give better suggestion for unused pattern

Author: Martijn Hoekstra

src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala

@@ -703,7 +703,7 @@ trait TypeDiagnostics {
       }
       if (settings.warnUnusedPatVars) {
         for (v <- unusedPrivates.unusedPatVars)
-          typer.context.warning(v.pos, s"pattern var ${v.name} in ${v.owner} is never used; `${v.name}@_` suppresses this warning")
+          typer.context.warning(v.pos, s"pattern var ${v.name} in ${v.owner} is never used: use a wildcard `_` or suppress this warning with `${v.name}@_`")
       }
       if (settings.warnUnusedParams) {
         def isImplementation(m: Symbol): Boolean = {

test/files/neg/unused.check

@@ -0,0 +1,21 @@
+unused.scala:7: warning: pattern var a in value patvars is never used: use a wildcard `_` or suppress this warning with `a@_`
+    case C(a, _, _, _) => 1
+           ^
+unused.scala:8: warning: pattern var b in value patvars is never used: use a wildcard `_` or suppress this warning with `b@_`
+    case C(_, b, c@_, _) => 2
+              ^
+unused.scala:9: warning: pattern var a in value patvars is never used: use a wildcard `_` or suppress this warning with `a@_`
+    case C(a, b@_, c, d@_) => 3
+           ^
+unused.scala:9: warning: pattern var c in value patvars is never used: use a wildcard `_` or suppress this warning with `c@_`
+    case C(a, b@_, c, d@_) => 3
+                   ^
+unused.scala:10: warning: pattern var e in value patvars is never used: use a wildcard `_` or suppress this warning with `e@_`
+    case e => 4
+         ^
+unused.scala:8: warning: unreachable code
+    case C(_, b, c@_, _) => 2
+                            ^
+error: No warnings can be incurred under -Werror.
+6 warnings found
+one error found

test/files/neg/unused.scala

@@ -0,0 +1,12 @@
+// scalac: -Wunused:patvars -Werror
+
+case class C(a: Int, b: Int, c: Int, d: Int)
+
+object Test {
+  val patvars = C(1, 2, 3, 4) match {
+    case C(a, _, _, _) => 1
+    case C(_, b, c@_, _) => 2
+    case C(a, b@_, c, d@_) => 3
+    case e => 4
+  }
+}