Tweak HiddenSet

Avoid var for owningCap

Author: odersky

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

@@ -138,8 +138,8 @@ object Capabilities:
    *                 for diagnostics
    */
   case class FreshCap private (owner: Symbol, origin: Origin)(using @constructorOnly ctx: Context) extends RootCapability:
-    val hiddenSet = CaptureSet.HiddenSet(owner)
-    hiddenSet.owningCap = this
+    val hiddenSet = CaptureSet.HiddenSet(owner, this: @unchecked)
+      // fails initialization check without the @unchecked
 
     override def equals(that: Any) = that match
       case that: FreshCap => this eq that

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

@@ -950,16 +950,15 @@ object CaptureSet:
    *  which are already subject through snapshotting and rollbacks in VarState.
    *  It's advantageous if we don't need to deal with other pieces of state there.
    */
-  class HiddenSet(initialOwner: Symbol)(using @constructorOnly ictx: Context)
+  class HiddenSet(initialOwner: Symbol, val owningCap: FreshCap)(using @constructorOnly ictx: Context)
   extends Var(initialOwner):
-    var owningCap: FreshCap = uninitialized // initialized when owning FreshCap is created
     var givenOwner: Symbol = initialOwner
 
     override def owner = givenOwner
 
-    //assert(id != 4)
+    //assert(id != 3)
 
-    description = i"elements subsumed by $owningCap"
+    description = i"of elements subsumed by a fresh cap in $initialOwner"
 
     private def aliasRef: FreshCap | Null =
       if myElems.size == 1 then

tests/neg-custom-args/captures/scope-extrude-mut.check

@@ -0,0 +1,9 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrude-mut.scala:9:8 ------------------------------
+9 |    a = a1  // error
+  |        ^^
+  |        Found:    A^{a1.rd}
+  |        Required: A^
+  |
+  |        where:    ^ refers to a fresh root capability in the type of variable a
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/scope-extrude-mut.scala

@@ -0,0 +1,9 @@
+import language.experimental.captureChecking
+
+class A extends caps.Mutable
+
+class B:
+  private var a: A^ = A()
+  def b() =
+    val a1 = A()
+    a = a1  // error