IDE: Guard against out of bounds positions in requests

This can happen when a line is removed at the end of a file immediately
after a request on the removed line and the messages are processed out of
order on the server.

Author: smarter

compiler/src/dotty/tools/dotc/util/SourceFile.scala

@@ -115,6 +115,13 @@ class SourceFile(val file: AbstractFile, computeContent: => Array[Char]) extends
   /** Map line to offset of first character in line */
   def lineToOffset(index: Int): Int = lineIndices(index)
 
+  /** Like `lineToOffset`, but doesn't crash if the index is out of bounds. */
+  def lineToOffsetOpt(index: Int): Option[Int] =
+    if (index < 0 || index >= lineIndices.length)
+      None
+    else
+      Some(lineToOffset(index))
+
   /** A cache to speed up offsetToLine searches to similar lines */
   private[this] var lastLine = 0
 

language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala

@@ -651,10 +651,14 @@ object DottyLanguageServer {
       if (isWorksheet(uri)) toWrappedPosition(pos)
       else pos
     val source = driver.openedFiles(uri)
-    if (source.exists) {
-      val p = Spans.Span(source.lineToOffset(actualPosition.getLine) + actualPosition.getCharacter)
-      new SourcePosition(source, p)
-    }
+    if (source.exists)
+      source.lineToOffsetOpt(actualPosition.getLine).map(_ + actualPosition.getCharacter) match {
+        case Some(offset) if offset < source.content().length =>
+          val p = Spans.Span(offset)
+          new SourcePosition(source, p)
+        case _ =>
+          NoSourcePosition
+      }
     else NoSourcePosition
   }