Well there are a few reasons not to have it in diesel itself. One is that the main crate already is really large. Adding another massive feature like that would make it make harder to maintain. I think about splitting up diesel into multiple parts instead from time to time. The other important reason is that diesel itself is stable, while diesel-async is still a bit in the flux due to the general instability of the async ecosystem and async rust in general. For example we still miss several language features like asynchronous drop or proper async closures that allow to reason about the future they return to provide an actual stable interface for diesel-async. While all of that likely will not result in a fundamental change in the provided interface it remains a breaking change to make adjustments as soon as these features exist and are stabilised. That’s incompatible with the stability guarantee given by diesel itself.

As for having it next to diesel itself: Technically these crates are still separate projects maintained by different set of peoples. Diesel itself is maintained by the diesel core team, where I happen to be a member of that team. That team has a limited amount of capacity. Diesel-async is maintained by myself in whatever time is left after the time I spend on diesel. Sure it would be possible to have it in the diesel organisation as well, but I honestly do not see the relevant capacity there yet. You are welcome to change that. See the linked blog post for details on how you can contribute to that.