Add -Wunused flag with -Wunused:nowarn option

Scala 2 has a bunch of options for `-Wunused` warnings (imports,
locals, ...). This commit adds the flag and a first warning, for
unused `@nowarn` annotations that don't silence any warnings.

Author: lrytz

compiler/src/dotty/tools/dotc/Run.scala

@@ -125,14 +125,25 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
       val source = sup.annotPos.source
       mySuppressions.getOrElseUpdate(source, mutable.ListBuffer.empty) += sup
 
-    def reportSuspendedMessages(unit: CompilationUnit)(using Context): Unit = {
+    def reportSuspendedMessages(source: SourceFile)(using Context): Unit = {
       // sort suppressions. they are not added in any particular order because of lazy type completion
-      for (sups <- mySuppressions.get(unit.source))
-        mySuppressions(unit.source) = sups.sortBy(sup => 0 - sup.start)
-      mySuppressionsComplete += unit.source
-      mySuspendedMessages.remove(unit.source).foreach(_.foreach(ctx.reporter.issueIfNotSuppressed))
+      for (sups <- mySuppressions.get(source))
+        mySuppressions(source) = sups.sortBy(sup => 0 - sup.start)
+      mySuppressionsComplete += source
+      mySuspendedMessages.remove(source).foreach(_.foreach(ctx.reporter.issueIfNotSuppressed))
     }
 
+    def warnUnusedSuppressions(): Unit =
+      // if we stop before typer completes (errors in parser, Ystop), report all suspended messages
+      mySuspendedMessages.keysIterator.toList.foreach(reportSuspendedMessages)
+      if ctx.settings.WunusedHas.nowarn then
+        for {
+          source <- mySuppressions.keysIterator.toList
+          sups   <- mySuppressions.remove(source)
+          sup    <- sups.reverse
+        } if (!sup.used)
+          report.warning("@nowarn annotation does not suppress any warnings", sup.annotPos)
+
   /** The compilation units currently being compiled, this may return different
    *  results over time.
    */
@@ -264,6 +275,7 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
       val action = finalizeActions.remove(0)
       action()
     }
+    suppressions.warnUnusedSuppressions()
     compiling = false
   }
 

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -1,11 +1,13 @@
 package dotty.tools.dotc
 package config
 
+import dotty.tools.dotc.config.PathResolver.Defaults
+import dotty.tools.dotc.config.Settings.{Setting, SettingGroup}
 import dotty.tools.dotc.core.Contexts._
-import dotty.tools.io.{ Directory, PlainDirectory, AbstractFile, JDK9Reflectors }
-import PathResolver.Defaults
-import rewrites.Rewrites
-import Settings.{ Setting, SettingGroup }
+import dotty.tools.dotc.rewrites.Rewrites
+import dotty.tools.io.{AbstractFile, Directory, JDK9Reflectors, PlainDirectory}
+
+import scala.util.chaining._
 
 class ScalaSettings extends SettingGroup with AllScalaSettings
 
@@ -132,6 +134,18 @@ private sealed trait WarningSettings:
   self: SettingGroup =>
   val Whelp: Setting[Boolean] = BooleanSetting("-W", "Print a synopsis of warning options.")
   val XfatalWarnings: Setting[Boolean] = BooleanSetting("-Werror", "Fail the compilation if there are any warnings.", aliases = List("-Xfatal-warnings"))
+
+  val Wunused: Setting[List[String]] = MultiChoiceSetting(
+    name = "-Wunused",
+    helpArg = "warning",
+    descr = "Enable or disable specific `unused` warnings",
+    choices = List("nowarn", "all"),
+    default = Nil
+  )
+  object WunusedHas:
+    def allOr(s: String)(using Context) = Wunused.value.pipe(us => us.contains("all") || us.contains(s))
+    def nowarn(using Context) = allOr("nowarn")
+
   val Wconf: Setting[List[String]] = MultiStringSetting(
     "-Wconf",
     "patterns",

