SI-8040 Warn unused flags

Introduce `-Ywarn-unused:x,y,z` and exploit `-Ywarn-unused:patvars`.

Although the tree attachment for shielding patvars from warnings
is not structural, sneaking the settings flag into the reflection
internal TreeGen is awkward.

Add test to ensure isolation of patvars warning from others.

`-Ywarn-unused-import` is an alias for `-Ywarn-unused:imports`.

`-Xlint:unused` is an alias for `-Ywarn-unused`, but not enabled
yet. The help text advises to use `-Ywarn-unused`. The future can
decide if `-Xlint:unused-imports` is warranted.

1 files changed, 35 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/settings/Warnings.scala b/src/compiler/scala/tools/nsc/settings/Warnings.scala
index 87534656f9..1a16480149 100644
--- a/src/compiler/scala/tools/nsc/settings/Warnings.scala
+++ b/src/compiler/scala/tools/nsc/settings/Warnings.scala
@@ -20,10 +20,35 @@ trait Warnings {
   val warnDeadCode         = BooleanSetting("-Ywarn-dead-code", "Warn when dead code is identified.")
   val warnValueDiscard     = BooleanSetting("-Ywarn-value-discard", "Warn when non-Unit expression results are unused.")
   val warnNumericWiden     = BooleanSetting("-Ywarn-numeric-widen", "Warn when numerics are widened.")
-  // SI-7712, SI-7707 warnUnused not quite ready for prime-time
-  val warnUnused           = BooleanSetting("-Ywarn-unused", "Warn when local and private vals, vars, defs, and types are unused.")
-  // currently considered too noisy for general use
-  val warnUnusedImport     = BooleanSetting("-Ywarn-unused-import", "Warn when imports are unused.")
+
+  object UnusedWarnings extends MultiChoiceEnumeration {
+    val Imports   = Choice("imports",   "Warn if an import selector is not referenced.")
+    val PatVars   = Choice("patvars",   "Warn if a variable bound in a pattern is unused.")
+    val Privates  = Choice("privates",  "Warn if a private member is unused.")
+    val Locals    = Choice("locals",    "Warn if a local definition is unused.")
+    val Params    = Choice("params",    "Warn if a value parameter is unused.")
+    val Implicits = Choice("implicits", "Warn if an implicit parameter is unused.")
+  }
+
+  // The -Ywarn-unused warning group.
+  val warnUnused = MultiChoiceSetting(
+    name    = "-Ywarn-unused",
+    helpArg = "warning",
+    descr   = "Enable or disable specific `unused' warnings",
+    domain  = UnusedWarnings,
+    default = Some(List("_"))
+  )
+
+  def warnUnusedImport    = warnUnused contains UnusedWarnings.Imports
+  def warnUnusedPatVars   = warnUnused contains UnusedWarnings.PatVars
+  def warnUnusedPrivates  = warnUnused contains UnusedWarnings.Privates
+  def warnUnusedLocals    = warnUnused contains UnusedWarnings.Locals
+  def warnUnusedParams    = warnUnused contains UnusedWarnings.Params
+  def warnUnusedImplicits = warnUnused contains UnusedWarnings.Implicits
+
+  BooleanSetting("-Ywarn-unused-import", "Warn when imports are unused.") withPostSetHook { s =>
+    warnUnused.add(s"${if (s) "" else "-"}imports")
+  } // withDeprecationMessage s"Enable -Ywarn-unused:imports"
 
   val warnExtraImplicit    = BooleanSetting("-Ywarn-extra-implicit", "Warn when more than one implicit parameter section is defined.")
 
@@ -60,6 +85,8 @@ trait Warnings {
     val UnsoundMatch           = LintWarning("unsound-match",             "Pattern match may not be typesafe.")
     val StarsAlign             = LintWarning("stars-align",               "Pattern sequence wildcard must align with sequence component.")
     val Constant               = LintWarning("constant",                  "Evaluation of a constant arithmetic expression results in an error.")
+    //val Unused                 = LintWarning("unused",                    "Warn when private and local definitions are unused.")
+    val Unused                 = LintWarning("unused",                    "Use -Ywarn-unused to warn when private and local definitions are unused.")
 
     def allLintWarnings = values.toSeq.asInstanceOf[Seq[LintWarning]]
   }
@@ -82,6 +109,7 @@ trait Warnings {
   def warnUnsoundMatch           = lint contains UnsoundMatch
   def warnStarsAlign             = lint contains StarsAlign
   def warnConstant               = lint contains Constant
+  def lintUnused                 = lint contains Unused
 
   // Lint warnings that are currently -Y, but deprecated in that usage
   @deprecated("Use warnAdaptedArgs", since="2.11.2")
@@ -101,7 +129,9 @@ trait Warnings {
     helpArg = "warning",
     descr   = "Enable or disable specific warnings",
     domain  = LintWarnings,
-    default = Some(List("_")))
+    default = Some(List("_"))) //.withPostSetHook (s => if (s contains Unused) warnUnused.add("_"))
+
+  // restore -Xlint:unused hook when SI-8040 is complete
 
   allLintWarnings foreach {
     case w if w.yAliased =>