategy.REFERENCED_NAME_BY_QUALIFIED)
+        val EXPERIMENTAL_CAN_ONLY_BE_USED_AS_ANNOTATION by error<PsiElement>()
+        val EXPERIMENTAL_MARKER_CAN_ONLY_BE_USED_AS_ANNOTATION_OR_ARGUMENT_IN_USE_EXPERIMENTAL by error<PsiElement>()
+        val USE_EXPERIMENTAL_WITHOUT_ARGUMENTS by warning<KtAnnotationEntry>()
+        val USE_EXPERIMENTAL_ARGUMENT_IS_NOT_MARKER by warning<KtAnnotationEntry> {
+            parameter<FqName>("notMarkerFqName")
+        }
+        val EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET by error<KtAnnotationEntry> {
+            parameter<String>("target")
+        }
+        val EXPERIMENTAL_ANNOTATION_WITH_WRONG_RETENTION by error<KtAnnotationEntry>()
+    }
+
     val EXPOSED_VISIBILITY by object : DiagnosticGroup("Exposed visibility") {
         val EXPOSED_TYPEALIAS_EXPANDED_TYPE by exposedVisibilityError<KtNamedDeclaration>(PositioningStrategy.DECLARATION_NAME)
         val EXPOSED_FUNCTION_RETURN_TYPE by exposedVisibilityError<KtNamedDeclaration>(PositioningStrategy.DECLARATION_NAME)

compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt

@@ -34,6 +34,7 @@ import org.jetbrains.kotlin.fir.symbols.impl.FirVariableSymbol
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.lexer.KtKeywordToken
 import org.jetbrains.kotlin.lexer.KtModifierKeywordToken
+import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.KtAnnotation
 import org.jetbrains.kotlin.psi.KtAnnotationEntry
@@ -200,6 +201,19 @@ object FirErrors {
     val REDUNDANT_ANNOTATION_TARGET by warning1<KtAnnotationEntry, String>()
     val INAPPLICABLE_FILE_TARGET by error0<KtAnnotationEntry>(SourceElementPositioningStrategies.ANNOTATION_USE_SITE)
 
+    // OptIn-related
+    val EXPERIMENTAL_API_USAGE by warning2<PsiElement, FqName, String>(SourceElementPositioningStrategies.REFERENCE_BY_QUALIFIED)
+    val EXPERIMENTAL_API_USAGE_ERROR by error2<PsiElement, FqName, String>(SourceElementPositioningStrategies.REFERENCE_BY_QUALIFIED)
+    val EXPERIMENTAL_OVERRIDE by warning2<PsiElement, FqName, String>()
+    val EXPERIMENTAL_OVERRIDE_ERROR by error2<PsiElement, FqName, String>()
+    val EXPERIMENTAL_IS_NOT_ENABLED by warning0<KtAnnotationEntry>(SourceElementPositioningStrategies.REFERENCED_NAME_BY_QUALIFIED)
+    val EXPERIMENTAL_CAN_ONLY_BE_USED_AS_ANNOTATION by error0<PsiElement>()
+    val EXPERIMENTAL_MARKER_CAN_ONLY_BE_USED_AS_ANNOTATION_OR_ARGUMENT_IN_USE_EXPERIMENTAL by error0<PsiElement>()
+    val USE_EXPERIMENTAL_WITHOUT_ARGUMENTS by warning0<KtAnnotationEntry>()
+    val USE_EXPERIMENTAL_ARGUMENT_IS_NOT_MARKER by warning1<KtAnnotationEntry, FqName>()
+    val EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET by error1<KtAnnotationEntry, String>()
+    val EXPERIMENTAL_ANNOTATION_WITH_WRONG_RETENTION by error0<KtAnnotationEntry>()
+
     // Exposed visibility
     val EXPOSED_TYPEALIAS_EXPANDED_TYPE by error3<KtNamedDeclaration, EffectiveVisibility, FirMemberDeclaration, EffectiveVisibility>(SourceElementPositioningStrategies.DECLARATION_NAME)
     val EXPOSED_FUNCTION_RETURN_TYPE by error3<KtNamedDeclaration, EffectiveVisibility, FirMemberDeclaration, EffectiveVisibility>(SourceElementPositioningStrategies.DECLARATION_NAME)

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

@@ -71,6 +71,7 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
     override val regularClassCheckers: Set<FirRegularClassChecker>
         get() = setOf(
             FirAnnotationClassDeclarationChecker,
+            FirOptInAnnotationClassChecker,
             FirCommonConstructorDelegationIssuesChecker,
             FirConstructorInInterfaceChecker,
             FirDelegationSuperCallInEnumConstructorChecker,

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

@@ -12,6 +12,7 @@ object CommonExpressionCheckers : ExpressionCheckers() {
     override val annotationCallCheckers: Set<FirAnnotationCallChecker>
         get() = setOf(
             FirAnnotationArgumentChecker,
+            FirOptInAnnotationCallChecker,
         )
 
