message: Simplify the ParametersSpec tracer
  Summary: This code is inherently unsafe, so simplifications are well worth it.
diff: @@ -496,7 +496,7 @@ impl<'v, V: ValueLike<'v>> ParametersSpec<V> {
  unsafe impl<'v, T: Trace<'v>> Trace<'v> for ParametersSpec<T> {
  fn trace(&mut self, tracer: &Tracer<'v>) {
  - self.kinds.iter_mut().for_each(|x| x.trace(tracer))
  + self.kinds.trace(tracer);
  }
  }
  