Fix runtime api impl detection by construct runtime

Construct runtime uses autoref-based specialization to fetch the
metadata about the implemented runtime apis. This is done to not fail to
compile when there are no runtime apis implemented. However, there was an issue
with detecting runtime apis when they were implemented in a different file. The problem
is solved by moving the trait implemented by `impl_runtime_apis!` to the metadata ir crate.

substrate/frame/support/procedural/src/construct_runtime/mod.rs

@@ -466,7 +466,6 @@ fn construct_runtime_final_expansion(
 		// Therefore, the `Deref` trait will resolve the `runtime_metadata` from `impl_runtime_apis!`
 		// when both macros are called; and will resolve an empty `runtime_metadata` when only the `construct_runtime!`
 		// is called.
-
 		#[doc(hidden)]
 		trait InternalConstructRuntime {
 			#[inline(always)]
@@ -477,6 +476,8 @@ fn construct_runtime_final_expansion(
 		#[doc(hidden)]
 		impl InternalConstructRuntime for &#name {}
 
+		use #scrate::__private::metadata_ir::InternalImplRuntimeApis;
+
 		#outer_event
 
 		#outer_error

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

@@ -80,34 +80,39 @@ sp_api::decl_runtime_apis! {
 	}
 }
 
-sp_api::impl_runtime_apis! {
-	impl self::Api<Block> for Runtime {
-		fn test(_data: u64) {
-			unimplemented!()
-		}
+// Module to emulate having the implementation in a different file.
+mod apis {
+	use super::{Block, BlockT, Runtime};
 
-		fn something_with_block(_: Block) -> Block {
-			unimplemented!()
-		}
+	sp_api::impl_runtime_apis! {
+		impl crate::Api<Block> for Runtime {
+			fn test(_data: u64) {
+				unimplemented!()
+			}
 
-		fn function_with_two_args(_: u64, _: Block) {
-			unimplemented!()
-		}
+			fn something_with_block(_: Block) -> Block {
+				unimplemented!()
+			}
 
-		fn same_name() {}
+			fn function_with_two_args(_: u64, _: Block) {
+				unimplemented!()
+			}
 
-		fn wild_card(_: u32) {}
-	}
+			fn same_name() {}
 
-	impl sp_api::Core<Block> for Runtime {
-		fn version() -> sp_version::RuntimeVersion {
-			unimplemented!()
-		}
-		fn execute_block(_: Block) {
-			unimplemented!()
+			fn wild_card(_: u32) {}
 		}
-		fn initialize_block(_: &<Block as BlockT>::Header) -> sp_runtime::ExtrinsicInclusionMode {
-			unimplemented!()
+
+		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!()
+			}
 		}
 	}
 }

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

@@ -298,18 +298,14 @@ pub fn generate_impl_runtime_metadata(impls: &[ItemImpl]) -> Result<TokenStream2
 	// Therefore, the `Deref` trait will resolve the `runtime_metadata` from `impl_runtime_apis!`
 	// when both macros are called; and will resolve an empty `runtime_metadata` when only the
 	// `construct_runtime!` is called.
-
 	Ok(quote!(
 		#crate_::frame_metadata_enabled! {
 			#[doc(hidden)]
-			trait InternalImplRuntimeApis {
-				#[inline(always)]
+			impl #crate_::metadata_ir::InternalImplRuntimeApis for #runtime_name {
 				fn runtime_metadata(&self) -> #crate_::vec::Vec<#crate_::metadata_ir::RuntimeApiMetadataIR> {
 					#crate_::vec![ #( #metadata, )* ]
 				}
 			}
-			#[doc(hidden)]
-			impl InternalImplRuntimeApis for #runtime_name {}
 		}
 	))
 }

substrate/primitives/metadata-ir/src/lib.rs

@@ -70,6 +70,16 @@ pub fn into_v14(metadata: MetadataIR) -> RuntimeMetadataPrefixed {
 	latest.into()
 }
 
+/// INTERNAL USE ONLY
+///
+/// Special trait that is used together with `InternalConstructRuntime` by `construct_runtime!` to
+/// fetch the runtime api metadata without exploding when there is no runtime api implementation
+/// available.
+#[doc(hidden)]
+pub trait InternalImplRuntimeApis {
+	fn runtime_metadata(&self) -> alloc::vec::Vec<RuntimeApiMetadataIR>;
+}
+
 #[cfg(test)]
 mod test {
 	use super::*;