Improve handling of language flags by Vulpix

compiler/test/dotty/tools/vulpix/TestFlags.scala

@@ -26,23 +26,17 @@ final case class TestFlags(
   private val languageFeatureFlag = "-language:"
   private def withoutLanguageFeaturesOptions = options.filterNot(_.startsWith(languageFeatureFlag))
 
-  // TODO simplify to add `-language:feature` to `options` once
-  //      https://github.com/lampepfl/dotty-feature-requests/issues/107 is implemented
-  def andLanguageFeature(feature: String) = {
-    val (languageFeatures, rest) = options.partition(_.startsWith(languageFeatureFlag))
-    val existingFeatures = if (languageFeatures.isEmpty) languageFeatures.mkString(",") + "," else ""
-    copy(options = rest ++ Array(languageFeatureFlag + existingFeatures + feature))
-  }
+  def andLanguageFeature(feature: String) =
+    copy(options = options ++ Array(s"$languageFeatureFlag$feature"))
 
-  def withoutLanguageFeature(feature: String) = {
+  def withoutLanguageFeature(feature: String) =
     val (languageFeatures, rest) = options.partition(_.startsWith(languageFeatureFlag))
-    val filteredFeatures = languageFeatures.filter(_ == feature)
+    val existingFeatures = languageFeatures.flatMap(_.stripPrefix(languageFeatureFlag).split(","))
+    val filteredFeatures = existingFeatures.filterNot(_ == feature)
     val newOptions =
-      if (filteredFeatures.isEmpty) rest
+      if filteredFeatures.isEmpty then rest
       else rest ++ Array(languageFeatureFlag + filteredFeatures.mkString(","))
-
     copy(options = newOptions)
-  }
 
   /** Subset of the flags that should be passed to javac. */
   def javacFlags: Array[String] = {

tests/neg-custom-args/i5498-postfixOps.scala

@@ -4,4 +4,4 @@ def test() = {
   1 second // error: usage of postfix operator
 
   Seq(1, 2).filter(List(1,2) contains) // error: usage of postfix operator
-}
+}

tests/pos-custom-args/i5498-postfixOps.scala

@@ -5,5 +5,5 @@ import scala.language.postfixOps
 def test() = {
   1 second
 
-  Seq(1, 2) filter (List(1,2) contains)
-}
+  Seq(1, 2) filter (List(1,2) contains) toList
+}