aura: remove unneeded SlotDuration struct and rename digest -> digests (#4958)

* aura: remove unneeded SlotDuration struct and rename digest -> digests

* aura: add alias type for SlotDuration

* aura: fix tests

* Fix missing parameters in get_or_compute

* Use special function for fetching aura slot_duration

substrate/client/consensus/aura/src/digests.rs

@@ -0,0 +1,65 @@
+// Copyright 2018-2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Aura (Authority-Round) digests
+//!
+//! This implements the digests for AuRa, to allow the private
+//! `CompatibleDigestItem` trait to appear in public interfaces.
+
+use sp_core::Pair;
+use sp_consensus_aura::AURA_ENGINE_ID;
+use sp_runtime::generic::{DigestItem, OpaqueDigestItemId};
+use codec::{Encode, Codec};
+use std::fmt::Debug;
+
+type Signature<P> = <P as Pair>::Signature;
+
+/// A digest item which is usable with aura consensus.
+pub trait CompatibleDigestItem<P: Pair>: Sized {
+	/// Construct a digest item which contains a signature on the hash.
+	fn aura_seal(signature: Signature<P>) -> Self;
+
+	/// If this item is an Aura seal, return the signature.
+	fn as_aura_seal(&self) -> Option<Signature<P>>;
+
+	/// Construct a digest item which contains the slot number
+	fn aura_pre_digest(slot_num: u64) -> Self;
+
+	/// If this item is an AuRa pre-digest, return the slot number
+	fn as_aura_pre_digest(&self) -> Option<u64>;
+}
+
+impl<P, Hash> CompatibleDigestItem<P> for DigestItem<Hash> where
+	P: Pair,
+	Signature<P>: Codec,
+	Hash: Debug + Send + Sync + Eq + Clone + Codec + 'static
+{
+	fn aura_seal(signature: Signature<P>) -> Self {
+		DigestItem::Seal(AURA_ENGINE_ID, signature.encode())
+	}
+
+	fn as_aura_seal(&self) -> Option<Signature<P>> {
+		self.try_to(OpaqueDigestItemId::Seal(&AURA_ENGINE_ID))
+	}
+
+	fn aura_pre_digest(slot_num: u64) -> Self {
+		DigestItem::PreRuntime(AURA_ENGINE_ID, slot_num.encode())
+	}
+
+	fn as_aura_pre_digest(&self) -> Option<u64> {
+		self.try_to(OpaqueDigestItemId::PreRuntime(&AURA_ENGINE_ID))
+	}
+}