message: Impl Trace for PhantomData
  Summary: Used in the following diff for `#[derive(Trace)]`.
diff: @@ -33,6 +33,7 @@ use std::{
  cell::{Cell, RefCell},
  cmp::Ordering,
  fmt::{Debug, Display, Write},
  + marker,
  };
  use derive_more::Display;
  @@ -139,6 +140,10 @@ unsafe impl<'v> Trace<'v> for std::time::Instant {
  fn trace(&mut self, _tracer: &Tracer<'v>) {}
  }
  +unsafe impl<'v, T> Trace<'v> for marker::PhantomData<T> {
  + fn trace(&mut self, _tracer: &Tracer<'v>) {}
  +}
  +
  /// A trait for values which are more complex - because they are either mutable,
  /// or contain references to other values.
  ///
  