Use LinearSet instead of mutable.HashSet in TailRec

Author: odersky

compiler/src/dotty/tools/dotc/transform/TailRec.scala

@@ -13,6 +13,7 @@ import core.StdNames.nme
 import core.Symbols._
 import reporting._
 import transform.MegaPhase.MiniPhase
+import util.LinearSet
 
 import scala.collection.mutable
 
@@ -258,7 +259,7 @@ class TailRec extends MiniPhase {
       }
 
     /** Symbols of Labeled blocks that are in tail position. */
-    private val tailPositionLabeledSyms = new mutable.HashSet[Symbol]()
+    private var tailPositionLabeledSyms = LinearSet.empty[Symbol]
 
     private var inTailPosition = true
 
@@ -283,7 +284,7 @@ class TailRec extends MiniPhase {
      *  a recursive call of a @tailrec annotated method (i.e. `isMandatory`).
      */
     private def isTraversalNeeded =
-      isMandatory || tailPositionLabeledSyms.nonEmpty
+      isMandatory || tailPositionLabeledSyms.size > 0
 
     def noTailTransform(tree: Tree)(using Context): Tree =
       if (isTraversalNeeded) transform(tree, tailPosition = false)

compiler/src/dotty/tools/dotc/util/LinearSet.scala

@@ -0,0 +1,46 @@
+package dotty.tools.dotc.util
+import collection.immutable
+
+/** A linear identity set is a set that uses `eq` as the underlying
+ *  equality where after a `+` the previous set value cannot be used anymore.
+ *  The set is implemented as an immutable set for
+ *  sizes <= 4 and as a HashSet for larger sizes.
+ */
+opaque type LinearSet[Elem >: Null <: AnyRef] =
+  immutable.Set[Elem] | HashSet[Elem]
+
+object LinearSet:
+
+  def empty[Elem >: Null <: AnyRef]: LinearSet[Elem] = immutable.Set.empty[Elem]
+
+  extension [Elem >: Null <: AnyRef](s: LinearSet[Elem]):
+
+    def contains(elem: Elem): Boolean = (s: @unchecked) match
+      case s: immutable.AbstractSet[Elem] @unchecked => s.contains(elem)
+      case s: HashSet[Elem] @unchecked => s.contains(elem)
+
+    def + (elem: Elem): LinearSet[Elem] = (s: @unchecked) match
+      case s: immutable.AbstractSet[Elem] @unchecked =>
+        if s.size < 4 then
+          s + elem
+        else
+          val s1 = HashSet[Elem](initialCapacity = 8)
+          s.foreach(s1 += _)
+          s1 += elem
+          s1
+      case s: HashSet[Elem] @unchecked =>
+        s += elem
+        s
+
+    def - (elem: Elem): LinearSet[Elem] = (s: @unchecked) match
+      case s: immutable.AbstractSet[Elem] @unchecked =>
+        s - elem
+      case s: HashSet[Elem] @unchecked =>
+        s -= elem
+        s
+
+    def size = (s: @unchecked) match
+      case s: immutable.AbstractSet[Elem] @unchecked => s.size
+      case s: HashSet[Elem] @unchecked => s.size
+
+end LinearSet