     override val basicExpressionCheckers: Set<FirBasicExpressionChecker>

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

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.fir.analysis.checkers
 
 import org.jetbrains.kotlin.builtins.StandardNames
 import org.jetbrains.kotlin.descriptors.annotations.KotlinTarget
+import org.jetbrains.kotlin.fir.FirAnnotationContainer
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.declarations.FirAnnotatedDeclaration
 import org.jetbrains.kotlin.fir.declarations.FirRegularClass
@@ -14,6 +15,7 @@ import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.references.FirResolvedNamedReference
 import org.jetbrains.kotlin.fir.resolve.fullyExpandedType
 import org.jetbrains.kotlin.fir.resolve.toSymbol
+import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
 import org.jetbrains.kotlin.fir.types.ConeClassLikeType
 import org.jetbrains.kotlin.fir.types.FirErrorTypeRef
 import org.jetbrains.kotlin.fir.types.coneType
@@ -49,11 +51,7 @@ fun FirAnnotationCall.getAllowedAnnotationTargets(session: FirSession): Set<Kotl
 fun FirRegularClass.getAllowedAnnotationTargets(): Set<KotlinTarget> {
     val targetAnnotation = getTargetAnnotation() ?: return defaultAnnotationTargets
     if (targetAnnotation.argumentList.arguments.isEmpty()) return emptySet()
-    val arguments = when (val targetArgument = targetAnnotation.findSingleArgumentByName(TARGET_PARAMETER_NAME)) {
-        is FirVarargArgumentsExpression -> targetArgument.arguments
-        is FirArrayOfCall -> targetArgument.arguments
-        else -> return defaultAnnotationTargets
-    }
+    val arguments = targetAnnotation.findSingleArgumentByName(TARGET_PARAMETER_NAME)?.unfoldArrayOrVararg().orEmpty()
 
     return arguments.mapNotNullTo(mutableSetOf()) { argument ->
         val targetExpression = argument as? FirQualifiedAccessExpression
@@ -70,7 +68,7 @@ fun FirAnnotatedDeclaration.getTargetAnnotation(): FirAnnotationCall? {
     return getAnnotationByFqName(StandardNames.FqNames.target)
 }
 
-fun FirAnnotatedDeclaration.getAnnotationByFqName(fqName: FqName): FirAnnotationCall? {
+fun FirAnnotationContainer.getAnnotationByFqName(fqName: FqName): FirAnnotationCall? {
     return annotations.find {
         (it.annotationTypeRef.coneType as? ConeClassLikeType)?.lookupTag?.classId?.asSingleFqName() == fqName
     }
@@ -79,7 +77,7 @@ fun FirAnnotatedDeclaration.getAnnotationByFqName(fqName: FqName): FirAnnotation
 fun FirAnnotationCall.findSingleArgumentByName(name: Name): FirExpression? {
     val argumentMapping = argumentMapping
     if (argumentMapping != null) {
-        return argumentMapping.keys.firstOrNull()?.takeIf { argumentMapping[it]?.name == name }
+        return argumentMapping.keys.firstOrNull()?.takeIf { argumentMapping[it]?.name == name }?.unwrapArgument()
     }
     // NB: we have to consider both cases, because deserializer does not create argument mapping
     val arguments = argumentList.arguments
@@ -87,3 +85,19 @@ fun FirAnnotationCall.findSingleArgumentByName(name: Name): FirExpression? {
     return firstArgument.takeIf { it.name == name }?.expression
 }
 
+fun FirExpression.extractClassesFromArgument(): List<FirRegularClassSymbol> {
+    return unfoldArrayOrVararg().mapNotNull {
+        if (it !is FirGetClassCall) return@mapNotNull null
+        val qualifier = it.argument as? FirResolvedQualifier ?: return@mapNotNull null
+        qualifier.symbol as? FirRegularClassSymbol
+    }
+}
+
+private fun FirExpression.unfoldArrayOrVararg(): List<FirExpression> {
+    return when (this) {
+        is FirVarargArgumentsExpression -> arguments
+        is FirArrayOfCall -> arguments
+        else -> return emptyList()
+    }
+}
+

