Resolver-related code cleanup

This commit introduces a new exit code (4) for when no system resolver can be found, instead of just crashing.

It also introduces ResolverType, which is similar to code that used to be present. While the previous code passed all its unit tests, there was system-level code (obtaining the resolvers) being called from those unit tests, which isn't something unit tests are meant to be doing — there's no guarantee that they'll continue to work on systems with different network conifgurations. Because ResolverType is just a type, it can be used in tests. (Resolver has had its PartialEq impl stripped, and there's now a warning, to stop this from happening again.)

The refactor lets us remove some Clippy allowances, which is always nice.

Author: ogham

man/dog.1.md

@@ -232,6 +232,9 @@ EXIT STATUSES
 3
 : If there was a problem with the command-line arguments.
 
+4
+: If there was a problem obtaining the system nameserver information.
+
 
 AUTHOR
 ======

src/main.rs

@@ -16,10 +16,7 @@
 #![allow(clippy::module_name_repetitions)]
 #![allow(clippy::option_if_let_else)]
 #![allow(clippy::too_many_lines)]
-#![allow(clippy::unit_arg)]
-#![allow(clippy::unused_self)]
 #![allow(clippy::upper_case_acronyms)]
-#![allow(clippy::useless_let_if_seq)]
 #![allow(clippy::wildcard_imports)]
 
 #![deny(unsafe_code)]
@@ -109,7 +106,16 @@ fn run(Options { requests, format, measure_time }: Options) -> i32 {
     let timer = if measure_time { Some(Instant::now()) } else { None };
 
     let mut errored = false;
-    for (request_list, transport) in requests.generate() {
+
+    let request_tuples = match requests.generate() {
+        Ok(rt) => rt,
+        Err(e) => {
+            eprintln!("Unable to obtain resolver: {}", e);
+            return exits::SYSTEM_ERROR;
+        }
+    };
+
+    for (request_list, transport) in request_tuples {
         let request_list_len = request_list.len();
         for (i, request) in request_list.into_iter().enumerate() {
             let result = transport.send(&request);
@@ -189,4 +195,7 @@ mod exits {
 
     /// Exit code for when the command-line options are invalid.
     pub const OPTIONS_ERROR: i32 = 3;
+
+    /// Exit code for when the system network configuration could not be determined.
+    pub const SYSTEM_ERROR: i32 = 4;
 }