Allow '@Serializable' on a type usage (for kotlinx.serialization/367)

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/TypeUtil.kt

@@ -113,6 +113,7 @@ fun AbstractSerialGenerator.findTypeSerializerOrContext(
 ): ClassDescriptor? {
     if (kType.isTypeParameter()) return null
     if (kType.isMarkedNullable) return findTypeSerializerOrContext(module, kType.makeNotNullable(), annotations, sourceElement)
+    annotations.serializableWith(module)?.let { return it.toClassDescriptor }
     additionalSerializersInScopeOfCurrentFile[kType]?.let { return it }
     if (kType in contextualKClassListInCurrentFile) return module.getClassFromSerializationPackage(SpecialBuiltins.contextSerializer)
     return analyzeSpecialSerializers(module, annotations) ?: findTypeSerializer(module, kType) ?: throw CompilationException(

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

@@ -78,7 +78,10 @@ internal fun extractKSerializerArgumentFromImplementation(implementationClass: C
 }
 
 internal val DeclarationDescriptor.serializableWith: KotlinType?
-    get() = annotations.findAnnotationKotlinTypeValue(SerializationAnnotations.serializableAnnotationFqName, module,"with")
+    get() = annotations.serializableWith(module)
+
+internal fun Annotations.serializableWith(module: ModuleDescriptor): KotlinType? =
+    this.findAnnotationKotlinTypeValue(SerializationAnnotations.serializableAnnotationFqName, module,"with")
 
 internal val DeclarationDescriptor.serializerForClass: KotlinType?
     get() = annotations.findAnnotationKotlinTypeValue(SerializationAnnotations.serializerAnnotationFqName, module, "forClass")