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

@@ -0,0 +1,98 @@
+/*
+ * Copyright 2010-2021 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
+
+import org.jetbrains.kotlin.config.ApiVersion
+import org.jetbrains.kotlin.fir.FirSession
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.declarations.FirAnnotatedDeclaration
+import org.jetbrains.kotlin.fir.declarations.FirConstructor
+import org.jetbrains.kotlin.fir.declarations.FirTypeAlias
+import org.jetbrains.kotlin.fir.expressions.FirConstExpression
+import org.jetbrains.kotlin.fir.expressions.arguments
+import org.jetbrains.kotlin.fir.languageVersionSettings
+import org.jetbrains.kotlin.fir.resolve.symbolProvider
+import org.jetbrains.kotlin.fir.resolve.toSymbol
+import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
+import org.jetbrains.kotlin.fir.types.ConeClassLikeType
+import org.jetbrains.kotlin.fir.types.coneType
+import org.jetbrains.kotlin.resolve.SINCE_KOTLIN_FQ_NAME
+import org.jetbrains.kotlin.resolve.checkers.OptInNames
+
+sealed class FirSinceKotlinAccessibility {
+    object Accessible : FirSinceKotlinAccessibility()
+
+    data class NotAccessibleButWasExperimental(
+        val version: ApiVersion,
+        val markerClasses: List<FirRegularClassSymbol>
+    ) : FirSinceKotlinAccessibility()
+
+    data class NotAccessible(
+        val version: ApiVersion
+    ) : FirSinceKotlinAccessibility()
+}
+
+private data class FirSinceKotlinValue(
+    val apiVersion: ApiVersion,
+    val wasExperimentalMarkerClasses: List<FirRegularClassSymbol>
+)
+
+fun FirAnnotatedDeclaration.checkSinceKotlinVersionAccessibility(context: CheckerContext): FirSinceKotlinAccessibility {
+    val value = getOwnSinceKotlinVersion(context.session)
+    val version = value?.apiVersion
+    val languageVersionSettings = context.session.languageVersionSettings
+
+    // Allow access in the following cases:
+    // 1) There's no @SinceKotlin annotation for this descriptor
+    // 2) There's a @SinceKotlin annotation but its value is some unrecognizable nonsense
+    // 3) The value as a version is not greater than our API version
+    if (version == null || version <= languageVersionSettings.apiVersion) return FirSinceKotlinAccessibility.Accessible
+
+    val wasExperimentalFqNames = value.wasExperimentalMarkerClasses
+    if (wasExperimentalFqNames.isNotEmpty()) {
+        return FirSinceKotlinAccessibility.NotAccessibleButWasExperimental(version, wasExperimentalFqNames)
+    }
+
+    return FirSinceKotlinAccessibility.NotAccessible(version)
+}
+
+private fun FirAnnotatedDeclaration.getOwnSinceKotlinVersion(session: FirSession): FirSinceKotlinValue? {
+    var result: FirSinceKotlinValue? = null
+
+    // TODO: use-site targeted annotations
+    fun FirAnnotatedDeclaration.consider() {
+        val sinceKotlinSingleArgument = getAnnotationByFqName(SINCE_KOTLIN_FQ_NAME)?.arguments?.singleOrNull()
+        val apiVersion = ((sinceKotlinSingleArgument as? FirConstExpression<*>)?.value as? String)?.let(ApiVersion.Companion::parse)
+        if (apiVersion != null) {
+            // TODO: combine wasExperimentalMarkerClasses in case of several associated declarations with the same maximal API version
+            if (result == null || apiVersion > result!!.apiVersion) {
+                result = FirSinceKotlinValue(apiVersion, loadWasExperimentalMarkerClasses())
+            }
+        }
+    }
+
+    this.consider()
+    if (this is FirConstructor) {
+        val classId = symbol.callableId.classId
+        if (classId != null) {
+            val classSymbol = session.symbolProvider.getClassLikeSymbolByFqName(classId)
+            classSymbol?.fir?.consider()
+        }
+    }
+
+    if (this is FirTypeAlias) {
+        (this.expandedTypeRef.coneType as? ConeClassLikeType)?.lookupTag?.toSymbol(session)?.fir?.consider()
+    }
+
+    return result
+}
+
+private fun FirAnnotatedDeclaration.loadWasExperimentalMarkerClasses(): List<FirRegularClassSymbol> {
+    val wasExperimental = getAnnotationByFqName(OptInNames.WAS_EXPERIMENTAL_FQ_NAME) ?: return emptyList()
+    val annotationClasses = wasExperimental.findSingleArgumentByName(OptInNames.WAS_EXPERIMENTAL_ANNOTATION_CLASS) ?: return emptyList()
+    return annotationClasses.extractClassesFromArgument()
+}
+

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

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2010-2021 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.descriptors.ClassKind
+import org.jetbrains.kotlin.descriptors.annotations.KotlinTarget
+import org.jetbrains.kotlin.fir.analysis.checkers.*
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
+import org.jetbrains.kotlin.fir.analysis.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.declarations.FirRegularClass
+import org.jetbrains.kotlin.resolve.checkers.Experimentality
+import org.jetbrains.kotlin.resolve.checkers.OptInNames
+
+object FirOptInAnnotationClassChecker : FirRegularClassChecker() {
+    override fun check(declaration: FirRegularClass, context: CheckerContext, reporter: DiagnosticReporter) {
+        if (declaration.classKind != ClassKind.ANNOTATION_CLASS) return
+        if (declaration.getAnnotationByFqName(OptInNames.REQUIRES_OPT_IN_FQ_NAME) == null) return
+        if (declaration.getRetention() == AnnotationRetention.SOURCE) {
+            val target = declaration.getRetentionAnnotation()
+            reporter.reportOn(target?.source, FirErrors.EXPERIMENTAL_ANNOTATION_WITH_WRONG_RETENTION, context)
+
+        }
+        val wrongTargets = declaration.getAllowedAnnotationTargets().intersect(Experimentality.WRONG_TARGETS_FOR_MARKER)
+        if (wrongTargets.isNotEmpty()) {
+            val target = declaration.getTargetAnnotation()
+            reporter.reportOn(
+                target?.source,
+                FirErrors.EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET,
+                wrongTargets.joinToString(transform = KotlinTarget::description),
+                context
+            )
+        }
+    }
+}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirClassLiteralChecker.kt

@@ -20,11 +20,19 @@ import org.jetbrains.kotlin.fir.scopes.impl.toConeType
 import org.jetbrains.kotlin.fir.symbols.impl.FirTypeParameterSymbol
 import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.lexer.KtTokens.QUEST
+import org.jetbrains.kotlin.resolve.checkers.OptInNames
 
 object FirClassLiteralChecker : FirGetClassCallChecker() {
     override fun check(expression: FirGetClassCall, context: CheckerContext, reporter: DiagnosticReporter) {
         val source = expression.source ?: return
         if (source.kind is FirFakeSourceElementKind) return
+        val argument = expression.argument
+        if (argument is FirResolvedQualifier) {
+            val fqName = argument.classId?.asSingleFqName()
+            if (fqName in OptInNames.EXPERIMENTAL_FQ_NAMES || fqName in OptInNames.USE_EXPERIMENTAL_FQ_NAMES) {
+                reporter.reportOn(argument.source, FirErrors.EXPERIMENTAL_CAN_ONLY_BE_USED_AS_ANNOTATION, context)
+            }
+        }
 
         // Note that raw FIR drops marked nullability "?" in, e.g., `A?::class`, `A<T?>::class`, or `A<T?>?::class`.
         // That is, AST structures for those expressions have token type QUEST, whereas FIR element doesn't have any information about it.
@@ -36,7 +44,6 @@ object FirClassLiteralChecker : FirGetClassCallChecker() {
         //
         // Only the 2nd example is valid, and we want to check if token type QUEST doesn't exist at the same level as COLONCOLON.
         val markedNullable = source.getChild(QUEST, depth = 1) != null
-        val argument = expression.argument
         val isNullable = markedNullable ||
                 (argument as? FirResolvedQualifier)?.isNullableLHSForCallableReference == true ||
                 argument.typeRef.coneType.isMarkedNullable ||

