refactor: Avoid copying util::Result values

Copying util::Result values is less efficient than moving them because they
allocate memory and contain strings. Also this is needed to avoid compile
errors in #25722 which adds a
std::unique_ptr member to util::Result which implicity disables copying.

src/util/result.h

@@ -39,6 +39,10 @@ class Result
 
     std::variant<bilingual_str, T> m_variant;
 
+    //! Disallow copy constructor, require Result to be moved for efficiency and
+    //! freedom to change the implementation in the future.
+    Result(const Result&) = delete;
+
     //! Disallow operator= and require explicit Result::Update() calls to avoid
     //! confusion in the future when the Result class gains support for richer
     //! error reporting, and callers should have ability to set a new result
@@ -53,7 +57,6 @@ class Result
     Result() : m_variant{std::in_place_index_t<1>{}, std::monostate{}} {}  // constructor for void
     Result(T obj) : m_variant{std::in_place_index_t<1>{}, std::move(obj)} {}
     Result(Error error) : m_variant{std::in_place_index_t<0>{}, std::move(error.message)} {}
-    Result(const Result&) = default;
     Result(Result&&) = default;
     ~Result() = default;
 

src/wallet/spend.cpp

@@ -818,7 +818,7 @@ util::Result<SelectionResult> AutomaticCoinSelection(const CWallet& wallet, Coin
     // Coin Selection attempts to select inputs from a pool of eligible UTXOs to fund the
     // transaction at a target feerate. If an attempt fails, more attempts may be made using a more
     // permissive CoinEligibilityFilter.
-    util::Result<SelectionResult> res = [&] {
+    {
         // Place coins eligibility filters on a scope increasing order.
         std::vector<SelectionFilter> ordered_filters{
                 // If possible, fund the transaction with confirmed UTXOs only. Prefer at least six
@@ -866,9 +866,9 @@ util::Result<SelectionResult> AutomaticCoinSelection(const CWallet& wallet, Coin
         if (CAmount total_amount = available_coins.GetTotalAmount() - total_discarded < value_to_select) {
             // Special case, too-long-mempool cluster.
             if (total_amount + total_unconf_long_chain > value_to_select) {
-                return util::Result<SelectionResult>({_("Unconfirmed UTXOs are available, but spending them creates a chain of transactions that will be rejected by the mempool")});
+                return util::Error{_("Unconfirmed UTXOs are available, but spending them creates a chain of transactions that will be rejected by the mempool")};
             }
-            return util::Result<SelectionResult>(util::Error()); // General "Insufficient Funds"
+            return util::Error{}; // General "Insufficient Funds"
         }
 
         // Walk-through the filters until the solution gets found.
@@ -885,19 +885,17 @@ util::Result<SelectionResult> AutomaticCoinSelection(const CWallet& wallet, Coin
                 // If any specific error message appears here, then something particularly wrong might have happened.
                 // Save the error and continue the selection process. So if no solutions gets found, we can return
                 // the detailed error to the upper layers.
-                if (HasErrorMsg(res)) res_detailed_errors.emplace_back(res);
+                if (HasErrorMsg(res)) res_detailed_errors.emplace_back(std::move(res));
             }
         }
 
         // Return right away if we have a detailed error
-        if (!res_detailed_errors.empty()) return res_detailed_errors.front();
+        if (!res_detailed_errors.empty()) return std::move(res_detailed_errors.front());
 
 
         // General "Insufficient Funds"
-        return util::Result<SelectionResult>(util::Error());
-    }();
-
-    return res;
+        return util::Error{};
+    }
 }
 
 static bool IsCurrentForAntiFeeSniping(interfaces::Chain& chain, const uint256& block_hash)

src/wallet/test/fuzz/coinselection.cpp

@@ -291,7 +291,10 @@ FUZZ_TARGET(coinselection)
     }
 
     std::vector<COutput> utxos;
-    std::vector<util::Result<SelectionResult>> results{result_srd, result_knapsack, result_bnb};
+    std::vector<util::Result<SelectionResult>> results;
+    results.emplace_back(std::move(result_srd));
+    results.emplace_back(std::move(result_knapsack));
+    results.emplace_back(std::move(result_bnb));
     CAmount new_total_balance{CreateCoins(fuzzed_data_provider, utxos, coin_params, next_locktime)};
     if (new_total_balance > 0) {
         std::set<std::shared_ptr<COutput>> new_utxo_pool;