Unused var message mentions unread or unset (scala#23719)

Fixes scala#23704 

Further distinguish whether a variable was "mutated but not read".
[Cherry-picked 212ab3f]

Author: som-snytt

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

@@ -3224,11 +3224,13 @@ extends Message(UnusedSymbolID):
 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 localVars(using Context): UnusedSymbol = UnusedSymbol(i"local variable was mutated but not read")
   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 privateVars(using Context): UnusedSymbol = UnusedSymbol(i"private variable was mutated but not read")
   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")

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

@@ -536,8 +536,11 @@ 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---"))
+
+    // non-local sym was target of assignment or has a sibling setter that was referenced
+    def isMutated(sym: Symbol): Boolean =
+         infos.asss(sym)
+      || infos.refs(sym.owner.info.member(sym.name.asTermName.setterName).symbol)
 
     def checkUnassigned(sym: Symbol, pos: SrcPos) =
       if sym.isLocalToBlock then
@@ -547,8 +550,7 @@ object CheckUnused:
         && sym.is(Mutable)
         && (sym.is(Private) || sym.isEffectivelyPrivate)
         && !sym.isSetter // tracks sym.underlyingSymbol sibling getter, check setter below
-        && !infos.asss(sym)
-        && !infos.refs(sym.owner.info.member(sym.name.asTermName.setterName).symbol)
+        && !isMutated(sym)
       then
         warnAt(pos)(UnusedSymbol.unsetPrivates)
 
@@ -564,7 +566,10 @@ object CheckUnused:
         )
         && !infos.nowarn(sym)
       then
-        warnAt(pos)(UnusedSymbol.privateMembers)
+        if sym.is(Mutable) && isMutated(sym) then
+          warnAt(pos)(UnusedSymbol.privateVars)
+        else
+          warnAt(pos)(UnusedSymbol.privateMembers)
 
     def checkParam(sym: Symbol, pos: SrcPos) =
       val m = sym.owner
@@ -667,7 +672,10 @@ object CheckUnused:
         && !sym.is(InlineProxy)
         && !sym.isCanEqual
       then
-        warnAt(pos)(UnusedSymbol.localDefs)
+        if sym.is(Mutable) && infos.asss(sym) then
+          warnAt(pos)(UnusedSymbol.localVars)
+        else
+          warnAt(pos)(UnusedSymbol.localDefs)
 
     def checkPatvars() =
       // convert the one non-synthetic span so all are comparable; filter NoSpan below

tests/warn/i23704.check

@@ -0,0 +1,20 @@
+-- [E198] Unused Symbol Warning: tests/warn/i23704.scala:9:8 -----------------------------------------------------------
+9 |    var position: Int = 0 // warn position is assigned but not read
+  |        ^^^^^^^^
+  |        private variable was mutated but not read
+-- [E198] Unused Symbol Warning: tests/warn/i23704.scala:16:14 ---------------------------------------------------------
+16 |  private var myvar: Int = 0 // warn for the same case with simpler syntax
+   |              ^^^^^
+   |              private variable was mutated but not read
+-- [E198] Unused Symbol Warning: tests/warn/i23704.scala:26:8 ----------------------------------------------------------
+26 |    var localvar = 0 // warn local variable was mutated but not read
+   |        ^^^^^^^^
+   |        local variable was mutated but not read
+-- [E198] Unused Symbol Warning: tests/warn/i23704.scala:33:8 ----------------------------------------------------------
+33 |    var settable: Int = 0 // warn private variable was mutated but not read
+   |        ^^^^^^^^
+   |        private variable was mutated but not read
+-- [E198] Unused Symbol Warning: tests/warn/i23704.scala:39:14 ---------------------------------------------------------
+39 |  private var myvar: Int = 0 // warn plain unused
+   |              ^^^^^
+   |              unused private member

tests/warn/i23704.scala

@@ -0,0 +1,39 @@
+//> using options -Wunused:all
+
+trait Test {
+  def incr(): Unit
+}
+
+object Test {
+  val test = new Test {
+    var position: Int = 0 // warn position is assigned but not read
+
+    def incr(): Unit = { position += 1 }
+  }
+}
+
+class C:
+  private var myvar: Int = 0 // warn for the same case with simpler syntax
+  def mine: Int =
+    myvar = 42
+    27
+
+class D:
+  private var myvar: Int = 0 // nowarn (although read is RHS of assignment)
+  def incr(): Unit = myvar = myvar + 1
+
+  def local(): Unit =
+    var localvar = 0 // warn local variable was mutated but not read
+    localvar += 1
+
+class E:
+  trait Setting:
+    def setting(): Unit
+  val test = new Test:
+    var settable: Int = 0 // warn private variable was mutated but not read
+    def setting(): Unit =
+      settable_=(42)
+    def incr() = setting()
+
+class F:
+  private var myvar: Int = 0 // warn plain unused