The cause for that is the re-spanning of the macro here https://github.com/dtolnay/async-trait/blob/master/src/expand.rs, but its using the brace spans (which cover the entire body), it feels like we shouldn't even be considering them at all?

It might be that this is not actionable but its something we should investigate whether we can make work better, as the macro has all the rights to do this for better diagnostics.