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, 12 insertions, 7 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
index f344364a75..b263a0fffd 100644
--- a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
@@ -505,7 +505,7 @@ trait TypeDiagnostics {
           // Only record type references which don't originate within the
           // definition of the class being referenced.
           if (t.tpe ne null) {
-            for (tp <- t.tpe ; if !treeTypes(tp) && !currentOwner.ownerChain.contains(tp.typeSymbol)) {
+            for (tp <- t.tpe if !treeTypes(tp) && !currentOwner.ownerChain.contains(tp.typeSymbol)) {
               tp match {
                 case NoType | NoPrefix    =>
                 case NullaryMethodType(_) =>
@@ -557,12 +557,17 @@ trait TypeDiagnostics {
         def unsetVars = localVars filter (v => !setVars(v) && !isUnusedTerm(v))
       }
 
-      def apply(unit: CompilationUnit) = {
+      private def warningsEnabled: Boolean = {
+        val ss = settings
+        import ss._
+        warnUnusedPatVars || warnUnusedPrivates || warnUnusedLocals || warnUnusedParams || warnUnusedImplicits
+      }
+
+      def apply(unit: CompilationUnit): Unit = if (warningsEnabled) {
         val p = new UnusedPrivates
         p traverse unit.body
-        val unused = p.unusedTerms
-        unused foreach { defn: DefTree =>
-          val sym             = defn.symbol
+        for (defn: DefTree <- p.unusedTerms) {
+          val sym = defn.symbol
           val pos = (
             if (defn.pos.isDefined) defn.pos
             else if (sym.pos.isDefined) sym.pos
@@ -591,10 +596,10 @@ trait TypeDiagnostics {
           )
           reporter.warning(pos, s"$why $what in ${sym.owner} is never used")
         }
-        p.unsetVars foreach { v =>
+        for (v <- p.unsetVars) {
           reporter.warning(v.pos, s"local var ${v.name} in ${v.owner} is never set - it could be a val")
         }
-        p.unusedTypes foreach { t =>
+        for (t <- p.unusedTypes) {
           val sym = t.symbol
           val why = if (sym.isPrivate) "private" else "local"
           reporter.warning(t.pos, s"$why ${sym.fullLocationString} is never used")