diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/AbstractLazyMemberScope.java b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/AbstractLazyMemberScope.java
index 3fbbd2bd7bf..a97414dfd39 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/AbstractLazyMemberScope.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/AbstractLazyMemberScope.java
@@ -39,6 +39,7 @@ import org.jetbrains.jet.storage.MemoizedFunctionToNotNull;
 import org.jetbrains.jet.storage.NotNullLazyValue;
 import org.jetbrains.jet.storage.StorageManager;
 import org.jetbrains.jet.utils.Printer;
+import org.jetbrains.jet.utils.UtilsPackage;
 
 import java.util.*;
 
@@ -106,7 +107,7 @@ public abstract class AbstractLazyMemberScope<D extends DeclarationDescriptor, D
     private List<ClassDescriptor> resolveClassDescriptor(@NotNull final Name name) {
         Collection<JetClassLikeInfo> classOrObjectDeclarations = declarationProvider.getClassOrObjectDeclarations(name);
 
-        return ContainerUtil.mapNotNull(classOrObjectDeclarations, new Function<JetClassLikeInfo, ClassDescriptor>() {
+        return UtilsPackage.toReadOnlyList(ContainerUtil.mapNotNull(classOrObjectDeclarations, new Function<JetClassLikeInfo, ClassDescriptor>() {
             @Override
             public ClassDescriptor fun(JetClassLikeInfo classLikeInfo) {
                 // SCRIPT: Creating a script class
@@ -115,7 +116,7 @@ public abstract class AbstractLazyMemberScope<D extends DeclarationDescriptor, D
                 }
                 return new LazyClassDescriptor(resolveSession, thisDescriptor, name, classLikeInfo);
             }
-        });
+        }));
     }
 
     @Override
@@ -153,7 +154,7 @@ public abstract class AbstractLazyMemberScope<D extends DeclarationDescriptor, D
 
         getNonDeclaredFunctions(name, result);
 
-        return result;
+        return UtilsPackage.toReadOnlySet(result);
     }
 
     @NotNull
@@ -189,7 +190,7 @@ public abstract class AbstractLazyMemberScope<D extends DeclarationDescriptor, D
 
         getNonDeclaredProperties(name, result);
 
-        return result;
+        return UtilsPackage.toReadOnlySet(result);
     }
 
     protected abstract void getNonDeclaredProperties(@NotNull Name name, @NotNull Set<VariableDescriptor> result);
@@ -252,8 +253,8 @@ public abstract class AbstractLazyMemberScope<D extends DeclarationDescriptor, D
                 throw new IllegalArgumentException("Unsupported declaration kind: " + declaration);
             }
         }
-        result.trimToSize();
-        return result;
+
+        return UtilsPackage.toReadOnlyList(result);
     }
 
     @NotNull
diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/LazyClassMemberScope.java b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/LazyClassMemberScope.java
index 2b23821ec36..021d2a75cfb 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/LazyClassMemberScope.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/LazyClassMemberScope.java
@@ -36,6 +36,7 @@ import org.jetbrains.jet.lang.types.DeferredType;
 import org.jetbrains.jet.lang.types.JetType;
 import org.jetbrains.jet.lang.types.lang.KotlinBuiltIns;
 import org.jetbrains.jet.storage.NullableLazyValue;
+import org.jetbrains.jet.utils.UtilsPackage;
 
 import java.util.*;
 
@@ -299,7 +300,7 @@ public class LazyClassMemberScope extends AbstractLazyMemberScope<LazyClassDescr
     @Override
     @NotNull
     protected Collection<DeclarationDescriptor> computeExtraDescriptors() {
-        ArrayList<DeclarationDescriptor> result = new ArrayList<DeclarationDescriptor>();
+        List<DeclarationDescriptor> result = new ArrayList<DeclarationDescriptor>();
         for (JetType supertype : thisDescriptor.getTypeConstructor().getSupertypes()) {
             for (DeclarationDescriptor descriptor : supertype.getMemberScope().getAllDescriptors()) {
                 if (descriptor instanceof FunctionDescriptor) {
@@ -314,8 +315,7 @@ public class LazyClassMemberScope extends AbstractLazyMemberScope<LazyClassDescr
 
         addDataClassMethods(result);
 
