Support serializable objects in FIR plugin

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/fir/SerializationFirResolveExtension.kt

@@ -132,8 +132,10 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
     @OptIn(SymbolInternals::class)
     override fun generateFunctions(callableId: CallableId, context: MemberGenerationContext?): List<FirNamedFunctionSymbol> {
         val owner = context?.owner ?: return emptyList()
-        if (owner.name == SpecialNames.DEFAULT_NAME_FOR_COMPANION_OBJECT && callableId.callableName == SerialEntityNames.SERIALIZER_PROVIDER_NAME)
-            return listOf(generateSerializerGetterInCompanion(owner, callableId))
+        if (callableId.callableName == SerialEntityNames.SERIALIZER_PROVIDER_NAME) {
+            val serializableClass = session.getSerializableClassDescriptorByCompanion(owner) ?: return emptyList()
+            return listOf(generateSerializerGetterInCompanion(owner, serializableClass, callableId))
+        }
         if (owner.name != SerialEntityNames.SERIALIZER_CLASS_NAME) return emptyList()
         if (callableId.callableName !in setOf(
                 SpecialNames.INIT,
@@ -154,7 +156,7 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
         return listOf(copy.symbol)
     }
 
-    private fun generateSerializerGetterInCompanion(owner: FirClassSymbol<*>, callableId: CallableId): FirNamedFunctionSymbol {
+    private fun generateSerializerGetterInCompanion(owner: FirClassSymbol<*>, serializableClassSymbol: FirClassSymbol<*>, callableId: CallableId): FirNamedFunctionSymbol {
         val f = buildSimpleFunction {
             moduleData = session.moduleData
             symbol = FirNamedFunctionSymbol(callableId)
@@ -167,10 +169,6 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
             name = callableId.callableName
             dispatchReceiverType = owner.defaultType()
 
-            // TODO: handle serializable objects
-            val serializableClassSymbol =
-                (owner.getContainingDeclarationSymbol(session) as? FirClassSymbol<*>) ?: error("Can't get outer class for $owner")
-
             typeParameters.addAll(serializableClassSymbol.typeParameterSymbols.map { newSimpleTypeParameter(session, symbol, it.name) })
             val parametersAsArguments = typeParameters.map { it.toConeType() }.toTypedArray<ConeTypeProjection>()
 
@@ -193,7 +191,7 @@ class SerializationFirResolveExtension(session: FirSession) : FirDeclarationGene
         return f.symbol
     }
 
-    @OptIn(SymbolInternals::class) // TODO: localSerializersFieldDescriptors
+    @OptIn(SymbolInternals::class)
     override fun generateProperties(callableId: CallableId, context: MemberGenerationContext?): List<FirPropertySymbol> {
         val owner = context?.owner ?: return emptyList()
         if (owner.name != SerialEntityNames.SERIALIZER_CLASS_NAME) return emptyList()

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/fir/SerializationFirUtils.kt

@@ -5,15 +5,22 @@
 
 package org.jetbrains.kotlinx.serialization.compiler.fir
 
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.descriptors.Modality
+import org.jetbrains.kotlin.fir.FirSession
+import org.jetbrains.kotlin.fir.analysis.checkers.getContainingDeclarationSymbol
+import org.jetbrains.kotlin.fir.analysis.checkers.outerClassSymbol
 import org.jetbrains.kotlin.fir.declarations.findArgumentByName
+import org.jetbrains.kotlin.fir.declarations.utils.isCompanion
 import org.jetbrains.kotlin.fir.declarations.utils.modality
 import org.jetbrains.kotlin.fir.expressions.classId
 import org.jetbrains.kotlin.fir.symbols.SymbolInternals
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassSymbol
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlinx.serialization.compiler.resolve.SerializationAnnotations
+import org.jetbrains.kotlinx.serialization.compiler.resolve.isSerializableObject
+import org.jetbrains.kotlinx.serialization.compiler.resolve.shouldHaveGeneratedMethodsInCompanion
 
