Add CustomDigestItem type for application-level digest items

cumulus/parachains/runtimes/bridge-hubs/common/Cargo.toml

@@ -11,6 +11,7 @@ codec = { package = "parity-scale-codec", version = "3.0.0", default-features =
 scale-info = { version = "2.10.0", default-features = false, features = ["derive"] }
 frame-support = { path = "../../../../../substrate/frame/support", default-features = false }
 sp-std = { path = "../../../../../substrate/primitives/std", default-features = false }
+sp-core = { path = "../../../../../substrate/primitives/core", default-features = false }
 sp-runtime = { path = "../../../../../substrate/primitives/runtime", default-features = false }
 cumulus-primitives-core = { path = "../../../../primitives/core", default-features = false }
 xcm = { package = "staging-xcm", path = "../../../../../polkadot/xcm", default-features = false}
@@ -24,6 +25,7 @@ std = [
 	"scale-info/std",
 	"frame-support/std",
 	"sp-std/std",
+	"sp-core/std",
 	"sp-runtime/std",
 	"cumulus-primitives-core/std",
 	"xcm/std",

cumulus/parachains/runtimes/bridge-hubs/common/src/digest_item.rs

@@ -0,0 +1,43 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//! Custom digest items
+
+use codec::{Decode, Encode};
+use sp_core::{RuntimeDebug, H256};
+use sp_runtime::generic::DigestItem;
+
+/// Custom header digest items, inserted as DigestItem::Other
+#[derive(Encode, Decode, Copy, Clone, Eq, PartialEq, RuntimeDebug)]
+pub enum CustomDigestItem {
+	/// Merkle root of outbound Snowbridge messages.
+	Snowbridge(H256),
+}
+
+/// Convert custom application digest item into a concrete digest item
+impl Into<DigestItem> for CustomDigestItem {
+	fn into(self) -> DigestItem {
+		match self {
+			// For snowbridge, we sidestep SCALE-encoding of `CustomDigestItem`, and insert the
+			// merkle root directly into the DigestItem::Other payload. This reduces complexity
+			// and gas costs on the Ethereum side.
+			//
+			// Other light clients can discriminate between custom digest items by checking the
+			// length of the encoded payload. If the length is greater than 32, then its a digest
+			// item inserted by some application other than Snowbridge.
+			CustomDigestItem::Snowbridge(merkle_root) =>
+				DigestItem::Other(merkle_root.to_fixed_bytes().into()),
+		}
+	}
+}

cumulus/parachains/runtimes/bridge-hubs/common/src/lib.rs

@@ -14,6 +14,8 @@
 // limitations under the License.
 #![cfg_attr(not(feature = "std"), no_std)]
 
+pub mod digest_item;
 pub mod message_queue;
 
+pub use digest_item::CustomDigestItem;
 pub use message_queue::{AggregateMessageOrigin, BridgeHubMessageRouter};