Fix #1536: do not require to construct a block for encoding it (#3813)

* Fix #1536: do not require to construct a block for encoding it

* Bump `impl_version`

* Improve `Block::encode_from` signature and rustdoc (from review by @bkchr)

substrate/core/client/src/client.rs

@@ -1059,10 +1059,10 @@ impl<B, E, Block, RA> Client<B, E, Block, RA> where
 					}
 				};
 
-				let encoded_block = <Block as BlockT>::new(
-					import_headers.pre().clone(),
-					body.unwrap_or_default(),
-				).encode();
+				let encoded_block = <Block as BlockT>::encode_from(
+					import_headers.pre(),
+					&body.unwrap_or_default()
+				);
 
 				let (_, storage_update, changes_update) = self.executor
 					.call_at_state::<_, _, NeverNativeValue, fn() -> _>(

substrate/core/sr-primitives/src/generic/block.rs

@@ -93,6 +93,9 @@ where
 	fn new(header: Self::Header, extrinsics: Vec<Self::Extrinsic>) -> Self {
 		Block { header, extrinsics }
 	}
+	fn encode_from(header: &Self::Header, extrinsics: &[Self::Extrinsic]) -> Vec<u8> {
+		(header, extrinsics).encode()
+	}
 }
 
 /// Abstraction over a substrate block and justification.

substrate/core/sr-primitives/src/testing.rs

@@ -259,6 +259,9 @@ impl<Xt: 'static + Codec + Sized + Send + Sync + Serialize + Clone + Eq + Debug
 	fn new(header: Self::Header, extrinsics: Vec<Self::Extrinsic>) -> Self {
 		Block { header, extrinsics }
 	}
+	fn encode_from(header: &Self::Header, extrinsics: &[Self::Extrinsic]) -> Vec<u8> {
+		(header, extrinsics).encode()
+	}
 }
 
 impl<'a, Xt> Deserialize<'a> for Block<Xt> where Block<Xt>: Decode {

substrate/core/sr-primitives/src/traits.rs

@@ -687,6 +687,8 @@ pub trait Block: Clone + Send + Sync + Codec + Eq + MaybeSerializeDebugButNotDes
 	fn hash(&self) -> Self::Hash {
 		<<Self::Header as Header>::Hashing as Hash>::hash_of(self.header())
 	}
+	/// Create an encoded block from the given `header` and `extrinsics` without requiring to create an instance.
+	fn encode_from(header: &Self::Header, extrinsics: &[Self::Extrinsic]) -> Vec<u8>;
 }
 
 /// Something that acts like an `Extrinsic`.

substrate/node/runtime/src/lib.rs

@@ -85,7 +85,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
 	spec_version: 175,
-	impl_version: 175,
+	impl_version: 176,
 	apis: RUNTIME_API_VERSIONS,
 };