This lets us avoid the use of `DiagnosticBuilder::into_diagnostic` in miri, when then means that `DiagnosticBuilder::into_diagnostic` can become private, being now only used by `stash` and `buffer`.
This lets us avoid the use of `DiagnosticBuilder::into_diagnostic` in miri, when then means that `DiagnosticBuilder::into_diagnostic` can become private, being now only used by `stash` and `buffer`.