Multi-span error reporting for consume errors

Author: Linyxus

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

@@ -449,7 +449,8 @@ class SepCheck(checker: CheckCaptures.CheckerAPI) extends tpd.TreeTraverser:
         |and therefore is no longer available.""",
     useLoc.sourcePos
   ):
-      addSubdiag(em"... $ref was consumed here.", consumedLoc.sourcePos)
+      addSubdiag(em"$ref was consumed here.", consumedLoc.sourcePos)
+      addPrimaryNote(em"... and it was used here")
 
   /** Report a failure where a previously consumed capability is used again,
    *  @param ref     the capability that is used after being consumed

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

@@ -123,9 +123,18 @@ class Diagnostic(
 
   private val subdiags: ArrayBuffer[Subdiagnostic] = ArrayBuffer.empty
 
+  private var primaryNote: Message | Null = null
+
   def addSubdiag(diag: Subdiagnostic): Unit =
     subdiags += diag
 
+  def addPrimaryNote(msg: Message): Unit =
+    assert(primaryNote eq null)
+    primaryNote = msg
+
+  def getPrimaryNote: Option[Message] =
+    if primaryNote eq null then None else Some(primaryNote.nn)
+
   def addSubdiag(msg: Message, pos: SourcePosition): Unit =
     addSubdiag(Subdiagnostic(msg, pos))
 

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

@@ -313,7 +313,7 @@ trait MessageRendering {
     val posString = posStr(pos1, msg, diagnosticLevel(dia))
     if posString.nonEmpty then sb.append(posString).append(EOL)
 
-    // Display primary error message before code snippet
+    // Always display primary error message before code snippet
     sb.append(msg.message)
     if !msg.message.endsWith(EOL) then sb.append(EOL)
 
@@ -342,7 +342,7 @@ trait MessageRendering {
     val lineNumberWidth = maxLineNumber.toString.length
 
     // Render each line with its markers and messages
-    for (lineNum <- minLine to maxLine) {
+    for (lineNum <- minLine to maxLine) do
       val lineIdx = lineNum - minLine
       if lineIdx < lines.length then
         val lineContent = lines(lineIdx)
@@ -360,10 +360,13 @@ trait MessageRendering {
           // Use '^' for primary error, '-' for sub-diagnostics
           val markerChar = if posAndMsg.isPrimary then '^' else '-'
           val marker = positionMarker(posAndMsg.pos, markerChar)
-          val err = errorMsg(posAndMsg.pos, posAndMsg.msg.message)
+          // For primary position: use PrimaryNote if available, otherwise use primary message
+          val messageToShow =
+            if posAndMsg.isPrimary then dia.getPrimaryNote.map(_.message).getOrElse(posAndMsg.msg.message)
+            else posAndMsg.msg.message
+          val err = errorMsg(posAndMsg.pos, messageToShow)
           sb.append(marker).append(EOL)
           sb.append(err).append(EOL)
-    }
 
     // Add explanation if needed
     if Diagnostic.shouldExplain(dia) then