Add ReadOnlySet

Author: odersky

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

@@ -2,10 +2,7 @@ package dotty.tools.dotc.util
 
 /** A common class for lightweight mutable sets.
  */
-abstract class MutableSet[T] {
-
-  /** The entry in the set such that `isEqual(x, entry)`, or else `null`. */
-  def lookup(x: T): T | Null
+abstract class MutableSet[T] extends ReadOnlySet[T]:
 
   /** Add element `x` to the set */
   def +=(x: T): Unit
@@ -17,14 +14,3 @@ abstract class MutableSet[T] {
 
   def clear(): Unit
 
-  def size: Int
-
-  def iterator: Iterator[T]
-
-  def contains(x: T): Boolean = lookup(x) != null
-
-  def foreach[U](f: T => U): Unit = iterator foreach f
-
-  def toList: List[T] = iterator.toList
-
-}

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

@@ -0,0 +1,24 @@
+package dotty.tools.dotc.util
+
+/** A class for the readonly part of mutable sets.
+ */
+abstract class ReadOnlySet[T]:
+
+  /** The entry in the set such that `isEqual(x, entry)`, or else `null`. */
+  def lookup(x: T): T | Null
+
+  def size: Int
+
+  def iterator: Iterator[T]
+
+  def contains(x: T): Boolean = lookup(x) != null
+
+  def foreach[U](f: T => U): Unit = iterator.foreach(f)
+
+  def toList: List[T] = iterator.toList
+
+  def isEmpty = size == 0
+
+object ReadOnlySet:
+  def empty[T]: ReadOnlySet[T] = HashSet[T](4)
+