Deny warnings on CI (#1372)

## Issue Addressed

Prevent CI from succeeding when there are warnings. Code can still be built and tested with warnings locally, but CI may fail during a Rust update (which is fine IMO).

## Proposed Changes

* Deny warnings for all stable compiler jobs on CI (excludes `cargo udeps`, which runs under nightly)
* Fix the warnings currently on `master` related to unnecessary `mem::replace`

.github/workflows/test-suite.yml

@@ -8,7 +8,9 @@ on:
       - trying
       - 'pr/*'
   pull_request:
-
+env:
+  # Deny warnings in CI
+  RUSTFLAGS: "-D warnings"
 jobs:
   cargo-fmt:
     name: cargo-fmt
@@ -149,3 +151,6 @@ jobs:
       run: cargo install cargo-udeps --locked
     - name: Run cargo udeps to identify unused crates in the dependency graph
       run: make udeps
+    env:
+      # Allow warnings on Nightly
+      RUSTFLAGS: ""

consensus/merkle_proof/src/lib.rs

@@ -81,7 +81,6 @@ impl MerkleTree {
     /// Push an element in the MerkleTree.
     /// MerkleTree and depth must be correct, as the algorithm expects valid data.
     pub fn push_leaf(&mut self, elem: H256, depth: usize) -> Result<(), MerkleTreeError> {
-        use std::mem;
         use MerkleTree::*;
 
         if depth == 0 {
@@ -91,7 +90,7 @@ impl MerkleTree {
         match self {
             Leaf(_) => return Err(MerkleTreeError::LeafReached),
             Zero(_) => {
-                mem::replace(self, MerkleTree::create(&[elem], depth));
+                *self = MerkleTree::create(&[elem], depth);
             }
             Node(ref mut hash, ref mut left, ref mut right) => {
                 let left: &mut MerkleTree = &mut *left;
@@ -107,11 +106,11 @@ impl MerkleTree {
                     }
                     // Both branches are zero, insert in left one
                     (Zero(_), Zero(_)) => {
-                        mem::replace(left, MerkleTree::create(&[elem], depth - 1));
+                        *left = MerkleTree::create(&[elem], depth - 1);
                     }
                     // Leaf on left branch and zero on right branch, insert on right side
                     (Leaf(_), Zero(_)) => {
-                        mem::replace(right, MerkleTree::create(&[elem], depth - 1));
+                        *right = MerkleTree::create(&[elem], depth - 1);
                     }
                     // Try inserting on the left node -> if it fails because it is full, insert in right side.
                     (Node(_, _, _), Zero(_)) => {
@@ -119,7 +118,7 @@ impl MerkleTree {
                             Ok(_) => (),
                             // Left node is full, insert in right node
                             Err(MerkleTreeError::MerkleTreeFull) => {
-                                mem::replace(right, MerkleTree::create(&[elem], depth - 1));
+                                *right = MerkleTree::create(&[elem], depth - 1);
                             }
                             Err(e) => return Err(e),
                         };

consensus/tree_hash/benches/benches.rs

@@ -5,7 +5,7 @@ use types::test_utils::{generate_deterministic_keypairs, TestingBeaconStateBuild
 use types::{BeaconState, EthSpec, Keypair, MainnetEthSpec, MinimalEthSpec};
 
 lazy_static! {
-    static ref KEYPAIRS: Vec<Keypair> = { generate_deterministic_keypairs(300_000) };
+    static ref KEYPAIRS: Vec<Keypair> = generate_deterministic_keypairs(300_000);
 }
 
 fn build_state<T: EthSpec>(validator_count: usize) -> BeaconState<T> {

consensus/types/src/beacon_state/tree_hash_cache.rs

@@ -211,7 +211,7 @@ impl ValidatorsListTreeHashCache {
             .list_cache
             .recalculate_merkle_root(&mut list_arena, leaves.into_iter())?;
 
-        std::mem::replace(&mut self.list_arena, list_arena);
+        self.list_arena = list_arena;
 
         Ok(mix_in_length(&list_root, validators.len()))
     }