Mention extension in unused param warning (#23132)

Fixes #23125 

Make the message more verbose by stating the extension method name.

This could be improved by only adding words if the param pos is on a
different line from the method pos.

Author: WojciechMazur

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

@@ -3312,23 +3312,29 @@ extends TypeMsg(ConstructorProxyNotValueID):
        |are not values themselves, they can only be referred to in selections."""
 
 class UnusedSymbol(errorText: String, val actions: List[CodeAction] = Nil)(using Context)
-extends Message(UnusedSymbolID) {
+extends Message(UnusedSymbolID):
   def kind = MessageKind.UnusedSymbol
 
   override def msg(using Context) = errorText
   override def explain(using Context) = ""
   override def actions(using Context) = this.actions
-}
 
 object UnusedSymbol:
   def imports(actions: List[CodeAction])(using Context): UnusedSymbol = UnusedSymbol(i"unused import", actions)
   def localDefs(using Context): UnusedSymbol = UnusedSymbol(i"unused local definition")
-  def explicitParams(using Context): UnusedSymbol = UnusedSymbol(i"unused explicit parameter")
-  def implicitParams(using Context): UnusedSymbol = UnusedSymbol(i"unused implicit parameter")
+  def explicitParams(sym: Symbol)(using Context): UnusedSymbol =
+    UnusedSymbol(i"unused explicit parameter${paramAddendum(sym)}")
+  def implicitParams(sym: Symbol)(using Context): UnusedSymbol =
+    UnusedSymbol(i"unused implicit parameter${paramAddendum(sym)}")
   def privateMembers(using Context): UnusedSymbol = UnusedSymbol(i"unused private member")
   def patVars(using Context): UnusedSymbol = UnusedSymbol(i"unused pattern variable")
-  def unsetLocals(using Context): UnusedSymbol = UnusedSymbol(i"unset local variable, consider using an immutable val instead")
-  def unsetPrivates(using Context): UnusedSymbol = UnusedSymbol(i"unset private variable, consider using an immutable val instead")
+  def unsetLocals(using Context): UnusedSymbol =
+    UnusedSymbol(i"unset local variable, consider using an immutable val instead")
+  def unsetPrivates(using Context): UnusedSymbol =
+    UnusedSymbol(i"unset private variable, consider using an immutable val instead")
+  private def paramAddendum(sym: Symbol)(using Context): String =
+    if sym.denot.owner.is(ExtensionMethod) then i" in extension ${sym.denot.owner}"
+    else ""
 
 class NonNamedArgumentInJavaAnnotation(using Context) extends SyntaxMsg(NonNamedArgumentInJavaAnnotationID):
 

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

@@ -500,6 +500,8 @@ object CheckUnused:
     val warnings = ArrayBuilder.make[MessageInfo]
     def warnAt(pos: SrcPos)(msg: UnusedSymbol, origin: String = ""): Unit = warnings.addOne((msg, pos, origin))
     val infos = refInfos
+    //println(infos.defs.mkString("DEFS\n", "\n", "\n---"))
+    //println(infos.refs.mkString("REFS\n", "\n", "\n---"))
 
     def checkUnassigned(sym: Symbol, pos: SrcPos) =
       if sym.isLocalToBlock then
@@ -540,7 +542,7 @@ object CheckUnused:
             if aliasSym.isAllOf(PrivateParamAccessor, butNot = CaseAccessor) && !infos.refs(alias.symbol) then
               if aliasSym.is(Local) then
                 if ctx.settings.WunusedHas.explicits then
-                  warnAt(pos)(UnusedSymbol.explicitParams)
+                  warnAt(pos)(UnusedSymbol.explicitParams(aliasSym))
               else
                 if ctx.settings.WunusedHas.privates then
                   warnAt(pos)(UnusedSymbol.privateMembers)
@@ -554,7 +556,7 @@ object CheckUnused:
           && !sym.name.isInstanceOf[DerivedName]
           && !ctx.platform.isMainMethod(m)
         then
-          warnAt(pos)(UnusedSymbol.explicitParams)
+          warnAt(pos)(UnusedSymbol.explicitParams(sym))
       end checkExplicit
       // begin
       if !infos.skip(m)
@@ -594,9 +596,9 @@ object CheckUnused:
                  aliasSym.isAllOf(PrivateParamAccessor, butNot = CaseAccessor)
               || aliasSym.isAllOf(Protected | ParamAccessor, butNot = CaseAccessor) && m.owner.is(Given)
             if checking && !infos.refs(alias.symbol) then
-              warnAt(pos)(UnusedSymbol.implicitParams)
+              warnAt(pos)(UnusedSymbol.implicitParams(aliasSym))
         else
-          warnAt(pos)(UnusedSymbol.implicitParams)
+          warnAt(pos)(UnusedSymbol.implicitParams(sym))
 
     def checkLocal(sym: Symbol, pos: SrcPos) =
       if ctx.settings.WunusedHas.locals

tests/warn/i15503g.check

@@ -0,0 +1,28 @@
+-- [E198] Unused Symbol Warning: tests/warn/i15503g.scala:8:17 ---------------------------------------------------------
+8 |  private def f2(a: Int) = default_int // warn
+  |                 ^
+  |                 unused explicit parameter
+-- [E198] Unused Symbol Warning: tests/warn/i15503g.scala:9:31 ---------------------------------------------------------
+9 |  private def f3(a: Int)(using Int) = a // warn
+  |                               ^
+  |                               unused implicit parameter
+-- [E198] Unused Symbol Warning: tests/warn/i15503g.scala:10:17 --------------------------------------------------------
+10 |  private def f4(a: Int)(using Int) = default_int // warn // warn
+   |                 ^
+   |                 unused explicit parameter
+-- [E198] Unused Symbol Warning: tests/warn/i15503g.scala:10:31 --------------------------------------------------------
+10 |  private def f4(a: Int)(using Int) = default_int // warn // warn
+   |                               ^
+   |                               unused implicit parameter
+-- [E198] Unused Symbol Warning: tests/warn/i15503g.scala:11:17 --------------------------------------------------------
+11 |  private def f6(a: Int)(using Int) = summon[Int] // warn
+   |                 ^
+   |                 unused explicit parameter
+-- [E198] Unused Symbol Warning: tests/warn/i15503g.scala:23:18 --------------------------------------------------------
+23 |    def isAnIssue(y: A): Boolean = x == x // warn
+   |                  ^
+   |                  unused explicit parameter in extension method isAnIssue
+-- [E198] Unused Symbol Warning: tests/warn/i15503g.scala:29:30 --------------------------------------------------------
+29 |  extension (s: String)(using Show) // warn not used in repeat
+   |                              ^
+   |                              unused implicit parameter in extension method repeat

tests/warn/i15503g.scala

@@ -1,4 +1,4 @@
-//> using options  -Wunused:params
+//> using options -Wunused:params
 
 /* This goes around the "trivial method" detection */
 object Foo {
@@ -15,10 +15,17 @@ object Foo {
   private def g2(x: Int) = ??? // OK
 }
 
-package foo.test.i17101:
+object i17101:
   type Test[A] = A
   extension[A] (x: Test[A]) { // OK
     def value: A = x
     def causesIssue: Unit = println("oh no")
     def isAnIssue(y: A): Boolean = x == x // warn
   }
+
+object i23125:
+  trait Show:
+    def show(s: String) = s
+  extension (s: String)(using Show) // warn not used in repeat
+    def echo = println(summon[Show].show(s))
+    def repeat = s * 2