[FRAME] Simplify pallet config definition: remove RuntimeEvent associated type

prdoc/pr_7229.prdoc

@@ -0,0 +1,32 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: "FRAME: `RuntimeEvent` as associated type bound`"
+doc:
+  - audience: Runtime Dev
+    description: |
+      Using stabilized feature of associated type bounds, we can reduce redundant types defined in the pallet's `Config` traits. This PR adds
+      support for `RuntimeEvent` as an associated type bound.
+
+      With this change, we can do this:
+
+      ```rs
+      #[pallet::config]
+      pub trait Config: frame_system::Config<RuntimeEvent: From<Event<Self>>> {
+      }
+      ```
+      instead of this:
+
+      ```rs
+      #[pallet::config]
+      pub trait Config: frame_system::Config {
+              /// Overarching event type.
+            type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
+      }
+      ```
+
+crates: 
+  - name: frame-support
+    bump: minor
+  - name: pallet-examples
+    bump: minor

substrate/frame/examples/basic/src/lib.rs

@@ -153,14 +153,13 @@ pub mod pallet {
 	///
 	/// `frame_system::Config` should always be included.
 	#[pallet::config]
-	pub trait Config: pallet_balances::Config + frame_system::Config {
+	pub trait Config:
+		pallet_balances::Config + frame_system::Config<RuntimeEvent: From<Event<Self>>>
+	{
 		// Setting a constant config parameter from the runtime
 		#[pallet::constant]
 		type MagicNumber: Get<Self::Balance>;
 
-		/// The overarching event type.
-		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
-
 		/// Type representing the weight of this pallet
 		type WeightInfo: WeightInfo;
 	}

substrate/frame/examples/basic/src/tests.rs

@@ -79,7 +79,6 @@ impl pallet_balances::Config for Test {
 
 impl Config for Test {
 	type MagicNumber = ConstU64<1_000_000_000>;
-	type RuntimeEvent = RuntimeEvent;
 	type WeightInfo = ();
 }
 

substrate/frame/examples/default-config/src/lib.rs

@@ -43,12 +43,7 @@ pub mod pallet {
 	///
 	/// It will be an identical, but won't have anything that is `#[pallet::no_default]`.
 	#[pallet::config(with_default)]
-	pub trait Config: frame_system::Config {
-		/// The overarching event type. This is coming from the runtime, and cannot have a default.
-		/// In general, `Runtime*`-oriented types cannot have a sensible default.
-		#[pallet::no_default] // optional. `RuntimeEvent` is automatically excluded as well.
-		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
-
+	pub trait Config: frame_system::Config<RuntimeEvent: From<Event<Self>>> {
 		/// The overarching task type.
 		#[pallet::no_default]
 		type RuntimeTask: Task;
@@ -196,8 +191,7 @@ pub mod tests {
 
 	#[derive_impl(TestDefaultConfig as pallet::DefaultConfig)]
 	impl pallet_default_config_example::Config for Runtime {
-		// These two both cannot have defaults.
-		type RuntimeEvent = RuntimeEvent;
+		// This cannot have default.
 		type RuntimeTask = RuntimeTask;
 
 		type HasNoDefault = frame_support::traits::ConstU32<1>;

substrate/frame/examples/dev-mode/src/lib.rs

@@ -51,9 +51,9 @@ pub mod pallet {
 	use frame_system::pallet_prelude::*;
 
 	#[pallet::config]
-	pub trait Config: pallet_balances::Config + frame_system::Config {
-		/// The overarching event type.
-		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
+	pub trait Config:
+		pallet_balances::Config + frame_system::Config<RuntimeEvent: From<Event<Self>>>
+	{
 	}
 
 	// Simple declaration of the `Pallet` type. It is placeholder we use to implement traits and

substrate/frame/examples/dev-mode/src/tests.rs

@@ -70,9 +70,7 @@ impl pallet_balances::Config for Test {
 	type AccountStore = System;
 }
 
