Add ability to evaluate selector during generation of contract from selector's name

[xgreenx]

It will be cool to have the ability to generate a selector inside of code and during the declaration of the selector.

During declaration of selector:

#[ink(message, selector = "IErc20::token_name")]
fn token_name(&self) -> Option<String> {
    Erc20::token_name(self)
}

Somewhere in code:

build_call::<DefaultEnvironment>()
  .callee(to)
  .exec_input(
    ExecutionInput::new(selector!("IErc721Receiver::on_erc721_received"))
        .push_arg(operator)
        .push_arg(from)
        .push_arg(id)
        .push_arg(data),
  )
  .returns::<ReturnType<Result<(), Erc721ReceiverError>>>()
  .fire()

[Robbepop]

With #928 merged and #665 in the review process and about to be merged soon we can actually start working on this cool feature request.

[Robbepop]

Note that this feature could be very confusing to users of ink! version 3.0-rc5 and older since until then selector = value:string was allowed with value being of the form 0xC0DECAFE or some other hex-encoded u32 value in this particular form.
If a user does not update their codebase while jumping from ink! version 3.0-rc5 and a future version that implements this new evaluation feature for selectors the user's selectors will just silently change without notice.

[Robbepop]

#947 implemented this. Closed.

[xgreenx]

Thanks=) You implemented the second case, but what about the first?

#[ink(message, selector = "IErc20::token_name")]
fn token_name(&self) -> Option<String> {
    Erc20::token_name(self)
}

Note that this feature could be very confusing to users of ink! version 3.0-rc5 and older since until then selector = value:string was allowed with value being of the form 0xC0DECAFE or some other hex-encoded u32 value in this particular form.

I agree with it. But I think we can create a workaround here. If we are able to parse the string into u32, then we should emit an error(that it is deprecated). But if it is not u32, so it is a user-defined string and we can calculate a hash form it.