Make capture-sets thread-safe again

Author: odersky

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

@@ -512,17 +512,13 @@ object CaptureSet:
   val emptyRefs: Refs = SimpleIdentitySet.empty
 
   /** The empty capture set `{}` */
+  @sharable // sharable since the set is empty, so setMutable is a no-op
   val empty: CaptureSet.Const = Const(emptyRefs)
 
   /** The universal capture set `{cap}` */
   def universal(using Context): Const =
     Const(SimpleIdentitySet(GlobalCap))
 
-  /** The same as {cap.rd} but generated implicitly for
-   * references of Capability subtypes
-   */
-  val csImpliedByCapability = Const(SimpleIdentitySet(GlobalCap.readOnly))
-
   def fresh(origin: Origin)(using Context): Const =
     FreshCap(origin).singletonCaptureSet
 
@@ -572,7 +568,8 @@ object CaptureSet:
     private var isComplete = true
 
     def setMutable()(using Context): Unit =
-      isComplete = false // delay computation of Mutability status
+      if !elems.isEmpty then
+        isComplete = false // delay computation of Mutability status
 
     override def mutability(using Context): Mutability =
       if !isComplete then
@@ -589,12 +586,17 @@ object CaptureSet:
       then i" under-approximating the result of mapping $ref to $mapped"
       else ""
 
+  /* The same as {cap.rd} but generated implicitly for references of Capability subtypes.
+   */
+  case class CSImpliedByCapability() extends Const(SimpleIdentitySet(GlobalCap.readOnly))
+
   /** A special capture set that gets added to the types of symbols that were not
    *  themselves capture checked, in order to admit arbitrary corresponding capture
    *  sets in subcapturing comparisons. Similar to platform types for explicit
    *  nulls, this provides more lenient checking against compilation units that
    *  were not yet compiled with capture checking on.
    */
+  @sharable // sharable since the set is empty, so setMutable is a no-op
   object Fluid extends Const(emptyRefs):
     override def isAlwaysEmpty(using Context) = false
     override def addThisElem(elem: Capability)(using Context, VarState) = true

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

@@ -367,7 +367,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
      */
     def stripImpliedCaptureSet(tp: Type): Type = tp match
       case tp @ CapturingType(parent, refs)
-      if (refs eq CaptureSet.csImpliedByCapability) && !tp.isBoxedCapturing =>
+      if refs.isInstanceOf[CaptureSet.CSImpliedByCapability] && !tp.isBoxedCapturing =>
         parent
       case tp: AliasingBounds =>
         tp.derivedAlias(stripImpliedCaptureSet(tp.alias))
@@ -430,7 +430,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
           && !t.isSingleton
           && (!sym.isConstructor || (t ne tp.finalResultType))
             // Don't add ^ to result types of class constructors deriving from Capability
-        then CapturingType(t, CaptureSet.csImpliedByCapability, boxed = false)
+        then CapturingType(t, CaptureSet.CSImpliedByCapability(), boxed = false)
         else normalizeCaptures(mapFollowingAliases(t))
 
       def innerApply(t: Type) =