Diesel maintainer here. So first of all we are aware of the sometimes bad error message and we are working on solutions. There is already an unstable feature flag that improves some error messages and the next diesel release will include additional tools to improve the error messages in additional cases. In addition we try to change the language to give us more tools to control certain error messages emitted by the compiler, because mostly the error is large, but has a well known underlying issue that can be described much shorter.

> I once (foolishly) tried to write a generic repository implementation for database entities which could either have sqlite or postgres as a backend. I thought this would be very convenient for testing, being able to just keep all of the code the same safe for using a sqlite backed repository instead of a Postgres one.

It's generally not advice to test against different databases than you run in production. This is one of the reasons why these kind of things are not simple in diesel. Prefer running the tests against the "real" database system and use `Connection::begin_test_transaction()` to prevent polluting the database with test data.

More generally speaking: Writing generic code involving diesel is likely an advanced topic, so don't expect it to work quickly. I usually advice people to not write this generic code, at least not if they just want to use diesel in their applications.