Of Architecture, Traits and Unit Testing

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Box<MyTrait> isn't Send/Sync, because it could be any implementation, not just the ones that meet that requirement. If you want to constraint the trait object to also be Send/Sync, add it to the type:

trait UserRepository {}
struct PostgresUserRepository {
    database: Box<Database + Send + Sync>,
}
impl UserRepository for PostgresUserRepository {}

See Sending trait objects between threads - #3 by Gankra for a bit more info.