like, you can also make functions like:

```rust
const fn calc_buffer_size<E: WriteReg>() -> usize {
    size_of::<E::AddrTy>() + size_of::<E::Val>()
}

const fn xfer_buffer<E: WriteReg>() -> [u8; calc_buffer_size::<E>()] {
    [0u8; calc_buffer_size::<E>()]
}

// in your write_reg funtion
let mut buf = xfer_buffer::<E>(); // magically the right size!
```