xcm-builder: describe local location

[muharem]

DescribeLocation implementation for local xcm locations.

Currently, our runtimes lack a conversion mechanism for translating local root/pallet/plurality xcm locations into account IDs. However, there are instances where this functionality is needed - polkadot-fellows/runtimes#125.

In this PR, I propose three options to address this issue:

1. Include (0, Here) description into the DescribeFamily type for the root location. Utilize terminals like DescribePalletTerminal and DescribeBodyTerminal independently for the remaining locations without any additional prefixes.
2. Introduce a type similar to DescribeFamily called DescribeLocal.
3. Introduce a more generic type called DescribeWithPrefix.

[gavofyork]

The Plurality and pallet instance stuff is probably fine (though plurality might better be described with readable language rather than just encoding some value (which isn't necessarily expected to stay the same between XCM versions)).

The rest totally misses the point. What has been added is merely a confusing duplicate of DescribeTerminus.

[muharem]

The Plurality and pallet instance stuff is probably fine (though plurality might better be described with readable language rather than just encoding some value (which isn't necessarily expected to stay the same between XCM versions)).

The rest totally misses the point. What has been added is merely a confusing duplicate of DescribeTerminus.

I see, I wasn't certain whether to utilize DescribeTerminus and other Terminals for local locations without a prefix. Specifically, DescribeTerminus results an empty byte slice. And DescribeLocal was intended to be used in conjunction with existing terminals, only to have a prefix.

However, if I can simply use DescribeTerminus to describe (0, Here), then we do not need any of this.

[gavofyork]

(And actually, we already have DescribePalletTerminal for pallet instances.)

[muharem]

(And actually, we already have DescribePalletTerminal for pallet instances.)

yes, and for pluralities - DescribeBodyTerminal. I meant only to add LocalChain prefix to them, something like,

• DescribeLocal<(DescribeTerminus, DescribePalletTerminal, DescribeBodyTerminal)>
• or DescribeWithPrefix<.., b"LocalChain", (DescribeTerminus, DescribePalletTerminal, DescribeBodyTerminal)>

but never mind, without prefix also looks good to me