Metadata V16 (unstable): Enrich metadata with associated types of config traits

[lexnv]

This feature is part of the upcoming metadata V16. The associated types of the Config trait that require the TypeInfo or Parameter bounds are included in the metadata of the pallet. The metadata is not yet exposed to the end-user, however the metadata intermediate representation (IR) contains these types.

Developers can opt out of metadata collection of the associated types by specifying without_metadata optional attribute to the #[pallet::config].

Furthermore, the without_metadata argument can be used in combination with the newly added #[pallet::include_metadata] attribute to selectively include only certain associated types in the metadata collection.

API Design

• There is nothing to collect from the associated types:

#[pallet::config]
pub trait Config: frame_system::Config {
		// Runtime events already propagated to the metadata.
		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;

		// Constants are already propagated.
		#[pallet::constant]
		type MyGetParam2: Get<u32>;
	}

• Default automatic collection of associated types that require TypeInfo or Parameter bounds:

	#[pallet::config]
	pub trait Config: frame_system::Config {
		// Runtime events already propagated to the metadata.
		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;

		// Constants are already propagated.
		#[pallet::constant]
		type MyGetParam2: Get<u32>;

		// Associated type included by default, because it requires TypeInfo bound.
		/// Nonce doc.
		type Nonce: TypeInfo;

		// Associated type included by default, because it requires
		// Parameter bound (indirect TypeInfo).
		type AccountData: Parameter;

		// Associated type without metadata bounds, not included.
		type NotIncluded: From<u8>;
	}

• Disable automatic collection

// Associated types are not collected by default.
	#[pallet::config(without_metadata)]
	pub trait Config: frame_system::Config {
		// Runtime events already propagated to the metadata.
		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;

		// Constants are already propagated.
		#[pallet::constant]
		type MyGetParam2: Get<u32>;

		// Explicitly include associated types.
		#[pallet::include_metadata]
		type Nonce: TypeInfo;

		type AccountData: Parameter;

		type NotIncluded: From<u8>;
	}

Builds on top of the PoC: https://github.com/paritytech/polkadot-sdk/pull/4358 Closes: https://github.com/paritytech/polkadot-sdk/issues/4519

cc @paritytech/subxt-team