compiler/src/dotty/tools/dotc/config/Settings.scala

@@ -56,7 +56,7 @@ object Settings:
     description: String,
     default: T,
     helpArg: String = "",
-    choices: Option[Seq[T]] = None,
+    choices: Option[Seq[?]] = None,
     prefix: String = "",
     aliases: List[String] = Nil,
     depends: List[(Setting[?], Any)] = Nil,
@@ -115,7 +115,13 @@ object Settings:
           update(Some(propertyClass.get.getConstructor().newInstance()), args)
         case (ListTag, _) =>
           if (argRest.isEmpty) missingArg
-          else update((argRest split ",").toList, args)
+          else
+            val strings = argRest.split(",").toList
+            choices match
+              case Some(valid) => strings.filterNot(valid.contains) match
+                case Nil => update(strings, args)
+                case invalid => fail(s"invalid choice(s) for $name: ${invalid.mkString(",")}", args)
+              case _ => update(strings, args)
         case (StringTag, _) if argRest.nonEmpty || choices.exists(_.contains("")) =>
           setString(argRest, args)
         case (StringTag, arg2 :: args2) =>
@@ -251,6 +257,9 @@ object Settings:
     def ChoiceSetting(name: String, helpArg: String, descr: String, choices: List[String], default: String, aliases: List[String] = Nil): Setting[String] =
       publish(Setting(name, descr, default, helpArg, Some(choices), aliases = aliases))
 
+    def MultiChoiceSetting(name: String, helpArg: String, descr: String, choices: List[String], default: List[String], aliases: List[String] = Nil): Setting[List[String]] =
+      publish(Setting(name, descr, default, helpArg, Some(choices), aliases = aliases))
+
     def IntSetting(name: String, descr: String, default: Int, aliases: List[String] = Nil): Setting[Int] =
       publish(Setting(name, descr, default, aliases = aliases))
 

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2110,7 +2110,9 @@ class Typer extends Namer
           else
             fs
     val range = mdef.sourcePos
-    ctx.run.suppressions.addSuppression(Suppression(tree.sourcePos, filters, range.start, range.end, verbose))
+    val sup = Suppression(tree.sourcePos, filters, range.start, range.end, verbose)
+    if filters == List(MessageFilter.None) then sup.markUsed()
+    ctx.run.suppressions.addSuppression(sup)
 
   def typedValDef(vdef: untpd.ValDef, sym: Symbol)(using Context): Tree = {
     val ValDef(name, tpt, _) = vdef

compiler/src/dotty/tools/dotc/typer/TyperPhase.scala

@@ -57,7 +57,7 @@ class TyperPhase(addRootImports: Boolean = true) extends Phase {
         typr.println("typed: " + unit.source)
         record("retained untyped trees", unit.untpdTree.treeSize)
         record("retained typed trees after typer", unit.tpdTree.treeSize)
-        ctx.run.suppressions.reportSuspendedMessages(unit)
+        ctx.run.suppressions.reportSuspendedMessages(unit.source)
     catch
       case ex: CompilationUnit.SuspendException =>
   }

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -129,7 +129,7 @@ class CompilationTests {
       compileFilesInDir("tests/neg-no-kind-polymorphism", defaultOptions and "-Yno-kind-polymorphism"),
       compileFilesInDir("tests/neg-custom-args/deprecation", defaultOptions.and("-Xfatal-warnings", "-deprecation")),
       compileFilesInDir("tests/neg-custom-args/fatal-warnings", defaultOptions.and("-Xfatal-warnings")),
-      compileFilesInDir("tests/neg-custom-args/nowarn", defaultOptions.and("-deprecation", "-Werror")),
+      compileFilesInDir("tests/neg-custom-args/nowarn", defaultOptions.and("-deprecation", "-Werror", "-Wunused:nowarn")),
       compileFilesInDir("tests/neg-custom-args/erased", defaultOptions.and("-language:experimental.erasedDefinitions")),
       compileFilesInDir("tests/neg-custom-args/allow-double-bindings", allowDoubleBindings),
       compileFilesInDir("tests/neg-custom-args/allow-deep-subtypes", allowDeepSubtypes),

tests/neg-custom-args/nowarn/nowarn-parser-typer.check

@@ -5,15 +5,15 @@
   |          its body in a block; no exceptions are handled.
 
 longer explanation available when compiling with `-explain`
--- [E000] Syntax Error: tests/neg-custom-args/nowarn/nowarn-parser-typer.scala:23:26 -----------------------------------
-23 |@nowarn("id=1") def t5d = try 1 // error, wrong id
-   |                          ^^^^^
-   |                          A try without catch or finally is equivalent to putting
-   |                          its body in a block; no exceptions are handled.
+-- [E000] Syntax Error: tests/neg-custom-args/nowarn/nowarn-parser-typer.scala:24:10 -----------------------------------
+24 |def t5d = try 1 // error, wrong id
+   |          ^^^^^
+   |          A try without catch or finally is equivalent to putting
+   |          its body in a block; no exceptions are handled.
 
