Automatically use enum discriminants as magic?

[drelatgithub]

In our code base we often have the following patterns, where we heavily use the discriminants of the enum variants elsewhere.

#[repr(u8}]
pub enum Enum {
    V1(T1) = 0x01,
    V2(T2), // 0x02
    V3(T3) = 0xFF,
    // ...
}

Now we want to equip this enum with binary serialization using binrw, and a natural way of discriminating among variants are via the magic value, so we would simply use the discriminant as the magic number as follows (assuming all Tx types are equipped with BinRead and BinWrite).

#[derive(BinRead, BinWrite)]
#[repr(u8)]
#[brw(big)]
pub enum Enum {
    #[brw(magic = 0x01u8)]
    V1(T1) = 0x01,
    #[brw(magic = 0x02u8)]
    V2(T2), // 0x02
    #[brw(magic = 0xFFu8)]
    V3(T3) = 0xFF,
    // ...
}

However, doing this is tedious and introduces two sources of truth for each magic number. Is there any option we can use in binrw that directly uses the discriminant of enum values as magic numbers?

If not, do you think adding this feature is possible, or useful?

[csnover]

Hi, thanks for writing! There is currently no support for that syntax in binrw. I’m not aware of a reason why it would not be possible to make it work but I have not ever checked. I don’t plan to implement this myself but if you or someone else wants to take a crack at it, feel free to send a PR. Thanks!