Add error code to diagnostics about unused code

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

@@ -211,6 +211,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case ConstructorProxyNotValueID // errorNumber: 195
   case ContextBoundCompanionNotValueID // errorNumber: 196
   case InlinedAnonClassWarningID // errorNumber: 197
+  case UnusedSymbolID // errorNumber: 198
 
   def errorNumber = ordinal - 1
 

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

@@ -21,6 +21,7 @@ enum MessageKind:
   case MatchCaseUnreachable
   case Compatibility
   case PotentialIssue
+  case UnusedSymbol
 
   /** Human readable message that will end up being shown to the user.
    *  NOTE: This is only used in the situation where you have multiple words
@@ -37,5 +38,6 @@ enum MessageKind:
       case PatternMatchExhaustivity => "Pattern Match Exhaustivity"
       case MatchCaseUnreachable => "Match case Unreachable"
       case PotentialIssue => "Potential Issue"
+      case UnusedSymbol => "Unused Symbol"
       case kind => kind.toString
 end MessageKind

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

@@ -3239,3 +3239,19 @@ extends TypeMsg(ConstructorProxyNotValueID):
        |companion value with the (term-)name `A`. However, these context bound companions
        |are not values themselves, they can only be referred to in selections."""
 
+class UnusedSymbol(errorText: String)(using Context)
+extends Message(UnusedSymbolID) {
+  def kind = MessageKind.UnusedSymbol
+
+  override def msg(using Context) = errorText
+  override def explain(using Context) = ""
+}
+
+object UnusedSymbol {
+    def imports(using Context): UnusedSymbol = new UnusedSymbol(i"unused import")
+    def localDefs(using Context): UnusedSymbol = new UnusedSymbol(i"unused local definition")
+    def explicitParams(using Context): UnusedSymbol = new UnusedSymbol(i"unused explicit parameter")
+    def implicitParams(using Context): UnusedSymbol = new UnusedSymbol(i"unused implicit parameter")
+    def privateMembers(using Context): UnusedSymbol = new UnusedSymbol(i"unused private member")
+    def patVars(using Context): UnusedSymbol = new UnusedSymbol(i"unused pattern variable")
+}

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -17,6 +17,7 @@ import dotty.tools.dotc.core.Phases.Phase
 import dotty.tools.dotc.core.StdNames
 import dotty.tools.dotc.report
 import dotty.tools.dotc.reporting.Message
+import dotty.tools.dotc.reporting.UnusedSymbol as UnusedSymbolMessage
 import dotty.tools.dotc.typer.ImportInfo
 import dotty.tools.dotc.util.{Property, SrcPos}
 import dotty.tools.dotc.core.Mode
@@ -295,21 +296,21 @@ class CheckUnused private (phaseMode: CheckUnused.PhaseMode, suffix: String, _ke
     res.warnings.toList.sortBy(_.pos.span.point)(using Ordering[Int]).foreach { s =>
       s match
         case UnusedSymbol(t, _, WarnTypes.Imports) =>
-          report.warning(s"unused import", t)
+          report.warning(UnusedSymbolMessage.imports, t)
         case UnusedSymbol(t, _, WarnTypes.LocalDefs) =>
-          report.warning(s"unused local definition", t)
+          report.warning(UnusedSymbolMessage.localDefs, t)
         case UnusedSymbol(t, _, WarnTypes.ExplicitParams) =>
-          report.warning(s"unused explicit parameter", t)
+          report.warning(UnusedSymbolMessage.explicitParams, t)
         case UnusedSymbol(t, _, WarnTypes.ImplicitParams) =>
-          report.warning(s"unused implicit parameter", t)
+          report.warning(UnusedSymbolMessage.implicitParams, t)
         case UnusedSymbol(t, _, WarnTypes.PrivateMembers) =>
-          report.warning(s"unused private member", t)
+          report.warning(UnusedSymbolMessage.privateMembers, t)
         case UnusedSymbol(t, _, WarnTypes.PatVars) =>
-          report.warning(s"unused pattern variable", t)
+          report.warning(UnusedSymbolMessage.patVars, t)
         case UnusedSymbol(t, _, WarnTypes.UnsetLocals) =>
-          report.warning(s"unset local variable, consider using an immutable val instead", t)
+          report.warning("unset local variable, consider using an immutable val instead", t)
         case UnusedSymbol(t, _, WarnTypes.UnsetPrivates) =>
-          report.warning(s"unset private variable, consider using an immutable val instead", t)
+          report.warning("unset private variable, consider using an immutable val instead", t)
     }
 
 end CheckUnused

compiler/test-resources/repl/i18383

@@ -4,7 +4,7 @@ scala> import scala.collection.*
 
 scala> class Foo { import scala.util.*; println("foo") }
 1 warning found
--- Warning: --------------------------------------------------------------------
+-- [E198] Unused Symbol Warning: -----------------------------------------------
 1 | class Foo { import scala.util.*; println("foo") }
   |                               ^
   |                               unused import