Merge pull request scala#8789 from mkeskells/2.12.x_SeqList1

avoid creation of ListBuffers for Seq.empty and Seq()

Author: retronym

src/library/scala/collection/generic/GenericCompanion.scala

@@ -39,7 +39,10 @@ abstract class GenericCompanion[+CC[X] <: GenTraversable[X]] {
   /** An empty collection of type `$Coll[A]`
    *  @tparam A      the type of the ${coll}'s elements
    */
-  def empty[A]: CC[A] = newBuilder[A].result()
+  def empty[A]: CC[A] = {
+    if ((this eq immutable.Seq) || (this eq collection.Seq)) Nil.asInstanceOf[CC[A]]
+    else newBuilder[A].result()
+  }
 
   /** Creates a $coll with the specified elements.
    *  @tparam A      the type of the ${coll}'s elements

test/junit/scala/collection/SeqTest.scala

@@ -0,0 +1,16 @@
+package scala.collection
+
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+import scala.tools.testing.AllocationTest
+
+@RunWith(classOf[JUnit4])
+class SeqTest extends AllocationTest{
+
+  @Test def emptyNonAllocating(): Unit = {
+    nonAllocating(Seq.empty)
+    nonAllocating(Seq())
+  }
+}

test/junit/scala/collection/immutable/ListTest.scala

@@ -5,9 +5,10 @@ import org.junit.runner.RunWith
 import org.junit.runners.JUnit4
 
 import scala.ref.WeakReference
+import scala.tools.testing.AllocationTest
 
 @RunWith(classOf[JUnit4])
-class ListTest {
+class ListTest extends AllocationTest{
   /**
    * Test that empty iterator does not hold reference
    * to complete List
@@ -46,4 +47,9 @@ class ListTest {
     // real assertion
     Assert.assertTrue(emptyIterators.exists(_._2.get.isEmpty))
   }
+
+  @Test def emptyNonAllocating(): Unit = {
+    nonAllocating(List.empty)
+    nonAllocating(List())
+  }
 }

test/junit/scala/collection/immutable/SeqTest.scala

@@ -0,0 +1,16 @@
+package scala.collection.immutable
+
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+import org.junit.{Assert, Test}
+
+import scala.tools.testing.AllocationTest
+
+@RunWith(classOf[JUnit4])
+class SeqTest extends AllocationTest{
+
+  @Test def emptyNonAllocating(): Unit = {
+    nonAllocating(Seq.empty)
+    nonAllocating(Seq())
+  }
+}