refactor: Enable clang-tidy bugprone-unused-return-value

This requires some small refactors to silence false-positive warnings.

Also, expand the bugprone-unused-return-value.CheckedReturnTypes option
to include util::Result, and util::Expected.

src/.clang-tidy

@@ -7,6 +7,7 @@ bugprone-string-constructor,
 bugprone-use-after-move,
 bugprone-lambda-function-name,
 bugprone-unhandled-self-assignment,
+bugprone-unused-return-value,
 misc-unused-using-decls,
 misc-no-recursion,
 modernize-deprecated-headers,
@@ -36,3 +37,5 @@ CheckOptions:
    value: false
  - key: bugprone-unhandled-self-assignment.WarnOnlyIfThisHasSuspiciousField
    value: false
+ - key: bugprone-unused-return-value.CheckedReturnTypes
+   value: '^::std::error_code$;^::std::error_condition$;^::std::errc$;^::std::expected$;^::util::Result$;^::util::Expected$'

src/bench/coin_selection.cpp

@@ -130,7 +130,7 @@ static void BnBExhaustion(benchmark::Bench& bench)
     bench.run([&] {
         // Benchmark
         CAmount target = make_hard_case(17, utxo_pool);
-        SelectCoinsBnB(utxo_pool, target, 0, MAX_STANDARD_TX_WEIGHT); // Should exhaust
+        [[maybe_unused]] auto _{SelectCoinsBnB(utxo_pool, target, /*cost_of_change=*/0, MAX_STANDARD_TX_WEIGHT)}; // Should exhaust
 
         // Cleanup
         utxo_pool.clear();

src/bitcoin-cli.cpp

@@ -899,8 +899,7 @@ static UniValue CallRPC(BaseRequestHandler* rh, const std::string& strMethod, co
             throw CConnectionFailed("uri-encode failed");
         }
     }
-    int r = evhttp_make_request(evcon.get(), req.get(), EVHTTP_REQ_POST, endpoint.c_str());
-    req.release(); // ownership moved to evcon in above call
+    int r = evhttp_make_request(evcon.get(), req.release(), EVHTTP_REQ_POST, endpoint.c_str());
     if (r != 0) {
         throw CConnectionFailed("send http request failed");
     }

src/httpserver.cpp

@@ -330,7 +330,7 @@ static void http_request_cb(struct evhttp_request* req, void* arg)
         std::unique_ptr<HTTPWorkItem> item(new HTTPWorkItem(std::move(hreq), path, i->handler));
         assert(g_work_queue);
         if (g_work_queue->Enqueue(item.get())) {
-            item.release(); /* if true, queue took ownership */
+            [[maybe_unused]] auto _{item.release()}; /* if true, queue took ownership */
         } else {
             LogWarning("Request rejected because http work queue depth exceeded, it can be increased with the -rpcworkqueue= setting");
             item->req->WriteReply(HTTP_SERVICE_UNAVAILABLE, "Work queue depth exceeded");

src/ipc/test/ipc_test.cpp

@@ -63,7 +63,9 @@ void IpcPipeTest()
         auto foo_client = std::make_unique<mp::ProxyClient<gen::FooInterface>>(
             connection_client->m_rpc_system->bootstrap(mp::ServerVatId().vat_id).castAs<gen::FooInterface>(),
             connection_client.get(), /* destroy_connection= */ true);
-        connection_client.release();
+        {
+            [[maybe_unused]] auto _{connection_client.release()};
+        }
         foo_promise.set_value(std::move(foo_client));
 
         auto connection_server = std::make_unique<mp::Connection>(loop, kj::mv(pipe.ends[1]), [&](mp::Connection& connection) {

src/wallet/test/fuzz/spend.cpp

@@ -98,7 +98,7 @@ FUZZ_TARGET(wallet_create_transaction, .init = initialize_setup)
 
     std::optional<unsigned int> change_pos;
     if (fuzzed_data_provider.ConsumeBool()) change_pos = fuzzed_data_provider.ConsumeIntegral<unsigned int>();
-    (void)CreateTransaction(*fuzzed_wallet.wallet, recipients, change_pos, coin_control);
+    [[maybe_unused]] auto _{CreateTransaction(*fuzzed_wallet.wallet, recipients, change_pos, coin_control)};
 }
 } // namespace
 } // namespace wallet