cleanup(rust): Simplify code using closure capture in Rust 2021 edition (#6737)

* Simplify code using closure capture in Rust 2021 edition

* clippy: manual_next_back and unit_arg

* cargo fmt --all

zebra-consensus/src/transaction/tests.rs

@@ -866,8 +866,7 @@ async fn v5_transaction_is_rejected_before_nu5_activation() {
         let verifier = Verifier::new(network, state_service);
 
         let transaction = fake_v5_transactions_for_network(network, blocks)
-            .rev()
-            .next()
+            .next_back()
             .expect("At least one fake V5 transaction in the test vectors");
 
         let result = verifier
@@ -918,8 +917,7 @@ fn v5_transaction_is_accepted_after_nu5_activation_for_network(network: Network)
         let verifier = Verifier::new(network, state_service);
 
         let mut transaction = fake_v5_transactions_for_network(network, blocks)
-            .rev()
-            .next()
+            .next_back()
             .expect("At least one fake V5 transaction in the test vectors");
         if transaction
             .expiry_height()

zebra-network/src/peer_set/set.rs

@@ -504,15 +504,12 @@ where
     /// Checks if the minimum peer version has changed, and disconnects from outdated peers.
     fn disconnect_from_outdated_peers(&mut self) {
         if let Some(minimum_version) = self.minimum_peer_version.changed() {
-            // TODO: Remove when the code base migrates to Rust 2021 edition (#2709).
-            let preselected_p2c_peer = &mut self.preselected_p2c_peer;
-
             self.ready_services.retain(|address, peer| {
                 if peer.remote_version() >= minimum_version {
                     true
                 } else {
-                    if *preselected_p2c_peer == Some(*address) {
-                        *preselected_p2c_peer = None;
+                    if self.preselected_p2c_peer == Some(*address) {
+                        self.preselected_p2c_peer = None;
                     }
 
                     false

zebra-rpc/src/server/tests/vectors.rs

@@ -1,5 +1,8 @@
 //! Fixed test vectors for the RPC server.
 
+// These tests call functions which can take unit arguments if some features aren't enabled.
+#![allow(clippy::unit_arg)]
+
 use std::{
     net::{Ipv4Addr, SocketAddrV4},
     time::Duration,

zebra-state/src/service/non_finalized_state.rs

@@ -549,7 +549,7 @@ impl NonFinalizedState {
 
     /// Return the non-finalized portion of the current best chain.
     pub fn best_chain(&self) -> Option<&Arc<Chain>> {
-        self.chain_set.iter().rev().next()
+        self.chain_iter().next()
     }
 
     /// Return the number of chains.