add the ability to declare multiple impl blocks

prdoc/pr_6795.prdoc

@@ -0,0 +1,21 @@
+# 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: ...
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Makes possible to split internals of `sp_api::impl_runtime_apis!` into multiple blocks.
+      to do this user is required to use `sp_api::impl_runtime_apis_ext` proc-macro attribute on a 
+      module that defines additional `impl` items that will be included in `impl_runtime_apis!` through `use <path>::*` declarations.
+
+
+crates: 
+- name: sp-api
+  bump: major
+- name: sp-api-proc-macro
+  bump: major
+- name: frame-support
+  bump: major
+

substrate/frame/support/test/tests/runtime_metadata.rs

@@ -82,7 +82,7 @@ sp_api::decl_runtime_apis! {
 
 // Module to emulate having the implementation in a different file.
 mod apis {
-	use super::{Block, BlockT, Runtime};
+	use super::{ext, Block, Runtime};
 
 	sp_api::impl_runtime_apis! {
 		impl crate::Api<Block> for Runtime {
@@ -103,16 +103,24 @@ mod apis {
 			fn wild_card(_: u32) {}
 		}
 
-		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) -> sp_runtime::ExtrinsicInclusionMode {
-				unimplemented!()
-			}
+		external_impls!{ext}
+	}
+}
+
+#[sp_api::impl_runtime_apis_ext]
+mod ext {
+
+	external_impls! {super, crate::apis}
+
+	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) -> sp_runtime::ExtrinsicInclusionMode {
+			unimplemented!()
 		}
 	}
 }