Transitively handle nested binds

Adding ability to process additional maps at the end of the original flatmap clause.

slick/src/main/scala/slick/compiler/UnrollTailBinds.scala

@@ -16,32 +16,46 @@ class UnrollTailBinds extends Phase {
   def tr(n: Node): Node = {
     n match {
       case bb@Bind(br,
-      Bind(bo, Filter(fa, ff1, where1), Filter(fb, ff2, where2)),
+      downstream:Bind,
       Bind(bi, bf, select)) => {
-          // make a new symbol
-          val bm = new AnonSymbol
-          def rep(node: Node) = {
-            def repInternal(node: Node): Node = {
-              val out = node.replace({
-                case p@Path(brs :: tail) if brs == br => {
-                  Path(List(bm) ++ tail)
-                }
-              }, bottomUp = true, keepType = true)
-              out.mapChildren(repInternal)
+
+        // make a new symbol
+        val bm = new AnonSymbol
+        def rep(node: Node) = {
+          def repInternal(node: Node): Node = {
+            val out = node.replace({
+              case p@Path(brs :: tail) if brs == br => {
+                Path(List(bm) ++ tail)
+              }
+            }, bottomUp = true, keepType = true)
+            out.mapChildren(repInternal)
+          }
+          repInternal(node)
+        }
+
+        def replaceDownstream(node:Bind):Option[Node] = {
+          node match {
+            case Bind(bo, Filter(fa, ff1, where1), Filter(fb, ff2, where2)) => {
+              val out = Bind(bo,
+                Filter(fa, rep(ff1), rep(where1)),
+                Bind(bm,
+                  Filter(fb, rep(ff2), rep(where2)),
+                  Bind(bi, rep(bf), rep(select))))
+              Some(out)
             }
-            repInternal(node)
+            case Bind(bo1, bi1:Bind, sel1) => replaceDownstream(bi1).map(
+              replacement => Bind(bo1, replacement, sel1))
+            case n => None
           }
-          // bind all needed elements to the new symbol
-          val bindCombo = Bind(bo,
-            Filter(fa, rep(ff1), rep(where1)), //replace s4 here with s5
-            Bind(bm,
-              Filter(fb, rep(ff2), rep(where2)), // replace s4 here with s5
-              Bind(bi, rep(bf), rep(select)))) // replace s4 here with s5
-
-          bindCombo.mapChildren(tr)
         }
-      case n => n.mapChildren(tr)
 
+        // bind all needed elements to the new symbol
+        val bindCombo = replaceDownstream(downstream).getOrElse(bb)
+
+        bindCombo.mapChildren(tr)
+      }
+
+      case n => n.mapChildren(tr)
     }
   }