FIR: add smartcast stability to FIR

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrImplicitCastInserter.kt

@@ -274,7 +274,11 @@ class Fir2IrImplicitCastInserter(
     }
 
     override fun visitExpressionWithSmartcast(expressionWithSmartcast: FirExpressionWithSmartcast, data: IrElement): IrExpression {
-        return implicitCastOrExpression(data as IrExpression, expressionWithSmartcast.typeRef)
+        return if (expressionWithSmartcast.smartcastStability == SmartcastStability.STABLE_VALUE) {
+            implicitCastOrExpression(data as IrExpression, expressionWithSmartcast.typeRef)
+        } else {
+            data as IrExpression
+        }
     }
 
     override fun visitExpressionWithSmartcastToNull(

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/ResolveUtils.kt

@@ -36,6 +36,7 @@ import org.jetbrains.kotlin.fir.types.builder.buildResolvedTypeRef
 import org.jetbrains.kotlin.fir.types.impl.ConeClassLikeTypeImpl
 import org.jetbrains.kotlin.name.*
 import org.jetbrains.kotlin.resolve.ForbiddenNamedArgumentsTarget
+import org.jetbrains.kotlin.types.SmartcastStability
 
 fun List<FirQualifierPart>.toTypeProjections(): Array<ConeTypeProjection> =
     asReversed().flatMap { it.typeArgumentList.typeArguments.map { typeArgument -> typeArgument.toConeTypeProjection() } }.toTypedArray()
@@ -285,8 +286,9 @@ fun BodyResolveComponents.transformQualifiedAccessUsingSmartcastInfo(
     }
     return buildExpressionWithSmartcast {
         originalExpression = qualifiedAccessExpression
-        typeRef = intersectedTypeRef
+        smartcastType = intersectedTypeRef
         this.typesFromSmartCast = typesFromSmartCast
+        smartcastStability = SmartcastStability.STABLE_VALUE
     }
 }
 

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/FirReceivers.kt

@@ -27,6 +27,7 @@ import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.builder.buildResolvedTypeRef
 import org.jetbrains.kotlin.fir.types.coneType
 import org.jetbrains.kotlin.fir.types.coneTypeSafe
+import org.jetbrains.kotlin.types.SmartcastStability
 
 interface Receiver
 
@@ -89,8 +90,9 @@ sealed class ImplicitReceiverValue<S : AbstractFirBasedSymbol<*>>(
         } else {
             buildExpressionWithSmartcast {
                 originalExpression = originalReceiverExpression
-                typeRef = originalReceiverExpression.typeRef.resolvedTypeFromPrototype(type)
+                smartcastType = originalReceiverExpression.typeRef.resolvedTypeFromPrototype(type)
                 typesFromSmartCast = listOf(type)
+                smartcastStability = SmartcastStability.STABLE_VALUE
             }
         }
         implicitScope = type.scope(useSiteSession, scopeSession, FakeOverrideTypeCalculator.DoNothing)

compiler/fir/tree/gen/org/jetbrains/kotlin/fir/expressions/FirExpressionWithSmartcast.kt

@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.fir.references.FirReference
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.FirTypeProjection
 import org.jetbrains.kotlin.fir.types.FirTypeRef
+import org.jetbrains.kotlin.types.SmartcastStability
 import org.jetbrains.kotlin.fir.visitors.*
 import org.jetbrains.kotlin.fir.FirImplementationDetail
 
@@ -31,6 +32,8 @@ abstract class FirExpressionWithSmartcast : FirQualifiedAccessExpression() {
     abstract val originalExpression: FirQualifiedAccessExpression
     abstract val typesFromSmartCast: Collection<ConeKotlinType>
     abstract val originalType: FirTypeRef
+    abstract val smartcastType: FirTypeRef
+    abstract val smartcastStability: SmartcastStability
 
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitExpressionWithSmartcast(this, data)
 

compiler/fir/tree/gen/org/jetbrains/kotlin/fir/expressions/FirExpressionWithSmartcastToNull.kt

@@ -11,6 +11,7 @@ import org.jetbrains.kotlin.fir.references.FirReference
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.FirTypeProjection
 import org.jetbrains.kotlin.fir.types.FirTypeRef
+import org.jetbrains.kotlin.types.SmartcastStability
 import org.jetbrains.kotlin.fir.visitors.*
 import org.jetbrains.kotlin.fir.FirImplementationDetail
 
@@ -31,6 +32,8 @@ abstract class FirExpressionWithSmartcastToNull : FirExpressionWithSmartcast() {
     abstract override val originalExpression: FirQualifiedAccessExpression
     abstract override val typesFromSmartCast: Collection<ConeKotlinType>
     abstract override val originalType: FirTypeRef
+    abstract override val smartcastType: FirTypeRef
+    abstract override val smartcastStability: SmartcastStability
 
