Yes, but be aware, if you have something like:

```rust
#[repr(c)]
struct MyNode {
    hdr: MyHeader,
    body: Rest
    // ...
}
```

You can NEVER go from `&mut MyHeader` -> `&mut MyNode`, ONLY `NonNull<MyHeader>` -> `NonNull<MyNode>`

edit: AND the `NonNull<MyHeader>` MUST have been created from a `NonNull<MyNode>`/`&mut MyNode`