Use [u8; 4] for well known cache keys (#2152)

* Use [u8; 4] for well known cache keys

* Use type alias

substrate/core/consensus/common/src/block_import.rs

@@ -20,6 +20,7 @@ use runtime_primitives::traits::{Block as BlockT, DigestItemFor, Header as Heade
 use runtime_primitives::Justification;
 use std::borrow::Cow;
 use std::collections::HashMap;
+use crate::well_known_cache_keys;
 
 /// Block import result.
 #[derive(Debug, PartialEq, Eq)]
@@ -182,7 +183,7 @@ pub trait BlockImport<B: BlockT> {
 	fn import_block(
 		&self,
 		block: ImportBlock<B>,
-		cache: HashMap<Vec<u8>, Vec<u8>>,
+		cache: HashMap<well_known_cache_keys::Id, Vec<u8>>,
 	) -> Result<ImportResult, Self::Error>;
 }
 

substrate/core/consensus/common/src/import_queue.rs

@@ -570,7 +570,7 @@ pub fn import_single_block<B: BlockT, V: Verifier<B>>(
 
 	let mut cache = HashMap::new();
 	if let Some(authorities) = new_authorities {
-		cache.insert(crate::well_known_cache_keys::AUTHORITIES.to_vec(), authorities.encode());
+		cache.insert(crate::well_known_cache_keys::AUTHORITIES, authorities.encode());
 	}
 
 	import_error(import_handle.import_block(import_block, cache))

substrate/core/consensus/common/src/lib.rs

@@ -120,6 +120,9 @@ impl<T: SyncOracle> SyncOracle for Arc<T> {
 
 /// A list of all well known keys in the cache.
 pub mod well_known_cache_keys {
+	/// The type representing cache keys.
+	pub type Id = [u8; 4];
+
 	/// A list of authorities.
-	pub const AUTHORITIES: &'static [u8] = b"auth";
+	pub const AUTHORITIES: Id = *b"auth";
 }