Currently, the unsafe Send + Sync implementations for Error are unsound, because the source: Box<dyn StdError> doesn't require Send + Sync. When fixing this (by adding dyn Send + Sync to source, and removing unsafe impl Synd and Sync) we encounter compile errors due to syntect exporting a dyn Error + Send (no Sync). I've opened trishume/syntect#304 to fix that; once complete, we can fix the issue in Zola as well.