* In reply to @andresovela:matrix.org
Then I have a BSP struct that looks like this:

pub struct Ui {
    io_expander: Aw9523b<I2cDeviceOnSharedBus>,
}
The question would be, how can I name E in my impl for Ui?
impl<E> Ui
where
    embedded_hal_async::i2c::I2c<Error = E>,
{
    pub async fn initialize(&mut self) -> Result<(), Error<E>> {
        // Stuff
    }
}


E is no longer generic. You've named the generic type `I2C` to be I2cDeviceOnSharedBus and you previously defined E to be equal to <I2C as I2c>::Error so you can just substitute the generic that you've named and you get <I2cDeviceOnSharedBus as I2c>::Error which is just a single type. So get rid of the E generic on your impl block and just replace it with that