OK, I made https://docs.rs/derive-mmio/0.1.0/derive_mmio/.  You can now write:

```rust
#[derive(derive_mmio::Mmio)]
#[repr(C)]
struct Uart {
    data: u32,
    status: u32,
}
```

This might be useful for the cortex-m crate, for example, which doesn't have an SVD file but has a lot of memory-mapped I/O peripherals (like the NVIC).