 internal val FirClassSymbol<*>.hasSerializableAnnotation
     get() = this.annotations.any {
@@ -59,4 +66,12 @@ private fun FirClassSymbol<*>.isFinalOrOpen(): Boolean {
     val modality = rawStatus.modality
     // null means default modality, final
     return (modality == null || modality == Modality.FINAL || modality == Modality.OPEN)
+}
+
+internal fun FirSession.getSerializableClassDescriptorByCompanion(thisDescriptor: FirClassSymbol<*>): FirClassSymbol<*>? {
+    if (thisDescriptor.isSerializableObject) return thisDescriptor
+    if (!thisDescriptor.isCompanion) return null
+    val classDescriptor = (thisDescriptor.getContainingDeclarationSymbol(this) as? FirClassSymbol<*>) ?: return null
+    if (!classDescriptor.shouldHaveGeneratedMethodsInCompanion) return null
+    return classDescriptor
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/test/org/jetbrains/kotlinx/serialization/SerializationFirBlackBoxTestGenerated.java

@@ -54,4 +54,10 @@ public class SerializationFirBlackBoxTestGenerated extends AbstractSerialization
     public void testPrivatePropertiesSerialization() throws Exception {
         runTest("plugins/kotlin-serialization/kotlin-serialization-compiler/testData/firMembers/privatePropertiesSerialization.kt");
     }
+
+    @Test
+    @TestMetadata("serializableObject.kt")
+    public void testSerializableObject() throws Exception {
+        runTest("plugins/kotlin-serialization/kotlin-serialization-compiler/testData/firMembers/serializableObject.kt");
+    }
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/test/org/jetbrains/kotlinx/serialization/SerializationFirMembersTestGenerated.java

@@ -53,4 +53,10 @@ public class SerializationFirMembersTestGenerated extends AbstractSerializationF
     public void testPrivatePropertiesSerialization() throws Exception {
         runTest("plugins/kotlin-serialization/kotlin-serialization-compiler/testData/firMembers/privatePropertiesSerialization.kt");
     }
+
+    @Test
+    @TestMetadata("serializableObject.kt")
+    public void testSerializableObject() throws Exception {
+        runTest("plugins/kotlin-serialization/kotlin-serialization-compiler/testData/firMembers/serializableObject.kt");
+    }
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/firMembers/serializableObject.fir.txt

@@ -0,0 +1,26 @@
+FILE: serializableObject.kt
+    @R|kotlinx/serialization/Serializable|() public final object FooBar : R|kotlin/Any| {
+        private constructor(): R|FooBar| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final fun serializer(): R|kotlinx/serialization/KSerializer<FooBar>|
+
+    }
+    public final fun box(): R|kotlin/String| {
+        lval encoded: R|kotlin/String| = Q|kotlinx/serialization/json/Json|.R|kotlinx/serialization/json/Json.encodeToString|<R|FooBar|>(Q|FooBar|.R|/FooBar.serializer|(), Q|FooBar|)
+        when () {
+            !=(R|<local>/encoded|, String({})) ->  {
+                ^box R|<local>/encoded|
+            }
+        }
+
+        lval decoded: R|FooBar| = Q|kotlinx/serialization/json/Json|.R|kotlinx/serialization/json/Json.decodeFromString|<R|FooBar|>(Q|FooBar|.R|/FooBar.serializer|(), R|<local>/encoded|)
+        when () {
+            !==(R|<local>/decoded|, Q|FooBar|) ->  {
+                ^box String(Incorrect object instance)
+            }
+        }
+
+        ^box String(OK)
+    }

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/firMembers/serializableObject.kt

@@ -0,0 +1,15 @@
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+@Serializable
+object FooBar
+
+fun box(): String {
+    val encoded = Json.encodeToString(FooBar.serializer(), FooBar)
+    if (encoded != "{}") return encoded
+    val decoded = Json.decodeFromString(FooBar.serializer(), encoded)
+    if (decoded !== FooBar) return "Incorrect object instance"
+    return "OK"
+}