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitExpressionWithSmartcastToNull(this, data)
 

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/builder/FirExpressionWithSmartcastBuilder.kt

@@ -10,14 +10,16 @@ import org.jetbrains.kotlin.fir.expressions.FirQualifiedAccessExpression
 import org.jetbrains.kotlin.fir.expressions.impl.FirExpressionWithSmartcastImpl
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.FirTypeRef
+import org.jetbrains.kotlin.types.SmartcastStability
 
 class FirExpressionWithSmartcastBuilder {
     lateinit var originalExpression: FirQualifiedAccessExpression
-    lateinit var typeRef: FirTypeRef
+    lateinit var smartcastType: FirTypeRef
     lateinit var typesFromSmartCast: Collection<ConeKotlinType>
+    lateinit var smartcastStability: SmartcastStability
 
     fun build(): FirExpressionWithSmartcast {
-        return FirExpressionWithSmartcastImpl(originalExpression, typeRef, typesFromSmartCast)
+        return FirExpressionWithSmartcastImpl(originalExpression, smartcastType, typesFromSmartCast, smartcastStability)
     }
 }
 

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/builder/FirExpressionWithSmartcastToNullBuilder.kt

@@ -10,14 +10,16 @@ import org.jetbrains.kotlin.fir.expressions.FirQualifiedAccessExpression
 import org.jetbrains.kotlin.fir.expressions.impl.FirExpressionWithSmartcastToNullImpl
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.FirTypeRef
+import org.jetbrains.kotlin.types.SmartcastStability
 
 class FirExpressionWithSmartcastToNullBuilder {
     lateinit var originalExpression: FirQualifiedAccessExpression
-    lateinit var typeRef: FirTypeRef
+    lateinit var smartcastType: FirTypeRef
     lateinit var typesFromSmartCast: Collection<ConeKotlinType>
+    lateinit var smartcastStability: SmartcastStability
 
     fun build(): FirExpressionWithSmartcastToNull {
-        return FirExpressionWithSmartcastToNullImpl(originalExpression, typeRef, typesFromSmartCast)
+        return FirExpressionWithSmartcastToNullImpl(originalExpression, smartcastType, typesFromSmartCast, smartcastStability)
     }
 }
 

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirExpressionWithSmartcastImpl.kt

@@ -19,11 +19,13 @@ import org.jetbrains.kotlin.fir.types.FirTypeRef
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 import org.jetbrains.kotlin.fir.visitors.FirVisitor
 import org.jetbrains.kotlin.fir.visitors.transformSingle
+import org.jetbrains.kotlin.types.SmartcastStability
 
 internal class FirExpressionWithSmartcastImpl(
     override var originalExpression: FirQualifiedAccessExpression,
-    override val typeRef: FirTypeRef,
+    override val smartcastType: FirTypeRef,
-    override val typesFromSmartCast: Collection<ConeKotlinType>
+    override val typesFromSmartCast: Collection<ConeKotlinType>,
+    override val smartcastStability: SmartcastStability
 ) : FirExpressionWithSmartcast() {
     init {
         assert(originalExpression.typeRef is FirResolvedTypeRef)
@@ -37,6 +39,10 @@ internal class FirExpressionWithSmartcastImpl(
     override val extensionReceiver: FirExpression get() = originalExpression.extensionReceiver
     override val calleeReference: FirReference get() = originalExpression.calleeReference
     override val originalType: FirTypeRef get() = originalExpression.typeRef
+    // A FirExpressionWithSmartcast is only an effective smartcast if `smartcastStability == SmartcastStability.STABLE_VALUE`. Otherwise,
+    // it's the same as the `originalExpression` under the hood. The reason we still create such a smartcast expression is for diagnostics
+    // purpose only.
+    override val typeRef: FirTypeRef get() = if (smartcastStability == SmartcastStability.STABLE_VALUE) smartcastType else originalType
 
     override fun <D> transformChildren(transformer: FirTransformer<D>, data: D): FirExpressionWithSmartcast {
         originalExpression = originalExpression.transformSingle(transformer, data)

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirExpressionWithSmartcastToNullImpl.kt

@@ -16,11 +16,13 @@ import org.jetbrains.kotlin.fir.types.FirTypeRef
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 import org.jetbrains.kotlin.fir.visitors.FirVisitor
 import org.jetbrains.kotlin.fir.visitors.transformSingle
+import org.jetbrains.kotlin.types.SmartcastStability
 
