Metadata V16: Enrich metadata with associated types of config traits

[lexnv]

Developers use associated types from pallet config traits to determine how to encode transactions (ie what hash is used by the node, the accountId, asset id etc). For example, subxt uses the following configs for Substrate and Polkadot to communicate with different chains.

This information can be exposed in the metadata V16, initially as unstable metadata (version u32::max).
Types included in the metadata must implement scale_info::TypeInfo.

Default to include associated types to metadata

By default, all associated types from config traits are captured in the metadata:

polkadot-sdk/substrate/frame/system/src/lib.rs

Lines 481 to 518 in 313fe0f

	type Nonce: Parameter
	+ Member
	+ MaybeSerializeDeserialize
	+ Debug
	+ Default
	+ MaybeDisplay
	+ AtLeast32Bit
	+ Copy
	+ MaxEncodedLen;
	/// The output of the `Hashing` function.
	type Hash: Parameter
	+ Member
	+ MaybeSerializeDeserialize
	+ Debug
	+ MaybeDisplay
	+ SimpleBitOps
	+ Ord
	+ Default
	+ Copy
	+ CheckEqual
	+ sp_std::hash::Hash
	+ AsRef<[u8]>
	+ AsMut<[u8]>
	+ MaxEncodedLen;
	/// The hashing system (algorithm) being used in the runtime (e.g. Blake2).
	type Hashing: Hash<Output = Self::Hash> + TypeInfo;
	/// The user account identifier type for the runtime.
	type AccountId: Parameter
	+ Member
	+ MaybeSerializeDeserialize
	+ Debug
	+ MaybeDisplay
	+ Ord
	+ MaxEncodedLen;

Opt-out of associated types from metadata

Developers that do not want to include types into the metadata can specify it by the following attribute (name tbd):

#[pallet::config(without_metadata)]
pub trait Config {
  type CustomParaType: ...
  
  type SecondType: ...
}

Selectively include associated types

There might be scenarios where only a single associated type is needed to express the pallet in the metadata.
One such case is the AssetId:

polkadot-sdk/substrate/frame/assets/src/lib.rs

Lines 294 to 295 in 313fe0f

	/// Identifier for the class of asset.
	type AssetId: Member + Parameter + Clone + MaybeSerializeDeserialize + MaxEncodedLen;

#[pallet::config(without_metadata)]
pub trait Config {
  
  #[pallet::include_metadata]
  type AssetId: ...
  
  type SecondType: ...
}

In this example, the AssetId is selectively included in the metadata, where the SecondType is not included.

PoC to capture associated types: #4358
PoC in subx to use associated metadata types: paritytech/subxt#1566

cc @jsdw @niklasad1 @bkchr @paritytech/subxt-team

[bkchr]

Looks like a reasonable proposal to me!