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirOptInAnnotationCallChecker.kt

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2010-2021 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.expression
+
+import org.jetbrains.kotlin.config.AnalysisFlags
+import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.extractClassesFromArgument
+import org.jetbrains.kotlin.fir.analysis.checkers.findSingleArgumentByName
+import org.jetbrains.kotlin.fir.analysis.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
+import org.jetbrains.kotlin.fir.analysis.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.expressions.*
+import org.jetbrains.kotlin.fir.languageVersionSettings
+import org.jetbrains.kotlin.fir.types.ConeClassLikeType
+import org.jetbrains.kotlin.fir.types.coneTypeSafe
+import org.jetbrains.kotlin.resolve.checkers.OptInNames
+
+object FirOptInAnnotationCallChecker : FirAnnotationCallChecker() {
+    override fun check(expression: FirAnnotationCall, context: CheckerContext, reporter: DiagnosticReporter) {
+        val lookupTag = expression.annotationTypeRef.coneTypeSafe<ConeClassLikeType>()?.lookupTag ?: return
+        val fqName = lookupTag.classId.asSingleFqName()
+        val isMarker = fqName == OptInNames.REQUIRES_OPT_IN_FQ_NAME
+        val isOptIn = fqName == OptInNames.OPT_IN_FQ_NAME
+        if (isMarker || isOptIn) {
+            checkUsageOfKotlinExperimentalOrUseExperimental(expression.source, context, reporter)
+            if (isOptIn) {
+                val arguments = expression.arguments
+                if (arguments.isEmpty()) {
+                    reporter.reportOn(expression.source, FirErrors.USE_EXPERIMENTAL_WITHOUT_ARGUMENTS, context)
+                } else {
+                    val annotationClasses = expression.findSingleArgumentByName(OptInNames.USE_EXPERIMENTAL_ANNOTATION_CLASS)
+                    for (classSymbol in annotationClasses?.extractClassesFromArgument().orEmpty()) {
+                        with(FirOptInUsageBaseChecker) {
+                            if (classSymbol.fir.loadExperimentalityForMarkerAnnotation() == null) {
+                                reporter.reportOn(
+                                    expression.source,
+                                    FirErrors.USE_EXPERIMENTAL_ARGUMENT_IS_NOT_MARKER,
+                                    classSymbol.classId.asSingleFqName(),
+                                    context
+                                )
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    private fun checkUsageOfKotlinExperimentalOrUseExperimental(
+        element: FirSourceElement?,
+        context: CheckerContext,
+        reporter: DiagnosticReporter
+    ) {
+        val useExperimentalFqNames = context.session.languageVersionSettings.getFlag(AnalysisFlags.useExperimental)
+        if (OptInNames.REQUIRES_OPT_IN_FQ_NAME.asString() !in useExperimentalFqNames) {
+            reporter.reportOn(element, FirErrors.EXPERIMENTAL_IS_NOT_ENABLED, context)
+        }
+    }
+}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirOptInUsageBaseChecker.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2010-2021 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.expression
+
+import org.jetbrains.kotlin.fir.analysis.checkers.findSingleArgumentByName
+import org.jetbrains.kotlin.fir.analysis.checkers.getAnnotationByFqName
+import org.jetbrains.kotlin.fir.declarations.FirRegularClass
+import org.jetbrains.kotlin.fir.expressions.FirConstExpression
+import org.jetbrains.kotlin.fir.expressions.FirQualifiedAccessExpression
+import org.jetbrains.kotlin.fir.references.FirResolvedNamedReference
+import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.resolve.checkers.Experimentality
+import org.jetbrains.kotlin.resolve.checkers.OptInNames
+
+object FirOptInUsageBaseChecker {
+    internal fun FirRegularClass.loadExperimentalityForMarkerAnnotation(): Experimentality? {
+        val experimental = getAnnotationByFqName(OptInNames.REQUIRES_OPT_IN_FQ_NAME) ?: return null
+
+        val levelArgument = experimental.findSingleArgumentByName(LEVEL) as? FirQualifiedAccessExpression
+        val levelName = (levelArgument?.calleeReference as? FirResolvedNamedReference)?.name?.asString()
+        val level = OptInLevel.values().firstOrNull { it.name == levelName } ?: OptInLevel.DEFAULT
+        val message = (experimental.findSingleArgumentByName(MESSAGE) as? FirConstExpression<*>)?.value as? String
+        return Experimentality(symbol.classId.asSingleFqName(), level.severity, message)
+    }
+
+    private val LEVEL = Name.identifier("level")
+    private val MESSAGE = Name.identifier("message")
+
+    private enum class OptInLevel(val severity: Experimentality.Severity) {
+        WARNING(Experimentality.Severity.WARNING),
+        ERROR(Experimentality.Severity.ERROR),
+        DEFAULT(Experimentality.DEFAULT_SEVERITY)
+    }
+}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.fir.analysis.diagnostics
 
