Completions should prepend, not replace as it is for Scala 2

Author: rochala

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

@@ -29,10 +29,9 @@ case class CompletionPos(
 ):
 
   def sourcePos: SourcePosition = cursorPos.withSpan(Spans.Span(start, end))
+  def stripSuffixEditRange: l.Range = new l.Range(cursorPos.offsetToPos(start), cursorPos.offsetToPos(end))
+  def toEditRange: l.Range = cursorPos.withStart(start).withEnd(cursorPos.point).toLsp
 
-  def toEditRange: l.Range =
-    new l.Range(cursorPos.offsetToPos(start), cursorPos.offsetToPos(end))
-  end toEditRange
 end CompletionPos
 
 object CompletionPos:

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

@@ -151,10 +151,7 @@ class CompletionProvider(
       indexedContext: IndexedContext
   )(using ctx: Context): CompletionItem =
     val printer =
-      ShortenedTypePrinter(search, IncludeDefaultParam.ResolveLater)(using
-        indexedContext
-      )
-    val editRange = completionPos.toEditRange
+      ShortenedTypePrinter(search, IncludeDefaultParam.ResolveLater)(using indexedContext)
 
     // For overloaded signatures we get multiple symbols, so we need
     // to recalculate the description
@@ -165,24 +162,24 @@ class CompletionProvider(
     val ident = completion.insertText.getOrElse(completion.label)
 
     def mkItem(
-        insertText: String,
+        newText: String,
         additionalEdits: List[TextEdit] = Nil,
         range: Option[LspRange] = None
     ): CompletionItem =
-      val nameEdit = new TextEdit(
-        range.getOrElse(editRange),
-        insertText
-      )
+      val oldText = params.text.substring(completionPos.start, completionPos.end)
+      val editRange = if newText.startsWith(oldText) then completionPos.stripSuffixEditRange
+        else completionPos.toEditRange
+
+      val textEdit = range
+        .map(lspRange => new TextEdit(lspRange, newText))
+        .getOrElse(new TextEdit(editRange, newText))
+
       val item = new CompletionItem(label)
       item.setSortText(f"${idx}%05d")
       item.setDetail(description)
-      item.setFilterText(
-        completion.filterText.getOrElse(completion.label)
-      )
-      item.setTextEdit(nameEdit)
-      item.setAdditionalTextEdits(
-        (completion.additionalEdits ++ additionalEdits).asJava
-      )
+      item.setFilterText(completion.filterText.getOrElse(completion.label))
+      item.setTextEdit(textEdit)
+      item.setAdditionalTextEdits((completion.additionalEdits ++ additionalEdits).asJava)
       completion.insertMode.foreach(item.setInsertTextMode)
 
       completion

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

@@ -1500,3 +1500,36 @@ class CompletionSuite extends BaseCompletionSuite:
         |""".stripMargin,
    )
 
+  @Test def `prepend-instead-of-replace` =
+    checkEdit(
+      """|object O:
+         |  printl@@println()
+         |""".stripMargin,
+      """|object O:
+         |  printlnprintln()
+         |""".stripMargin,
+      assertSingleItem = false
+    )
+
+  @Test def `prepend-instead-of-replace-duplicate-word` =
+    checkEdit(
+      """|object O:
+         |  println@@println()
+         |""".stripMargin,
+      """|object O:
+         |  printlnprintln()
+         |""".stripMargin,
+      assertSingleItem = false
+    )
+
+  @Test def `prepend-instead-of-replace-exact-same` =
+    checkEdit(
+      """|object O:
+         |  println@@()
+         |""".stripMargin,
+      """|object O:
+         |  println()
+         |""".stripMargin,
+      assertSingleItem = false
+    )
+