refactor: Replace BResult with util::Result

Rename `BResult` class to `util::Result` and update the class interface to be
more compatible with `std::optional` and with a full-featured result class
implemented in https://github.com/bitcoin/bitcoin/pull/25665. Motivation for
this change is to update existing `BResult` usages now so they don't have to
change later when more features are added in #25665.

This change makes the following improvements originally implemented in #25665:

- More explicit API. Drops potentially misleading `BResult` constructor that
  treats any bilingual string argument as an error. Adds `util::Error`
  constructor so it is never ambiguous when a result is being assigned an error
  or non-error value.

- Better type compatibility. Supports `util::Result<bilingual_str>` return
  values to hold translated messages which are not errors.

- More standard and consistent API. `util::Result` supports most of the same
  operators and methods as `std::optional`. `BResult` had a less familiar
  interface with `HasRes`/`GetObj`/`ReleaseObj` methods. The Result/Res/Obj
  naming was also not internally consistent.

- Better code organization. Puts `src/util/` code in the `util::` namespace so
  naming reflects code organization and it is obvious where the class is coming
  from. Drops "B" from name because it is undocumented what it stands for
  (bilingual?)

- Has unit tests.

5 files changed, 16 insertions, 19 deletions

diff --git a/src/wallet/test/availablecoins_tests.cpp b/src/wallet/test/availablecoins_tests.cpp
index 01d24da981..3356128112 100644
--- a/src/wallet/test/availablecoins_tests.cpp
+++ b/src/wallet/test/availablecoins_tests.cpp
@@ -33,7 +33,7 @@ public:
             constexpr int RANDOM_CHANGE_POSITION = -1;
             auto res = CreateTransaction(*wallet, {recipient}, RANDOM_CHANGE_POSITION, dummy);
             BOOST_CHECK(res);
-            tx = res.GetObj().tx;
+            tx = res->tx;
         }
         wallet->CommitTransaction(tx, {}, {});
         CMutableTransaction blocktx;
@@ -57,7 +57,7 @@ public:
 BOOST_FIXTURE_TEST_CASE(BasicOutputTypesTest, AvailableCoinsTestingSetup)
 {
     CoinsResult available_coins;
-    BResult<CTxDestination> dest;
+    util::Result<CTxDestination> dest{util::Error{}};
     LOCK(wallet->cs_wallet);
 
     // Verify our wallet has one usable coinbase UTXO before starting
@@ -75,28 +75,28 @@ BOOST_FIXTURE_TEST_CASE(BasicOutputTypesTest, AvailableCoinsTestingSetup)
 
     // Bech32m
     dest = wallet->GetNewDestination(OutputType::BECH32M, "");
-    BOOST_ASSERT(dest.HasRes());
-    AddTx(CRecipient{{GetScriptForDestination(dest.GetObj())}, 1 * COIN, /*fSubtractFeeFromAmount=*/true});
+    BOOST_ASSERT(dest);
+    AddTx(CRecipient{{GetScriptForDestination(*dest)}, 1 * COIN, /*fSubtractFeeFromAmount=*/true});
     available_coins = AvailableCoins(*wallet);
     BOOST_CHECK_EQUAL(available_coins.bech32m.size(), 2U);
 
     // Bech32
     dest = wallet->GetNewDestination(OutputType::BECH32, "");
-    BOOST_ASSERT(dest.HasRes());
-    AddTx(CRecipient{{GetScriptForDestination(dest.GetObj())}, 2 * COIN, /*fSubtractFeeFromAmount=*/true});
+    BOOST_ASSERT(dest);
+    AddTx(CRecipient{{GetScriptForDestination(*dest)}, 2 * COIN, /*fSubtractFeeFromAmount=*/true});
     available_coins = AvailableCoins(*wallet);
     BOOST_CHECK_EQUAL(available_coins.bech32.size(), 2U);
 
     // P2SH-SEGWIT
     dest = wallet->GetNewDestination(OutputType::P2SH_SEGWIT, "");
-    AddTx(CRecipient{{GetScriptForDestination(dest.GetObj())}, 3 * COIN, /*fSubtractFeeFromAmount=*/true});
+    AddTx(CRecipient{{GetScriptForDestination(*dest)}, 3 * COIN, /*fSubtractFeeFromAmount=*/true});
     available_coins = AvailableCoins(*wallet);
     BOOST_CHECK_EQUAL(available_coins.P2SH_segwit.size(), 2U);
 
