Implement diagnostic about nullable serializer for non-nullable type

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/DeclarationCheckers.kt

@@ -17,6 +17,7 @@ import org.jetbrains.kotlin.resolve.BindingTrace
 import org.jetbrains.kotlin.resolve.checkers.DeclarationChecker
 import org.jetbrains.kotlin.resolve.checkers.DeclarationCheckerContext
 import org.jetbrains.kotlin.types.KotlinType
+import org.jetbrains.kotlin.types.typeUtil.supertypes
 import org.jetbrains.kotlin.util.slicedMap.Slices
 import org.jetbrains.kotlin.util.slicedMap.WritableSlice
 import org.jetbrains.kotlinx.serialization.compiler.backend.common.AbstractSerialGenerator
@@ -79,6 +80,7 @@ class SerializationPluginDeclarationChecker : DeclarationChecker {
             val ktType = (it.descriptor.findPsi() as? KtCallableDeclaration)?.typeReference ?: return@forEach
             if (serializer != null) {
                 val element = ktType.typeElement ?: return
+                checkSerializerNullability(it.type, serializer.defaultType, element, trace)
                 generatorContextForAnalysis.checkTypeArguments(it.module, it.type, element, trace)
                 trace.record(SERIALIZER_FOR_PROPERTY, it.descriptor, serializer)
             } else {
@@ -104,8 +106,9 @@ class SerializationPluginDeclarationChecker : DeclarationChecker {
     ) {
         if (type.genericIndex != null) return
         val element = ktType.typeElement ?: return
-        val serializerForType = findTypeSerializerOrContext(module, type)
-        if (serializerForType != null) {
+        val serializer = findTypeSerializerOrContext(module, type)
+        if (serializer != null) {
+            checkSerializerNullability(type, serializer.defaultType, element, trace)
             checkTypeArguments(module, type, element, trace)
         } else {
             trace.reportFromPlugin(
@@ -115,6 +118,22 @@ class SerializationPluginDeclarationChecker : DeclarationChecker {
         }
     }
 
+    private fun checkSerializerNullability(
+        classType: KotlinType,
+        serializerType: KotlinType,
+        element: KtTypeElement,
+        trace: BindingTrace
+    ) {
+        // @Serializable annotation has proper signature so this error would be caught in type checker
+        val castedToKSerial = serializerType.supertypes().find { isKSerializer(it) } ?: return
+
+        if (!classType.isMarkedNullable && castedToKSerial.arguments.first().type.isMarkedNullable)
+            trace.reportFromPlugin(
+                SerializationErrors.SERIALIZER_NULLABILITY_INCOMPATIBLE.on(element, serializerType),
+                SerializationPluginErrorsRendering
+            )
+    }
+
     private inline fun ClassDescriptor.safeReport(report: (KtAnnotationEntry) -> Unit) {
         findSerializableAnnotationDeclaration()?.let(report)
     }

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationErrors.java

@@ -10,6 +10,7 @@ import org.jetbrains.kotlin.diagnostics.DiagnosticFactory0;
 import org.jetbrains.kotlin.diagnostics.DiagnosticFactory1;
 import org.jetbrains.kotlin.diagnostics.Errors;
 import org.jetbrains.kotlin.psi.KtAnnotationEntry;
+import org.jetbrains.kotlin.types.KotlinType;
 
 import static org.jetbrains.kotlin.diagnostics.Severity.ERROR;
 
@@ -18,6 +19,7 @@ public interface SerializationErrors {
     DiagnosticFactory0<KtAnnotationEntry> PRIMARY_CONSTRUCTOR_PARAMETER_IS_NOT_A_PROPERTY = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory1<KtAnnotationEntry, String> DUPLICATE_SERIAL_NAME = DiagnosticFactory1.create(ERROR);
     DiagnosticFactory0<PsiElement> SERIALIZER_NOT_FOUND = DiagnosticFactory0.create(ERROR);
+    DiagnosticFactory1<PsiElement, KotlinType> SERIALIZER_NULLABILITY_INCOMPATIBLE = DiagnosticFactory1.create(ERROR);
 
     @SuppressWarnings("UnusedDeclaration")
     Object _initializer = new Object() {

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationPluginErrorsRendering.kt

@@ -33,5 +33,10 @@ object SerializationPluginErrorsRendering : DefaultErrorMessages.Extension {
             "Serializer has not been found for type of this property. " +
                     "To use context serializer as fallback, explicitly annotate element with @ContextualSerialization"
         )
+        MAP.put(
+            SerializationErrors.SERIALIZER_NULLABILITY_INCOMPATIBLE,
+            "This type is not-nullable and therefore can not be serialized with serializer for nullable type ''{0}''",
+            Renderers.RENDER_TYPE
+        )
     }
 }

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/KSerializationUtil.kt

@@ -160,21 +160,11 @@ internal val ClassDescriptor.hasCompanionObjectAsSerializer: Boolean
 
 internal fun ClassDescriptor.isSerializerWhichRequiersKClass() = classId in setOf(enumSerializerId, contextSerializerId, polymorphicSerializerId)
 
-internal fun checkSerializerNullability(classType: KotlinType, serializerType: KotlinType): KotlinType {
-    val castedToKSerial = requireNotNull(
-        serializerType.supertypes().find { isKSerializer(it) },
-        { "${KSERIALIZER_CLASS} is not a supertype of $serializerType" }
-    )
-    if (!classType.isMarkedNullable && castedToKSerial.arguments.first().type.isMarkedNullable)
-        throw IllegalStateException("Can't serialize non-nullable field of type ${classType} with nullable serializer ${serializerType}")
-    return serializerType
-}
-
 // returns only user-overriden Serializer
 internal val KotlinType.overridenSerializer: KotlinType?
     get() {
         val desc = this.toClassDescriptor ?: return null
-        (desc.serializableWith)?.let { return checkSerializerNullability(this, it) }
+        desc.serializableWith?.let { return it }
         if (desc.annotations.hasAnnotation(SerializationAnnotations.polymorphicFqName))
             return desc.module.getClassFromSerializationPackage(SpecialBuiltins.polymorphicSerializer).defaultType
         return null