Refactoring: Capabilities as a separate type

Split out Capability from Type. For now we keep most of the overall structure. E.g. capability handling code is
in file CaptureRef.scala. We will change this afterwards in separate refactorings.

Author: odersky

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

@@ -10,6 +10,7 @@ import Decorators.*
 import config.Printers.capt
 import printing.Printer
 import printing.Texts.Text
+import cc.Capabilities.{Capability, RootCapability}
 
 /** An annotation representing a capture set and whether it is boxed.
  *  It simulates a normal @retains annotation except that it is more efficient,
@@ -39,10 +40,10 @@ case class CaptureAnnotation(refs: CaptureSet, boxed: Boolean)(cls: Symbol) exte
   /** Reconstitute annotation tree from capture set */
   override def tree(using Context) =
     val elems = refs.elems.toList.map {
-      case cr: TermRef => ref(cr)
-      case cr: TermParamRef => untpd.Ident(cr.paramName).withType(cr)
-      case cr: ThisType => This(cr.cls)
-      case root(_) => ref(root.cap)
+      case c: TermRef => ref(c)
+      case c: TermParamRef => untpd.Ident(c.paramName).withType(c)
+      case c: ThisType => This(c.cls)
+      case c: RootCapability => ref(defn.captureRoot)
       // TODO: Will crash if the type is an annotated type, for example `cap.rd`
     }
     val arg = repeated(elems, TypeTree(defn.AnyType))
@@ -66,9 +67,9 @@ case class CaptureAnnotation(refs: CaptureSet, boxed: Boolean)(cls: Symbol) exte
     val elems1 = elems.mapConserve(tm.mapCapability(_))
     if elems1 eq elems then this
     else if elems1.forall:
-      case elem1: CaptureRef => elem1.isTrackableRef
+      case elem1: Capability => elem1.isWellformed
       case _ => false
-    then derivedAnnotation(CaptureSet(elems1.asInstanceOf[List[CaptureRef]]*), boxed)
+    then derivedAnnotation(CaptureSet(elems1.asInstanceOf[List[Capability]]*), boxed)
     else EmptyAnnotation
 
   override def refersToParamOf(tl: TermLambda)(using Context): Boolean =