clippy 0.0.208

A bunch of helpful lints to avoid common pitfalls in Rust
error: `--x` could be misinterpreted as pre-decrement by C programmers, is usually a no-op
 --> $DIR/double_neg.rs:9:5
  |
9 |     --x;
  |     ^^^
  |
  = note: `-D double-neg` implied by `-D warnings`

error: aborting due to previous error