Merge bitcoin/bitcoin#29906: Disable util::Result copying and assignment

6a8b2befeab25e4e92d8e947a23e78014695e06c refactor: Avoid copying util::Result values (Ryan Ofsky)
834f65e82405bbed336f98996bc8cef366bbed0f refactor: Drop util::Result operator= (Ryan Ofsky)

Pull request description:

  This PR just contains the first two commits of #25665.

  It disables copying of `util::Result` objects because unnecessary copies are inefficient and not possible after #25665, which makes `util::Result` object move-only.

  It disables the assignment operator and replaces it with an `Update()` method, because #25665 adds more information to `util::Result` objects (warning and error messages and failure values) and having an assignment operator that overwrites data instead of merging it would make it easy to accidentally erase existing information while trying to assign new information.

ACKs for top commit:
  stickies-v:
    re-ACK 6a8b2befeab25e4e92d8e947a23e78014695e06c
  achow101:
    ACK 6a8b2befeab25e4e92d8e947a23e78014695e06c
  furszy:
    re-ACK https://github.com/bitcoin/bitcoin/commit/6a8b2befeab25e4e92d8e947a23e78014695e06c

Tree-SHA512: 3f21af9031d50d6c68cca69133de03080f69b1ddcf8b140bdeb762069f14645209b2586037236d15b6ebd8973af0fbefd7e83144aeb7b84078a4cb4df812f984

3 files changed, 8 insertions, 9 deletions

diff --git a/src/wallet/test/fuzz/coinselection.cpp b/src/wallet/test/fuzz/coinselection.cpp
index 297432de9e..331590df7f 100644
--- a/src/wallet/test/fuzz/coinselection.cpp
+++ b/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;
diff --git a/src/wallet/test/fuzz/notifications.cpp b/src/wallet/test/fuzz/notifications.cpp
index 9a515828fe..792079e6c6 100644
--- a/src/wallet/test/fuzz/notifications.cpp
+++ b/src/wallet/test/fuzz/notifications.cpp
@@ -106,13 +106,11 @@ struct FuzzedWallet {
     CTxDestination GetDestination(FuzzedDataProvider& fuzzed_data_provider)
     {
         auto type{fuzzed_data_provider.PickValueInArray(OUTPUT_TYPES)};
-        util::Result<CTxDestination> op_dest{util::Error{}};
         if (fuzzed_data_provider.ConsumeBool()) {
-            op_dest = wallet->GetNewDestination(type, "");
+            return *Assert(wallet->GetNewDestination(type, ""));
         } else {
-            op_dest = wallet->GetNewChangeDestination(type);
+            return *Assert(wallet->GetNewChangeDestination(type));
         }
-        return *Assert(op_dest);
     }
     CScript GetScriptPubKey(FuzzedDataProvider& fuzzed_data_provider) { return GetScriptForDestination(GetDestination(fuzzed_data_provider)); }
     void FundTx(FuzzedDataProvider& fuzzed_data_provider, CMutableTransaction tx)
diff --git a/src/wallet/test/spend_tests.cpp b/src/wallet/test/spend_tests.cpp
index 3509bc116f..963c0f838b 100644
--- a/src/wallet/test/spend_tests.cpp
+++ b/src/wallet/test/spend_tests.cpp
@@ -97,13 +97,11 @@ BOOST_FIXTURE_TEST_CASE(wallet_duplicated_preset_inputs_test, TestChain100Setup)
     // so that the recipient's amount is no longer equal to the user's selected target of 299 BTC.
 
     // First case, use 'subtract_fee_from_outputs=true'
-    util::Result<CreatedTransactionResult> res_tx = CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control);
-    BOOST_CHECK(!res_tx.has_value());
+    BOOST_CHECK(!CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control));
 
     // Second case, don't use 'subtract_fee_from_outputs'.
     recipients[0].fSubtractFeeFromAmount = false;
-    res_tx = CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control);
-    BOOST_CHECK(!res_tx.has_value());
+    BOOST_CHECK(!CreateTransaction(*wallet, recipients, /*change_pos=*/std::nullopt, coin_control));
 }
 
 BOOST_AUTO_TEST_SUITE_END()