message: Using std::iter::once.
diff: @@ -6,6 +6,7 @@ use crate::schema::{Document, Field};
  use crate::termdict::TermOrdinal;
  use crate::DocId;
  use fnv::FnvHashMap;
  +use std::iter::once;
  use std::io;
  /// Writer for multi-valued (as in, more than one value per document)
  @@ -133,7 +134,7 @@ impl MultiValuedFastFieldWriter {
  .doc_index
  .windows(2)
  .map(|interval| interval[0] as usize..interval[1] as usize)
  - .chain(Some(last_interval).into_iter())
  + .chain(once(last_interval))
  {
  doc_vals.clear();
  let remapped_vals = self.vals[range]
  