-impl Config for Test {
-	type RuntimeEvent = RuntimeEvent;
-}
+impl Config for Test {}
 
 // This function basically just builds a genesis storage key/value store according to
 // our desired mockup.

substrate/frame/examples/frame-crate/src/lib.rs

@@ -22,9 +22,7 @@ pub mod pallet {
 	use super::*;
 
 	#[pallet::config]
-	pub trait Config: frame_system::Config {
-		type RuntimeEvent: IsType<<Self as frame_system::Config>::RuntimeEvent> + From<Event<Self>>;
-	}
+	pub trait Config: frame_system::Config<RuntimeEvent: From<Event<Self>>> {}
 
 	#[pallet::pallet]
 	pub struct Pallet<T>(_);
@@ -60,7 +58,5 @@ mod tests {
 		type Block = MockBlock<Self>;
 	}
 
-	impl my_pallet::Config for Runtime {
-		type RuntimeEvent = RuntimeEvent;
-	}
+	impl my_pallet::Config for Runtime {}
 }

substrate/frame/examples/kitchensink/src/lib.rs

@@ -68,10 +68,7 @@ pub mod pallet {
 	/// * `#[pallet::disable_frame_system_supertrait_check]` would remove the need for
 	///   `frame_system::Config` to exist, which you should almost never need.
 	#[pallet::config]
-	pub trait Config: frame_system::Config {
-		/// The overarching runtime event type.
-		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
-
+	pub trait Config: frame_system::Config<RuntimeEvent: From<Event<Self>>> {
 		/// Type representing the weight of this pallet
 		type WeightInfo: WeightInfo;
 

substrate/frame/examples/kitchensink/src/tests.rs

@@ -58,7 +58,6 @@ parameter_types! {
 }
 
 impl Config for Test {
-	type RuntimeEvent = RuntimeEvent;
 	type WeightInfo = ();
 
 	type Currency = Balances;

substrate/frame/examples/offchain-worker/src/lib.rs

@@ -125,14 +125,13 @@ pub mod pallet {
 	/// This pallet's configuration trait
 	#[pallet::config]
 	pub trait Config:
-		CreateSignedTransaction<Call<Self>> + CreateInherent<Call<Self>> + frame_system::Config
+		CreateSignedTransaction<Call<Self>>
+		+ CreateInherent<Call<Self>>
+		+ frame_system::Config<RuntimeEvent: From<Event<Self>>>
 	{
 		/// The identifier type for an offchain worker.
 		type AuthorityId: AppCrypto<Self::Public, Self::Signature>;
 
-		/// The overarching event type.
-		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
-
 		// Configuration parameters
 
 		/// A grace period after we send transaction.

substrate/frame/examples/offchain-worker/src/tests.rs

@@ -129,7 +129,6 @@ parameter_types! {
 }
 
 impl Config for Test {
-	type RuntimeEvent = RuntimeEvent;
 	type AuthorityId = crypto::TestAuthId;
 	type GracePeriod = ConstU64<5>;
 	type UnsignedInterval = ConstU64<128>;

substrate/frame/examples/split/src/lib.rs

@@ -57,9 +57,7 @@ pub mod pallet {
 
 	/// Configure the pallet by specifying the parameters and types on which it depends.
 	#[pallet::config]
-	pub trait Config: frame_system::Config {
-		/// Because this pallet emits events, it depends on the runtime's definition of an event.
-		type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
+	pub trait Config: frame_system::Config<RuntimeEvent: From<Event<Self>>> {
 		/// Type representing the weight of this pallet
 		type WeightInfo: WeightInfo;
 	}

substrate/frame/examples/split/src/mock.rs

@@ -37,7 +37,6 @@ impl frame_system::Config for Test {
 }
 
 impl pallet_template::Config for Test {
-	type RuntimeEvent = RuntimeEvent;
 	type WeightInfo = ();
 }
 