 longer explanation available when compiling with `-explain`
--- [E000] Syntax Error: tests/neg-custom-args/nowarn/nowarn-parser-typer.scala:25:28 -----------------------------------
-25 |@nowarn("verbose") def t6 = try 1 // error with details
+-- [E000] Syntax Error: tests/neg-custom-args/nowarn/nowarn-parser-typer.scala:26:28 -----------------------------------
+26 |@nowarn("verbose") def t6 = try 1 // error with details
    |                            ^^^^^
    |                            A try without catch or finally is equivalent to putting
    |                            its body in a block; no exceptions are handled.
@@ -33,3 +33,7 @@ longer explanation available when compiling with `-explain`
    |        ^^^^^^
    |        Invalid message filter
    |        unknown filter: wat?
+-- Error: tests/neg-custom-args/nowarn/nowarn-parser-typer.scala:23:1 --------------------------------------------------
+23 |@nowarn("id=1") // error, unused nowarn
+   |^^^^^^^^^^^^^^^
+   |@nowarn annotation does not suppress any warnings

tests/neg-custom-args/nowarn/nowarn-parser-typer.scala

@@ -20,10 +20,7 @@ def t3 = { 1; 2 } // error, the invalid nowarn doesn't silence this warning
 @nowarn("id=E000") def t5a = try 1
 @nowarn("id=E0") def t5b = try 1
 @nowarn("id=0") def t5c = try 1
-@nowarn("id=1") def t5d = try 1 // error, wrong id
+@nowarn("id=1") // error, unused nowarn
+def t5d = try 1 // error, wrong id
 
 @nowarn("verbose") def t6 = try 1 // error with details
-
-@nowarn("cat=unchecked") def  t7(x: Any) = x match
-  case _: List[Int] => 0
-  case _ => 1

tests/neg-custom-args/nowarn/nowarn-refchecks.check

@@ -2,11 +2,23 @@
 5 |def t1 = f // error
   |         ^
   |         method f is deprecated
--- Error: tests/neg-custom-args/nowarn/nowarn-refchecks.scala:9:29 -----------------------------------------------------
-9 |@nowarn("msg=fish") def t4 = f // error
-  |                             ^
-  |                             method f is deprecated
--- Error: tests/neg-custom-args/nowarn/nowarn-refchecks.scala:15:9 -----------------------------------------------------
-15 |def t9 = f: @nowarn("msg=fish") // error
+-- Error: tests/neg-custom-args/nowarn/nowarn-refchecks.scala:10:9 -----------------------------------------------------
+10 |def t4 = f // error
    |         ^
    |         method f is deprecated
+-- Error: tests/neg-custom-args/nowarn/nowarn-refchecks.scala:17:9 -----------------------------------------------------
+17 |def t9 = f: // error
+   |         ^
+   |         method f is deprecated
+-- Error: tests/neg-custom-args/nowarn/nowarn-refchecks.scala:9:1 ------------------------------------------------------
+9 |@nowarn("msg=fish") // error, unused nowarn
+  |^^^^^^^^^^^^^^^^^^^
+  |@nowarn annotation does not suppress any warnings
+-- Error: tests/neg-custom-args/nowarn/nowarn-refchecks.scala:18:3 -----------------------------------------------------
+18 |  @nowarn("msg=fish") // error, unused nowarn
+   |  ^^^^^^^^^^^^^^^^^^^
+   |  @nowarn annotation does not suppress any warnings
+-- Error: tests/neg-custom-args/nowarn/nowarn-refchecks.scala:23:1 -----------------------------------------------------
+23 |@nowarn("cat=unchecked") def t7b(x: Any) = x match // error
+   |^^^^^^^^^^^^^^^^^^^^^^^^
+   |@nowarn annotation does not suppress any warnings

tests/neg-custom-args/nowarn/nowarn-refchecks.scala

@@ -6,12 +6,20 @@ def t1 = f // error
 
 @nowarn("cat=deprecation") def t2 = f
 @nowarn("msg=deprecated") def t3 = f
-@nowarn("msg=fish") def t4 = f // error
+@nowarn("msg=fish") // error, unused nowarn
+def t4 = f // error
 @nowarn("") def t5 = f
 @nowarn def t6 = f
 
 def t7 = f: @nowarn("cat=deprecation")
-def t8 = f: @nowarn("msg=deprecated")
-def t9 = f: @nowarn("msg=fish") // error
+def t8 = f:
+  @nowarn("msg=deprecated")
+def t9 = f: // error
+  @nowarn("msg=fish") // error, unused nowarn
 def t10 = f: @nowarn("")
 def t11 = f: @nowarn
+
+// unused nowarn; this test only runs until refchecks with Werror, the unchecked warning is issued in a later phase
+@nowarn("cat=unchecked") def t7b(x: Any) = x match // error
+  case _: List[Int] => 0
+  case _ => 1