bugfix: add multiline comment completion (#18703)

metals backport: scalameta/metals#5739
CC: @tgodzik

presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala

@@ -286,6 +286,15 @@ class Completions(
     path match
       case ScalaCliCompletions(dependency) =>
         (ScalaCliCompletions.contribute(dependency), true)
+
+      case _
+          if MultilineCommentCompletion.isMultilineCommentCompletion(
+            pos,
+            text,
+          ) =>
+        val values = MultilineCommentCompletion.contribute(config)
+        (values, true)
+
       case _ if ScaladocCompletions.isScaladocCompletion(pos, text) =>
         val values = ScaladocCompletions.contribute(pos, text, config)
         (values, true)

presentation-compiler/src/main/dotty/tools/pc/completions/MultilineCommentCompletion.scala

@@ -0,0 +1,16 @@
+package dotty.tools.pc.completions
+
+import scala.meta.pc.PresentationCompilerConfig
+
+import dotty.tools.dotc.util.SourcePosition
+
+object MultilineCommentCompletion:
+
+  def contribute(config: PresentationCompilerConfig): List[CompletionValue] =
+    val newText = if config.isCompletionSnippetsEnabled then " $0 */" else " */"
+    List(CompletionValue.document("/* */", newText, "Multiline Comment"))
+
+  def isMultilineCommentCompletion(pos: SourcePosition, text: String): Boolean =
+    pos.point >= 2 &&
+      text.charAt(pos.point - 2) == '/' &&
+      text.charAt(pos.point - 1) == '*'

presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala

@@ -1486,3 +1486,17 @@ class CompletionSuite extends BaseCompletionSuite:
          |""".stripMargin,
     )
 
+  @Test def `multiline-comment` =
+   checkEdit(
+     """|package a
+        |object O:
+        |  /*@@
+        |  def f = 1
+        |""".stripMargin,
+     """|package a
+        |object O:
+        |  /* $0 */
+        |  def f = 1
+        |""".stripMargin,
+   )
+