Genesis API

[mversic]

          I'm not at all a fan of this API. I'd rather we had a similarly named method in two different traits. The user is left guessing as to what `<true>` means in this context. It was slightly better when we had an enum, but now I'm convinced we really need two trait extension methods and only one in scope.

Originally posted by @appetrosyan in #3356 (comment)

[Erigara]

IMO we can use wrapper around function with const generic functions like:

#[inline]
fn validate<const IS_GENESIS: bool>() {
     // ...
}

fn validate_in_block() {
    validate::<false>()
}

fn validate_in_genesis() {
    validate::<true>()
}

I checked similar pattern once using godbolt and emitted assembly looks as we want (link)

[appetrosyan]

The inline on a generic function is redundant. Also, the trait-based solution is arguably more descriptive, while allowing for idiomatic usage:

fn do_stuff_in_genesis( ... ) {
    use InGenesis as _;
    // ... 
    thing.validate(args);
}

fn do_stuff_in_both( ... ) {
    use InGenesis;
    use InBlock;

    // ... 
    <Thing as InGenesis>::validate(thing, args);
    <Thing as InBlock>::validate(thing, args);
}

and generating the same assembly as the const generic.

If at some point we need to change the semantics of validate in fn do_stuff_in_genesis all we need to do is import a different trait. In cases where we need to disambiguate, we can do that too, except now instead of wondering what <true> or <false> mean in this context, you know that you need to validate X in the gensis context, and something else in the regular block context.

Moreover, I doubt that we'd need to validate with genesis semantics outside the core crate, while other forms of validation might need to be exported. With a const generic, both functions are inline and you can't expose only one. With a trait, if the trait is private to a crate, you can only use it inside the crate.

[Erigara]

Nice, i like this solution because we combine best of both worlds: DRY and descriptive behavior of the function.

[mversic]

There was this interesting comment by @appetrosyan on how we can extract genesis into a bootstrap process
I think we should explore all these options in the scope of this ticket