src/Pure/Tools/bibtex.scala

--- a/src/Pure/Tools/bibtex.scala	Mon Jun 19 20:32:06 2017 +0200
+++ b/src/Pure/Tools/bibtex.scala	Mon Jun 19 20:44:48 2017 +0200
@@ -422,4 +422,31 @@
     }
     (chunks.toList, ctxt)
   }
+
+
+  /* completion */
+
+  def completion(history: Completion.History, rendering: Rendering, caret: Text.Offset,
+    complete: String => List[String]): Option[Completion.Result] =
+  {
+    for {
+      Text.Info(r, name) <- rendering.citation(rendering.before_caret_range(caret))
+      name1 <- Completion.clean_name(name)
+
+      original <- rendering.model.try_get_text(r)
+      original1 <- Completion.clean_name(Library.perhaps_unquote(original))
+
+      entries = complete(name1).filter(_ != original1)
+      if entries.nonEmpty
+
+      items =
+        entries.sorted.map({
+          case entry =>
+            val full_name = Long_Name.qualify(Markup.CITATION, entry)
+            val description = List(entry, "(BibTeX entry)")
+            val replacement = quote(entry)
+          Completion.Item(r, original, full_name, description, replacement, 0, false)
+        }).sorted(history.ordering).take(rendering.options.int("completion_limit"))
+    } yield Completion.Result(r, original, false, items)
+  }
 }