message: UnpackValue for NoneType
  Summary:
  For consistency.
  Someone might want to declare a parameter like `Vec<NoneType>`, they should be allowed to.
diff: @@ -35,6 +35,7 @@ use crate::values::FrozenHeap;
  use crate::values::FrozenValue;
  use crate::values::Heap;
  use crate::values::StarlarkValue;
  +use crate::values::UnpackValue;
  use crate::values::Value;
  /// Define the None type, use [`NoneType`] in Rust.
  @@ -115,3 +116,13 @@ impl AllocFrozenValue for NoneType {
  FrozenValue::new_none()
  }
  }
  +
  +impl<'v> UnpackValue<'v> for NoneType {
  + fn unpack_value(value: Value<'v>) -> Option<Self> {
  + if value.is_none() {
  + Some(NoneType)
  + } else {
  + None
  + }
  + }
  +}
  