substrate/frame/parameters/src/tests/mock.rs

@@ -140,8 +140,6 @@ impl Config for Runtime {
 impl pallet_example_basic::Config for Runtime {
 	// Use the dynamic key in the pallet config:
 	type MagicNumber = dynamic_params::pallet1::Key1;
-
-	type RuntimeEvent = RuntimeEvent;
 	type WeightInfo = ();
 }
 

substrate/frame/parameters/src/tests/test_renamed.rs

@@ -90,8 +90,6 @@ impl Config for Runtime {
 impl pallet_example_basic::Config for Runtime {
 	// Use the dynamic key in the pallet config:
 	type MagicNumber = dynamic_params::pallet1::Key1;
-
-	type RuntimeEvent = RuntimeEvent;
 	type WeightInfo = ();
 }
 

substrate/frame/support/procedural/src/pallet/expand/event.rs

@@ -143,17 +143,25 @@ pub fn expand_event(def: &mut Def) -> proc_macro2::TokenStream {
 
 		let PalletEventDepositAttr { fn_vis, fn_span, .. } = deposit_event;
 
+		// `RuntimeEvent` can be defined either as associated type in `Config` or as a type bound in
+		// system supertrait.
+		let runtime_event_path = if def.config.has_event_bound {
+			quote::quote! { <T as #frame_system::Config>::RuntimeEvent }
+		} else {
+			quote::quote! { <T as Config #trait_use_gen>::RuntimeEvent }
+		};
+
 		quote::quote_spanned!(*fn_span =>
 			impl<#type_impl_gen> #pallet_ident<#type_use_gen> #completed_where_clause {
 				#fn_vis fn deposit_event(event: Event<#event_use_gen>) {
 					let event = <
-						<T as Config #trait_use_gen>::RuntimeEvent as
+						#runtime_event_path as
 						From<Event<#event_use_gen>>
 					>::from(event);
 
 					let event = <
-						<T as Config #trait_use_gen>::RuntimeEvent as
-						Into<<T as #frame_system::Config>::RuntimeEvent>
+						#runtime_event_path as
+						Into<#runtime_event_path>
 					>::into(event);
 
 					<#frame_system::Pallet<T>>::deposit_event(event)

substrate/frame/support/procedural/src/pallet/parse/config.rs

@@ -63,6 +63,8 @@ pub struct ConfigDef {
 	/// * `IsType<Self as frame_system::Config>::RuntimeEvent`
 	/// * `From<Event>` or `From<Event<T>>` or `From<Event<T, I>>`
 	pub has_event_type: bool,
+	/// Whether the supertrait `frame_system::Config` defines associated type `RuntimeEvent`.
+	pub has_event_bound: bool,
 	/// The where clause on trait definition but modified so `Self` is `T`.
 	pub where_clause: Option<syn::WhereClause>,
 	/// Whether a default sub-trait should be generated.
@@ -366,6 +368,38 @@ fn contains_type_info_bound(ty: &TraitItemType) -> bool {
 	})
 }
 
+/// Check that supertrait `Config` contains `RuntimeEvent` associated type bound with
+/// `From<Event<Self>>`.
+///
+/// NOTE: Does not check if the supertrait path is valid system config path.
+///
+/// ```rs
+/// pub trait Config: frame_system::Config<RuntimeEvent: From<Event<Self>>> {
+/// ```
+fn contains_runtime_event_associated_type_bound(supertrait: &syn::Path) -> bool {
+	if let Some(args) = supertrait.segments.iter().find(|s| s.ident == "Config") {
+		if let syn::PathArguments::AngleBracketed(args) = &args.arguments {
+			for arg in &args.args {
+				if let syn::GenericArgument::Constraint(c) = arg {
+					if c.ident != "RuntimeEvent" {
+						continue;
+					}
+
+					// Check `From<Event<Self>>` bound
+					let from_event_bound = c
+						.bounds
+						.iter()
+						.find_map(|s| syn::parse2::<FromEventParse>(s.to_token_stream()).ok());
+
+					return from_event_bound.is_some();
+				}
+			}
+		}
+	}
+
+	false
+}
+
 impl ConfigDef {
 	pub fn try_from(
 		frame_system: &syn::Path,
@@ -406,10 +440,20 @@ impl ConfigDef {
 			false
 		};
 
