basically, using it to "bundle" a bunch of metadata into a single entity. For I2C registers, you might have something like:

```
pub trait WriteReg {
    type AddrTy;
    type Val
    type Endianness: EndianTy;
    const RegAddr: Self::AddrTy;
}

impl WriteReg for SampleRate {
    type AddrTy = u8; // 8-bit register addr
    type Val = u16; // 16-bit register value
    type Endianness = BigEndian; // data in reg is big endian
    const RegAddr: Self::AddrTy = 0x45; // register address
}
```

Then do:

```rust
// set sample rate to 4khz
i2c.write_reg::<SampleRate>(DEV_I2C_ADDR, 4000u16).await?;
```