message: ColorMap::lookup() argument shouldn't start with an underscore
diff: @@ -184,7 +184,8 @@ pub trait ColorMap {
  fn index_of(&self, color: &Self::Color) -> usize;
  /// Looks up color by index in the color map. If `idx` is out of range for the color map, or
  /// ColorMap doesn't implement `lookup` `None` is returned.
  - fn lookup(&self, _idx: usize) -> Option<Self::Color> {
  + fn lookup(&self, index: usize) -> Option<Self::Color> {
  + let _ = index;
  None
  }
  /// Determine if this implementation of ColorMap overrides the default `lookup`.
  