avoid unnecessary creation of empty containers

core/descriptors/src/org/jetbrains/kotlin/resolve/scopes/ChainedScope.kt

@@ -19,6 +19,7 @@ package org.jetbrains.kotlin.resolve.scopes
 import org.jetbrains.kotlin.descriptors.*
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.utils.Printer
+import org.jetbrains.kotlin.util.collectionUtils.concat
 import java.util.*
 
 public open class ChainedScope(
@@ -38,15 +39,11 @@ public open class ChainedScope(
         return null
     }
 
-    private inline fun getFromAllScopes<T>(callback: (JetScope) -> Collection<T>): Set<T> {
+    private inline fun getFromAllScopes<T>(callback: (JetScope) -> Collection<T>): Collection<T> {
         if (scopeChain.isEmpty()) return emptySet()
-        var result: MutableSet<T>? = null
+        var result: Collection<T>? = null
         for (scope in scopeChain) {
-            val fromScope = callback(scope)
-            if (result == null) {
-                result = LinkedHashSet<T>()
-            }
-            result.addAll(fromScope)
+            result = result.concat(callback(scope))
         }
         return result ?: emptySet()
     }
@@ -57,13 +54,13 @@ public open class ChainedScope(
     override fun getPackage(name: Name): PackageViewDescriptor?
             = getFirstMatch { it.getPackage(name) }
 
-    override fun getProperties(name: Name): Set<VariableDescriptor>
+    override fun getProperties(name: Name): Collection<VariableDescriptor>
             = getFromAllScopes { it.getProperties(name) }
 
     override fun getLocalVariable(name: Name): VariableDescriptor?
             = getFirstMatch { it.getLocalVariable(name) }
 
-    override fun getFunctions(name: Name): Set<FunctionDescriptor>
+    override fun getFunctions(name: Name): Collection<FunctionDescriptor>
             = getFromAllScopes { it.getFunctions(name) }
 
     override fun getImplicitReceiversHierarchy(): List<ReceiverParameterDescriptor> {

core/descriptors/src/org/jetbrains/kotlin/util/collectionUtils.kt

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2010-2015 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.kotlin.util.collectionUtils
+
+import java.util.*
+
+/**
+ * Concatenates the contents of this collection with the given collection, avoiding allocations if possible.
+ * Can modify `this` if it is a mutable collection.
+ */
+fun <T> Collection<T>?.concat(collection: Collection<T>): Collection<T>? {
+    if (collection.isEmpty()) {
+        return this
+    }
+    if (this == null) {
+        return collection
+    }
+    if (this is LinkedHashSet<*>) {
+        addAll(collection)
+        return this
+    }
+
+    val result = LinkedHashSet(this)
+    result.addAll(collection)
+    return result
+}
+
+fun concatInOrder<T>(c1: Collection<T>?, c2: Collection<T>?): Collection<T> {
+    val result = if (c1 == null || c1.isEmpty())
+        c2
+    else if (c2 == null || c2.isEmpty())
+        c1
+    else {
+        val result = LinkedHashSet<T>()
+        result.addAll(c1)
+        result.addAll(c2)
+        result
+    }
+    return result ?: emptySet()
+}