message: Add note about constant reduction
diff: @@ -128,7 +128,12 @@ impl<'me> Context<'me> {
  semantics::ItemData::StructFormat(field_declarations) => {
  self.read_struct_format(&field_declarations)
  }
  - _ => Err(ReadError::InvalidDataDescription),
  + // NOTE: We expect that all constants should be reduced
  + // during evaluation, but this assumption could be
  + // invalidated if we ever introduce 'opaque' constants.
  + ItemData::Constant(_) | ItemData::StructType(_) => {
  + Err(ReadError::InvalidDataDescription)
  + }
  },
  (Some(_), _) | (None, _) => Err(ReadError::InvalidDataDescription),
  }
  