Use enum for error messages IDs.

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

@@ -5,7 +5,7 @@ package reporting
 import core.Contexts.Context
 import core.Decorators._
 import printing.Highlighting.{Blue, Red}
-import diagnostic.{Message, MessageContainer, NoExplanation}
+import diagnostic.{ErrorMessageID, Message, MessageContainer, NoExplanation}
 import diagnostic.messages._
 import util.SourcePosition
 
@@ -95,9 +95,10 @@ trait MessageRendering {
     if (pos.exists) Blue({
       val file = pos.source.file.toString
       val errId =
-        if (message.errorId != NoExplanation.ID)
-          s"[E${"0" * (3 - message.errorId.toString.length) + message.errorId}] "
-        else ""
+        if (message.errorId ne ErrorMessageID.NoExplanationID) {
+          val errorNumber = message.errorId.errorNumber()
+          s"[E${"0" * (3 - errorNumber.toString.length) + errorNumber}] "
+        } else ""
       val kind =
         if (message.kind == "") diagnosticLevel
         else s"${message.kind} $diagnosticLevel"

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -0,0 +1,57 @@
+package dotty.tools.dotc.reporting.diagnostic;
+
+/** Unique IDs identifying the messages */
+public enum ErrorMessageID {
+
+    // IMPORTANT: Add new IDs only at the end and never remove IDs
+
+    LazyErrorId, // // errorNumber: -2
+    NoExplanationID, // errorNumber: -1
+
+    EmptyCatchOrFinallyBlockID, // errorNumber: 0
+    EmptyCatchBlockID, // errorNumber: 1
+    EmptyCatchAndFinallyBlockID, // errorNumber: 2
+    DeprecatedWithOperatorID,
+    CaseClassMissingParamListID,
+    DuplicateBindID,
+    MissingIdentID,
+    TypeMismatchID,
+    NotAMemberID,
+    EarlyDefinitionsNotSupportedID,
+    TopLevelImplicitClassID,
+    ImplicitCaseClassID,
+    ObjectMayNotHaveSelfTypeID,
+    TupleTooLongID,
+    RepeatedModifierID,
+    InterpolatedStringErrorID,
+    UnboundPlaceholderParameterID,
+    IllegalStartSimpleExprID,
+    MissingReturnTypeID,
+    YieldOrDoExpectedInForComprehensionID,
+    ProperDefinitionNotFoundID,
+    ByNameParameterNotSupportedID,
+    WrongNumberOfTypeArgsID,
+    IllegalVariableInPatternAlternativeID,
+    TypeParamsTypeExpectedID,
+    IdentifierExpectedID,
+    AuxConstructorNeedsNonImplicitParameterID,
+    IncorrectRepeatedParameterSyntaxID,
+    IllegalLiteralID,
+    PatternMatchExhaustivityID,
+    MatchCaseUnreachableID,
+    SeqWildcardPatternPosID,
+    IllegalStartOfSimplePatternID,
+    PkgDuplicateSymbolID,
+    ExistentialTypesNoLongerSupportedID,
+    UnboundWildcardTypeID,
+    DanglingThisInPathID,
+    OverridesNothingID,
+    OverridesNothingButNameExistsID,
+    ForwardReferenceExtendsOverDefinitionID,
+    ExpectedTokenButFoundID;
+
+    public int errorNumber() {
+        return ordinal() - 2;
+    }
+
+}

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

@@ -30,11 +30,10 @@ object Message {
   * Instead use the `persist` method to create an instance that does not keep a
   * reference to these contexts.
   *
-  * @param errorId a unique number identifying the message, this will later be
+  * @param errorId a unique id identifying the message, this will later be
   *                used to reference documentation online
   */
-abstract class Message(val errorId: Int) { self =>
-  import messages._
+abstract class Message(val errorId: ErrorMessageID) { self =>
 
   /** The `msg` contains the diagnostic message e.g:
     *
@@ -116,7 +115,7 @@ class ExtendMessage(_msg: () => Message)(f: String => String) { self =>
 }
 
 /** The fallback `Message` containing no explanation and having no `kind` */
-class NoExplanation(val msg: String) extends Message(NoExplanation.ID) {
+class NoExplanation(val msg: String) extends Message(ErrorMessageID.NoExplanationID) {
   val explanation = ""
   val kind = ""
 
@@ -127,8 +126,6 @@ class NoExplanation(val msg: String) extends Message(NoExplanation.ID) {
   * lacks an explanation
   */
 object NoExplanation {
-  final val ID = -1
-
   def unapply(m: Message): Option[Message] =
     if (m.explanation == "") Some(m)
     else None