Hash-cons FreshCaps

Have at most one FreshCap for a given prefix and hidden set.

Author: odersky

compiler/src/dotty/tools/dotc/cc/Capability.scala

@@ -159,7 +159,11 @@ object Capabilities:
 
     def derivedFreshCap(newPrefix: Type)(using Context): FreshCap =
       if newPrefix eq prefix then this
-      else FreshCap(newPrefix)(owner, origin, hiddenSet)
+      else if newPrefix eq hiddenSet.owningCap.prefix then
+        hiddenSet.owningCap
+      else
+        hiddenSet.derivedCaps
+          .getOrElseUpdate(newPrefix, FreshCap(newPrefix)(owner, origin, hiddenSet))
 
     //assert(rootId != 10, i"fresh $prefix, ${ctx.owner}")
 

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -12,7 +12,7 @@ import annotation.internal.sharable
 import reporting.trace
 import printing.{Showable, Printer}
 import printing.Texts.*
-import util.{SimpleIdentitySet, Property}
+import util.{SimpleIdentitySet, Property, EqHashMap}
 import typer.ErrorReporting.Addenda
 import scala.collection.{mutable, immutable}
 import TypeComparer.ErrorNote
@@ -1168,6 +1168,9 @@ object CaptureSet:
 
     override def owner = givenOwner
 
+    /** The FreshCaps generated by derivedFreshCap, indexed by prefix */
+    val derivedCaps = new EqHashMap[Type, FreshCap]()
+
     //assert(id != 3)
 
     description = i"of elements subsumed by a fresh cap in $initialOwner"