Use HashSet to record seen items

Use HashSet to record seen items in type size and covering set computations.

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -5577,11 +5577,11 @@ object Types {
   }
 
   class TypeSizeAccumulator(using Context) extends TypeAccumulator[Int] {
-    val seen = new java.util.IdentityHashMap[Type, Type]
+    var seen = util.HashSet[Type](initialCapacity = 8)
     def apply(n: Int, tp: Type): Int =
-      if (seen.get(tp) != null) n
+      if seen.contains(tp) then n
       else {
-        seen.put(tp, tp)
+        seen += tp
         tp match {
           case tp: AppliedType =>
             foldOver(n + 1, tp)
@@ -5598,11 +5598,11 @@ object Types {
   }
 
   class CoveringSetAccumulator(using Context) extends TypeAccumulator[Set[Symbol]] {
-    val seen = new java.util.IdentityHashMap[Type, Type]
+    var seen = util.HashSet[Type](initialCapacity = 8)
     def apply(cs: Set[Symbol], tp: Type): Set[Symbol] =
-      if (seen.get(tp) != null) cs
+      if seen.contains(tp) then cs
       else {
-        seen.put(tp, tp)
+        seen += tp
         tp match {
           case tp if tp.isTopType || tp.isBottomType =>
             cs

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

@@ -1,10 +1,9 @@
 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.
+/** A linear set is a set here 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]