-		let has_frame_system_supertrait = item.supertraits.iter().any(|s| {
-			syn::parse2::<syn::Path>(s.to_token_stream())
-				.map_or(false, |b| has_expected_system_config(b, frame_system))
-		});
+		let (has_frame_system_supertrait, has_event_bound) = item
+			.supertraits
+			.iter()
+			.filter_map(|supertrait| syn::parse2::<syn::Path>(supertrait.to_token_stream()).ok())
+			.fold((false, false), |(mut has_system, mut has_event_bound), supertrait| {
+				if has_expected_system_config(supertrait.clone(), frame_system) {
+					has_system = true;
+					// only check for `RuntimeEvent` bound if `frame_system::Config` is found.
+					has_event_bound = has_event_bound ||
+						contains_runtime_event_associated_type_bound(&supertrait);
+				}
+
+				(has_system, has_event_bound)
+			});
 
 		let mut has_event_type = false;
 		let mut consts_metadata = vec![];
@@ -589,6 +633,7 @@ impl ConfigDef {
 			consts_metadata,
 			associated_types_metadata,
 			has_event_type,
+			has_event_bound,
 			where_clause,
 			default_sub_trait,
 		})
@@ -712,4 +757,56 @@ mod tests {
 		let path = syn::parse2::<syn::Path>(quote::quote!(something::Config)).unwrap();
 		assert!(!has_expected_system_config(path, &frame_system));
 	}
+
+	#[test]
+	fn contains_runtime_event_associated_type_bound_no_bound() {
+		let supertrait = syn::parse2::<syn::Path>(quote::quote!(frame_system::Config)).unwrap();
+		assert!(!contains_runtime_event_associated_type_bound(&supertrait));
+	}
+
+	#[test]
+	fn contains_runtime_event_associated_type_bound_works() {
+		let supertrait = syn::parse2::<syn::Path>(quote::quote!(
+			Config<RuntimeEvent: From<Event<Self>>>
+		));
+		assert!(contains_runtime_event_associated_type_bound(&supertrait.unwrap()));
+	}
+	#[test]
+	fn contains_runtime_event_associated_type_bound_works_interface() {
+		let supertrait = syn::parse2::<syn::Path>(quote::quote!(
+			Config<RuntimeEvent: From<Event<Self, I>>>
+		));
+		assert!(contains_runtime_event_associated_type_bound(&supertrait.unwrap()));
+	}
+
+	#[test]
+	fn contains_runtime_event_associated_type_bound_works_full_path() {
+		let supertrait = syn::parse2::<syn::Path>(quote::quote!(
+			frame_system::Config<RuntimeEvent: From<Event<Self>>>
+		));
+		assert!(contains_runtime_event_associated_type_bound(&supertrait.unwrap()));
+	}
+
+	#[test]
+	fn contains_runtime_event_associated_type_bound_invalid_supertrait() {
+		let supertrait =
+			syn::parse2::<syn::Path>(quote::quote!(SystemConfig<RuntimeEvent: From<Event<Self>>>))
+				.unwrap();
+		assert!(!contains_runtime_event_associated_type_bound(&supertrait));
+	}
+
+	#[test]
+	fn contains_runtime_event_associated_type_bound_invalid_assoc_type_name() {
+		let supertrait =
+			syn::parse2::<syn::Path>(quote::quote!(Config<NonRuntimeEvent: From<Event<Self>>>))
+				.unwrap();
+		assert!(!contains_runtime_event_associated_type_bound(&supertrait));
+	}
+	#[test]
+	fn contains_runtime_event_associated_type_bound_invalid_trait_bound() {
+		let supertrait =
+			syn::parse2::<syn::Path>(quote::quote!(Config<RuntimeEvent: TryFrom<Event<Self>>>))
+				.unwrap();
+		assert!(!contains_runtime_event_associated_type_bound(&supertrait));
+	}
 }