improvement: Add case keyword completion

mtags/src/main/scala-2/scala/meta/internal/pc/completions/MatchCaseCompletions.scala

@@ -195,7 +195,7 @@ trait MatchCaseCompletions { this: MetalsGlobal =>
               sortSubclasses(sealedMembers, selectorSym.tpe, source).map(_._2)
 
             sortedMembers match {
-              case Nil => edits
+              case Nil => edits ++ completionGenerator.caseKeywordOnly
               case head :: tail =>
                 val (newLine, addIndent) =
                   if (moveToNewLine) ("\n\t", "\t") else ("", "")
@@ -225,9 +225,9 @@ trait MatchCaseCompletions { this: MetalsGlobal =>
                   additionalTextEdits =
                     sortedMembers.toList.flatMap(_.additionalTextEdits)
                 )
-                exhaustive :: edits
+                exhaustive :: edits ++ completionGenerator.caseKeywordOnly
             }
-          case None => edits
+          case None => edits ++ completionGenerator.caseKeywordOnly
         }
       }
     }
@@ -460,6 +460,22 @@ trait MatchCaseCompletions { this: MetalsGlobal =>
       )
     }
 
+    def caseKeywordOnly: List[TextEditMember] =
+      if (patternOnly.isEmpty) {
+        val label = "case"
+        val suffix =
+          if (clientSupportsSnippets) " $0"
+          else ""
+        List(
+          new TextEditMember(
+            label,
+            new l.TextEdit(editRange, label + suffix),
+            NoSymbol.newErrorSymbol(TermName("case")).setInfo(NoType),
+            label = Some(label)
+          )
+        )
+      } else Nil
+
     private def tryInfixPattern(sym: Symbol): Option[String] = {
       sym.primaryConstructor.paramss match {
         case (a :: b :: Nil) :: Nil =>

mtags/src/main/scala-3/scala/meta/internal/pc/completions/MatchCaseCompletions.scala

@@ -183,7 +183,7 @@ object CaseKeywordCompletion:
               search,
             )
           sealedMembers match
-            case Nil => caseItems
+            case Nil => caseItems ++ completionGenerator.caseKeywordOnly
             case (_, label) :: tail =>
               val (newLine, addIndent) =
                 if moveToNewLine then ("\n\t", "\t") else ("", "")
@@ -207,9 +207,9 @@ object CaseKeywordCompletion:
                 importEdit.toList,
                 s" ${selectorSym.decodedName} (${res.length} cases)",
               )
-              exhaustive :: caseItems
+              exhaustive :: caseItems ++ completionGenerator.caseKeywordOnly
           end match
-        case None => caseItems
+        case None => caseItems ++ completionGenerator.caseKeywordOnly
       end match
     end if
 
@@ -443,6 +443,20 @@ class CompletionValueGenerator(
     end if
   end labelForCaseMember
 
+  def caseKeywordOnly: List[CompletionValue.Keyword] =
+    if patternOnly.isEmpty then
+      val label = "case"
+      val suffix =
+        if clientSupportsSnippets then " $0"
+        else ""
+      List(
+        CompletionValue.Keyword(
+          label,
+          Some(label + suffix),
+        )
+      )
+    else Nil
+
   def toCompletionValue(
       sym: Symbol,
       label: String,

tests/cross/src/test/scala/tests/pc/CompletionCaseSuite.scala

@@ -24,6 +24,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |""".stripMargin,
   )
 
@@ -54,6 +55,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
        |case _: HasMouth[_] => pkg
        |case HasWings(e) => pkg
        |case Seal => pkg
+       |case
        |""".stripMargin,
     compat = Map(
       "3" -> """|case _: Animal => pkg
@@ -65,6 +67,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
                 |case _: HasMouth[?] => pkg
                 |case HasWings(e) => pkg
                 |case Seal => pkg
+                |case
                 |""".stripMargin
     ),
   )
@@ -79,6 +82,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |""".stripMargin,
   )
 
@@ -93,6 +97,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |""".stripMargin,
   )
 
@@ -107,6 +112,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |""".stripMargin,
   )
 
@@ -120,6 +126,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case Left(value) => scala.util
        |case Right(value) => scala.util
+       |case
        |""".stripMargin,
   )
 
@@ -134,6 +141,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case Failure(exception) => scala.util
        |case Success(value) => scala.util
+       |case
        |""".stripMargin,
   )
 
@@ -153,9 +161,12 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     // Assert we don't include AdtTwo in the results.
     """|case Cls(a, b) => `sealed-two`.Outer
+       |case
        |""".stripMargin,
     compat = Map(
-      "3" -> "case Cls(a, b) => sealed-two.Outer"
+      "3" -> """|case Cls(a, b) => sealed-two.Outer
+                |case
+                |""".stripMargin
     ),
   )
 
