The readme for smol has an example for stdout. I assume stdin is the same:

        let mut stdout = Unblock::new(std::io::stdout());
        io::copy(stream, &mut stdout).await?;

And there is more information about how to use it for reading/writing on the Unblock docs.

I've not looked too much into it, but it seems AsyncRead/AsyncBufRead (through IoSafe) is implemented for stdin. So i think you should be able to use it like that.