1. I can't tell you how unconvinced I am with the error being less confusing. A good error message tells you what's wrong and ideally what to do to remedy it if possible... and to me there isn't really a practical difference between "set this environment variable" and "run this command". It seems like you basically add one extra step, but you prevent people from choosing a probably suboptimal workflow that they almost certainly don't want to use anyways... Either way, I don't think it's more confusing, and for someone new it's better to only have one way to do something, especially if it's the obviously superior thing anyways.
2. Sure, the database will probably be running locally, when you're working on database stuff. However, the trouble here is that while I almost definitely will have a local database running somehow, it is not necessarily going to be accessible from where the compiler would normally run. It might be in a VM or a Docker container where the database port isn't actually directly accessible. Plus, the state of the database schema in that environment is not guaranteed to match the code.
If I'm going to have something pull my database schema to do some code generation I'd greatly prefer it to be set up in such a way that I can easily wrap it so I can hermetically set up a database and run migrations from scratch so it's going to always match the code. It's not obvious what kinds of issues could be caused by a mismatch other than compilation errors, but personally I would prefer if it just wasn't possible.
The error message is a fair point, I do still think that making caching the default is premature.
I would definitely recommend writing a Compose file that applies your migrations to a fresh RDBMS and allows you to connect from the host device, regardless of what libraries you're using. Applying your migrations will vary by what tools you use, but the port forwarding is 2 simple lines. (Note that SQLx has a migration facility, but it's quite bare bones.)
2. Sure, the database will probably be running locally, when you're working on database stuff. However, the trouble here is that while I almost definitely will have a local database running somehow, it is not necessarily going to be accessible from where the compiler would normally run. It might be in a VM or a Docker container where the database port isn't actually directly accessible. Plus, the state of the database schema in that environment is not guaranteed to match the code.
If I'm going to have something pull my database schema to do some code generation I'd greatly prefer it to be set up in such a way that I can easily wrap it so I can hermetically set up a database and run migrations from scratch so it's going to always match the code. It's not obvious what kinds of issues could be caused by a mismatch other than compilation errors, but personally I would prefer if it just wasn't possible.