-        result.trimToSize();
-        return result;
+        return UtilsPackage.toReadOnlyList(result);
     }
 
     private void addDataClassMethods(@NotNull Collection<DeclarationDescriptor> result) {
diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/LazyScriptClassMemberScope.java b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/LazyScriptClassMemberScope.java
index 456b297af8f..3e625bfc14b 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/LazyScriptClassMemberScope.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/lazy/descriptors/LazyScriptClassMemberScope.java
@@ -30,6 +30,7 @@ import org.jetbrains.jet.lang.resolve.lazy.data.JetScriptInfo;
 import org.jetbrains.jet.lang.resolve.lazy.declarations.ClassMemberDeclarationProvider;
 import org.jetbrains.jet.lang.resolve.name.Name;
 import org.jetbrains.jet.storage.NotNullLazyValue;
+import org.jetbrains.jet.utils.UtilsPackage;
 
 import java.util.Collection;
 import java.util.Set;
@@ -61,13 +62,13 @@ public class LazyScriptClassMemberScope extends LazyClassMemberScope {
     @NotNull
     @Override
     protected Collection<DeclarationDescriptor> computeExtraDescriptors() {
-        return KotlinPackage.plus(
+        return UtilsPackage.toReadOnlyList(KotlinPackage.plus(
                 super.computeExtraDescriptors(),
                 KotlinPackage.plus(
                     getProperties(Name.identifier(ScriptDescriptor.LAST_EXPRESSION_VALUE_FIELD_NAME)),
                     getPropertiesForScriptParameters()
                 )
-        );
+        ));
     }
 
     private Collection<VariableDescriptor> getPropertiesForScriptParameters() {
diff --git a/core/descriptor.loader.java/src/org/jetbrains/jet/lang/resolve/java/lazy/descriptors/LazyJavaMemberScope.kt b/core/descriptor.loader.java/src/org/jetbrains/jet/lang/resolve/java/lazy/descriptors/LazyJavaMemberScope.kt
index 626c3a4b2e9..fc53aa80885 100644
--- a/core/descriptor.loader.java/src/org/jetbrains/jet/lang/resolve/java/lazy/descriptors/LazyJavaMemberScope.kt
+++ b/core/descriptor.loader.java/src/org/jetbrains/jet/lang/resolve/java/lazy/descriptors/LazyJavaMemberScope.kt
@@ -100,7 +100,7 @@ public abstract class LazyJavaMemberScope(
             }
         }
 
-        ArrayList(functions)
+        functions.toReadOnlyList()
     }
 
     data class MethodSignatureData(
@@ -237,8 +237,7 @@ public abstract class LazyJavaMemberScope(
 
         computeNonDeclaredProperties(name, properties)
 
-        properties.trimToSize()
-        properties
+        properties.toReadOnlyList()
     }
 
     private fun resolveProperty(field: JavaField): PropertyDescriptor {
@@ -303,7 +302,7 @@ public abstract class LazyJavaMemberScope(
     override fun getOwnDeclaredDescriptors() = getAllDescriptors()
     override fun getAllDescriptors() = _allDescriptors()
 
-    private fun computeAllDescriptors(): MutableCollection<DeclarationDescriptor> {
+    private fun computeAllDescriptors(): List<DeclarationDescriptor> {
         val result = LinkedHashSet<DeclarationDescriptor>()
 
         for (name in getAllClassNames()) {
@@ -324,7 +323,7 @@ public abstract class LazyJavaMemberScope(
 
         addExtraDescriptors(result)
 
