[Compiler plugin] Introduce singleton for PluginDataFrameSchema(emptyList())

Author: koperagen

plugins/kotlin-dataframe/src/org/jetbrains/kotlinx/dataframe/plugin/impl/SimpleCol.kt

@@ -18,6 +18,9 @@ import org.jetbrains.kotlinx.dataframe.plugin.utils.Names
 data class PluginDataFrameSchema(
     private val columns: List<SimpleCol>
 ) : DataFrameLikeContainer<SimpleCol> {
+    companion object {
+        val EMPTY = PluginDataFrameSchema(emptyList())
+    }
     override fun columns(): List<SimpleCol> {
         return columns
     }

plugins/kotlin-dataframe/src/org/jetbrains/kotlinx/dataframe/plugin/impl/api/groupBy.kt

@@ -92,7 +92,7 @@ fun KotlinTypeFacade.aggregate(
         }
         PluginDataFrameSchema(cols)
     } else {
-        PluginDataFrameSchema(emptyList())
+        PluginDataFrameSchema.EMPTY
     }
 }
 

plugins/kotlin-dataframe/src/org/jetbrains/kotlinx/dataframe/plugin/impl/api/insert.kt

@@ -132,7 +132,7 @@ internal fun PluginDataFrameSchema.insertImpl(
         columns.firstOrNull()?.referenceNode?.getRoot(),
         0,
         factory = { PluginDataFrameSchema(it) },
-        empty = PluginDataFrameSchema(emptyList()),
+        empty = PluginDataFrameSchema.EMPTY,
         rename = { rename(it) },
         createColumnGroup = { name, columns ->
             SimpleColumnGroup(name, columns)

plugins/kotlin-dataframe/src/org/jetbrains/kotlinx/dataframe/plugin/impl/api/toDataFrame.kt

@@ -288,12 +288,12 @@ internal fun KotlinTypeFacade.toDataFrame(
             }
     }
 
-    val receiver = explicitReceiver ?: return PluginDataFrameSchema(emptyList())
-    val arg = receiver.resolvedType.typeArguments.firstOrNull() ?: return PluginDataFrameSchema(emptyList())
+    val receiver = explicitReceiver ?: return PluginDataFrameSchema.EMPTY
+    val arg = receiver.resolvedType.typeArguments.firstOrNull() ?: return PluginDataFrameSchema.EMPTY
     return when {
-        arg.isStarProjection -> PluginDataFrameSchema(emptyList())
+        arg.isStarProjection -> PluginDataFrameSchema.EMPTY
         else -> {
-            val classLike = arg.type as? ConeClassLikeType ?: return PluginDataFrameSchema(emptyList())
+            val classLike = arg.type as? ConeClassLikeType ?: return PluginDataFrameSchema.EMPTY
             val columns = convert(classLike, 0)
             PluginDataFrameSchema(columns)
         }

plugins/kotlin-dataframe/src/org/jetbrains/kotlinx/dataframe/plugin/interpret.kt

@@ -314,16 +314,16 @@ interface InterpretationErrorReporter {
 
 fun KotlinTypeFacade.pluginDataFrameSchema(schemaTypeArg: ConeTypeProjection): PluginDataFrameSchema {
     val schema = if (schemaTypeArg.isStarProjection) {
-        PluginDataFrameSchema(emptyList())
+        PluginDataFrameSchema.EMPTY
     } else {
-        val coneClassLikeType = schemaTypeArg.type as? ConeClassLikeType ?: return PluginDataFrameSchema(emptyList())
+        val coneClassLikeType = schemaTypeArg.type as? ConeClassLikeType ?: return PluginDataFrameSchema.EMPTY
         pluginDataFrameSchema(coneClassLikeType)
     }
     return schema
 }
 
 fun KotlinTypeFacade.pluginDataFrameSchema(coneClassLikeType: ConeClassLikeType): PluginDataFrameSchema {
-    val symbol = coneClassLikeType.toSymbol(session) as? FirRegularClassSymbol ?: return PluginDataFrameSchema(emptyList())
+    val symbol = coneClassLikeType.toSymbol(session) as? FirRegularClassSymbol ?: return PluginDataFrameSchema.EMPTY
     val declarationSymbols = if (symbol.isLocal && symbol.resolvedSuperTypes.firstOrNull() != session.builtinTypes.anyType.type) {
         val rootSchemaSymbol = symbol.resolvedSuperTypes.first().toSymbol(session) as? FirRegularClassSymbol
         rootSchemaSymbol?.declaredMemberScope(session, FirResolvePhase.DECLARATIONS)