 import org.jetbrains.kotlin.diagnostics.rendering.*
 import org.jetbrains.kotlin.diagnostics.rendering.Renderers.RENDER_POSITION_VARIANCE
+import org.jetbrains.kotlin.diagnostics.rendering.Renderers.STRING
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirDiagnosticRenderers.AMBIGUOUS_CALLS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirDiagnosticRenderers.DECLARATION_NAME
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirDiagnosticRenderers.FIR
@@ -117,6 +118,15 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPECTED_DELEGATE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPECTED_LATEINIT_PROPERTY
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPECTED_PRIVATE_DECLARATION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPECTED_PROPERTY_INITIALIZER
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPERIMENTAL_ANNOTATION_WITH_WRONG_RETENTION
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPERIMENTAL_API_USAGE
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPERIMENTAL_API_USAGE_ERROR
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPERIMENTAL_CAN_ONLY_BE_USED_AS_ANNOTATION
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPERIMENTAL_IS_NOT_ENABLED
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPERIMENTAL_MARKER_CAN_ONLY_BE_USED_AS_ANNOTATION_OR_ARGUMENT_IN_USE_EXPERIMENTAL
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPERIMENTAL_OVERRIDE
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPERIMENTAL_OVERRIDE_ERROR
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPLICIT_DELEGATION_CALL_REQUIRED
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPOSED_FUNCTION_RETURN_TYPE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPOSED_PARAMETER_TYPE
@@ -336,6 +346,8 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.USELESS_ELVIS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.USELESS_ELVIS_RIGHT_IS_NULL
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.USELESS_IS_CHECK
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.USELESS_VARARG_ON_PARAMETER
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.USE_EXPERIMENTAL_ARGUMENT_IS_NOT_MARKER
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.USE_EXPERIMENTAL_WITHOUT_ARGUMENTS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VALUE_CLASS_CANNOT_BE_CLONEABLE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VALUE_PARAMETER_WITH_NO_TYPE_ANNOTATION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VAL_OR_VAR_ON_CATCH_PARAMETER
@@ -363,7 +375,6 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.WRONG_MODIFIER_TA
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.WRONG_NUMBER_OF_TYPE_ARGUMENTS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.WRONG_SETTER_PARAMETER_TYPE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.WRONG_SETTER_RETURN_TYPE
-import org.jetbrains.kotlin.resolve.VarianceConflictDiagnosticData
 
 @Suppress("unused")
 class FirDefaultErrorMessages : DefaultErrorMessages.Extension {
@@ -399,10 +410,10 @@ class FirDefaultErrorMessages : DefaultErrorMessages.Extension {
             map.put(VARIABLE_EXPECTED, "Variable expected")
             map.put(DELEGATION_IN_INTERFACE, "Interfaces cannot use delegation")
             map.put(NESTED_CLASS_NOT_ALLOWED, "{0} is not allowed here", TO_STRING)
-            map.put(VAL_OR_VAR_ON_LOOP_PARAMETER, "''{0}'' on loop parameter is not allowed", TO_STRING);
-            map.put(VAL_OR_VAR_ON_FUN_PARAMETER, "''{0}'' on function parameter is not allowed", TO_STRING);
-            map.put(VAL_OR_VAR_ON_CATCH_PARAMETER, "''{0}'' on catch parameter is not allowed", TO_STRING);
-            map.put(VAL_OR_VAR_ON_SECONDARY_CONSTRUCTOR_PARAMETER, "''{0}'' on secondary constructor parameter is not allowed", TO_STRING);
+            map.put(VAL_OR_VAR_ON_LOOP_PARAMETER, "''{0}'' on loop parameter is not allowed", TO_STRING)
+            map.put(VAL_OR_VAR_ON_FUN_PARAMETER, "''{0}'' on function parameter is not allowed", TO_STRING)
+            map.put(VAL_OR_VAR_ON_CATCH_PARAMETER, "''{0}'' on catch parameter is not allowed", TO_STRING)
+            map.put(VAL_OR_VAR_ON_SECONDARY_CONSTRUCTOR_PARAMETER, "''{0}'' on secondary constructor parameter is not allowed", TO_STRING)
 
             // Unresolved
             map.put(INVISIBLE_REFERENCE, "Symbol {0} is invisible", SYMBOL)
@@ -528,6 +539,36 @@ class FirDefaultErrorMessages : DefaultErrorMessages.Extension {
                 TO_STRING
             )
 
+            // OptIn
+            map.put(EXPERIMENTAL_API_USAGE, "{1}", TO_STRING, STRING)
+            map.put(EXPERIMENTAL_API_USAGE_ERROR, "{1}", TO_STRING, STRING)
+
+            map.put(EXPERIMENTAL_OVERRIDE, "{1}", TO_STRING, STRING)
+            map.put(EXPERIMENTAL_OVERRIDE_ERROR, "{1}", TO_STRING, STRING)
+
+            map.put(EXPERIMENTAL_IS_NOT_ENABLED, "This class can only be used with the compiler argument '-Xopt-in=kotlin.RequiresOptIn'")
+            map.put(EXPERIMENTAL_CAN_ONLY_BE_USED_AS_ANNOTATION, "This class can only be used as an annotation")
+            map.put(
+                EXPERIMENTAL_MARKER_CAN_ONLY_BE_USED_AS_ANNOTATION_OR_ARGUMENT_IN_USE_EXPERIMENTAL,
+                "This class can only be used as an annotation or as an argument to @OptIn"
+            )
+            map.put(USE_EXPERIMENTAL_WITHOUT_ARGUMENTS, "@OptIn without any arguments has no effect")
+            map.put(
+                USE_EXPERIMENTAL_ARGUMENT_IS_NOT_MARKER,
+                "Annotation ''{0}'' is not an opt-in requirement marker, therefore its usage in @OptIn is ignored",
+                TO_STRING
+            )
+            map.put(
+                EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET,
+                "Opt-in requirement marker annotation cannot be used on the following code elements: {0}. Please remove these targets",
+                STRING
+            )
+            map.put(
+                EXPERIMENTAL_ANNOTATION_WITH_WRONG_RETENTION,
+                "Opt-in requirement marker annotation cannot be used with SOURCE retention. Please replace retention with BINARY"
+            )
+
+
             // Exposed visibility group // #
             map.put(
                 EXPOSED_TYPEALIAS_EXPANDED_TYPE,
@@ -585,7 +626,13 @@ class FirDefaultErrorMessages : DefaultErrorMessages.Extension {
             map.put(TYPE_MISMATCH, "Type mismatch: inferred type is {1} but {0} was expected", TO_STRING, TO_STRING)
             map.put(THROWABLE_TYPE_MISMATCH, "Throwable type mismatch: actual type is {0}", TO_STRING)
             map.put(CONDITION_TYPE_MISMATCH, "Condition type mismatch: inferred type is {0} but Boolean was expected", TO_STRING)
-            map.put(ARGUMENT_TYPE_MISMATCH, "Argument type mismatch: actual type is {1} but {0} was expected", RENDER_TYPE, RENDER_TYPE, NOT_RENDERED)
+            map.put(
+                ARGUMENT_TYPE_MISMATCH,
+                "Argument type mismatch: actual type is {1} but {0} was expected",
+                RENDER_TYPE,
+                RENDER_TYPE,
+                NOT_RENDERED
+            )
             map.put(ASSIGNMENT_TYPE_MISMATCH, "Assignment type mismatch: actual type is {1} but {0} was expected", RENDER_TYPE, RENDER_TYPE)
             map.put(
                 RESULT_TYPE_MISMATCH,