     // Legacy (P2PKH)
     dest = wallet->GetNewDestination(OutputType::LEGACY, "");
-    BOOST_ASSERT(dest.HasRes());
-    AddTx(CRecipient{{GetScriptForDestination(dest.GetObj())}, 4 * COIN, /*fSubtractFeeFromAmount=*/true});
+    BOOST_ASSERT(dest);
+    AddTx(CRecipient{{GetScriptForDestination(*dest)}, 4 * COIN, /*fSubtractFeeFromAmount=*/true});
     available_coins = AvailableCoins(*wallet);
     BOOST_CHECK_EQUAL(available_coins.legacy.size(), 2U);
 }
diff --git a/src/wallet/test/coinselector_tests.cpp b/src/wallet/test/coinselector_tests.cpp
index cd7fd3f4dd..a4a7216436 100644
--- a/src/wallet/test/coinselector_tests.cpp
+++ b/src/wallet/test/coinselector_tests.cpp
@@ -74,9 +74,7 @@ static void add_coin(CoinsResult& available_coins, CWallet& wallet, const CAmoun
     tx.vout.resize(nInput + 1);
     tx.vout[nInput].nValue = nValue;
     if (spendable) {
-        auto op_dest = wallet.GetNewDestination(OutputType::BECH32, "");
-        assert(op_dest.HasRes());
-        tx.vout[nInput].scriptPubKey = GetScriptForDestination(op_dest.GetObj());
+        tx.vout[nInput].scriptPubKey = GetScriptForDestination(*Assert(wallet.GetNewDestination(OutputType::BECH32, "")));
     }
     uint256 txid = tx.GetHash();
 
diff --git a/src/wallet/test/fuzz/notifications.cpp b/src/wallet/test/fuzz/notifications.cpp
index 5c173773e4..5e9cd4001b 100644
--- a/src/wallet/test/fuzz/notifications.cpp
+++ b/src/wallet/test/fuzz/notifications.cpp
@@ -69,14 +69,13 @@ struct FuzzedWallet {
     CScript GetScriptPubKey(FuzzedDataProvider& fuzzed_data_provider)
     {
         auto type{fuzzed_data_provider.PickValueInArray(OUTPUT_TYPES)};
-        BResult<CTxDestination> op_dest;
+        util::Result<CTxDestination> op_dest{util::Error{}};
         if (fuzzed_data_provider.ConsumeBool()) {
             op_dest = wallet->GetNewDestination(type, "");
         } else {
             op_dest = wallet->GetNewChangeDestination(type);
         }
-        assert(op_dest.HasRes());
-        return GetScriptForDestination(op_dest.GetObj());
+        return GetScriptForDestination(*Assert(op_dest));
     }
 };
 
diff --git a/src/wallet/test/spend_tests.cpp b/src/wallet/test/spend_tests.cpp
index 53c3b5d2ae..dc935a1a04 100644
--- a/src/wallet/test/spend_tests.cpp
+++ b/src/wallet/test/spend_tests.cpp
@@ -35,7 +35,7 @@ BOOST_FIXTURE_TEST_CASE(SubtractFee, TestChain100Setup)
         coin_control.m_change_type = OutputType::LEGACY;
         auto res = CreateTransaction(*wallet, {recipient}, RANDOM_CHANGE_POSITION, coin_control);
         BOOST_CHECK(res);
-        const auto& txr = res.GetObj();
+        const auto& txr = *res;
         BOOST_CHECK_EQUAL(txr.tx->vout.size(), 1);
         BOOST_CHECK_EQUAL(txr.tx->vout[0].nValue, recipient.nAmount + leftover_input_amount - txr.fee);
         BOOST_CHECK_GT(txr.fee, 0);
diff --git a/src/wallet/test/wallet_tests.cpp b/src/wallet/test/wallet_tests.cpp
index d3a760742d..1d2235be3d 100644
--- a/src/wallet/test/wallet_tests.cpp
+++ b/src/wallet/test/wallet_tests.cpp
@@ -536,7 +536,7 @@ public:
             constexpr int RANDOM_CHANGE_POSITION = -1;
             auto res = CreateTransaction(*wallet, {recipient}, RANDOM_CHANGE_POSITION, dummy);
             BOOST_CHECK(res);
-            tx = res.GetObj().tx;
+            tx = res->tx;
         }
         wallet->CommitTransaction(tx, {}, {});
         CMutableTransaction blocktx;
@@ -918,8 +918,8 @@ BOOST_FIXTURE_TEST_CASE(wallet_sync_tx_invalid_state_test, TestingSetup)
 
     // Add tx to wallet
     const auto& op_dest = wallet.GetNewDestination(OutputType::BECH32M, "");
-    BOOST_ASSERT(op_dest.HasRes());
-    const CTxDestination& dest = op_dest.GetObj();
+    BOOST_ASSERT(op_dest);
+    const CTxDestination& dest = *op_dest;
 
     CMutableTransaction mtx;
     mtx.vout.push_back({COIN, GetScriptForDestination(dest)});