 class FirExpressionWithSmartcastToNullImpl(
     override var originalExpression: FirQualifiedAccessExpression,
-    override val typeRef: FirTypeRef,
+    override val smartcastType: FirTypeRef,
-    override val typesFromSmartCast: Collection<ConeKotlinType>
+    override val typesFromSmartCast: Collection<ConeKotlinType>,
+    override val smartcastStability: SmartcastStability
 ) : FirExpressionWithSmartcastToNull() {
     init {
         assert(originalExpression.typeRef is FirResolvedTypeRef)
@@ -34,6 +36,10 @@ class FirExpressionWithSmartcastToNullImpl(
     override val extensionReceiver: FirExpression get() = originalExpression.extensionReceiver
     override val calleeReference: FirReference get() = originalExpression.calleeReference
     override val originalType: FirTypeRef get() = originalExpression.typeRef
+    // A FirExpressionWithSmartcast is only an effective smartcast if `smartcastStability == SmartcastStability.STABLE_VALUE`. Otherwise,
+    // it's the same as the `originalExpression` under the hood. The reason we still create such a smartcast expression is for diagnostics
+    // purpose only.
+    override val typeRef: FirTypeRef get() = if (smartcastStability == SmartcastStability.STABLE_VALUE) smartcastType else originalType
 
     override fun <D> transformChildren(transformer: FirTransformer<D>, data: D): FirExpressionWithSmartcastToNull {
         originalExpression = originalExpression.transformSingle(transformer, data)

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/FieldSets.kt

@@ -83,4 +83,6 @@ object FieldSets {
     val modality = field(modalityType, nullable = true)
 
     val scopeProvider = field("scopeProvider", firScopeProviderType)
+
+    val smartcastStability = field(smartcastStabilityType)
 }

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -19,6 +19,7 @@ import org.jetbrains.kotlin.fir.tree.generator.FieldSets.name
 import org.jetbrains.kotlin.fir.tree.generator.FieldSets.receivers
 import org.jetbrains.kotlin.fir.tree.generator.FieldSets.returnTypeRef
 import org.jetbrains.kotlin.fir.tree.generator.FieldSets.scopeProvider
+import org.jetbrains.kotlin.fir.tree.generator.FieldSets.smartcastStability
 import org.jetbrains.kotlin.fir.tree.generator.FieldSets.status
 import org.jetbrains.kotlin.fir.tree.generator.FieldSets.superTypeRefs
 import org.jetbrains.kotlin.fir.tree.generator.FieldSets.symbol
@@ -458,6 +459,8 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
             +field("originalExpression", qualifiedAccessExpression)
             +field("typesFromSmartCast", "Collection<ConeKotlinType>", null, customType = coneKotlinTypeType)
             +field("originalType", typeRef)
+            +field("smartcastType", typeRef)
+            +smartcastStability
         }
 
         expressionWithSmartcastToNull.configure {

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/Types.kt

@@ -17,6 +17,7 @@ import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
+import org.jetbrains.kotlin.types.SmartcastStability
 import org.jetbrains.kotlin.types.Variance
 
 val sourceElementType = type("fir", "FirSourceElement")
@@ -31,6 +32,7 @@ val nameType = type(Name::class)
 val visibilityType = type(Visibility::class)
 val effectiveVisibilityType = type("descriptors", "EffectiveVisibility")
 val modalityType = type(Modality::class)
+val smartcastStabilityType = type(SmartcastStability::class)
 val fqNameType = type(FqName::class)
 val classIdType = type(ClassId::class)
 val annotationUseSiteTargetType = type(AnnotationUseSiteTarget::class)

core/compiler.common/src/org/jetbrains/kotlin/types/SmartcastStability.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.types
+
+/**
+ * See https://kotlinlang.org/spec/type-inference.html#smart-cast-sink-stability for explanation on smartcast stability. The "mutable value
+ * capturing" and "concurrent writes" categories in the spec are covered by [CAPTURED_VARIABLE] and [MUTABLE_PROPERTY] together.
+ * Specifically, we only do capture analysis on local mutable properties. Non-local ones are considered to be always unstable (assuming
+ * some concurrent writes are always present).
+ */
+enum class SmartcastStability(private val str: String, val description: String = str) {
+    // Local value, or parameter, or private / internal member value without open / custom getter,
+    // or protected / public member value from the same module without open / custom getter
+    // Smart casts are completely safe
+    STABLE_VALUE("stable val"),
+
+    // Member value with open / custom getter
+    // Smart casts are not safe
+    PROPERTY_WITH_GETTER("custom getter", "property that has open or custom getter"),
+
+    // Protected / public member value from another module
+    // Smart casts are not safe
+    ALIEN_PUBLIC_PROPERTY("alien public", "public API property declared in different module"),
+
+    // Local variable already captured by a changing closure
+    // Smart casts are not safe
+    CAPTURED_VARIABLE("captured var", "local variable that is captured by a changing closure"),
+
+    // Member variable regardless of its visibility
+    // Smart casts are not safe
+    MUTABLE_PROPERTY("member", "mutable property that could have been changed by this time"),
+
+    // A delegated property.
+    // Smart casts are not safe
+    DELEGATED_PROPERTY("delegate", "delegated property that could have been changed by this time"),
+}