decl_runtime_apis! - check that a method without changed_in exists (#5635)

* `decl_runtime_apis!` - check that a method without `changed_in` exists

This adds another check to the macro that ensures that not only methods
with `changed_in` exists, but there are also the default methods exist.

* Update primitives/api/proc-macro/src/decl_runtime_apis.rs

Co-Authored-By: Nikolay Volf <nikvolf@gmail.com>

* Fix test

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

substrate/primitives/api/test/tests/ui/changed_in_no_default_method.rs

@@ -0,0 +1,19 @@
+use sp_runtime::traits::GetNodeBlockType;
+use substrate_test_runtime_client::runtime::Block;
+
+/// The declaration of the `Runtime` type and the implementation of the `GetNodeBlockType`
+/// trait are done by the `construct_runtime!` macro in a real runtime.
+struct Runtime {}
+impl GetNodeBlockType for Runtime {
+	type NodeBlock = Block;
+}
+
+sp_api::decl_runtime_apis! {
+	#[api_version(2)]
+	pub trait Api {
+		#[changed_in(2)]
+		fn test(data: u64);
+	}
+}
+
+fn main() {}

substrate/primitives/api/test/tests/ui/changed_in_no_default_method.stderr

@@ -0,0 +1,6 @@
+error: There is no 'default' method with this name (without `changed_in` attribute).
+The 'default' method is used to call into the latest implementation.
+  --> $DIR/changed_in_no_default_method.rs:15:6
+   |
+15 |         fn test(data: u64);
+   |            ^^^^