-        return ArrayList(result)
+        return result.toReadOnlyList()
     }
 
     protected open fun addExtraDescriptors(result: MutableSet<DeclarationDescriptor>) {
diff --git a/core/serialization/src/org/jetbrains/jet/descriptors/serialization/descriptors/DeserializedMemberScope.java b/core/serialization/src/org/jetbrains/jet/descriptors/serialization/descriptors/DeserializedMemberScope.java
index c1f4e76d381..31bed719ab6 100644
--- a/core/serialization/src/org/jetbrains/jet/descriptors/serialization/descriptors/DeserializedMemberScope.java
+++ b/core/serialization/src/org/jetbrains/jet/descriptors/serialization/descriptors/DeserializedMemberScope.java
@@ -31,6 +31,7 @@ import org.jetbrains.jet.storage.MemoizedFunctionToNotNull;
 import org.jetbrains.jet.storage.NotNullLazyValue;
 import org.jetbrains.jet.storage.StorageManager;
 import org.jetbrains.jet.utils.Printer;
+import org.jetbrains.jet.utils.UtilsPackage;
 
 import java.util.*;
 
@@ -135,7 +136,7 @@ public abstract class DeserializedMemberScope implements JetScope {
     private Collection<FunctionDescriptor> computeFunctions(@NotNull Name name) {
         Collection<FunctionDescriptor> descriptors = computeMembersByName(name, FUNCTION);
         computeNonDeclaredFunctions(name, descriptors);
-        return new ArrayList<FunctionDescriptor>(descriptors);
+        return UtilsPackage.toReadOnlyList(descriptors);
     }
 
     protected void computeNonDeclaredFunctions(@NotNull Name name, @NotNull Collection<FunctionDescriptor> functions) {
@@ -152,7 +153,7 @@ public abstract class DeserializedMemberScope implements JetScope {
         Collection<PropertyDescriptor> descriptors = computeMembersByName(name, PROPERTY);
         computeNonDeclaredProperties(name, descriptors);
         //noinspection unchecked
-        return new ArrayList<VariableDescriptor>((Collection) descriptors);
+        return UtilsPackage.<VariableDescriptor>toReadOnlyList(descriptors);
     }
 
     protected void computeNonDeclaredProperties(@NotNull Name name, @NotNull Collection<PropertyDescriptor> descriptors) {
@@ -211,7 +212,7 @@ public abstract class DeserializedMemberScope implements JetScope {
 
         addAllClassDescriptors(result);
 
-        return result;
+        return UtilsPackage.toReadOnlyList(result);
     }
 
     protected abstract void addNonDeclaredDescriptors(@NotNull Collection<DeclarationDescriptor> result);
diff --git a/core/util.runtime/src/org/jetbrains/jet/utils/collections.kt b/core/util.runtime/src/org/jetbrains/jet/utils/collections.kt
index 1278d44c9c2..5b97947b864 100644
--- a/core/util.runtime/src/org/jetbrains/jet/utils/collections.kt
+++ b/core/util.runtime/src/org/jetbrains/jet/utils/collections.kt
@@ -20,6 +20,7 @@ import java.util.LinkedHashMap
 import java.util.ArrayList
 import java.util.HashMap
 import java.util.HashSet
+import java.util.Collections
 
 public fun <K, V> Stream<V>.valuesToMap(key: (V) -> K): Map<K, V> {
     val map = LinkedHashMap<K, V>()
@@ -94,3 +95,9 @@ public fun <K, V> newHashMapWithExpectedSize(expectedSize: Int): HashMap<K, V> {
 public fun <E> newHashSetWithExpectedSize(expectedSize: Int): HashSet<E> {
     return HashSet(if (expectedSize < 3) 3 else expectedSize + expectedSize / 3 + 1)
 }
+
+public fun <T> Collection<T>.toReadOnlyList(): List<T> =
+        if (isEmpty()) Collections.emptyList() else ArrayList(this)
+
+public fun <T> Collection<T>.toReadOnlySet(): Set<T> =
+        if (isEmpty()) Collections.emptySet() else HashSet(this)
\ No newline at end of file