backend computes line number from source of position

[Cherry-picked c6b23cc]

Author: bishabosha

compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala

@@ -623,7 +623,13 @@ trait BCodeSkelBuilder extends BCodeHelpers {
       }
 
       if (emitLines && tree.span.exists && !tree.hasAttachment(SyntheticUnit)) {
-        val nr = ctx.source.offsetToLine(tree.span.point) + 1
+        val nr =
+          val sourcePos = tree.sourcePos
+          (
+            if sourcePos.exists then sourcePos.finalPosition.line
+            else ctx.source.offsetToLine(tree.span.point) // fallback
+          ) + 1
+
         if (nr != lastEmittedLineNr) {
           lastEmittedLineNr = nr
           getNonLabelNode(lastInsn) match {

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

@@ -118,7 +118,8 @@ class SourceFile(val file: AbstractFile, computeContent: => Array[Char]) extends
    *  For regular source files, simply return the argument.
    */
   def positionInUltimateSource(position: SourcePosition): SourcePosition =
-    SourcePosition(underlying, position.span shift start)
+    if isSelfContained then position // return the argument
+    else SourcePosition(underlying, position.span shift start)
 
   private def calculateLineIndicesFromContents() = {
     val cs = content()

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

@@ -79,6 +79,10 @@ extends SrcPos, interfaces.SourcePosition, Showable {
     rec(this)
   }
 
+  def finalPosition: SourcePosition = {
+    source.positionInUltimateSource(this)
+  }
+
 
   override def toString: String =
     s"${if (source.exists) source.file.toString else "(no source)"}:$span"