create some vectors with initial capacities (#3701)

Signed-off-by: Benjamin Coenen <benjamin.coenen@hotmail.com>

substrate/core/client/db/src/lib.rs

@@ -1048,24 +1048,22 @@ impl<Block: BlockT<Hash=H256>> Backend<Block> {
 
 		operation.apply_aux(&mut transaction);
 
-		let mut meta_updates = Vec::new();
+		let mut meta_updates = Vec::with_capacity(operation.finalized_blocks.len());
 		let mut last_finalized_hash = self.blockchain.meta.read().finalized_hash;
 
-		if !operation.finalized_blocks.is_empty() {
-			for (block, justification) in operation.finalized_blocks {
-				let block_hash = self.blockchain.expect_block_hash_from_id(&block)?;
-				let block_header = self.blockchain.expect_header(BlockId::Hash(block_hash))?;
+		for (block, justification) in operation.finalized_blocks {
+			let block_hash = self.blockchain.expect_block_hash_from_id(&block)?;
+			let block_header = self.blockchain.expect_header(BlockId::Hash(block_hash))?;
 
-				meta_updates.push(self.finalize_block_with_transaction(
-					&mut transaction,
-					&block_hash,
-					&block_header,
-					Some(last_finalized_hash),
-					justification,
-					&mut finalization_displaced_leaves,
-				)?);
-				last_finalized_hash = block_hash;
-			}
+			meta_updates.push(self.finalize_block_with_transaction(
+				&mut transaction,
+				&block_hash,
+				&block_header,
+				Some(last_finalized_hash),
+				justification,
+				&mut finalization_displaced_leaves,
+			)?);
+			last_finalized_hash = block_hash;
 		}
 
 		let imported = if let Some(pending_block) = operation.pending_block {

substrate/core/phragmen/benches/phragmen.rs

@@ -61,8 +61,7 @@ fn do_phragmen(
 	// prefix to distinguish the validator and nominator account ranges.
 	let np = 10_000;
 
-	let mut candidates = vec![];
-	let mut voters = vec![];
+	let mut candidates = Vec::with_capacity(num_vals as usize);
 	let mut slashable_balance_of: BTreeMap<AccountId, Balance> = BTreeMap::new();
 
 	(1 ..= num_vals)
@@ -71,6 +70,7 @@ fn do_phragmen(
 			slashable_balance_of.insert(acc, STAKE + rr(10, 50));
 		});
 
+	let mut voters = Vec::with_capacity(num_noms as usize);
 	(np ..= (np + num_noms))
 		.for_each(|acc| {
 			let mut stashes_to_vote = candidates.clone();

substrate/core/sr-io/without_std.rs

@@ -737,8 +737,8 @@ impl StorageApi for () {
 	}
 
 	fn blake2_256_ordered_trie_root(input: Vec<Vec<u8>>) -> H256 {
-		let mut values = Vec::new();
-		let mut lengths = Vec::new();
+		let mut values = Vec::with_capacity(input.len());
+		let mut lengths = Vec::with_capacity(input.len());
 		for v in input {
 			values.extend_from_slice(&v);
 			lengths.push((v.len() as u32).to_le());