Support multi-span error reporting for CC

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

@@ -461,10 +461,7 @@ class SepCheck(checker: CheckCaptures.CheckerAPI) extends tpd.TreeTraverser:
    */
   def consumeError(ref: Capability, loc: (SrcPos, TypeRole), pos: SrcPos)(using Context): Unit =
     val (locPos, role) = loc
-    report.error(
-      em"""Separation failure: Illegal access to $ref, which was ${role.howConsumed}
-          |on line ${locPos.line + 1} and therefore is no longer available.""",
-      pos)
+    report.error(reporting.UseAfterConsume(ref, locPos.sourcePos, pos.sourcePos, role.howConsumed), pos)
 
   /** Report a failure where a capability is consumed in a loop.
    *  @param ref     the capability

compiler/src/dotty/tools/dotc/reporting/Diagnostic.scala

@@ -117,6 +117,6 @@ class Diagnostic(
     msg.message.replaceAll("\u001B\\[[;\\d]*m", "")
   override def diagnosticRelatedInformation: JList[interfaces.DiagnosticRelatedInformation] =
     Collections.emptyList()
-
   override def toString: String = s"$getClass at $pos L${pos.line+1}: $message"
 end Diagnostic
+

compiler/src/dotty/tools/dotc/reporting/Message.scala

@@ -300,6 +300,13 @@ object Message:
       super.toText(sym)
   end Printer
 
+  /** A part of a multi-span message, associating text with a source position.
+   *  @param text       the message text for this part
+   *  @param srcPos     the source position where this part applies
+   *  @param isPrimary  whether this is the primary message (true) or a secondary note (false)
+   */
+  case class MessagePart(text: String, srcPos: util.SourcePosition, isPrimary: Boolean)
+
 end Message
 
 /** A `Message` contains all semantic information necessary to easily
@@ -370,6 +377,17 @@ abstract class Message(val errorId: ErrorMessageID)(using Context) { self =>
     */
   protected def explain(using Context): String
 
+  /** Optional leading text to be displayed before the source snippet.
+    * If present along with parts, triggers multi-span rendering.
+    */
+  def leading(using Context): Option[String] = None
+
+  /** Optional list of message parts for multi-span error messages.
+    * Each part associates text with a source position and indicates
+    * whether it's a primary message or a secondary note.
+    */
+  def parts(using Context): List[MessagePart] = Nil
+
   /** What gets printed after the message proper */
   protected def msgPostscript(using Context): String =
     if ctx eq NoContext then ""