[nomaster] duplicate tailImpl as a private method

Reworks d526f8b.
This is necessary to maintain binary compatibility with 2.10.0.

    matchName="scala.collection.mutable.MutableList.tailImpl"
    problemName=MissingMethodProblem

Author: adriaanm

bincompat-forward.whitelist.conf

@@ -1,10 +1,10 @@
 filter {
     problems=[
         # rework d526f8bd74 to duplicate tailImpl as a private method
-        {
-            matchName="scala.collection.mutable.MutableList.tailImpl"
-            problemName=MissingMethodProblem
-        },
+        # {
+        #     matchName="scala.collection.mutable.MutableList.tailImpl"
+        #     problemName=MissingMethodProblem
+        # },
         {
             # can only be called from Stream::distinct, which cannot itself be inlined, so distinct is the only feasible call-site
             matchName="scala.collection.immutable.Stream.scala$collection$immutable$Stream$$loop$6"

src/library/scala/collection/mutable/MutableList.scala

@@ -61,7 +61,8 @@ extends AbstractSeq[A]
     tl
   }
 
-  protected final def tailImpl(tl: MutableList[A]) {
+  // this method must be private for binary compatibility
+  private final def tailImpl(tl: MutableList[A]) {
     require(nonEmpty, "tail of empty list")
     tl.first0 = first0.tail
     tl.len = len - 1

src/library/scala/collection/mutable/Queue.scala

@@ -167,6 +167,13 @@ extends MutableList[A]
    */
   def front: A = head
 
+  // this method (duplicated from MutableList) must be private for binary compatibility
+  private final def tailImpl(tl: Queue[A]) {
+    require(nonEmpty, "tail of empty list")
+    tl.first0 = first0.tail
+    tl.len = len - 1
+    tl.last0 = if (tl.len == 0) tl.first0 else last0
+  }
 
   // TODO - Don't override this just for new to create appropriate type....
   override def tail: Queue[A] = {