[Feature] Can you implement the Compact trait for U256?

[atenjin]

For current version, codec provide the compact for u8 ~ u128, while in substrate, there are also lots of cases need to use U256.

In my case, we use lots of u256 in some structs, which are the types in some extrinsic and storage. I hope I can add #[compact] derive on the fields so that it can save some bytes.

[bkchr]

I think this would need to be done downstream in the primitive-types crate. It already implements Encode + Decode and it should be possible to implement Compact there as well.

[atenjin]

But I think U256 is a 4 bytes u8 array, why it can implement Compact? while I will have a try later

[atenjin]

anyway, you are right, I should ask this question in primitives-types repo, not here...

[atenjin]

paritytech/parity-common#895