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: #4358
Closes: #4519

cc @paritytech/subxt-team

[lexnv]

@bkchr could you please have a quick look whenever you have some time? 🙏

Thanks everyone for the reviews here 🙏

[gui1117]

nitpick, maybe we can say: does not explicitly bound TypeInfo or Parameter.

[gui1117]

I think that this will not match scale::TypeInfo but only TypeInfo, isn't it?

What I mean is that we do the match on the whole path, not the last segment, no?

If we want to support only this. Then we can say in the error message of include_metadata something like: "Note: only ident is supported, e.g. scale::TypeInfo is not detected, only TypeInfo.

[bkchr]

We should just take the last element of the path to improve the detection?

[bkchr]

Mainly left some nitpicks. Generally looking good. Good work 👍

[bkchr]

Could we implement here a mix of:

https://docs.rs/static_assertions/1.1.0/src/static_assertions/assert_impl.rs.html#329-356

and

https://doc.rust-lang.org/beta/reference/attributes/diagnostics.html#the-diagnosticon_unimplemented-attribute

This should give some very nice error message.

[bkchr]

Ahh but maybe not possible. Not sure, but worth a try.

[bkchr]

There should be an error if the attribute is passed multiple times.

[bkchr]

We should just take the last element of the path to improve the detection?

[bkchr]

Maybe just having some options would be easier here? But yeah, the code works the way it is written.