The following code triggers RUF029 (unused async):

from fastapi import FastAPI


def setup_app(app: FastAPI) -> None:
    @app.get("/")
    async def get_root() -> str:  # RUF029 here
        return "Hello World!"


app = FastAPI()


@app.get("/foo")
async def get_foo() -> str:  # no error
    return "Hello this is foo"

Note that it doesn't trigger on the top-level function.

That's because there is special handling in the code for FastAPI, implemented by @TomerBin in #12925/#12938, but I guess the underlying cause for the different outcome is that one of the is_fastapi_* checks in rules/fastapi/rules/mod.rs doesn't detect this case correctly.