message: TODO: remove extra_memory
  Summary:
  The plan is to remove `extra_memory` function and require `Allocative` for `StarlarkValue`.
  So `Allocative` can be used instead of `extra_memory`, but also to fill the gap in `buck2 debug allocative` (which currently ignores pointers in starlark values).
diff: @@ -327,6 +327,7 @@ pub trait StarlarkValue<'v>: 'v + ProvidesStaticType + Debug + Display + Seriali
  /// Return how much extra memory is consumed by this data type, in bytes, in addition to the
  /// direct `size_of` measurements. Used for profiling, so best effort rather than precise. Defaults to 0.
  /// Should not reported any memory held on to by a [`Value`].
  + // TODO(nga): require `Allocative` and remove this function.
  fn extra_memory(&self) -> usize {
  0
  }
  