K2: introduce & use MutableOrEmptyList.EMPTY singleton

compiler/fir/tree/gen/org/jetbrains/kotlin/fir/expressions/impl/FirErrorLoopImpl.kt

@@ -32,7 +32,7 @@ internal class FirErrorLoopImpl(
     override val diagnostic: ConeDiagnostic,
 ) : FirErrorLoop() {
     override var block: FirBlock = FirEmptyExpressionBlock()
-    override var condition: FirExpression = FirErrorExpressionImpl(source, MutableOrEmptyList(), ConeStubDiagnostic(diagnostic), null, null)
+    override var condition: FirExpression = FirErrorExpressionImpl(source, MutableOrEmptyList.empty(), ConeStubDiagnostic(diagnostic), null, null)
 
     override fun <R, D> acceptChildren(visitor: FirVisitor<R, D>, data: D) {
         annotations.forEach { it.accept(visitor, data) }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/Utils.kt

@@ -14,10 +14,7 @@ import org.jetbrains.kotlin.fir.declarations.impl.FirDeclarationStatusImpl
 import org.jetbrains.kotlin.fir.declarations.impl.FirResolvedDeclarationStatusImpl
 import org.jetbrains.kotlin.fir.expressions.FirBlock
 import org.jetbrains.kotlin.fir.expressions.FirExpression
-import org.jetbrains.kotlin.fir.references.FirReference
-import org.jetbrains.kotlin.fir.references.FirResolvedNamedReference
 import org.jetbrains.kotlin.fir.renderer.FirRenderer
-import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.fir.types.builder.*
 import org.jetbrains.kotlin.fir.types.impl.*
@@ -146,7 +143,10 @@ inline fun <R> withFileAnalysisExceptionWrapping(file: FirFile, block: () -> R):
 }
 
 @JvmInline
-value class MutableOrEmptyList<T>(val list: MutableList<T>? = null) : List<T> {
+value class MutableOrEmptyList<T>(val list: MutableList<T>?) : List<T> {
+
+    private constructor(list: Nothing?) : this(list as MutableList<T>?)
+
     override val size: Int
         get() = list?.size ?: 0
 
@@ -189,4 +189,11 @@ value class MutableOrEmptyList<T>(val list: MutableList<T>? = null) : List<T> {
     override fun contains(element: T): Boolean {
         return list?.contains(element) ?: false
     }
+
+    companion object {
+        private val EMPTY = MutableOrEmptyList<Nothing>(null)
+
+        @Suppress("UNCHECKED_CAST")
+        fun <T> empty(): MutableOrEmptyList<T> = EMPTY as MutableOrEmptyList<T>
+    }
 }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/builder/FirBuilderDsl.kt

@@ -11,8 +11,8 @@ import org.jetbrains.kotlin.fir.MutableOrEmptyList
 annotation class FirBuilderDsl
 
 fun <T> MutableList<T>.toMutableOrEmpty(): MutableOrEmptyList<T> =
-    if (isEmpty()) MutableOrEmptyList() else MutableOrEmptyList(this)
+    if (isEmpty()) MutableOrEmptyList.empty() else MutableOrEmptyList(this)
 
 @JvmName("toMutableOrEmptyForImmutable")
 fun <T> List<T>.toMutableOrEmpty(): MutableOrEmptyList<T> =
-    if (isEmpty()) MutableOrEmptyList() else MutableOrEmptyList(this.toMutableList())
+    if (isEmpty()) MutableOrEmptyList.empty() else MutableOrEmptyList(this.toMutableList())

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirDefaultPropertyAccessor.kt

@@ -61,7 +61,7 @@ abstract class FirDefaultPropertyAccessor(
     symbol,
     propertySymbol,
     isGetter,
-    annotations = MutableOrEmptyList(),
+    annotations = MutableOrEmptyList.empty(),
     typeParameters = mutableListOf(),
 ) {
     override val dispatchReceiverType: ConeSimpleKotlinType?

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

@@ -25,7 +25,7 @@ class FirSingleExpressionBlock(
 ) : FirBlock() {
     override val source: KtSourceElement?
         get() = statement.source?.fakeElement(KtFakeSourceElementKind.SingleExpressionBlock)
-    override var annotations: MutableOrEmptyList<FirAnnotation> = MutableOrEmptyList()
+    override var annotations: MutableOrEmptyList<FirAnnotation> = MutableOrEmptyList.empty()
     override val statements: List<FirStatement> get() = listOf(statement)
     override var typeRef: FirTypeRef = FirImplicitTypeRefImpl(null)
 

compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirErrorTypeRefImpl.kt

@@ -32,7 +32,7 @@ internal class FirErrorTypeRefImpl(
         isFromStubType
     )
 
-    override val annotations: MutableOrEmptyList<FirAnnotation> = MutableOrEmptyList()
+    override val annotations: MutableOrEmptyList<FirAnnotation> = MutableOrEmptyList.empty()
 
     override fun <R, D> acceptChildren(visitor: FirVisitor<R, D>, data: D) {
         annotations.forEach { it.accept(visitor, data) }

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

@@ -195,7 +195,7 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
 
         impl(errorLoop) {
             default("block", "FirEmptyExpressionBlock()")
-            default("condition", "FirErrorExpressionImpl(source, MutableOrEmptyList(), ConeStubDiagnostic(diagnostic), null, null)")
+            default("condition", "FirErrorExpressionImpl(source, MutableOrEmptyList.empty(), ConeStubDiagnostic(diagnostic), null, null)")
             useTypes(emptyExpressionBlock, coneStubDiagnosticType)
         }