@@ -173,6 +184,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case scala.util.Left(value) =>
        |case Right(value) => scala.util
+       |case
        |""".stripMargin,
   )
 
@@ -234,6 +246,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |""".stripMargin,
   )
 
@@ -248,6 +261,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |""".stripMargin,
   )
 
@@ -274,11 +288,13 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |case (exhaustive) Option[A] (2 cases)
        |""".stripMargin,
     compat = Map("3" -> """|case None => scala
                            |case Some(value) => scala
                            |case (exhaustive) Option (2 cases)
+                           |case
                            |""".stripMargin),
   )
 
@@ -293,6 +309,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |""".stripMargin,
   )
 
@@ -307,6 +324,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |""".stripMargin,
   )
 
@@ -320,11 +338,13 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |case (exhaustive) Option[A] (2 cases)
        |""".stripMargin,
     compat = Map("3" -> """|case None => scala
                            |case Some(value) => scala
                            |case (exhaustive) Option (2 cases)
+                           |case
                            |""".stripMargin),
   )
 
@@ -338,11 +358,13 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |case (exhaustive) Option[A] (2 cases)
        |""".stripMargin,
     compat = Map("3" -> """|case None => scala
                            |case Some(value) => scala
                            |case (exhaustive) Option (2 cases)
+                           |case
                            |""".stripMargin),
   )
 
@@ -357,6 +379,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |""".stripMargin,
   )
 
@@ -371,6 +394,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case None => scala
        |case Some(value) => scala
+       |case
        |""".stripMargin,
   )
 
@@ -384,11 +408,13 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |}""".stripMargin,
     """|case head :: next => scala.collection.immutable
        |case Nil => scala.collection.immutable
+       |case
        |""".stripMargin,
     compat = Map(
       "2.12" ->
         """|case head :: tl => scala.collection.immutable
            |case Nil => scala.collection.immutable
+           |case
            |""".stripMargin
     ),
   )
@@ -463,6 +489,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |""".stripMargin,
     "cas@@",
     "case a :+: b => $0",
+    filter = _.contains(":+:"),
   )
 
   checkEditLine(
@@ -482,6 +509,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
     "cas@@",
     // Assert we don't use infix syntax because `::` resolves to conflicting symbol in scope.
     "case Outer.::(a, b) => $0",
+    filter = _.contains("::"),
   )
 
   check(
@@ -499,6 +527,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
     """|case Color.Blue =>
        |case Color.Green =>
        |case Color.Red =>
+       |case
        |""".stripMargin,
   )
 
@@ -563,6 +592,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |  List(Foo(1,2)).map{ cas@@ }
       |}""".stripMargin,
     """|case Foo(a, b) => example
+       |case
        |""".stripMargin,
   )
 
@@ -577,7 +607,7 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |    ca@@  
       |  }
       |}""".stripMargin,
-    "",
+    "case",
   )
 
   check(
@@ -592,7 +622,9 @@ class CompletionCaseSuite extends BaseCompletionSuite {
       |      cas@@
       |}""".stripMargin,
     """|case A.B =>
-       |case A.C =>""".stripMargin,
+       |case A.C =>
+       |case
+       |""".stripMargin,
   )
 
   check(
@@ -632,7 +664,9 @@ class CompletionCaseSuite extends BaseCompletionSuite {
         |""".stripMargin,
     """|case Cleaning =>Activity & Physical & Chore
        |case DishWashing(amount) => exhaustive-enum-tags.Activity
-       |case Sports(time, intensity) => exhaustive-enum-tags.Activity""".stripMargin,
+       |case Sports(time, intensity) => exhaustive-enum-tags.Activity
+       |case
+       |""".stripMargin,
   )
 
   check(
@@ -660,7 +694,9 @@ class CompletionCaseSuite extends BaseCompletionSuite {
         |
         |""".stripMargin,
     """|case Cleaning =>Activity & Physical & Chore
-       |case Sports(time, intensity) => exhaustive-enum-tags2.Activity""".stripMargin,
+       |case Sports(time, intensity) => exhaustive-enum-tags2.Activity
+       |case
+       |""".stripMargin,
   )
 
   check(
@@ -690,7 +726,9 @@ class CompletionCaseSuite extends BaseCompletionSuite {
     """|case Cleaning =>Activity & Physical & Chore
        |case Reading(book, author) => exhaustive-enum-tags3.Activity
        |case Singing(song) => exhaustive-enum-tags3.Activity
-       |case Sports(time, intensity) => exhaustive-enum-tags3.Activity""".stripMargin,
+       |case Sports(time, intensity) => exhaustive-enum-tags3.Activity
+       |case
+       |""".stripMargin,
   )
 
 }