Remove as frame_system::DefaultConfig from the required syntax in derive_impl

prdoc/pr_3505.prdoc

@@ -0,0 +1,36 @@
+# 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: Removes `as [disambiguation_path]` from the required syntax in `derive_impl`
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      This PR removes the need to specify `as [disambiguation_path]` for cases where the trait 
+      definition resides within the same scope as default impl path. 
+
+      For example, in the following macro invocation
+      ```rust
+      #[derive_impl(frame_system::config_preludes::TestDefaultConfig as frame_system::DefaultConfig)]
+      impl frame_system::Config for Runtime {
+        ...
+      }
+      ```
+      the trait `DefaultConfig` lies within the `frame_system` scope and `TestDefaultConfig` impls
+      the `DefaultConfig` trait. 
+
+      Using this information, we can compute the disambiguation path internally, thus removing the
+      need of an explicit specification:
+      ```rust
+      #[derive_impl(frame_system::config_preludes::TestDefaultConfig)]
+      impl frame_system::Config for Runtime {
+        ...
+      }
+      ```
+
+      In cases where the trait lies outside this scope, we would still need to specify it explicitly, 
+      but this should take care of most (if not all) uses of `derive_impl` within FRAME's context.
+
+crates: 
+  - name: frame-support-procedural
+  - name: pallet-default-config-example

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

@@ -149,7 +149,7 @@ pub mod tests {
 		}
 	);
 
-	#[derive_impl(frame_system::config_preludes::TestDefaultConfig as frame_system::DefaultConfig)]
+	#[derive_impl(frame_system::config_preludes::TestDefaultConfig)]
 	impl frame_system::Config for Runtime {
 		// these items are defined by frame-system as `no_default`, so we must specify them here.
 		type Block = Block;

substrate/frame/support/procedural/src/derive_impl.rs

@@ -172,6 +172,33 @@ fn combine_impls(
 	final_impl
 }
 
+/// Computes the disambiguation path for the `derive_impl` attribute macro.
+///
+/// When specified explicitly using `as [disambiguation_path]` in the macro attr, the
+/// disambiguation is used as is. If not, we infer the disambiguation path from the
+/// `foreign_impl_path` and the computed scope.
+fn compute_disambiguation_path(
+	disambiguation_path: Option<Path>,
+	foreign_impl: ItemImpl,
+	default_impl_path: Path,
+) -> Result<Path> {
+	match (disambiguation_path, foreign_impl.clone().trait_) {
+		(Some(disambiguation_path), _) => Ok(disambiguation_path),
+		(None, Some((_, foreign_impl_path, _))) =>
+			if default_impl_path.segments.len() > 1 {
+				let scope = default_impl_path.segments.first();
+				Ok(parse_quote!(#scope :: #foreign_impl_path))
+			} else {
+				Ok(foreign_impl_path)
+			},
+		_ => Err(syn::Error::new(
+			default_impl_path.span(),
+			"Impl statement must have a defined type being implemented \
+			for a defined type such as `impl A for B`",
+		)),
+	}
+}
+
 /// Internal implementation behind [`#[derive_impl(..)]`](`macro@crate::derive_impl`).
 ///
 /// `default_impl_path`: the module path of the external `impl` statement whose tokens we are
@@ -194,18 +221,11 @@ pub fn derive_impl(
 	let foreign_impl = parse2::<ItemImpl>(foreign_tokens)?;
 	let default_impl_path = parse2::<Path>(default_impl_path)?;
 
-	// have disambiguation_path default to the item being impl'd in the foreign impl if we
-	// don't specify an `as [disambiguation_path]` in the macro attr
-	let disambiguation_path = match (disambiguation_path, foreign_impl.clone().trait_) {
-		(Some(disambiguation_path), _) => disambiguation_path,
-		(None, Some((_, foreign_impl_path, _))) => foreign_impl_path,
-		_ =>
-			return Err(syn::Error::new(
-				foreign_impl.span(),
-				"Impl statement must have a defined type being implemented \
-				for a defined type such as `impl A for B`",
-			)),
-	};
+	let disambiguation_path = compute_disambiguation_path(
+		disambiguation_path,
+		foreign_impl.clone(),
+		default_impl_path.clone(),
+	)?;
 
 	// generate the combined impl
 	let combined_impl = combine_impls(
@@ -257,3 +277,27 @@ fn test_runtime_type_with_doc() {
 		}
 	}
 }
+
+#[test]
+fn test_disambugation_path() {
+	let foreign_impl: ItemImpl = parse_quote!(impl SomeTrait for SomeType {});
+	let default_impl_path: Path = parse_quote!(SomeScope::SomeType);
+
+	// disambiguation path is specified
+	let disambiguation_path = compute_disambiguation_path(
+		Some(parse_quote!(SomeScope::SomePath)),
+		foreign_impl.clone(),
+		default_impl_path.clone(),
+	);
+	assert_eq!(disambiguation_path.unwrap(), parse_quote!(SomeScope::SomePath));
+
+	// disambiguation path is not specified and the default_impl_path has more than one segment
+	let disambiguation_path =
+		compute_disambiguation_path(None, foreign_impl.clone(), default_impl_path.clone());
+	assert_eq!(disambiguation_path.unwrap(), parse_quote!(SomeScope::SomeTrait));
+
+	// disambiguation path is not specified and the default_impl_path has only one segment
+	let disambiguation_path =
+		compute_disambiguation_path(None, foreign_impl.clone(), parse_quote!(SomeType));
+	assert_eq!(disambiguation_path.unwrap(), parse_quote!(SomeTrait));
+}

substrate/frame/support/procedural/src/lib.rs

@@ -639,6 +639,11 @@ pub fn storage_alias(attributes: TokenStream, input: TokenStream) -> TokenStream
 /// default to `A` from the `impl A for B` part of the default impl. This is useful for scenarios
 /// where all of the relevant types are already in scope via `use` statements.
 ///
+/// In case the `default_impl_path` is scoped to a different module such as
+/// `some::path::TestTraitImpl`, the same scope is assumed for the `disambiguation_path`, i.e.
+/// `some::A`. This enables the use of `derive_impl` attribute without having to specify the
+/// `disambiguation_path` in most (if not all) uses within FRAME's context.
+///
 /// Conversely, the `default_impl_path` argument is required and cannot be omitted.
 ///
 /// Optionally, `no_aggregated_types` can be specified as follows: