Writing enums: pre_assert?

Nobody has claimed this yet.

I have code like this:

```
#[derive(BinRead, BinWrite, Debug)]
#[brw(import(message_type: u8))]
enum MessageData {
#[br(pre_assert(message_type == 0x0u8))] Nil {
},
...
}
```

`bw` / `brw` don't allow `pre_assert` so they can't serialize this. Is there a way to reverse parsing an enum like this? Is it feasible to implement `pre_assert` for `brw`?... I don't _think_ using `assert` is correct.

No repository file or test is named. Start by tracing how enum attributes such as #[br(pre_assert(...))] and #[brw(...)] are handled, then compare the parsing and writing paths. Done means determining whether pre_assert can support brw serialization and documenting or testing the resulting behavior.

Written by the indexing model from the issue text.