I don't think this is acceptable, the spans need to be rewritten to at least refer to the `deriving(Default)` (but preferably refer to the `#[default]` string itself).

This should be as simple as writing an `syntax::fold::Folder` that only overrides [`new_span`](https://github.com/mozilla/rust/blob/e10fd317211511c3fb4656aee10eb4fd17dc8586/src/libsyntax/fold.rs#L315-L317), and then calling it on the parsed expression.