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

@@ -33,15 +33,6 @@ import org.jetbrains.kotlin.resolve.descriptorUtil.getAnnotationRetention
 import org.jetbrains.kotlin.utils.addToStdlib.safeAs
 
 class ExperimentalMarkerDeclarationAnnotationChecker(private val module: ModuleDescriptor) : AdditionalAnnotationChecker {
-    companion object {
-        private val WRONG_TARGETS_FOR_MARKER = setOf(
-            KotlinTarget.EXPRESSION,
-            KotlinTarget.FILE,
-            KotlinTarget.TYPE,
-            KotlinTarget.TYPE_PARAMETER
-        )
-    }
-
     override fun checkEntries(
         entries: List<KtAnnotationEntry>,
         actualTargets: List<KotlinTarget>,
@@ -154,7 +145,7 @@ class ExperimentalMarkerDeclarationAnnotationChecker(private val module: ModuleD
         if (targetEntry != null) {
             val (entry, descriptor) = targetEntry
             val allowedTargets = AnnotationChecker.loadAnnotationTargets(descriptor!!) ?: return
-            val wrongTargets = allowedTargets.intersect(WRONG_TARGETS_FOR_MARKER)
+            val wrongTargets = allowedTargets.intersect(Experimentality.WRONG_TARGETS_FOR_MARKER)
             if (wrongTargets.isNotEmpty()) {
                 trace.report(
                     Errors.EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET.on(

compiler/testData/diagnostics/testsWithStdLib/experimental/experimentalIsNotEnabled.fir.kt

@@ -1,6 +1,6 @@
 // FILE: api.kt
 
-@RequiresOptIn
+@<!EXPERIMENTAL_IS_NOT_ENABLED!>RequiresOptIn<!>
 @Retention(AnnotationRetention.BINARY)
 annotation class Marker
 
@@ -18,7 +18,7 @@ fun use2() {
     f()
 }
 
-@OptIn(Marker::class)
+@<!EXPERIMENTAL_IS_NOT_ENABLED!>OptIn<!>(Marker::class)
 fun use3() {
     f()
 }

compiler/testData/diagnostics/testsWithStdLib/experimental/incorrectTargetsForExperimentalAnnotation.fir.kt

@@ -12,21 +12,21 @@ import kotlin.annotation.AnnotationTarget.*
 annotation class E1
 
 @RequiresOptIn(level = RequiresOptIn.Level.WARNING)
-@Target(FILE)
+<!EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET!>@Target(FILE)<!>
 annotation class E2
 
 @RequiresOptIn(level = RequiresOptIn.Level.WARNING)
-@Target(EXPRESSION)
-@Retention(AnnotationRetention.SOURCE)
+<!EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET!>@Target(EXPRESSION)<!>
+<!EXPERIMENTAL_ANNOTATION_WITH_WRONG_RETENTION!>@Retention(AnnotationRetention.SOURCE)<!>
 annotation class E3
 
 @RequiresOptIn(level = RequiresOptIn.Level.WARNING)
-@Target(TYPE_PARAMETER)
+<!EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET!>@Target(TYPE_PARAMETER)<!>
 @Retention(AnnotationRetention.BINARY)
 annotation class E3A
 
 @RequiresOptIn(level = RequiresOptIn.Level.WARNING)
-@Target(TYPE)
+<!EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET!>@Target(TYPE)<!>
 @Retention(AnnotationRetention.BINARY)
 annotation class E3B
 

compiler/testData/diagnostics/testsWithStdLib/experimental/incorrectUseExperimental.fir.kt

@@ -1,9 +0,0 @@
-// !USE_EXPERIMENTAL: kotlin.RequiresOptIn
-
-annotation class NotAMarker
-
-@OptIn
-fun f1() {}
-
-@OptIn(NotAMarker::class)
-fun f2() {}

compiler/testData/diagnostics/testsWithStdLib/experimental/incorrectUseExperimental.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !USE_EXPERIMENTAL: kotlin.RequiresOptIn
 
 annotation class NotAMarker

compiler/testData/diagnostics/testsWithStdLib/experimental/usageNotAsAnnotation.fir.kt

