Auto impl display for Abigen Event Structs

[gakonst]

Is your feature request related to a problem? Please describe.

Right now, printing events in a nice human readable way requires matching on each event variant and then formatting the output. This is suboptimal because if there's a large amount of events, it means there's a lot of manual work to be done.

Relatedly, I can see the same issue arising on printing function args related to #506, so it might be helpful to tackle these 2 together.

Describe the solution you'd like

Define some encoding rules (e.g. all Bytes should be hex::encode'd before printing them) and implement Display for all event and function structs emitted by abigen.

[mattsse]

it really boils down to rules for some Token https://docs.rs/ethabi/15.0.0/ethabi/token/enum.Token.html

so essefntially

• bytes | address => 0x + hex
• fixedbytes => as rust array [....]? or something like [fixed length; .......]

[gakonst]

Maybe erring on the side of simplicity / consistency:

1. Numbers => decimal representations
2. Anything bytes related -> 0x encoding
3. Arrays (independently of fixed/variable length) -> [ <..>, <..> ] (where <..> is the inner type's Display impl)

[mattsse]

I suppose we should limit this to abigen! only and not generate Display when using EthAbiType and EthEvent directly, which could be unexpected and not intuitive.
Also, should we only print the events inputs or EventName(val1, val2, val3) style?

[gakonst]

My thinking was that theDisplay impl could be a comma separated list of val1,...val_n. We could make the Debug impl include the name perhaps if we wanted?

I think that doing it for EthAbiType/EthEvent also makes sense, so i'd be supportive of doing that too - unless you think that it's a footgun in any way

[mattsse]

I think we could go for an EthDisplay derive proc macro that would implement Display for the type.
By default abigen! would add this to all events, and for types a user could do

#[derive(Debug, Clone, PartialEq, EthAbiType, EthDisplay)]
struct ValueChanged {
    old_author: Address,
    new_author: Address,
}

assert_eq!("0x.....,0x....", format!("{}", ValueChanged{...}));