fix #8289 (#9828)

compiler/docgen.nim

@@ -400,7 +400,13 @@ proc extractImports(n: PNode; result: PNode) =
   for i in 0..<n.safeLen: extractImports(n[i], result)
 
 proc prepareExamples(d: PDoc; n: PNode) =
+
+  var docComment = newTree(nkCommentStmt)
+  let loc = d.conf.toFileLineCol(n.info)
+  docComment.comment = "autogenerated by docgen from " & loc
+
   var runnableExamples = newTree(nkStmtList,
+      docComment,
       newTree(nkImportStmt, newStrNode(nkStrLit, d.filename)))
   runnableExamples.info = n.info
   let imports = newTree(nkStmtList)

compiler/semexprs.nim

@@ -2046,6 +2046,7 @@ proc semMagic(c: PContext, n: PNode, s: PSym, flags: TExprFlags): PNode =
   of mRunnableExamples:
     if c.config.cmd == cmdDoc and n.len >= 2 and n.lastSon.kind == nkStmtList:
       when false:
+        # some of this dead code was moved to `prepareExamples`
         if sfMainModule in c.module.flags:
           let inp = toFullPath(c.config, c.module.info)
           if c.runnableExamples == nil: