Improve toReadOnlyList(), consider the case when size = 1

core/descriptors/src/org/jetbrains/kotlin/descriptors/impl/FunctionDescriptorImpl.java

@@ -26,6 +26,7 @@ import org.jetbrains.kotlin.types.DescriptorSubstitutor;
 import org.jetbrains.kotlin.types.JetType;
 import org.jetbrains.kotlin.types.TypeSubstitutor;
 import org.jetbrains.kotlin.types.Variance;
+import org.jetbrains.kotlin.utils.UtilsPackage;
 
 import java.util.*;
 
@@ -64,7 +65,7 @@ public abstract class FunctionDescriptorImpl extends DeclarationDescriptorNonRoo
             @Nullable Modality modality,
             @NotNull Visibility visibility
     ) {
-        this.typeParameters = new ArrayList<TypeParameterDescriptor>(typeParameters);
+        this.typeParameters = UtilsPackage.toReadOnlyList(typeParameters);
         this.unsubstitutedValueParameters = unsubstitutedValueParameters;
         this.unsubstitutedReturnType = unsubstitutedReturnType;
         this.modality = modality;

core/deserialization/src/org/jetbrains/kotlin/serialization/deserialization/MemberDeserializer.kt

@@ -28,6 +28,7 @@ import org.jetbrains.kotlin.serialization.ProtoBuf.Callable.CallableKind.FUN
 import org.jetbrains.kotlin.serialization.ProtoBuf.Callable.CallableKind.VAL
 import org.jetbrains.kotlin.serialization.ProtoBuf.Callable.CallableKind.VAR
 import org.jetbrains.kotlin.serialization.deserialization.descriptors.*
+import org.jetbrains.kotlin.utils.toReadOnlyList
 
 public class MemberDeserializer(private val c: DeserializationContext) {
     public fun loadCallable(proto: Callable): CallableMemberDescriptor {
@@ -187,7 +188,7 @@ public class MemberDeserializer(private val c: DeserializationContext) {
                     if (proto.hasVarargElementType()) c.typeDeserializer.type(proto.getVarargElementType()) else null,
                     SourceElement.NO_SOURCE
             )
-        }
+        }.toReadOnlyList()
     }
 
     private fun getParameterAnnotations(

core/util.runtime/src/org/jetbrains/kotlin/utils/collections.kt

@@ -16,12 +16,10 @@
 
 package org.jetbrains.kotlin.utils
 
-import java.util.LinkedHashMap
 import java.util.ArrayList
 import java.util.HashMap
 import java.util.HashSet
-import java.util.Collections
-import java.util.LinkedHashSet
+import java.util.LinkedHashMap
 
 public fun <K, V> Sequence<V>.valuesToMap(key: (V) -> K): Map<K, V> {
     val map = LinkedHashMap<K, V>()
@@ -102,6 +100,11 @@ public fun <E> newHashSetWithExpectedSize(expectedSize: Int): HashSet<E> {
 }
 
 public fun <T> Collection<T>.toReadOnlyList(): List<T> =
-        if (isEmpty()) Collections.emptyList() else ArrayList(this)
+        when (size()) {
+            0 -> emptyList()
+            1 -> listOf(first())
+            else -> ArrayList(this)
+        }
 
-public fun <T: Any> T?.singletonOrEmptyList(): List<T> = if (this != null) Collections.singletonList(this) else Collections.emptyList()
+public fun <T: Any> T?.singletonOrEmptyList(): List<T> =
+        if (this != null) listOf(this) else emptyList()