Add optional binrw/BinRead implementation

[caitp]

The goal here is ultimately to teach serde to deserialize raw bitfields from a binary file format, being mindful of the expected endian-ness of the structs. This doesn't get it there from the start, afaik -- but it does seem like a usable starting point.

I don't write a ton of Rust code, so I won't be surprised if this isn't the best way to get this done. Please let me know if there's a better way to do this.

[KodrAus]

Hi @caitp 👋

Thanks for working on this!

Since binrw is unstable, we unfortunately can't add direct support for it in bitflags, but you can still define bitflags types that you can apply its attributes to. We have an example using the zerocopy library here. I think the principle should be the same for binrw.

[DCNick3]

This is an unfortunate degradation of ergonomics compared to 1.x release, where the derive(BinRead) worked without the impl mode

[KodrAus]

@DCNick3 Yes it is a difference with 1.x, but trade-offs have to be made. The 1.x behavior of simply forwarding to the underlying integer type is only useful for cases like this where you're treating them as transparent.

[DCNick3]

Is it possible to have the textual Debug and Display implementations when using the impl mode?

[KodrAus]

You can yes, you just have to implement traits for formatting and parsing based on the functions in the bitflags::parser module. We've got an example that shows it.

[KodrAus]

It's been a while here so I'll go ahead and close this one to keep our PR list current. I'd be happy to take this with unstable support based on the approach we have for zerocopy in the future though.

Thanks again for the PR!

[DCNick3]

Until the Debug impl can be derived without boilerplate (tracked in #395), I'll have to stick to bitflags 1.x.