need to know how use a wildcard output with Spidev -> io::Result in Rust

Question

Would be grateful for some help here with my Rust code! I'm trying to set up the MCP 3002 SPI chip with the raspberry pi 5 using the spidev crate, to show the output from the MCP3002 on an egui screen. I need to replace the OK(-1) which is the output from the io::Result with a wildcard, such that I can then output this as a string to the egui screen. The code ONLY compiles and outputs to the egui screen if I put an integer into the OK() such as OK(2) , but the egui screen will only show the integer after the Ok that I put between the () brackets. No matter what I try, I can't get the wildcard to work, I have tried OK(_) and many other combinations and when using the (_) the compiler responds with the error "in expressions, _ can only be used on the left-hand side of an assignment".

Here is the code:

//Here is the code:
fn full_duplex(spi: &mut Spidev) -> io::Result<i8> {
    let tx_buf = [0x0D, 0x02];
    //let pop = io::Result<i8> ;
    //let pop = io::Result();

    let mut rx_buf = [0, 1]; //array size 1 and the type is i8
    let mut transfer = SpidevTransfer::read_write(&tx_buf, &mut rx_buf);
    spi.transfer(&mut transfer)?;

    //println!("{:?}", rx_buf);
    //Ok(u8)
    //Ok(io::Result::<i8>)
    //Ok(_:i8)
    //Ok(-1000..=1000)
    //Ok(())
    //Ok(<i8>::2)
    //Ok(<i8>:2)
    //Ok(i8:2)
    Ok(_)
}