warning: this pattern takes a reference on something that is being de-referenced
 --> needless_borrowed_ref.rs:7:35
  |
7 |     let _ = v.iter_mut().filter(|&ref a| a.is_empty());
  |                                   ^^^^^
  |
  = note: #[warn(needless_borrowed_reference)] on by default
  = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#needless_borrowed_reference