Refactorings for CC error reporting

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

@@ -3,7 +3,7 @@ package dotc
 package cc
 
 import core.*
-import CaptureSet.{CompareResult, CompareFailure, VarState}
+import CaptureSet.VarState
 import collection.mutable
 import reporting.Message
 import Contexts.Context
@@ -16,24 +16,6 @@ class CCState:
 
   // ------ Error diagnostics -----------------------------
 
-  /** Error reprting notes produces since the last call to `test` */
-  var notes: List[ErrorNote] = Nil
-
-  def addNote(note: ErrorNote): Unit =
-    if !notes.exists(_.getClass == note.getClass) then
-      notes = note :: notes
-
-  def test(op: => CompareResult): CompareResult =
-    val saved = notes
-    notes = Nil
-    try op match
-      case res: CompareFailure => res.withNotes(notes)
-      case res => res
-    finally notes = saved
-
-  def testOK(op: => Boolean): CompareResult =
-    test(if op then CompareResult.OK else CompareResult.Fail(Nil))
-
   /** Warnings relating to upper approximations of capture sets with
    *  existentially bound variables.
    */

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

@@ -58,6 +58,7 @@ object Capabilities:
   trait RootCapability extends Capability:
     val rootId = nextRootId
     nextRootId += 1
+    def descr(using Context): String
 
   /** The base trait of all capabilties represented as types */
   trait CoreCapability extends TypeProxy, Capability:
@@ -120,6 +121,7 @@ object Capabilities:
    */
   @sharable // We override below all operations that access internal capability state
   object GlobalCap extends RootCapability:
+    def descr(using Context) = "the universal root capability"
     override val maybe = Maybe(this)
     override val readOnly = ReadOnly(this)
     override def reach = unsupported("cap.reach")
@@ -136,13 +138,21 @@ 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
       case _ => false
 
+    def descr(using Context) =
+      val originStr = origin match
+        case Origin.InDecl(sym) if sym.exists =>
+          origin.explanation
+        case _ =>
+          i" created in ${hiddenSet.owner.sanitizedDescription}${origin.explanation}"
+      i"a fresh root capability$originStr"
+
   object FreshCap:
     def apply(origin: Origin)(using Context): FreshCap | GlobalCap.type =
       if ccConfig.useSepChecks then FreshCap(ctx.owner, origin)
@@ -225,6 +235,9 @@ object Capabilities:
             rcap.myOrigin = primary
             primary.variants += rcap
             rcap
+
+    def descr(using Context) =
+      i"a root capability associated with the result type of $binder"
   end ResultCap
 
   /** A trait for references in CaptureSets. These can be NamedTypes, ThisTypes or ParamRefs,
@@ -545,7 +558,7 @@ object Capabilities:
             case y: ResultCap => vs.unify(x, y)
             case _ => y.derivesFromSharedCapability
           if !result then
-            ccState.addNote(CaptureSet.ExistentialSubsumesFailure(x, y))
+            TypeComparer.addErrorNote(CaptureSet.ExistentialSubsumesFailure(x, y))
           result
         case GlobalCap =>
           y match

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

@@ -41,9 +41,6 @@ def depFun(args: List[Type], resultType: Type, isContextual: Boolean, paramNames
 /** An exception thrown if a @retains argument is not syntactically a Capability */
 class IllegalCaptureRef(tpe: Type)(using Context) extends Exception(tpe.show)
 
-/** A base trait for data producing addenda to error messages */
-trait ErrorNote
-
 /** The currently valid CCState */
 def ccState(using Context): CCState =
   Phases.checkCapturesPhase.asInstanceOf[CheckCaptures].ccState1
@@ -157,7 +154,7 @@ extension (tp: Type)
    *  the two capture sets are combined.
    */
   def capturing(cs: CaptureSet)(using Context): Type =
-    if (cs.isAlwaysEmpty || cs.isConst && cs.subCaptures(tp.captureSet, VarState.Separate).isOK)
+    if (cs.isAlwaysEmpty || cs.isConst && cs.subCaptures(tp.captureSet, VarState.Separate))
         && !cs.keepAlways
     then tp
     else tp match