Rule request: prefer async for FastAPI dependencies

[spladug]

When a synchronous function is used as a dependency factory for FastAPI, the framework runs the function on a threadpool. This is done even if the factory never does IO, which can be quite wasteful. Since it's super tempting to not async-color the function when it's doing something super simple, it seems like it'd be useful to lint for this to avoid potential performance pitfalls.

See https://github.com/zhanymkanov/fastapi-best-practices?tab=readme-ov-file#prefer-async-dependencies for more info.

[MichaReiser]

Thanks.

I think the challenge with a rule such as this is how Ruff should quantify "a small non-IO operation"

And threads here also come with a price and limitations, that are redundant, if you just make a small non-I/O operation.

I worry that suggest a rule would have a high false positive rate. It might be a better fit for Ruff once we have a "suggestion" severity that only applies to the IDE context.

[spladug]

Yeah, that's totally fair. Difficult to tell without some way of identifying a function as pure.

I could maybe see it being an opt-in rule as in an asyncio-primary codebase it's definitely the exception to want the synchronous style for IO anyway.