@@ -35,10 +35,10 @@ fun f4(u: OptIn0 /* TODO */) {}
 annotation class VarargKClasses(vararg val k: KClass<*>)
 
 @VarargKClasses(
-    RequiresOptIn::class,
-    OptIn::class,
-    kotlin.RequiresOptIn::class,
-    kotlin.OptIn::class
+    <!EXPERIMENTAL_CAN_ONLY_BE_USED_AS_ANNOTATION!>RequiresOptIn<!>::class,
+    <!EXPERIMENTAL_CAN_ONLY_BE_USED_AS_ANNOTATION!>OptIn<!>::class,
+    <!EXPERIMENTAL_CAN_ONLY_BE_USED_AS_ANNOTATION!>kotlin.RequiresOptIn<!>::class,
+    <!EXPERIMENTAL_CAN_ONLY_BE_USED_AS_ANNOTATION!>kotlin.OptIn<!>::class
 )
 fun f5() {}
 

compiler/testData/diagnostics/testsWithStdLib/regression/kt34391.fir.kt

@@ -15,5 +15,5 @@ class A {
     }
 }
 
-@RequiresOptIn(level = RequiresOptIn.Level.ERROR)
+@<!EXPERIMENTAL_IS_NOT_ENABLED!>RequiresOptIn<!>(level = RequiresOptIn.Level.ERROR)
 annotation class ExperimentalTime

core/compiler.common/src/org/jetbrains/kotlin/resolve/checkers/Experimentality.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.resolve.checkers
 
+import org.jetbrains.kotlin.descriptors.annotations.KotlinTarget
 import org.jetbrains.kotlin.name.FqName
 
 data class Experimentality(val annotationFqName: FqName, val severity: Severity, val message: String?) {
@@ -12,5 +13,12 @@ data class Experimentality(val annotationFqName: FqName, val severity: Severity,
 
     companion object {
         val DEFAULT_SEVERITY = Severity.ERROR
+
+        val WRONG_TARGETS_FOR_MARKER = setOf(
+            KotlinTarget.EXPRESSION,
+            KotlinTarget.FILE,
+            KotlinTarget.TYPE,
+            KotlinTarget.TYPE_PARAMETER
+        )
     }
 }

idea/idea-frontend-fir/idea-frontend-fir-generator/src/org/jetbrains/kotlin/idea/frontend/api/fir/generator/HLDiagnosticConverter.kt

@@ -26,6 +26,7 @@ import org.jetbrains.kotlin.idea.frontend.api.symbols.*
 import org.jetbrains.kotlin.idea.frontend.api.types.KtType
 import org.jetbrains.kotlin.lexer.KtKeywordToken
 import org.jetbrains.kotlin.lexer.KtModifierKeywordToken
+import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.KtExpression
 import org.jetbrains.kotlin.resolve.ForbiddenNamedArgumentsTarget
@@ -252,7 +253,8 @@ private object FirToKtConversionCreator {
         ForbiddenNamedArgumentsTarget::class,
         LanguageFeature::class,
         LanguageVersionSettings::class,
-        Variance::class
+        Variance::class,
+        FqName::class,
     )
 
     private val KType.kClass: KClass<*>

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -717,6 +717,82 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.EXPERIMENTAL_API_USAGE) { firDiagnostic ->
+        ExperimentalApiUsageImpl(
+            firDiagnostic.a,
+            firDiagnostic.b,
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.EXPERIMENTAL_API_USAGE_ERROR) { firDiagnostic ->
+        ExperimentalApiUsageErrorImpl(
+            firDiagnostic.a,
+            firDiagnostic.b,
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.EXPERIMENTAL_OVERRIDE) { firDiagnostic ->
+        ExperimentalOverrideImpl(
+            firDiagnostic.a,
+            firDiagnostic.b,
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.EXPERIMENTAL_OVERRIDE_ERROR) { firDiagnostic ->
+        ExperimentalOverrideErrorImpl(
+            firDiagnostic.a,
+            firDiagnostic.b,
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.EXPERIMENTAL_IS_NOT_ENABLED) { firDiagnostic ->
+        ExperimentalIsNotEnabledImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.EXPERIMENTAL_CAN_ONLY_BE_USED_AS_ANNOTATION) { firDiagnostic ->
+        ExperimentalCanOnlyBeUsedAsAnnotationImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.EXPERIMENTAL_MARKER_CAN_ONLY_BE_USED_AS_ANNOTATION_OR_ARGUMENT_IN_USE_EXPERIMENTAL) { firDiagnostic ->
+        ExperimentalMarkerCanOnlyBeUsedAsAnnotationOrArgumentInUseExperimentalImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.USE_EXPERIMENTAL_WITHOUT_ARGUMENTS) { firDiagnostic ->
+        UseExperimentalWithoutArgumentsImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.USE_EXPERIMENTAL_ARGUMENT_IS_NOT_MARKER) { firDiagnostic ->
+        UseExperimentalArgumentIsNotMarkerImpl(
+            firDiagnostic.a,
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.EXPERIMENTAL_ANNOTATION_WITH_WRONG_TARGET) { firDiagnostic ->
+        ExperimentalAnnotationWithWrongTargetImpl(
+            firDiagnostic.a,
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.EXPERIMENTAL_ANNOTATION_WITH_WRONG_RETENTION) { firDiagnostic ->
+        ExperimentalAnnotationWithWrongRetentionImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.EXPOSED_TYPEALIAS_EXPANDED_TYPE) { firDiagnostic ->
         ExposedTypealiasExpandedTypeImpl(
             firDiagnostic.a,

