sp-api: Set correct where bound in the generated code (#14252)

The where bound for the `create_metadata` function wasn't correct. This pr fixes this by using the
where bound declared at the type declaration augmented with the manual where bound.

substrate/Cargo.lock

@@ -10654,6 +10654,7 @@ dependencies = [
  "parity-scale-codec",
  "rustversion",
  "sc-block-builder",
+ "scale-info",
  "sp-api",
  "sp-consensus",
  "sp-core",

substrate/primitives/api/proc-macro/src/runtime_metadata.rs

@@ -162,15 +162,18 @@ pub fn generate_decl_runtime_metadata(decl: &ItemTrait) -> TokenStream2 {
 		ty.default = None;
 	}
 
-	let where_clause = where_clause
-		.iter()
-		.map(|ty| quote!(#ty: #crate_::scale_info::TypeInfo + 'static));
+	where_clause
+		.into_iter()
+		.map(|ty| parse_quote!(#ty: #crate_::scale_info::TypeInfo + 'static))
+		.for_each(|w| generics.make_where_clause().predicates.push(w));
+
+	let (impl_generics, _, where_clause) = generics.split_for_impl();
 
 	quote!(
 		#( #attrs )*
 		#[inline(always)]
-		pub fn runtime_metadata #generics () -> #crate_::metadata_ir::RuntimeApiMetadataIR
-		where #( #where_clause, )*
+		pub fn runtime_metadata #impl_generics () -> #crate_::metadata_ir::RuntimeApiMetadataIR
+		#where_clause
 		{
 			#crate_::metadata_ir::RuntimeApiMetadataIR {
 				name: #trait_name,

substrate/primitives/api/test/Cargo.toml

@@ -23,6 +23,7 @@ codec = { package = "parity-scale-codec", version = "3.2.2" }
 sp-state-machine = { version = "0.13.0", path = "../../state-machine" }
 trybuild = "1.0.74"
 rustversion = "1.0.6"
+scale-info = { version = "2.5.0", default-features = false, features = ["derive"] }
 
 [dev-dependencies]
 criterion = "0.4.0"

substrate/primitives/api/test/tests/ui/positive_cases/custom_where_bound.rs

@@ -0,0 +1,46 @@
+use codec::{Decode, Encode};
+use scale_info::TypeInfo;
+use sp_runtime::traits::{Block as BlockT, GetNodeBlockType};
+use substrate_test_runtime_client::runtime::Block;
+
+struct Runtime {}
+impl GetNodeBlockType for Runtime {
+	type NodeBlock = Block;
+}
+
+pub trait CustomTrait: Encode + Decode + TypeInfo {}
+
+#[derive(Encode, Decode, TypeInfo)]
+pub struct SomeImpl;
+impl CustomTrait for SomeImpl {}
+
+#[derive(Encode, Decode, TypeInfo)]
+pub struct SomeOtherType<C: CustomTrait>(C);
+
+sp_api::decl_runtime_apis! {
+	pub trait Api<A> where A: CustomTrait {
+		fn test() -> A;
+		fn test2() -> SomeOtherType<A>;
+	}
+}
+
+sp_api::impl_runtime_apis! {
+	impl self::Api<Block, SomeImpl> for Runtime {
+		fn test() -> SomeImpl { SomeImpl }
+		fn test2() -> SomeOtherType<SomeImpl> { SomeOtherType(SomeImpl) }
+	}
+
+	impl sp_api::Core<Block> for Runtime {
+		fn version() -> sp_version::RuntimeVersion {
+			unimplemented!()
+		}
+		fn execute_block(_: Block) {
+			unimplemented!()
+		}
+		fn initialize_block(_: &<Block as BlockT>::Header) {
+			unimplemented!()
+		}
+	}
+}
+
+fn main() {}