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -26,6 +26,7 @@ import org.jetbrains.kotlin.idea.frontend.api.symbols.KtVariableSymbol
 import org.jetbrains.kotlin.idea.frontend.api.types.KtType
 import org.jetbrains.kotlin.lexer.KtKeywordToken
 import org.jetbrains.kotlin.lexer.KtModifierKeywordToken
+import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.KtAnnotation
 import org.jetbrains.kotlin.psi.KtAnnotationEntry
@@ -513,6 +514,60 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = InapplicableFileTarget::class
     }
 
+    abstract class ExperimentalApiUsage : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = ExperimentalApiUsage::class
+        abstract val optInMarkerFqName: FqName
+        abstract val message: String
+    }
+
+    abstract class ExperimentalApiUsageError : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = ExperimentalApiUsageError::class
+        abstract val optInMarkerFqName: FqName
+        abstract val message: String
+    }
+
+    abstract class ExperimentalOverride : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = ExperimentalOverride::class
+        abstract val optInMarkerFqName: FqName
+        abstract val message: String
+    }
+
+    abstract class ExperimentalOverrideError : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = ExperimentalOverrideError::class
+        abstract val optInMarkerFqName: FqName
+        abstract val message: String
+    }
+
+    abstract class ExperimentalIsNotEnabled : KtFirDiagnostic<KtAnnotationEntry>() {
+        override val diagnosticClass get() = ExperimentalIsNotEnabled::class
+    }
+
+    abstract class ExperimentalCanOnlyBeUsedAsAnnotation : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = ExperimentalCanOnlyBeUsedAsAnnotation::class
+    }
+
+    abstract class ExperimentalMarkerCanOnlyBeUsedAsAnnotationOrArgumentInUseExperimental : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = ExperimentalMarkerCanOnlyBeUsedAsAnnotationOrArgumentInUseExperimental::class
+    }
+
+    abstract class UseExperimentalWithoutArguments : KtFirDiagnostic<KtAnnotationEntry>() {
+        override val diagnosticClass get() = UseExperimentalWithoutArguments::class
+    }
+
+    abstract class UseExperimentalArgumentIsNotMarker : KtFirDiagnostic<KtAnnotationEntry>() {
+        override val diagnosticClass get() = UseExperimentalArgumentIsNotMarker::class
+        abstract val notMarkerFqName: FqName
+    }
+
+    abstract class ExperimentalAnnotationWithWrongTarget : KtFirDiagnostic<KtAnnotationEntry>() {
+        override val diagnosticClass get() = ExperimentalAnnotationWithWrongTarget::class
+        abstract val target: String
+    }
+
+    abstract class ExperimentalAnnotationWithWrongRetention : KtFirDiagnostic<KtAnnotationEntry>() {
+        override val diagnosticClass get() = ExperimentalAnnotationWithWrongRetention::class
+    }
+
     abstract class ExposedTypealiasExpandedType : KtFirDiagnostic<KtNamedDeclaration>() {
         override val diagnosticClass get() = ExposedTypealiasExpandedType::class
         abstract val elementVisibility: EffectiveVisibility

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -28,6 +28,7 @@ import org.jetbrains.kotlin.idea.frontend.api.tokens.ValidityToken
 import org.jetbrains.kotlin.idea.frontend.api.types.KtType
 import org.jetbrains.kotlin.lexer.KtKeywordToken
 import org.jetbrains.kotlin.lexer.KtModifierKeywordToken
+import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.KtAnnotation
 import org.jetbrains.kotlin.psi.KtAnnotationEntry
@@ -832,6 +833,93 @@ internal class InapplicableFileTargetImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class ExperimentalApiUsageImpl(
+    override val optInMarkerFqName: FqName,
+    override val message: String,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExperimentalApiUsage(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class ExperimentalApiUsageErrorImpl(
+    override val optInMarkerFqName: FqName,
+    override val message: String,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExperimentalApiUsageError(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class ExperimentalOverrideImpl(
+    override val optInMarkerFqName: FqName,
+    override val message: String,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExperimentalOverride(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class ExperimentalOverrideErrorImpl(
+    override val optInMarkerFqName: FqName,
+    override val message: String,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExperimentalOverrideError(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class ExperimentalIsNotEnabledImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExperimentalIsNotEnabled(), KtAbstractFirDiagnostic<KtAnnotationEntry> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class ExperimentalCanOnlyBeUsedAsAnnotationImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExperimentalCanOnlyBeUsedAsAnnotation(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class ExperimentalMarkerCanOnlyBeUsedAsAnnotationOrArgumentInUseExperimentalImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExperimentalMarkerCanOnlyBeUsedAsAnnotationOrArgumentInUseExperimental(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class UseExperimentalWithoutArgumentsImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.UseExperimentalWithoutArguments(), KtAbstractFirDiagnostic<KtAnnotationEntry> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class UseExperimentalArgumentIsNotMarkerImpl(
+    override val notMarkerFqName: FqName,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.UseExperimentalArgumentIsNotMarker(), KtAbstractFirDiagnostic<KtAnnotationEntry> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class ExperimentalAnnotationWithWrongTargetImpl(
+    override val target: String,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExperimentalAnnotationWithWrongTarget(), KtAbstractFirDiagnostic<KtAnnotationEntry> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class ExperimentalAnnotationWithWrongRetentionImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExperimentalAnnotationWithWrongRetention(), KtAbstractFirDiagnostic<KtAnnotationEntry> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class ExposedTypealiasExpandedTypeImpl(
     override val elementVisibility: EffectiveVisibility,
     override val restrictingDeclaration: KtSymbol,